Genetic Therapies for Alzheimer’s Disease: A Scoping Review

Abstract

Effective, disease modifying therapies for Alzheimer’s disease (AD) remain a quandary, following a panoply of expensive failures in human clinical trials. Given the stagnation in therapeutics, alternative approaches are needed. Recent successes of genetic therapies in other neurodegenerative diseases may highlight the way forward. This scoping review explores suggested targets of genetic therapy in AD, with a focus on vector-based approaches in pre-clinical and clinical trials. Putative targets of genetic therapies tested in pre-clinical trials include amyloid pathway intermediates and enzymes modulation, tau protein downregulation, APOE4 downregulation and APOE2 upregulation, neurotrophin expression (nerve growth factor (NGF) and brain-derived neurotrophic factor), and inflammatory cytokine alteration, among several other approaches. There have been three completed human clinical trials for genetic therapy in AD patients, all of which upregulated NGF in AD patients, showing some mixed evidence of benefit. Several impediments remain to be surpassed before genetic therapies can be successfully applied to AD, including the challenge of delivering monogenic genetic therapies for complex polygenic disorders, risks in the dominant delivery method (intracranial injection), stability of genetic therapies in vivo, poor translatability of pre-clinical AD models, and the expense of genetic therapy production. Genetic therapies represent an exciting opportunity within the world of AD therapeutics, but clinical applications likely remain a long term, rather than short term, possibility.

Keywords

Alzheimer’s disease amyloid dementia gene genetic therapy review tau therapy

INTRODUCTION

Alzheimer’s disease (AD) is the leading cause of dementia and is considered one of the foremost public health challenges in the world, affecting over 44 million individuals globally in 2020 [1, 2]. Currently available therapies focus on symptom management and include acetylcholinesterase inhibitors (donepezil, galantamine, rivastigmine) and an NMDA receptor antagonist (memantine). These pharmacological agents have been shown to only delay progression of cognitive decline for 3–6 months in mild-moderate AD [3]. A considerable number of disease modifying pharmacological and biological therapies have shown promise in pre-clinical models only to fail at great expense in human trials [4]. Several decades of trials using passive [5] and active [6] immunization against amyloid and tau targets have been shown to engage the target in the brain but have largely failed to show cognitive benefit (e.g., solanezumab [7], gantenerumab [8], bapineuzumab [9], gosuranemab [10], semorinemab [11], and CAD106 [12]). The recent approval of aducanumab by the Federal Drug Administration (FDA) has kept the hope of the potential benefit of this strategy alive, although convincing clinical benefit of aducanumab remains to be demonstrated [13, 14].

Considering the stagnation in therapeutics, more diverse approaches to AD therapy are required, and recent successes of genetic therapies in other neuro-degenerative diseases may pave a way forward. The first genetic therapy for a neurological disease approved by the FDA was nusinersen for spino-muscular atrophy, an antisense oligonucleotide (ASO), given intrathecally monthly from birth [15]. This was followed by approvals for therapy for Duchenne muscular dystrophy and familial amyloid polyneuropathy [16]. There have been a number of human trials examining application of ASOs for Huntington’s disease, with some showing promise [17] and others failing recently in trials [18].

This review will explore the suggested targets of genetic therapy in AD, the preclinical and clinical trials to date, and examine the key hurdles that remain to be surpassed. This review focuses on animal and human trials rather than cell-based models of AD. There are two main modalities of gene therapy: vector based and genetically modified cell replacement. Cell replacement therapies are comprehensively reviewed elsewhere [19] and thus this review will focus on vector based therapies.

WHAT IS A GENETIC THERAPY?

Clinically, genetic therapies are defined as therapeutic interventions that direct genetic material (DNA or RNA) to an individual’s cells to rectify defective genes or gene products [20]. In vivo delivery of genetic material via a viral vector or nanoparticles and ex-vivo genetically altered stem cells are the two central interventions for gene therapy [21]. There are a number of different approaches to gene therapies (see Table 1) that fall into the categories of either inserting/deleting genes themselves [22] or modifying their expression [23, 24].

Table 1

Approaches to gene therapeutics in AD

Approach	Mechanism	Gene effect location	Approved for human therapeutics
CRISPR/Cas9	Insertion and/or deletion of genes	Gene editing	No
Direct Gene Delivery	Expression of alternative gene	Gene addition	Yes
Antisense oligonucleotides	Gene suppression or splicing regulation	Gene expression	Yes
Interfering RNA (e.g., siRNA)	Gene suppression	Gene expression	Yes
Engineered zinc fingers	Transcriptional activation	Gene expression	No
Spliceosome-mediated RNA trans-splicing (SMaRT)	Gene suppression or splicing regulation	Gene expression	No
Micro-RNA regulation	Gene suppression or splicing regulation	Gene expression	No
Antagomirs	miRNA suppression/ upregulation of miR targets/Splicing regulation	Gene expression	No

CRISPR, Clustered Regularly Interspaced Short Palindromic Repeats; siRNA, small interfering RNA; miRNA, microRNA. Table adapted from Loera Valencia et al. (2018) [19].

Fig. 1

Summary of vector types and delivery methods applied in genetic therapies. AAV, adeno associated virus; HSV, herpes simplex virus; MLV, Moloney leukemia virus; iPSCs, induced pluripotent stem cells; i, induced.

VECTORS AND DELIVERY METHODS

The most common vector in pre-clinical trials of AD model mice has been adeno-associated virus (AAV), a single-strand DNA parvovirus. It has the advantages of low immunogenicity, low pathogenicity, and ability to infect a wide range of cells (both dividing and non-dividing). The key issues with AAV is that it has a small packaging capacity and may have off-target effects, given the range of cells it can infect [25]. The Lentivirus vector, a ssRNA HIV-like retrovirus, has also been frequently used. Rather than remaining episomal, genetic material is incorporated into the host cell genome, meaning that the therapy lasts longer but possesses the potential for oncogenicity [26]. The final commonly used viral vector in preclinical AD models is herpes simplex virus (HSV), a double strand DNA herpesvirus. HSV has the benefits of being targeted to neurons and having a large packaging capacity but has high immunogenicity and is more likely to stimulate an antibody response [27]. In saying this, all viral vectors (Table 2) face the challenge of neutralizing antibodies [28]. This is partly why the dominant delivery method in preclinical AD trials continues to be direct stereotactic injection into the CNS, where there is diminished immune patrol and a lower likelihood of an antibody response [28]. Given there has been at least one fatality caused by direct CNS stereotactic injection of gene therapies [29] into AD patients, it is unlikely to become a viable, scalable, human delivery method, representing a considerable conundrum for the use of viral vectors.

Table 2

Summary of key viral vectors used for AD gene therapy

Vector type	Virus type	Immunogenicity	Tropism	Maximum	Transgene location
gene size
AAV	Parvovirus (ssDNA)	Low	Dividing and non-dividing cells	4.5 kB	Episomes
Adenovirus	Adenovirus (dsDNA)	High	Dividing and non-dividing cells	7 kB	Episomes
HSV	Herpesvirus (dsDNA)	High	High in Neurons	50 kB	Episomes
Lentivirus	Retrovirus (ssRNA)	Low	Dividing and non-dividing cells	10 kB	Integration into chromosomes
MLV	Alphavirus (ssRNA)	Low	Dividing cells only	7 kB	Integration into chromosomes

AAV, adeno-associated virus; HSV, herpes simplex virus; MLV, Moloney leukemia virus; ss, single strand; ds, double strand.

Nanoparticles appear increasingly promising as vehicles for clinical gene therapy given their customizable size, shape, targeting and biological acti-vity [30]. No nanoparticle vectored AD gene therapy has been trialed in humans yet, but liposomal and polymeric nanoparticles have been attempted in preclinical models [31, 32]. They have demonstrated an ability to pass through the blood-brain barrier (BBB) and target subtypes of CNS cells, meaning they can be administered intravenously. Some challenges in utilizing nanoparticle approaches include stability of nanoparticles, degradation while in circulation, endocytosis by target cells and delivery efficiency [33].

GENE THERAPY TARGETS IN ALZHEIMER’S DISEASE

The following section summarizes results of pre-clinical and clinical trials examining putative genetic therapy targets for AD. A summary table of these studies can be found in the Supplementary Material.

