Cellular Therapies for Muscular Dystrophies: Frustrations and Clinical Successes

Abstract

Cell-based therapy for muscular dystrophies was initiated in humans after promising results obtained in murine models. Early trials failed to show substantial clinical benefit, sending researchers back to the bench, which led to the discovery of many hurdles as well as many new venues to optimize this therapeutic strategy. In this review we summarize progress in preclinical cell therapy approaches, with a special emphasis on human cells potentially attractive for human clinical trials. Future perspectives for cell therapy in skeletal muscle are discussed, including the perspective of combined therapeutic approaches.

Adult Skeletal Muscle: An Ideal Target for Cell Therapy

The American Society of Gene and Cell Therapy defines “cell therapy” as the administration of live whole cells or maturation of a specific cell population in a patient for the treatment of a disease. Historically, the first successful cell therapy trial was obtained as early as 1665 in the United Kingdom with the first blood transfusion in dogs.¹ Blood transfusions are now considered routine. Bone marrow transplantation has also become a well-established protocol, commonly performed for many kinds of blood disorders.² The key to successful bone marrow transplantation is the identification of a good “immunologically matched” donor, who is usually a close relative, such as a sibling. Bone marrow cells of the patient (recipient) are destroyed by chemotherapy or radiation, and cells from the matched donor are infused: the self-renewing stem cells find their way to the bone marrow and begin to replicate and produce cells that mature into the various types of blood cells. The first description of skeletal muscle cell transplantation was performed in mice and published by Partridge and colleagues in 1978,³ and the first clinical trials in humans were initiated in the early 1990s to restore the missing protein dystrophin in patients with Duchenne muscular dystrophy (DMD).

Skeletal muscle is the most abundant human tissue. It is a highly specialized postmitotic tissue assuring many physiological functions including locomotion, maintenance of posture, as well as metabolic activity. Skeletal muscle is also a dynamic tissue retaining a remarkable capacity to adapt to physiological demands such as growth and exercise, and to repair and regenerate after injury or disease.⁴ This ability is due to the presence of resident mononucleated muscle precursor cells, named satellite cells.⁵ In mature skeletal muscle, satellite cells are located in a specialized niche, between the basal lamina and the sarcolemma of muscle fibers.⁴ Normally quiescent in uninjured muscle, satellite cells are able in response to severe damage or disease to activate; express myogenic regulatory factors such as MyoD, Myf5, and MRF4⁶; and undergo proliferative expansion, providing a pool of muscle progenitors that fuse to generate multinucleated myofibers. Discovered by electron microscopy⁵ on the basis of their position—beneath the basal lamina—satellite cells are today identified by various protein markers, using various techniques (immunofluorescence, muscle tissue digestion, fluorescence-activated cell sorting), allowing their identification in various cell states (quiescent, activated, and proliferating) in mice and in humans (for more details see Boldrin et al.⁷ and Negroni et al.⁸). Thus skeletal muscle is an adequate target for cell therapy because (1) most of its nuclei are postmitotic with (2) limited cell turnover—15.1 years⁹—and (3) muscle stem cells are well characterized. Once restored by therapeutic cells, muscle fibers can produce the therapeutic protein for a long time.

Cell-Mediated Strategies for Muscle Diseases

Muscular dystrophies (MDs) are a heterogeneous group of disorders, characterized by progressive muscle wasting and weakness, with a wide clinical presentation and severity.¹⁰ More than 30 genetically distinct types of MD have been identified^11,12 according to the age at onset (pediatric or adults), severity, mode of inheritance, rate of progression, prognosis, and the specific muscle groups initially affected. MDs are caused by mutations in genes encoding a broad range of proteins located in the extracellular matrix (ECM), at the plasma membrane, in the cytoplasm, at the sarcomere, and in the nucleus of striated muscle cells (see the Gene Table of Neuromuscular Disorders [www.musclegenetable.fr/]). Although for many of these MDs the genetic defect is now identified and considerable progress has been made to increase our understanding of muscle genetics, pathophysiology, and molecular/cellular partners involved in these pathologies, the pathophysiological mechanisms leading from the genetic mutation to the development of the dystrophic features are still often unknown, with no cure available. Several therapeutic strategies have been investigated, including pharmacological, gene-based, and cell-based approaches (for pharmacological and gene therapy approaches we suggest to the readers several reviews^13

