Distinct Patterns Link the BDNF Val66Met Polymorphism to Alzheimer’s Disease Pathology

1 INTRODUCTION

Brain-derived neurotropic growth factor (BDNF), a protein encoded by the BDNF gene, plays an important role in neuronal maintenance, neuronal survival, neurotransmitter regulation, and in long term potentiation and plasticity, especially in the hippocampus, a structure that plays a critical role in memory formation and Alzheimer’s disease (AD) [1–3]. The single nucleotide polymorphisms (SNP) of BDNF Val66Met may be a genetic risk factor for dementia. Previous research has linked BDNF SNPs with AD and observed worse cognitive performance in Met allele carriers diagnosed with autosomal dominantly inherited AD, caused by mutations in the amyloid precursor protein, presenilin-1, and presenilin-2 [4]. Likewise, in a large middle-aged cognitively healthy cohort enriched for AD risk, carriage of the Met-allele was associated with steeper decline in episodic memory and executive function [5].

However, so far studies linking BDNF to AD pathophysiology remain inconclusive. Previous studies have shown an association of lower serum BDNF to widespread Aβ₄₂ in PET studies [6] and faster disease progression in Met carriers, and this effect is magnified by the presence of an Apolipoprotein E (APOE) ɛ4 allele [7, 8]. On the other hand, clinically healthy Val-Val individuals showed lower hippocampal volume compared to heterozygotes and with a dose-response effect with the number of APOE ɛ4 alleles [9]. Older meta-analyses in both healthy and neuropsychiatric populations found no effects of Val66Met on brain volume. It should be noted that the role of amyloid-β (Aβ) positivity was not considered in the meta-analyses, and one recurring observation across several recent studies is that Aβ₄₂ interacts with Met carriership on cognitive decline and hippocampal volume [10, 11].

We hypothesize that these inconsistencies may also be partly due to variability in immune responses. Higher immune response in Met carriers was associated with higher depression symptoms within the cognitive dimension [12]. Interleukin 6 (IL-6) is a pleiotropic cytokine with both pro- and anti-inflammatory actions [13] that in animal models also plays a role in memory [14] and neuronal differentiation and maintenance [15]. Plasma IL-6 has been associated with chronic neuroinflammation, is elevated in AD [16], plays a central role in the activation of microglia, negatively affects the clearance system of Aβ₄₂ [17, 18], and modulates the relationship between AD pathology and vascular/blood brain barrier damage [19]. IL-6 was shown in vitro to have a reciprocal relation supporting neuronal survival in combination with endogenous BDNF [20]. Animal models showed that chronically increasing IL-6 markedly decreases neurogenesis [21].

Inflammation is associated with, and possibly exacerbates, AD pathology, and based on the above observations, we posit that BDNF Val66Met may moderate this association [19, 22–24]. Thus, we sought to examine the relationship of the BDNF polymorphism with Aβ₄₂ versus phosphorylated tau (p-tau), and whether these associations were modified by APOE ɛ4, or inflammation (specifically IL-6) in a memory clinic population.

2 MATERIALS AND METHODS

A convenience sample of 114) subjects was recruited from the memory clinic of the Maastricht University Medical Center (MUMC) for this cross-sectional study. This group includes individuals diagnosed with subjective cognitive decline (SCD), mild cognitive impairment (MCI), and AD dementia. Diagnoses were made by experienced physicians based on the Petersen core clinical criteria for MCI [25] and AD dementia. Criteria for SCD included self-reported presence of subjective cognitive complaints and endorsing the question “Do you think your memory is becoming worse” and absence of impairments on cognitive tests (defined as a score below –1.5 SD of the age-, sex-, and education-adjusted mean) [26]. Exclusion criteria were major neurological disease, clinical diagnosis of other neurodegenerative disorders (e.g., frontotemporal dementia), recent transient ischemic attack or cerebrovascular accident (< 2 years), history of psychiatric disorders, and alcohol or drug abuse. All patients provided informed written consent and the study protocols were approved by the Medical Ethics Committee of the MUMC in confirmation with the declaration of Helsinki.

Demographics stratified by BDNF allele are listed in Table 1. Our sample had a mean age of 62.97 (SD = 9.04) years, 71% of the participants were male, 46.6% of Val-Met, and 49.2% Val-Val carried at least one APOE ɛ4 allele (no difference p = 0.935) There was no difference in demographics between BDNF Val-Val and Val-Met carriers (Table 1).