Amyloid pathway

Aβ/APP

Increases in extracellular soluble Aβ oligomers and insoluble Aβ plaques are the pathological hallmarks of both familial and sporadic AD, and have long been thought to play a critical role in pathogenesis [34]. Over the last few decades there have been a number of immunotherapy clinical trials aimed at reducing amyloid load. While some of them effectively diminished cerebral amyloid burden [35], none of them ameliorated patient clinical symptoms, with the possible exception of aducanumab [35]. These multiple failures have variously been attributed to poor cohort design, intervention at too late a stage, and poor passage through the BBB, although some have argued that these failures point away from the amyloid hypothesis of AD.

As a way of addressing these issues, Elmer et al. (2020) [36] performed intrahippocampal injections of an AAV vector encoding a transcript for anti-Aβ IgG into 2-month-old APP mutation mice, just prior to the time of Aβ plaque deposition in this model. They found that CNS expression of anti-Aβ IgG was maintained at levels higher than peak concentration achieved with IV injection and that it resulted in reduced cortical and hippocampal Aβ load. Similarly, in an experiment using direct hippocampal injection of a HSV vector containing a short hairpin RNA that reduced translation of APP RNA, there was a considerable reduction in Aβ accumulation [37]. While reduction in excessive accumulation of AβPP or Aβ may be helpful, pre-clinical attempts to completely knockdown the APP gene have generated pathological phenotypes including locomotor and behavioral dysfunction in mice [38]. Indeed, Fol et al. (2016) [39] found that injecting an AAV2 containing the APPs α gene (a subcomponent of APP) into the hippocampus of APP/PS1ΔE9 AD model mice resulted in a recovery of dendritic spine density deficits, improvements in spatial working memory and reduction in soluble and insoluble Aβ levels. Another approach used by Park et al. (2021) [40] has been to inject AAV encoding Aβ variants (F20P and F19D/L34P) that inhibit Aβ aggregation and reduce oligomer toxicity. They found that after intraventricular injection, mice expressing F20P, but not F19D/L34P, showed decreased Aβ levels, plaque burden, and plaque-associated neuroinflammation[40].

Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1)

BACE1 cleaves Aβ at its N-terminal end from AβPP and works in concert with gamma secretase to produce Aβ_40–42 peptides. Singer et al. (2005) [41] performed one of the first pre-clinical trials of BACE1 suppression in which they injected a Lentivirus expressing BACE1 suppressing siRNA into the hippocampi of 10-month-old APP mice. There was significantly diminished amyloid load and neurodegeneration as well as improvements in behavioral deficits. In the last three years, there has been a number of interesting in vivo demonstrations of BACE1 suppression using intravenously (IV) administered nanocomplexes. Wang et al. (2018) [42] used IV delivery of a PEGylated acylate polymer nanocomplex to deliver BACE1 suppressing siRNA to 8-month-old APP/PS1 mice. The nanocomplex was modified with both the Cingulin peptide, which allows for BBB penetration, and the Tet1 peptide, which facilitates neuron-specific binding. There was a reduction in amyloid plaques and p-tau, improved hippocampal neurogenesis, and better spatial working memory performance in transgenic mice, returning to the level of the wild-type controls. These results have been corroborated by other studies using IV injections of BACE1 suppressing gene therapies to similar effects [31, 43]. Park et al. (2019) [44] took a different approach and, using 6-month-old 5XFAD transgenic mice, stereotactically injected into the CA3 region amphiphilic nanocomplexes containing CRISPR-Cas9 constructs designed to delete BACE1 gene. They found there was a 70%decrease in BACE1 expression in the CA3 region and there was no genome wide elevation in mutation rates. Associative learning and spatial working memory performance were improved compared to controls, demonstrating in-principle that targeted deletion of a gene may hold therapeutic potential. However, similar to AβPP, BACE1 serves an adaptive function and complete knockdown of the gene causes hypomyelination and seizures in mice, indicating that its therapeutic suppression must be carried out with caution [45].

Aβ degrading enzymes: Neprilysin and endothelin converting enzyme (ECE)

Neprilysin is a membrane-bound metallo-endo-peptidase and a critical Aβ-degrading enzyme [46, 47]. One study found that knockdown of neprilysin in AD model mice doubles the degree of Aβ pathology [46]. Neprilysin levels decrease with age (in mice and humans) but have been found to be comparatively increased in brains of AD patients [48, 49], indicating a possible adaptive response to more rapidly degrade accumulating Aβ. Genetic approaches to upregulate neprilysin were first trialed by Marr et al. (2003) who injected a lentivirus encoding human neprilysin into the hippocampus of APP transgenic mice. They found that Aβ deposits were reduced by 50%and neurodegeneration was significantly diminished. Three similar trials [50 –52] using hippocampally injected AAV or lentivirus vectors have since corroborated these results, finding additionally that unilateral injection diminished Aβ levels bilaterally. Furthermore, they found that neprilysin upregulation causes Aβ reduction in young and old mice and that there were significant improvements on murine memory tests. Guan et al. (2009) [53] took a different approach and performed a lentiviral gene transfer of neprilysin to mouse allogenic hemopoetic stem cells in 8-month-old 3×Tg-AD mice. They then gave the mice a partial bone marrow transplant and later found that treated mice showed a 30%reduction in soluble brain Aβ peptide and 50–60%reduction in accumulation of Aβ plaques as well as recovery of previous spatial working memory deficits. This study highlights the importance of systemic Aβ peptide for AD progression and indicates that therapy may not need to penetrate the CNS. ECE is another enzyme critical to the degradation of Aβ [54] and, similar to the studies of neprilysin, it was found that after ECE gene delivery there were decreases in Aβ plaques around the CNS injection sites [55].

Tau

Hyperphosphorylated tau aggregates are a pathological hallmark of AD; they may precede amyloid pathology and are thought to be the dominant cause of AD neurodegeneration [56, 57]. Mutations in the MAPT gene have been associated with a range of neurodegenerative diseases but interestingly not with AD [58 –61]. Three studies [62 –64] have examined the use of CNS injected AAVs encoding components of anti-tau antibodies in tau mice models. They achieved reductions of p-tau (of up to 90%) and considerable slowing of neurodegenerative processes. DeVos et al. (2017) [65] examined the use of CNS injected ASOs in both tau model mice and non-human primates. They found that a 1-month long intracerebroventricular infusion of anti-tau ASOs in 6–9-month-old PS19 mice reversed p-tau pathology, prevented hippocampal volume loss and neurodegeneration and extended survival. Then, in 2–8-year-old Cynomologus monkeys, they gave either one or two doses of an ASO targeting tau via lumbar puncture. They demonstrated that anti-tau ASOs moved through the entire CNS and effectively downregulated tau mRNA and protein in the brain and spinal cord. The studies examining effects of genetic therapy reducing tau were all in tau transgenic mice, and given the lack of genetic association between MAPT and AD, it is unlikely these experiments effectively model a viable therapeutic approach for AD.

APOE

Apolipoprotein E (APOE) ɛ4 allele is the strongest genetic risk factor for AD [66], with one allele bringing a 2-3 fold increased risk and two alleles bringing a 12–15 fold risk as well as a reduction in age of onset [67]. The population frequencies of the predominant APOE alleles are ɛ2; 8.4%, ɛ3; 77.9%and ɛ4; 13.7%[68], with 58.5%of the AD population being at least heterozygous for APOE ɛ4 [69]. APOE is expressed in astrocytes and microglia and forms lipoprotein particles with cholesterol after it is lipidated by the ABCA1 transporter [70]. These APOE lipoprotein particles bind soluble Aβ, are taken up by cellular surface receptors (e.g., LRP1, LDLR, and HSPG [71]) and are degraded in lysosomes. APOE isoforms have different affinities to Aβ and to the aforementioned internalization receptors (ɛ2 > ɛ3 > ɛ4) [70]. Considering this, several research groups have trialed various forms of APOE ɛ2 insertion or APOE ɛ4 suppression [72]. Hu et al. (2015) [73] found in mice that after intracerebroventricular injection of AAV particles encoding APOE ɛ4 there was an increasingly poor lipidation of lipoprotein particles and an increase in endogenous Aβ levels, whereas injecting APOE ɛ2 had the reverse effects. Hyunh et al. (2017) [74] applied a different approach and, using APP/PS1-21 mice homozygous for APOE ɛ4, they gave intracerebroventricular injections of anti-APOE ɛ4 ASOs. They found that, when given at birth (prior to amyloid plaque development), this ASO treatment decreased Aβ plaque number by 50%, whereas when given at 6 weeks (around the start of amyloid plaque development), it had no effect on plaque number but did increase the plaque size. By contrast, elevation of APOE ɛ4 using an AAV vector [75] in mouse models of tauopathy (rTg4510 and PS19 lines) caused no change in tau pathology. An APOE gene therapy trial in African Green monkeys (n = 11) [76] determined that intracisternal injection was the least invasive, effective delivery method. This work is currently being translated into a Phase 1, open label, dose ranging trial. Having started in 2018, Lexeo Therapeutics (NCT03634007) [77] are examining the safety and maximum tolerable dose of intracisternally injected AAV10 encoding APOE ɛ2 in Aβ positive patients with MCI or dementia who are APOE ɛ4 homozygotes (n expected = 15). It is predicted to finish in January 2023. This trial represents the first iteration of a personalized genetic medicine approach to AD, that is treating APOE ɛ4 homozygotes with APOE ɛ2 replacement. It is precisely this approach of identifying and correcting specific monogenic abnormalities that have produced successes in genetic medicine in the past [15, 16] and researchers are hopeful that this will be what brings new therapeutics to AD in the future.