–18). Cell therapy for MD is based on the delivery of precursor cells to muscle tissue that will contribute to regeneration and tissue repair. This can be seen as a form of gene therapy, in which the functional gene is provided by cells used as vectors. There are basically two ways to obtain therapeutic cells: (1) from a healthy donor (allograft); these cells will express the missing protein but will also induce an immune response when grafted into the patient, requiring immune suppression treatment; (2) from the patient him- or herself (autograft); these cells will be genetically modified to express the missing protein, avoiding immune suppression (unless the reintroduced protein becomes immunogenic). A third possible option, that is, the use of nonmodified cells in an autologous graft, can be envisaged for particular MDs, such as oculo-pharyngeal muscular dystrophy (OPMD); this is detailed in the section “Human Cell Therapy Candidates: Preclinical and Clinical Studies.” Concerning the route of administration, at least two ways have been explored: intramuscular and systemic delivery. Intramuscular injection has been widely used for satellite cells, and seems appropriate if the target is limited. For broader distribution to the whole body or a range of muscles, systemic or locoregional delivery via the blood is more often done. This requires that the therapeutic cells are able to extravasate and distribute within the target tissue. Various criteria for an ideal cell candidate for muscle cell therapy have been identified. An ideal cell must first have stable myogenic potential: able to fuse with host myofibers after transplantation, the therapeutic cell will synthesize donor-derived proteins; its colonization of the host satellite cell niche will extend the therapeutic effect, because the donor cell will then be able to participate to new cycles of muscle degeneration/regeneration. This stable myogenic profile is essential in order to avoid differentiation into an inappropriate cell type/tissue. Three additional properties for the ideal cell candidate to be used under clinical GMP conditions are as follows: It should be available in sufficient numbers, easy to isolate, and straightforwardly amplified. If the candidate cell is compatible with a systemic or locoregional route of administration, homing capacities to the site of muscle regeneration are also highly desirable. Finally, if autologous cell therapy is envisaged, a genetically modifiable cell candidate will be required to correct the deregulated gene. In the last few decades, many stem/progenitor cells other than satellite cells/myoblasts have been studied as potential candidates for cell therapy in MD.^8,19
–21 In this review we focus on human cell candidates for skeletal muscle (Table 1) (excluding cardiac and smooth muscle studies) that either are already in clinical trials or represent future candidates (induced pluripotent stem cells and embryonic stem cells). We highlight problems that need to be solved for efficient clinical trials, including some genetic strategies to eventually modify cells before injection.

Table 1.

Myogenic cell candidates

Cell type	Origin	Physiological function	Myogenic differentiation	Alternative lineage potential	Systemic delivery in preclinical models	Participation in skeletal muscle regeneration and colonization of satellite cell niche	Clinical trials	Ref.
Myoblast	Skeletal muscle	Myogenesis and skeletal muscle regeneration	Spontaneous	Osteogenic and adipogenic differentiation	No	Yes	Phase II: completed (DMD); Phase I/IIa: on-going (OPMD)	19, 61
Mesoangioblast-like/pericyte	Vessel/skeletal muscle	Blood flow regulation, microvessel contractility, regulation and control of angiogenesis	Spontaneous for skeletal muscle/vessel-derived pericytes	Chondrogenic, osteogenic, and adipogenic differentiation	Yes	Yes	Phase I/II: intraarterial delivery of HLA-identical allogeneic mesoangioblasts. Results not yet published (DMD)	—
CD133⁺	Blood/skeletal muscle	Unknown	Spontaneous but limited (m); induced by C2C12 coculture (b)	Myeloid (b) and endothelial differentiation	Yes	Yes	Phase I: intramuscular injection, neither local nor systemic adverse effects were reported (DMD)	52
ALDH⁺	Skeletal muscle	Unknown	Spontaneous	Adipogenic differentiation	Not tested	Yes	NA	—
MuStem (c)	Skeletal muscle	Unknown	Spontaneous	Osteogenic and adipogenic differentiation	Yes	Yes	NA	—

ALDH, aldehyde dehydrogenase 1A1; b, peripheral blood; c, canine; DMD, Duchenne muscular dystrophy; m, skeletal muscle; MuStem, muscle-derived stem cells isolated from golden retriever muscular dystrophy (GRMD) dogs; NA, not available; OPMD, oculo-pharyngeal muscular dystrophy.