3.2 BDNF by APOE ɛ4 interaction (covaried for age and sex)

When looking at the interaction of BDNF and APOE ɛ4, we found lower CSF Aβ₄₂ in Val-Met carriers who also carried the APOE ɛ4 allele (t₁₀₈ = 2.19, p = 0.03 (p_FDR = 0.03), Fig. 1C), compared to Val-Val carriers with or without APOE ɛ4 carriage. When adding p-tau as covariate, the interaction be-tween BDNF and APOE ɛ4 remained significant (t₁₀₇ = 2.62, p = 0.009 (p_FDR = 0.018)), indicating that this effect was independent of p-tau. If the sample was post hoc restricted to MCI and SCD, the interaction between BDNF and APOE ɛ4 was not significant (t₈₅ = 1.83, p_FDR = 0.11). The interaction between BDNF and APOE ɛ4 was not significant for t-tau (t₁₀₈ = 1.09, p = 0.275) or p-tau (t₁₀₈ = 0.648 p_FDR = 0.56).

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Fig. 1

A) BDNF polymorphism SNP show different associations between IL-6 and CSF p-tau, dependent on Aβ₄₂ (continuous variable, plotted marginal effects at mean±1SD). B) Johnson-Neyman plot showing the level of CSF Aβ₄₂ (< 963 pg/ml) where the interaction BDNF^*IL6^*Aβ₄₂ becomes significant. Clinical threshold for Aβ₄₂ is 500 pg/ml and p-tau (85 pg/ml). C) CSF Aβ₄₂ levels differ among BDNF by APOE ɛ4 SNPs.

4 DISCUSSION

In this study we found distinct relationships of BDNF polymorphism to either Aβ₄₂ or p-tau. We observed that individuals with Val-Met genotype displayed lower CSF Aβ₄₂ when also carrying at least one APOE ɛ4 allele, independent of p-tau. However, Val-Val individuals exhibited higher p-tau under higher levels of IL-6, and subthreshold levels of Aβ₄₂ (< 963 pg/ml; clinical threshold for Aβ₄₂ = 500). BDNF plays a crucial role in synaptic plasticity and long-term potentiation in the hippocampus and therefore SNPs of the BNDF gene have been of interest for dementia research. These findings could provide new inroads to specific interventions for Aβ₄₂ and p-tau depending on BDNF polymorphism and immune response.

While only few studies examined the effect of BDNF on AD pathology, the literature is consistent in that the relationship between BDNF and memory is dependent on amyloid, but potential effect-modification by APOE is less clear. In in Aβ- SCD participants, no increased risk of dementia was found, but combined Aβ+ and Val66Met+ did cognitively worse than Aβ+ only [8], with similar findings in unimpaired but increased AD risk [11] suggesting a synergistic effect of Met+ and Aβ. However, no moderating effect of APOE ɛ4 was found on cognition. In the AIBL study, Aβ+ APOE ɛ4+ and Val66Met+ had worse memory performance [7] but interaction effects of APOE by BNDF on Aβ are not described. Two studies took a similar approach as ours and examined the relationship between BDNF Val66Met on AD pathology in cognitively normal older individuals [31, 32] and reported higher PET Aβ₄₂ in APOE ɛ4+ in Val66Met+ [32]. In a combined autosomal dominant AD group, lower CSF Aβ₄₂ in APOE ɛ4+ participants but no interaction with BDNF was found [2]; however, Val66Met+ was associated with hippocampus-frontal connectivity in autosomal dominant AD and AD and stronger associations with Aβ in sporadic AD but not in SCD [33]. Reasons underlying these differences between sporadic AD, preclinical autosomal dominant AD, and SCD might be related to different ages of onset and disease progression.

It is increasingly accepted that inflammation plays an important role in linking concurrent pathologies in AD dementia [19, 35]. IL-6 is a pleiotropic cytokine that has both pro- and anti-inflammatory effects, plays an important part in the innate immune system, can induce hyperphosphorylation of tau in animal models [36], and can interfere with Aβ₄₂ clearance [17, 37]. Inflammation and microglial activation lead to production of multiple neurotrophic factors, including BDNF. Sustained immune activation, however, strongly reduces the generation of these neurotropic factors possibly interfering with neuroplasticity [38].

This study has some limitations. This is a me-dium sized cross-sectional convenience sample from a referral-only university hospital memory clinic in northern Europe. The Val66Met prevalence is extremely dependent (0 to > 70%) on genetic background [39] and the prevalence of the Val66Met polymorphism is approximately 20% in Europe. The locale should be considered when comparing these results with studies conducted in a different genetic background. Both IL-6 and BDNF are not specific to any single disease complicating the interpretation of associations in various populations. Larger studies are needed to better model interactions between pathology, genetics, and the innate immune system. We tentatively hypothesize based on this data that trials targeting AD pathology (both Aβ and p-tau), especially using monoclonal antibodies, may have to consider APOE status, BDNF polymorphisms as well as innate immune system activation. Substantially larger cohorts are needed in other to be able to investigate the downstream effects of BDNF on cognition.