Neurotrophins

Neurotrophins are a group of structurally similar peptides that influence plasticity, synaptogenesis, differentiation, and longevity of neurons [78]. Studies examining the role of neurotrophins in AD have predominantly explored the role of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) in the cholinergic basal forebrain (CBF) and the hippocampus, and hence these have been the subjects of most of the pre-clinical and clinical studies.

NGF

CBF neurons require an ongoing supply of NGF for maintenance and survival through their lifespan. NGF is expressed by CBF neurons and are targeted to hippocampus and cerebral cortical neurons that express p75^NTR and TrkA [79]. In the hippocampus and cerebral cortex, NGF forms a complex with TrkA, supported by p75^NTR, and this NGF/TrkA construct is transferred back to the basal forebrain where it mediates signal transduction [80, 81]. Transgenic mice expressing anti-NGF antibodies manifest a more rapid age-dependent loss of CBF neurons [82,83, 82,83]. Thus, studies of genetic therapies in AD have attempted to increase NGF signaling using various techniques.

Nagahara et al. (2008) [84] using elderly rhesus monkeys (mean age = 23.9±1.8 years, ntreated = 3, ncontrols = 8) injected a lentivirus encoding NGF into the CBF region. They found there were no identified adverse effects, that NGF expression was maintained for at least 1 year, and that cholinergic p75-labeled neuron quantity and size in aged monkeys recovered to the level of young monkeys. NGF has been trialed in three completed clinical trials with mixed results. Tuszynski et al. (2005) [85] conducted a Phase 1 open label trial involving 8 patients with probable mild-moderate AD (mean age 67.2±2.6 years, range 54–76). They injected autologous fibroblasts modified to express NGF into the basal forebrain adjacent to nucleus basalis of Meynert and measured disease progression using the Alzheimer’s Disease Assessment Scale-Cognitive subcomponent (ADAS-C). After a mean follow-up of 22 months, there were no adverse effects of NGF, significant slowing in the rate of cognitive decline after compared to before the intervention, and a significant increase in cortical 18-fluorodeoxyglucose uptake (i.e., brain metabolic activity) on serial PET scans. Regrettably, one of the participants died of an intracranial hemorrhage following the injection, highlighting the considerable risk of such invasive procedures, particularly for the elderly population. Eriksdotter-Jönhagen et al. (2012) [86] performed a similar trial but instead used a modified human retinal epithelial cell as the vector for NGF and inserted them into the nucleus basalis of Meynert of 6 AD patients using a catheter like device over 12 months. They found that no patients suffered adverse events related to the therapy, and in 2 of 6 patients, there were positive findings in cognition (as measured by ADAS-C and Mini-Mental State Examination), EEG morphology, and nicotinic receptor binding. By contrast, Rafii et al. (2018) [87] sterotactically injected AAV2–NGF into the nucleus basalis of Meynert and found that there were no differences in cognitive outcomes (ADAS-C) (N_treated =25, N_controls = 24). However, postmortem analysis of a subset of these patients (n = 3) found that NGF expression had not reached the cholinergic neurons because of poor spread and ineffectual targeting. They argued that their study did not demonstrate that NGF was an ineffective therapy for AD [88]. Taken together, the studies do not illustrate clear therapeutic potential of NGF, but they do highlight the risks and operator dependency of the intracranial injection method. Thus, there have been recent attempts to move away from such delivery methods. Recent studies in APP/PS1 mice have illustrated that IV injections of nanoparticles encoding NGF can downregulate soluble Aβ peptides and increase CNS neuronal proliferation [32].

BDNF

BDNF is present throughout the brain but it is highly expressed in the cerebral cortex and hippocampus [89]. BDNF regulates cellular specification and synaptic plasticity, especially in dopaminergic and cholinergic neurons by acting on high-affinity receptor TrKB and its low-affinity receptor p75^NTR [90, 91]. BDNF has a role in long-term potentiation (LTP), with animal studies showing that TrkB overexpression augments LTP in the hippocampus [92]. Within AD, diminished cortical concentrations of BDNF has been correlated to higher Braak stages [93]. Further, BDNF expression has been shown to be protective against Aβ neurotoxicity in rat cells, in vivo and in vitro, and ameliorate Aβ-induced LTP impairment [96, 97].

One study found that in 6-month-old APP transgenic mice injected with a Lentivirus encoding the BDNF gene there was amelioration of spatial memory deficits and a supranormal response to contextual fear conditioning. This was in combination with diminished neurodegeneration and reduced cell atrophy without changes in amyloid load [98, 99]. The authors argued that BNDF represents a therapeutic approach independent of the amyloid cascade. Contrarily, Eremenko et al. (2019) [100] argue that the therapy may be pleiotropic and at least part of the benefit is derived from amyloid cascade alterations. Using 12-month-old 5XFAD mice, they injected BDNF expressing Aβ-specific CD4 T cells intracerebroventricularly. They found that increased BDNF levels reduced levels of beta-secretase 1 (BACE1) and improved amyloid pathology/inflammation, suggesting that BDNF, when targeted to Aβ, promotes its clearance.

The Tuszynski group [98] also found that that BDNF gene therapy reduced neuronal cell loss in rhesus monkeys with entorhinal lesions (14.6%in treated versus 45.9%in untreated monkeys) and, when given to aged mice, led to significant improvements in a visuospatial discrimination task. Recruitment for a three-year human clinical trial [101] that will consist of 24 participants (N_AD = 12, N_controls = 12) stereotactically injected with AAV2-BDNF began in 2021 with the hope that this therapy may ameliorate AD symptoms. Such treatment strategies that avoid directly treating pathology raise interesting possibilities of their use in improving healthy individuals rather than just mitigating disease.

INFLAMMATORY CYTOKINES

A number of recent genome wide studies have emphasized the importance of immune-inflammatory pathways in late-onset, spontaneous AD (e.g., TREM2, CD33) [102, 103]. Recent evidence has suggested that prior to amyloid or tau deposition, immune-inflammatory activity is manifested in astrocytosis, gliosis, and pro-inflammatory cytokine upregulation [104, 105]. Whether components of this inflammatory response are adaptive or harmful is still contested [106]. Regardless, inflammatory pathways seem to play a critical role in AD and thus targeted genetic therapies have been attempted in preclinical models, albeit with mixed results.

CD33

CD33 is an anti-inflammatory immune signaling protein involved in cell adhesion and endocytosis [107]. Griciuc et al. (2021) [108] used APP/PS1 mice and injected an AAV construct intracerebroventricularly encoding miRNA designed to downregulate CD33. They found there were significant decreases in CD33 mRNA, proinflammatory cytokines (e.g., Tlr4, Ccl2, and Tnfα), soluble Aβ_40 - 42 and insoluble Aβ plaque, with treatment at 2 months being more effective than at 8 months.

TREM2

Triggering receptor expressed on myeloid cell 2 (TREM2) expression is elevated in microglia surrounding amyloid plaques of APP mice [109]. In 7-month-old APPswe/PS1dE9 mice [110], after stereotactic injection of lentiviral particles encoding TREM2 into the cerebral cortex and hippocampus, there were reductions in Aβ deposition, neuroinflammation, and neuronal and synaptic losses as well as improvements in spatial working memory. The same intervention in 18-month-old APPswe/PS1dE9 mice produced no change in neuropathology or behavior, indicating that TREM2 upregulation is unlikely to mollify firmly established disease.

IL-2

IL-2 is expressed by antigen-stimulated T cells and induces T-cell proliferation, B-cell growth and antibody production amongst various other roles [111]. Alves et al. (2017) [112] took a different approach to most gene therapy deliveries, injecting an AAV8 encoding IL-2 intraperitoneally into APP/PS1 mice. They found that there was an increase in systemic and brain T cell expansion and activation as well as diminished Aβ_42/40 ratio, reduced amyloid plaque load and improved spatial working memory.

IL-4

IL-4 is secreted by CD4 + T cells, is broadly anti-inflammatory and is involved in Ig class switching recombination in B cells [113]. Two similar studies [114, 115] that involved injecting an AAV-IL4 construct into the hippocampi of 3-month-old APP + PS1 mice found that it resulted in diminished astro/microgliosis, Aβ deposition and one found improved behavioral outcomes. But contrarily Chakrabarty et al. (2012) [116], in an almost analogous experiment using AAV-IL4 in 4-month-old APP mice, found there was worsened Aβ deposition, increased microglial “M2 phenotype” and reduced soluble Aβ₄₀ uptake by microglia.

IL-10

IL-10 is another anti-inflammatory cytokine that suppresses the synthesis of a collection of pro-inflammatory cytokines including IFN-gamma, IL-2, IL-3, TNF, and GM-CSF [117]. As in studies in IL-4, there are several experiments in this area, that despite similar designs, have directly contrary results. Kiyota et al. (2012) [118] used 3-month-old APP/PS1 mice and found that hippocampal injection of AAV2/1-IL-10 resulted in sustained increases in CNS IL-10 expression, diminished astro/microgliosis, improved neurogenesis and spatial learning. By contrast, Chakrabarty et al. (2015) [105] injected AAV2/1-IL-10 into the cerebral ventricles of neonatal APP mice and found that upregulation of IL-10 caused elevated Aβ accumulation, increased APOE expression and impaired spatial working memory.

TNFα

Tumor necrosis factor-alpha (TNFα) is a pro-inflammatory cytokine expressed by a range of cells including macrophages, endothelial cells [119], neurons [120], and glia [121]. TNFα and its receptors are elevated in mouse models of AD [122] as well as human AD patients [123]. Two studies examining TNF-alpha genetic therapies in AD have also produced directly contradictory results. Janelsins et al. (2008) [124] used 2-month-old 3xTg-AD mice and injected an AAV vector encoding murine TNFα into the hippocampus. They found in treated mice there was increased Aβ, p-tau, and microglial activation. Chakrabarty et al. (2011) [125] used the same design, but used 4-month-old APP mice (rather than 2-month-olds) and they found rather in mice treated with TNFα there was a significant reduction in Aβ plaque burden.

The contradictory findings between studies of inflammatory cytokine gene therapy in AD may reflect small differences in timing, dosage, and delivery in the studies. These contradictions highlight that both excessive pro- and anti-inflammatory activity are likely to worsen AD pathology and thus careful titration of the dosage and inflammatory response is a critical, albeit complex, therapeutic priority. The evidence taken together supports the idea that modulating neuroinflammation is a promising therapeutic target but a better understanding of the inflammatory pathogenesis of AD is needed before its fruitful manipulation can be managed.

OPPORTUNITIES AND CHALLENGES AHEAD FOR GENETIC THERAPIES IN AD

Up to this point, successful genetic therapies have almost universally been applied to monogenic disorders with a relatively simple deficit to correct [15]. By contrast, AD is a complex multigenic disorder with considerable environmental influences [126]. In saying this, there seems to be final common pathways in all AD cases that present themselves as hopeful targets. Furthermore, given that genetic material (particularly compared to proteins) is compact, some groups have been trialing delivery of multiple therapies for AD in single vectors [31] with encouraging results. This multi-targeted approach to AD particularly makes sense given that excessive downregulation of common therapy targets (e.g., AβPP and BACE1) is dangerous and harmful, and thus a more titrated multi-pronged approach may produce best outcomes.

As discussed previously, all human clinical trials and most animal trials have used direct intracranial injection as the delivery method, partly because this is what is needed for viral vectors to reach their targets with appropriate concentrations and specificity. Not only is this dangerous in the elderly, it is also costly and unlikely to be scalable to any reasonable proportion of the AD population. Intravenously injected nanoparticles can be modified to pass through the BBB and be targeted to specific subpopulations of neurons. These techniques have been used in the last few years with great success in several pre-clinical trials and may pave the way away from viral vectors and intracranial injections. Viruses have other specific risks, including oncogenesis, that make them less favorable as a therapeutic vector [127]. In a trial of 20 patients with severe combined immuno-deficiency, when using a retroviral vector to insert a working gene, 3 patients developed leukemia secondary to insertional mutagenesis. Other high-profile deaths in clinical trials of genetic therapies have historically generated a general skepticism and fear of trialing and implementing them in the future.

Finally, the cost of genetic therapies is a considerable impediment to development. Zolgensma, the AAV9-based gene therapy for spinal muscular atrophy, in 2020 cost £1.79 m per dose [128]. Incredibly, this has been publicly funded in several developed countries, but funding would not be feasible if spino-muscular atrophy was not so rare. Even if the cost of genetic therapies for AD were a fraction of this, there would be no health system in the world that could afford such a therapy for all AD sufferers. Thus, development of an effective but costly genetic therapy could generate further ethical and clinical conundrums, raising questions of who gets treated, when, and why. Economies of scale are likely to reduce cost of genetic therapies, but it seems that further technological innovation is needed before such therapies become widely available.

CONCLUSION

Clinical trials for disease modifying AD treatments have had a sobering history, and genetic therapies are a promising avenue of treatment. Vector based genetic therapies targeting the amyloid pathway, tau, APOE, neurotrophins, and inflammatory cytokines have all shown exciting results in mouse models, but convincing evidence of their benefit is yet to be illustrated in human clinical trials. Critical challenges, including the risks of vectors and delivery methods, as well as the considerable cost of therapy and need for multiple drug targets, must be addressed before clinical applications can be realized.

Footnotes

ACKNOWLEDGMENTS

Dr. Matthew Lennon was funded in his studies by the John Monash Scholarship. Mr. Grant Rigney was funded in his studies by the Rhodes Scholarship.

Authors’ disclosures available online ().

The supplementary material is available in the electronic version of this article: .

References

GBD 2016 Dementia Collaborators (2019) Global, regional, and national burden of Alzheimer’s disease and other dementias, 1990-2016: A systematic analysis for the Global Burden of Disease Study 2016. Lancet Neurol 18, 88–106.

Fleming

, Zeisel

, Bennet

(2020) World Alzheimer Report 2020. Design, Dignity, Dementia: Dementia related design and the built environment. Volume I. Alzheimer ’s Disease International, London.

Yiannopoulou

, Papageorgiou

(2020) Current and future treatments in Alzheimer disease: An update. J Cent Nerv Syst Dis 12, 1179573520907397.

Huang

L-K

, Chao

S-P

, Hu

C-J

(2020) Clinical trials of new drugs for Alzheimer disease. J Biomed Sci 27, 18.

Loureiro

, Pais

, Stella

, Radanovic

, Teixeira

, Forlenza

, de Souza

(2020) Passive antiamyloid immunotherapy for Alzheimer’s disease. Curr Opin Psychiatry 33, 284–291.

Winblad

, Graf

, Riviere

M-E

, Andreasen

, Ryan

(2014) Active immunotherapy options for Alzheimer’s disease. Alzheimers Res Ther 6, 7.

Honig

, Vellas

, Woodward

, Boada

, Bullock

, Borrie

, Hager

, Andreasen

, Scarpini

, Liu-Seifert

, Case

, Dean

, Hake

, Sundell

, Poole Hoffmann

, Carlson

, Khanna

, Mintun

, DeMattos

, Selzler

, Siemers

(2018) Trial of solanezumab for mild dementia due to Alzheimer’s disease. N Engl J Med 378, 321–330.

Alzforum.org, Topline Result for First DIAN-TU Clinical Trial: Negative on Primary. https://www.alzforum.org/news/research-news/topline-result-first-dian-tu-clinical-trial-negative-primary.

Abushouk

, Elmaraezy

, Aglan

, Salama

, Fouda

, AlSafadi

(2017) Bapineuzumab for mild to moderate Alzheimer’s disease: A meta-analysis of randomized controlled trials. BMC Neurol 17, 1–13.

10.

Ratti

, Kong

, O’Gorman

, Rajagovindan

, Racine

, Graham

, Schrempp

, Viollet

, Jones

, Mirabile

, Walsh

, von Rosenstiel

, Shulman

(2020) Baseline characteristics from TANGO: Phase 2 study to evaluate gosuranemab (BIIB092) in patients with early Alzheimer’s disease. Alzheimers Dement 16, e044910.

11.

Mullard

(2020) Failure of first anti-tau antibody in Alzheimer disease highlights risks of history repeating. Nat Rev Drug Discov 20, 3–5.

12.

clinicaltrials.gov, A Study of CAD106 and CNP520 Versus Placebo in Participants at Risk for the Onset of Clinical Symptoms of Alzheimer’s Disease (GS1).

13.

Walsh

, Merrick

, Milne

, Brayne

(2021) Aducanumab for Alzheimer’s disease? BMJ 374, n1682.

14.

Schneider

(2020) A resurrection of aducanumab for Alzheimer’s disease. Lancet Neurol 19, 111–112.

15.

Finkel

, Mercuri

, Darras

, Connolly

, Kuntz

, Kirschner

, Chiriboga

, Saito

, Servais

, Tizzano

, Topaloglu

, Tulinius

, Montes

, Glanzman

, Bishop

, Zhong

, Gheuens

, Bennett

, Schneider

, Farwell

, De Vivo

(2017) Nusinersen versus sham control in infantile-onset spinal muscular atrophy. N Engl J Med 377, 1723–1732.

16.

Brenner

, Ludolph

, Weishaupt

(2020) Gene specific therapies - the next therapeutic milestone in neurology. Neurol Res Pract 2, 25.

17.

Business Wire, Triplet Therapeutics Presents Preclinical Data for TTX-3360, Announces Equity Pledge at the CHDI Foundation’s 16th Annual HD Therapeutics Conference, https://www.businesswire.com/news/home/20210427006117/en/Triplet-Therapeutics-Presents-Preclinical-Data-for-TTX-3360-Announces-Equity-Pledge-at-the-CHDI-Foundation%E2%80%99s-16th-Annual-HD-Therapeutics-Conference.

18.

Kwon

(2021) Failure of genetic therapies for Huntington’s devastates community. Nature 593, 180.

19.

Loera-Valencia

, Piras

, Ismail

MAM

, Manchanda

, Eyjolfsdottir

, Saido

, Johansson

, Eriksdotter

, Winblad

, Nilsson

(2018) Targeting Alzheimer’s disease with gene and cell therapies. J Intern Med 284, 2–36.

20.

Kumar

, Markusic

, Biswas

, High

, Herzog

(2016) Clinical development of gene therapy: Results and lessons from recent successes. Mol Ther Methods Clin Dev 3, 16034.

21.

Dunbar

, High

, Joung

, Kohn

, Ozawa

, Sadelain

(2018) Gene therapy comes of age.eaan. Science 359, 4672.

22.

Guttinger

(2018) Trust in science: CRISPR-Cas9 and the ban on human germline editing. Sci Eng Ethics 24, 1077–1096.

23.

Ura

, Okuda

, Shimada

(2014) Developments in viral vector-based vaccines. Vaccines 2, 624–641.

24.

Bulaklak

, Gersbach

(2020) The once and future gene therapy. Nat Commun 11, 5820.

25.

Ferreira

, Petry

, Salmon

(2014) Immune responses to AAV-vectors, the Glybera example from bench to bedside. Front Immunol 5, 82.

26.

Dropulić

, Hěrmánková

, Pitha

(1996) A conditionally replicating HIV-1 vector interferes with wild-type HIV-1 replication and spread. Proc Natl Acad Sci U S A 93, 11103–11108.

27.

Manservigi

, Argnani

, Marconi

(2010) HSV recombinant vectors for gene therapy. Open Virol J 4, 123–156.

28.

Gadani

, Walsh

, Lukens

, Kipnis

(2015) Dealing with danger in the CNS: The response of the immune system to injury. Neuron 87, 47–62.

29.

Nilsson

, Iwata

, Muramatsu ichi

, Tjernberg

, Winblad

, Saido

(2010) Gene therapy in Alzheimer’s disease - potential for disease modification. J Cell Mol Med 14, 741–757.

30.

Chen

, Guo

, Tian

, Chen

(2016) Production and clinical development of nanoparticles for gene delivery. Mol Ther Methods Clin Dev 3, 16023.

31.

, Yang

, Li

, Yuan

(2020) Brain-targeted co-delivery of β-amyloid converting enzyme 1 shRNA and epigallocatechin-3-gallate by multifunctional nanocarriers for Alzheimer’s disease treatment. IUBMB Life 72, 1819–1829.

32.

Rodrigues

BDS

, Kanekiyo

, Singh

(2020) Nerve growth factor gene delivery across the blood-brain barrier to reduce beta amyloid accumulation in AD mice. Mol Pharm 17, 2054–2063.

33.

Pensado

, Seijo

, Sanchez

(2014) Current strategies for DNA therapy based on lipid nanocarriers. Expert Opin Drug Deliv 11, 1721–1731.

34.

Winblad

, Amouyel

, Andrieu

, Ballard

, Brayne

, Brodaty

, Cedazo-Minguez

, Dubois

, Edvardsson

, Feldman

, Fratiglioni

, Frisoni

, Gauthier

, Georges

, Graff

, Iqbal

, Jessen

, Johansson

, Jönsson

, Kivipelto

, Knapp

, Mangialasche

, Melis

, Nordberg

, Rikkert

, Qiu

, Sakmar

, Scheltens

, Schneider

, Sperling

, Tjernberg

, Waldemar

, Wimo

, Zetterberg

(2016) Defeating Alzheimer’s disease and other dementias: A priority for European science and society. Lancet Neurol 15, 455–532.

35.

Knopman

, Jones

, Greicius

(2021) Failure to demonstrate efficacy of aducanumab: An analysis of the EMERGE and ENGAGE trials as reported by Biogen, December 2019. Alzheimers Dement 17, 696–701.

36.

Elmer

, Swanson

, Bangari

, Piepenhagen

, Roberts

, Taksir

, Guo

, Obinu

M-C

, Barneoud

, Ryan

, Zhang

, Pradier

, Yang

Z-Y

, Nabel

(2020) Gene delivery of a modified antibody to Aβ reduces progression of murine Alzheimer’s disease. PLoS One 14, e0226245.

37.

Hong

C-S

, Goins

, Goss

, Burton

, Glorioso

(2006) Herpes simplex virus RNAi and neprilysin gene transfer vectors reduce accumulation of Alzheimer’s disease-related amyloid-beta peptide in vivo. Gene Ther 13, 1068–1079.

38.

Zheng

, Jiang

, Trumbauer

, Sirinathsinghji

, Hopkins

, Smith

, Heavens

, Dawson

, Boyce

, Conner

, Stevens

, Slunt

, Sisoda

, Chen

, Van der Ploeg

(1995) beta-Amyloid precursor protein-deficient mice show reactive gliosis and decreased locomotor activity. Cell 81, 525–531.

39.

Fol

, Braudeau

, Ludewig

, Abel

, Weyer

, Roederer

J-P

, Brod

, Audrain

, Bemelmans

A-P

, Buchholz

, Korte

, Cartier

, Müller

(2016) Viral gene transfer of APPsα rescues synaptic failure in an Alzheimer’s disease mouse model. Acta Neuropathol 131, 247–266.

40.

Park

K-W

, Wood

, Li

, Taylor

, Oh

, Young

, Jankowsky

(2021) Gene therapy using Aβ variants for amyloid reduction. Mol Ther 29, 2294–2307.

41.

Singer

, Marr

, Rockenstein

, Crews

, Coufal

, Gage

, Verma

, Masliah

(2005) Targeting BACE1 with siRNAs ameliorates Alzheimer disease neuropathology in a transgenic model. Nat Neurosci 8, 1343–1349.

42.

Wang

, Zheng

, Guo

, Yang

, Pang

, Qian

, Lu

, Zhang

, Jiang

(2018) Systemic delivery of BACE1 siRNA through neuron-targeted nanocomplexes for treatment of Alzheimer’s disease. J Control Release 279, 220–233.

43.

Haroon

, Saba

, Boddedda

, Kumar

, Patel

, Gopal

(2019) Delivery of BACE1 siRNA mediated by TARBP-BTP fusion protein reduces β-amyloid deposits in a transgenic mouse model of Alzheimer’s disease. J Biosci 44, 1.

44.

Park

, Oh

, Shim

, Cho

, Chang

, Kim

, Baek

, Kim

, Shin

, Choi

, Yoo

, Kim

, Jun

, Lee

, Lengner

, Oh

Y-K

, Kim

(2019) In vivo neuronal gene editing via CRISPR-Cas9 amphiphilic nanocomplexes alleviates deficits in mouse models of Alzheimer’s disease. Nat Neurosci 22, 524–528.

45.

, Zhou

, He

, Yang

, Xiong

, Wong

, Wilson

, Yan

(2010) BACE1 deficiency causes altered neuronal activity and neurodegeneration. J Neurosci 30, 8819–8829.

46.

Iwata

, Tsubuki

, Takaki

, Shirotani

, Lu

, Gerard

, Hama

, Lee

, Saido

(2001) Metabolic regulation of brain Abeta by neprilysin. Science 292, 1550–1552.

47.

Takaki

, Iwata

, Tsubuki

, Taniguchi

, Toyoshima

, Lu

, Gerard

, Lee

, Shirotani

, Saido

(2000) Biochemical identification of the neutral endopeptidase family member responsible for the catabolism of amyloid beta peptide in the brain. J Biochem 128, 897–902.

48.

Yasojima

, Akiyama

, McGeer

(2001) Reduced neprilysin in high plaque areas of Alzheimer brain: A possible relationship to deficient degradation of beta-amyloid peptide. Neurosci Lett 297, 97–100.

49.

Akiyama

, Kondo

, Ikeda

, Kato

, McGeer

(2001) Immunohistochemical localization of neprilysin in the human cerebral cortex: Inverse association with vulnerability to amyloid beta-protein (Abeta) deposition. Brain Res 902, 277–281.

50.

Iwata

, Mizukami

, Shirotani

, Takaki

, Muramatsu

, Lu

, Gerard

, Ozawa

, Saido

(2004) Presynaptic localization of neprilysin contributes to efficient clearance of amyloid-beta peptide in mouse brain. J Neurosci 24, 991–998.

51.

Spencer

, Marr

, Rockenstein

, Crews

, Adame

, Potkar

, Patrick

, Gage

, Verma

, Masliah

(2008) Long-term neprilysin gene transfer is associated with reduced levels of intracellular Abeta and behavioral improvement in APP transgenic mice. BMC Neurosci 9, 109.

52.

Liu

, Studzinski

, Beckett

, Guan

, Hersh

, Murphy

, Klein

, Hersh

(2009) Expression of neprilysin in skeletal muscle reduces amyloid burden in a transgenic mouse model of Alzheimer disease. Mol Ther 17, 1381–1386.

53.

Guan

, Liu

, Daily

, Police

, Kim

M-H

, Oddo

, LaFerla

, Pauly

, Murphy

, Hersh

(2009) Peripherally expressed neprilysin reduces brain amyloid burden: A novel approach for treating Alzheimer’s disease. J Neurosci Res 87, 1462–1473.

54.

Eckman

, Reed

, Eckman

(2001) Degradation of the Alzheimer’s amyloid beta peptide by endothelin-converting enzyme. J Biol Chem 276, 24540–24548.

55.

Carty

, Nash

, Lee

, Mercer

, Gottschall

, Meyers

, Muzyczka

, Gordon

, Morgan

(2008) Adeno-associated viral (AAV) serotype 5 vector mediated gene delivery of endothelin-converting enzyme reduces Abeta deposits in APP+PS1 transgenic mice. Mol Ther 16, 1580–1586.

56.

KIDD

(1963) Paired helical filaments in electron microscopy of Alzheimer’s disease. Nature 197, 192–193.

57.

Kametani

, Hasegawa

(2018) Reconsideration of amyloid hypothesis and tau hypothesis in Alzheimer’s disease. Front Neurosci 12, 25.

58.

Iovino

, Pfisterer

, Holton

, Lashley

, Swingler

, Calo

, Treacy

, Revesz

, Parmar

, Goedert

, Muqit

MMK

, Spillantini

(2014) The novel MAPT mutation K298E: Mechanisms of mutant tau toxicity, brain pathology and tau expression in induced fibroblast-derived neurons. Acta Neuropathol 127, 283–295.

59.

Spillantini

, Murrell

, Goedert

, Farlow

, Klug

, Ghetti

(1998) Mutation in the tau gene in familial multiple system tauopathy with presenile dementia. Proc Natl Acad Sci U S A 95, 7737–7741.

60.

Spillantini

, Crowther

, Kamphorst

, Heutink

, van Swieten

(1998) Tau pathology in two Dutch families with mutations in the microtubule-binding region of tau. Am J Pathol 153, 1359–1363.

61.

Poorkaj

, Bird

, Wijsman

, Nemens

, Garruto

, Anderson

, Andreadis

, Wiederholt

, Raskind

, Schellenberg

(1998) Tau is a candidate gene for chromosome 17 frontotemporal dementia. Ann Neurol 43, 815–825.

62.

Liu

, Zhao

, Blackman

, Parmar

, Wong

, Woo

, Yu

, Chiuchiolo

, Sondhi

, Kaminsky

, Crystal

, Paul

(2016) Vectored intracerebral immunization with the anti-tau monoclonal antibody PHF1 markedly reduces tau pathology in mutant tau transgenic mice. J Neurosci 36, 12425–12435.

63.

Ising

, Gallardo

, Leyns

CEG

, Wong

, Jiang

, Stewart

, Koscal

, Roh

, Robinson

, Remolina Serrano

, Holtzman

(2017) AAV-mediated expression of anti-tau scFvs decreases tau accumulation in a mouse model of tauopathy. J Exp Med 214, 1227–1238.

64.

Goodwin

, Sinyavskaya

, Burg

, O’Neal

, Ceballos-Diaz

, Cruz

, Lewis

, Giasson

, Davies

, Golde

, Levites

(2021) Anti-tau scFvs targeted to the cytoplasm or secretory pathway variably modify pathology and neurodegenerative phenotypes. Mol Ther 29, 859–872.

65.

DeVos

, Miller

, Schoch

, Holmes

, Kebodeaux

, Wegener

, Chen

, Shen

, Tran

, Nichols

, Zanardi

, Kordasiewicz

, Swayze

, Bennett

, Diamond

, Miller

(2017) Tau reduction prevents neuronal loss and reverses pathological tau deposition and seeding in mice with tauopathy. Sci Transl Med 9, eaag0481.

66.

Kingwell

(2013) Alzheimer disease: Joining the dots between APOE ɛ4 and late-onset Alzheimer disease via integrative genomics. Nat Rev Neurol 9, 483.

67.

Corder

, Saunders

, Strittmatter

, Schmechel

, Gaskell

, Small

, Roses

, Haines

, Pericak-Vance

(1993) Gene dose of apolipoprotein E type 4 allele and the risk of Alzheimer’s disease in late onset families. Science 261, 921–923.

68.

Farrer

, Cupples

, Haines

, Hyman

, Kukull

, Mayeux

, Myers

, Pericak-Vance

, Risch

, van Duijn

(1997) Effects of age, sex, and ethnicity on the association between apolipoprotein E genotype and Alzheimer disease. A meta-analysis. APOE and Alzheimer Disease Meta Analysis Consortium. JAMA 278, 1349–1356.

69.

Ward

, Crean

, Mercaldi

, Collins

, Boyd

, Cook

, Arrighi

(2012) Prevalence of apolipoprotein E4 genotype and homozygotes (APOE e4/4) among patients diagnosed with Alzheimer’s disease: A systematic review and meta-analysis. Neuroepidemiology 38, 1–17.

70.

Liu

C-C

, Liu

C-C

, Kanekiyo

, Xu

, Bu

(2013) Apolipoprotein E and Alzheimer disease: Risk, mechanisms, and therapy. Nat Rev Neurol 9, 106–118.

71.

Kanekiyo

, Zhang

, Liu

C-C

, Zhang

, Bu

(2011) Heparan sulphate proteoglycan and the low-density lipoprotein receptor-related protein 1 constitute major pathways for neuronal amyloid-beta uptake. J Neurosci 31, 1644–1651.

72.

Hudry

, Dashkoff

, Roe

, Takeda

, Koffie

, Hashimoto

, Scheel

, Spires-Jones

, Arbel-Ornath

, Betensky

, Davidson

, Hyman

(2013) Gene transfer of human Apoe isoforms results in differential modulation of amyloid deposition and neurotoxicity in mouse brain. Sci Transl Med 5, 212ra161.

73.

, Liu

C-C

, Chen

X-F

, Zhang

Y-W

, Xu

, Bu

(2015) Opposing effects of viral mediated brain expression of apolipoprotein E2 (apoE2) and apoE4 on apoE lipidation and Aβ metabolism in apoE4-targeted replacement mice. Mol Neurodegener 10, 6.

74.

Huynh

T-P V

, Liao

, Francis

, Robinson

, Serrano

, Jiang

, Roh

, Finn

, Sullivan

, Esparza

, Stewart

, Mahan

, Ulrich

, Cole

, Holtzman

(2017) Age-dependent effects of apoE reduction using antisense oligonucleotides in a model of β-amyloidosis. Neuron 96, 1013–1023.e4.

75.

Koller

, Gonzalez De La Cruz

, Weinrich

, Williams

, Cruz

, Ryu

, Golde

, Sullivan

, Lewis

, Borchelt

, Chakrabarty

(2020) Intracerebral expression of AAV-APOE4 is not sufficient to alter tau burden in two distinct models of tauopathy. Mol Neurobiol 57, 1986–2001.

76.

Rosenberg

, Kaplitt

, De

, Chen

, Flagiello

, Salami

, Pey

, Zhao

, Ricart Arbona

, Monette

, Dyke

, Ballon

, Kaminsky

, Sondhi

, Petsko

, Paul

, Crystal

(2018) AAVrh.10-mediated APOE2 central nervous system gene therapy for APOE4-associated Alzheimer’s disease. Hum Gene Ther Clin Dev 29, 24–47.

77.

clinicaltrials.gov, Gene therapy for APOE4 homozygote of Alzheimer’s disease.

78.

Reichardt

(2006) Neurotrophin-regulated signalling pathways. Philos Trans R Soc Lond B Biol Sci 361, 1545–1564.

79.

Korsching

, Auburger

, Heumann

, Scott

, Thoenen

(1985) Levels of nerve growth factor and its mRNA in the central nervous system of the rat correlate with cholinergic innervation. EMBO J 4, 1389–1393.

80.

Kaplan

, Miller

(2004) Neurobiology: A move to sort life from death. Nature 427, 798–799.

81.

Mufson

, Kroin

, Sendera

, Sobreviela

(1999) Distribution and retrograde transport of trophic factors in the central nervous system: Functional implications for the treatment of neurodegenerative diseases. Prog Neurobiol 57, 451–484.

82.

Ruberti

, Capsoni

, Comparini

, Di Daniel

, Franzot

, Gonfloni

, Rossi

, Berardi

, Cattaneo

(2000) Phenotypic knockout of nerve growth factor in adult transgenic mice reveals severe deficits in basal forebrain cholinergic neurons, cell death in the spleen, and skeletal muscle dystrophy. J Neurosci 20, 2589–2601.

83.

Capsoni

, Ugolini

, Comparini

, Ruberti

, Berardi

, Cattaneo

(2000) Alzheimer-like neurodegeneration in aged antinerve growth factor transgenic mice. Proc Natl Acad Sci U S A 97, 6826–6831.

84.

Nagahara

, Bernot

, Moseanko

, Brignolo

, Blesch

, Conner

, Ramirez

, Gasmi

, Tuszynski

(2009) Long-term reversal of cholinergic neuronal decline in aged non-human primates by lentiviral NGF gene delivery. Exp Neurol 215, 153–159.

85.

Tuszynski

, Thal

, Pay

, Salmon

, U

, Bakay

, Patel

, Blesch

, Vahlsing

, Ho

, Tong

, Potkin

, Fallon

, Hansen

, Mufson

, Kordower

, Gall

, Conner

(2005) A phase 1 clinical trial of nerve growth factor gene therapy for Alzheimer disease. Nat Med 11, 551–555.

86.

Eriksdotter-Jönhagen

, Linderoth

, Lind

, Aladellie

, Almkvist

, Andreasen

, Blennow

, Bogdanovic

, Jelic

, Kadir

, Nordberg

, Sundström

, Wahlund

L-O

, Wall

, Wiberg

, Winblad

, Seiger

, Almqvist

, Wahlberg

(2012) Encapsulated cell biodelivery of nerve growth factor to the basal forebrain in patients with Alzheimer’s disease. Dement Geriatr Cogn Disord 33, 18–28.

87.

Rafii

, Tuszynski

, Thomas

, Barba

, Brewer

, Rissman

, Siffert

, Aisen

(2018) Adeno-associated viral vector (Serotype 2)-nerve growth factor for patients with Alzheimer disease: A randomized clinical trial. JAMA Neurol 75, 834–841.

88.

Castle

, Baltanás

, Kovacs

, Nagahara

, Barba

, Tuszynski

(2020) Postmortem analysis in a clinical trial of AAV2-NGF gene therapy for Alzheimer’s disease identifies a need for improved vector delivery. Hum Gene Ther 31, 415–422.

89.

Phillips

, Hains

, Laramee

, Rosenthal

, Winslow

(1990) Widespread expression of BDNF but not NT3 by target areas of basal forebrain cholinergic neurons. Science 250, 290–294.

90.

Ghosh

, Carnahan

, Greenberg

(1994) Requirement for BDNF in activity-dependent survival of cortical neurons. Science 263, 1618–1623.

91.

Hyman

, Hofer

, Barde

, Juhasz

, Yancopoulos

, Squinto

, Lindsay

(1991) BDNF is a neurotrophic factor for dopaminergic neurons of the substantia nigra. Nature 350, 230–232.

92.

Koponen

, Võikar

, Riekki

, Saarelainen

, Rauramaa

, Rauvala

, Taira

, Castrén

(2004) Transgenic mice overexpressing the full-length neurotrophin receptor trkB exhibit increased activation of the trkB-PLCgamma pathway, reduced anxiety, and facilitated learning. Mol Cell Neurosci 26, 166–181.

93.

Tanila

(2017) The role of BDNF in Alzheimer’s disease. Neurobiol Dis 97, 114–118.

94.

Kitiyanant

, Kitiyanant

, Svendsen

, Thangnipon

(2012) BDNF-, IGF-1- and GDNF-secreting human neural progenitor cells rescue amyloid β-induced toxicity in cultured rat septal neurons. Neurochem Res 37, 143–152.

95.

Zeng

, Zhao

, Xie

C-W

(2010) Neurotrophins enhance CaMKII activity and rescue amyloid-β-induced deficits in hippocampal synaptic plasticity. J Alzheimers Dis 21, 823–831.

96.

Arancibia

, Silhol

, Moulière

, Meffre

, Höllinger

, Maurice

, Tapia-Arancibia

(2008) Protective effect of BDNF against beta-amyloid induced neurotoxicity in vitro and in vivo in rats. Neurobiol Dis 31, 316–326.

97.

Criscuolo

, Fabiani

, Bonadonna

, Origlia

, Domenici

(2015) BDNF prevents amyloid-dependent impairment of LTP in the entorhinal cortex by attenuating p38 MAPK phosphorylation. Neurobiol Aging 36, 1303–1309.

98.

Nagahara

, Merrill

, Coppola

, Tsukada

, Schroeder

, Shaked

, Wang

, Blesch

, Kim

, Conner

, Rockenstein

, Chao

M V

, Koo

, Geschwind

, Masliah

, Chiba

, Tuszynski

(2009) Neuroprotective effects of brain-derived neurotrophic factor in rodent and primate models of Alzhei-mer’s disease. Nat Med 15, 331–337.

99.

Nagahara

, Mateling

, Kovacs

, Wang

, Eggert

, Rockenstein

, Koo

, Masliah

, Tuszynski

(2013) Early BDNF treatment ameliorates cell loss in the entorhinal cortex of APP transgenic mice. J Neurosci 33, 15596–15602.

100.

Eremenko

, Mittal

, Berner

, Kamenetsky

, Nemirovsky

, Elyahu

, Monsonego

(2019) BDNF-producing, amyloid β-specific CD4 T cells as targeted drug-delivery vehicles in Alzheimer’s disease. EBio-Medicine 43, 424–434.

101.

UC San Diego Health, First-in-Human Clinical Trial to Assess Gene Therapy for Alzheimer’s Disease. https://health.ucsd.edu/news/releases/Pages/2021-02-18-first-in-human-clinical-trial-to-assess-gene-therapy-for-alzheimers-disease.aspx.

102.

Jansen

, Savage

, Watanabe

, Bryois

, Williams

, Steinberg

, Sealock

, Karlsson

, Hägg

, Athanasiu

, Voyle

, Proitsi

, Witoelar

, Stringer

, Aarsland

, Almdahl

, Andersen

, Bergh

, Bettella

, Bjornsson

, Brækhus

, Bråthen

, de Leeuw

, Desikan

, Djurovic

, Dumitrescu

, Fladby

, Hohman

, Jonsson

, Kiddle

, Rongve

, Saltvedt

, Sando

, Selbæk

, Shoai

, Skene

, Snaedal

, Stordal

, Ulstein

, Wang

, White

, Hardy

, Hjerling-Leffler

, Sullivan

, van der Flier

, Dobson

, Davis

, Stefansson

, Pedersen

, Ripke

, Andreassen

, Posthuma

(2019) Genome-wide meta-analysis identifies new loci and functional pathways influencing Alzheimer’s disease risk. Nat Genet 51, 404–413.

103.

Sirkis

, Bonham

, Aparicio

, Geier

, Ramos

, Wang

, Karydas

, Miller

, Coppola

, Yokoyama

(2016) Rare TREM2 variants associated with Alzheimer’s disease display reduced cell surface expression. Acta Neuropathol Commun 4, 98.

104.

Rodriguez-Vieitez

, Saint-Aubert

, Carter

, Almkvist

, Farid

, Schöll

, Chiotis

, Thordardottir

, Graff

, Wall

, Långström

, Nordberg

(2016) Diverging longitudinal changes in astrocytosis and amyloid PET in autosomal dominant Alzheimer’s disease. Brain 139, 922–936.

105.

Chakrabarty

, Li

, Ceballos-Diaz

, Eddy

, Funk

, Moore

, DiNunno

, Rosario

, Cruz

, Verbeeck

, Sacino

, Nix

, Janus

, Price

, Das

, Golde

(2015) IL-10 alters immunoproteostasis in APP mice, increasing plaque burden and worsening cognitive behavior. Neuron 85, 519–533.

106.

Rivers-Auty

, Mather

, Peters

, Lawrence

, Brough

, Alzheimer’s Disease Neuroimaging Initiative (2020) Anti-inflammatories in Alzheimer’s disease—potential therapy or spurious correlate? Brain Commun 2, fcaa109.

107.

Walker

, Whetzel

, Serrano

, Sue

, Beach

, Lue

L-F

(2015) Association of CD33 polymorphism rs3865444 with Alzheimer’s disease pathology and CD33 expression in human cerebral cortex. Neurobiol Aging 36, 571–582.

108.

Griciuc

, Federico

, Natasan

, Forte

, McGinty

, Nguyen

, Volak

, LeRoy

, Gandhi

, Lerner

, Hudry

, Tanzi

, Maguire

(2020) Gene therapy for Alzheimer’s disease targeting CD33 reduces amyloid beta accumulation and neuroinflammation. Hum Mol Genet 29, 2920–2935.

109.

Edison

, Archer

, Gerhard

, Hinz

, Pavese

, Turkheimer

, Hammers

, Tai

, Fox

, Kennedy

, Rossor

, Brooks

(2008) Microglia, amyloid, and cognition in Alzheimer’s disease: An [11C](R)PK11195-PET and [11C]PIB-PET study. Neurobiol Dis 32, 412–419.

110.

Jiang

, Tan

, Zhu

X-C

, Zhang

Q-Q

, Cao

, Tan

M-S

, Gu

L-Z

, Wang

H-F

, Ding

Z-Z

, Zhang

Y-D

, Yu

J-T

(2014) Upregulation of TREM2 ameliorates neuropathology and rescues spatial cognitive impairment in a transgenic mouse model of Alzheimer’s disease. Neuropsychopharmacology 39, 2949–2962.

111.

Zheng

, Zhou

X-W

, Wang

J-Z

(2016) The dual roles of cytokines in Alzheimer’s disease: Update on interleukins, TNF-α, TGF-β and IFN-γ. Transl Neurodegener 5, 7.

112.

Alves

, Churlaud

, Audrain

, Michaelsen-Preusse

, Fol

, Souchet

, Braudeau

, Korte

, Klatzmann

, Cartier

(2017) Interleukin-2 improves amyloid pathology, synaptic failure and memory in Alzheimer’s disease mice. Brain 140, 826–842.

113.

Luzina

, Keegan

, Heller

, Rook

GAW

, Shea-Donohue

, Atamas

(2012) Regulation of inflammation by interleukin-4: A review of “alternatives”. J Leukoc Biol 92, 753–764.

114.

Latta

, Sudduth

, Weekman

, Brothers

, Abner

, Popa

, Mendenhall

, Gonzalez-Oregon

, Braun

, Wilcock

(2015) Determining the role of IL-4 induced neuroinflammation in microglial activity and amyloid-β using BV2 microglial cells and APP/PS1 transgenic mice. J Neuroinflammation 12, 41.

115.

Kiyota

, Okuyama

, Swan

, Jacobsen

, Gendelman

, Ikezu

(2010) CNS expression of anti-inflammatory cytokine interleukin-4 attenuates Alzheimer’s disease-like pathogenesis in APP+PS1 bigenic mice. FASEB J 24, 3093–3102.

116.

Chakrabarty

, Tianbai

, Herring

, Ceballos-Diaz

, Das

, Golde

(2012) Hippocampal expression of murine IL-4 results in exacerbation of amyloid deposition. Mol Neurodegener 7, 36.

117.

Burmeister

, Marriott

(2018) The interleukin-10 family of cytokines and their role in the CNS. Front Cell Neurosci 12, 458.

118.

Kiyota

, Ingraham

, Swan

, Jacobsen

, Andrews

, Ikezu

(2012) AAV serotype 2/1-mediated gene delivery of anti-inflammatory interleukin-10 enhances neurogenesis and cognitive function in APP+PS1 mice. Gene Ther 19, 724–733.

119.

Wajant

, Pfizenmaier

, Scheurich

(2003) Tumor necrosis factor signaling. Cell Death Differ 10, 45–65.

120.

Covey

, Ignatowski

, Renauld

, Knight

, Nader

, Spengler

(2002) Expression of neuron-associated tumor necrosis factor alpha in the brain is increased during persistent pain. Reg Anesth Pain Med 27, 357–366.

121.

Park

, Bowers

(2010) Tumor necrosis factor-alpha mediated signaling in neuronal homeostasis and dysfunction. Cell Signal 22, 977–983.

122.

Janelsins

, Mastrangelo

, Oddo

, LaFerla

, Federoff

, Bowers

(2005) Early correlation of microglial activation with enhanced tumor necrosis factor-alpha and monocyte chemoattractant protein-1 expression specifically within the entorhinal cortex of triple transgenic Alzheimer’s disease mice. J Neuroinflammation 2, 23.

123.

Bongioanni

, Romano

, Sposito

, Castagna

, Boccardi

, Borgna

(1997) T-cell tumour necrosis factor-alpha receptor binding in demented patients. J Neurol 244, 418–425.

124.

Janelsins

, Mastrangelo

, Park

, Sudol

, Narrow

, Oddo

, LaFerla

, Callahan

, Federoff

, Bowers

(2008) Chronic neuron-specific tumor necrosis factor-alpha expression enhances the local inflammatory environment ultimately leading to neuronal death in 3xTg-AD mice. Am J Pathol 173, 1768–1782.

125.

Chakrabarty

, Herring

, Ceballos-Diaz

, Das

, Golde

(2011) Hippocampal expression of murine TNFα results in attenuation of amyloid deposition in vivo. Mol Neurodegener 6, 16.

126.

Sanabria-Castro

, Alvarado-Echeverría

, Monge-Bonilla

(2017) Molecular pathogenesis of Alzheimer’s disease: An update. Ann Neurosci 24, 46–54.

127.

Goswami

, Subramanian

, Silayeva

, Newkirk

, Doctor

, Chawla

, Chattopadhyay

, Chandra

, Chilukuri

, Betapudi

(2019) Gene therapy leaves a vicious cycle. Front Oncol 9, 297.

128.

Dangouloff

, Servais

(2019) Clinical evidence supporting early treatment of patients with spinal muscular atrophy: Current perspectives. Ther Clin Risk Manag 15, 1153–1161.