Human Cell Therapy Candidates: Preclinical and Clinical Studies

Satellite cells and myoblasts

Discovered more than 50 years ago, physiological muscle precursors, that is, satellite cells/myoblasts, represented the first obvious candidate for cell therapy in MD. On the basis of pioneer studies showing that intramuscular injection of mouse myoblasts could restore dystrophin in the mdx model (a mouse model for DMD),²² isolation, characterization, and preclinical studies of human myoblasts were initiated in the early 1990s,^23,24 followed by clinical trials. When heterologous myoblasts were injected into patients with DMD in human clinical trials, this strategy was shown to be safe and provided evidence that the injected myoblasts were able to graft within host tissue, but with transient and limited dystrophin expression and no clinical benefit.^8,19,25 Immunosuppression of patients, and low migration and poor survival of human-injected myoblasts, were parameters that began to be taken into account.^19,26,27 Since then, several research groups have started to develop xenotransplantation strategies in immunodeficient preclinical mouse models in order to analyze human myoblast behavior and the regulation of key biological events occurring in the host muscle after intramuscular myoblast transplantation, in order to design rational strategies to improve grafting.^28
–30 For example, we showed that human myoblasts do not disappear massively during the first days after injection as previously described for mouse myoblast injection.³¹ A significant portion of grafted human myoblasts survives and engrafts host muscle, although a cell death peak (occurring 12–24 hr after transplantation) is noted (see Riederer et al.³² and our unpublished data). Interestingly, we observed in situ proliferation of human myoblasts during the first 3 days posttransplantation that was able to compensate for the loss of cells occurring after injection.³² Preclinical studies have indicated poor migration of engrafted myoblasts in both mouse^33,34 and monkey³⁵ muscles, although some migration has been detected in more recent studies in nonhuman primates.³⁶ Interestingly, we have shown that the migration of grafted human myoblasts is limited to the first 3 days after transplantation, and then proliferation is downregulated and grafted myoblasts start to differentiate in situ, suggesting that proliferation, migration, and differentiation are tightly linked and that modifying one of these three processes has an impact on the others.³² In preclinical models, muscle damage or irradiation³⁷ can improve the migration capacities of grafted myoblasts, but such a protocol is not clinically applicable. In clinical settings, Skuk and colleagues proposed to improve cell dispersion by using a protocol of “high-density” injections based on multiple 1-mm-spaced local injections.^38,39 More recent clinical trials in patients with DMD, conducted between 2004 and 2007, were based on these settings. Such a protocol remains applicable to small and accessible muscles in patients with DMD or in the context of autologous myoblast transplantation for localized muscles in less extended diseases such as OPMD, but seems inappropriate to treat numerous large targets.

Other myogenic stem cells

Myoblasts are often exhausted in dystrophic conditions (such as DMD),⁴⁰ which hampers their isolation, modification, and amplification for autografts. Furthermore, myoblasts, whatever their origin, cannot be injected systemically, which is the ideal route to target large amounts of tissue.^41,42 These reasons have led to the investigation and identification of other types of progenitor/stem cells, distinct from satellite cells but exhibiting myogenic capacities, in a search for candidates with better performance/efficacy to restore muscle function in MD.

Mesangioblasts/pericytes

Mesoangioblasts, mesodermal progenitors originating from the dorsal aorta of embryonic day 9.5 mouse embryos, contribute to postnatal muscle growth and regeneration.^43,44 In 2007 their human adult counterpart was identified in a subpopulation of muscle-resident cells, previously defined as pericytes.⁴¹ Since their isolation, human mesangioblasts/pericytes have been extensively characterized. They have been isolated from the muscle microvasculature of healthy subjects or young patients with DMD on the basis of the expression of pericyte markers such as alkaline phosphatase. In vivo, human healthy or DMD mesoangioblasts/pericytes transduced in vitro with a lentiviral vector expressing human minidystrophin were able, after intraarterial delivery in mice (SCID/mdx), to cross the vessel barrier and migrate into dystrophic muscle, where they participated in muscle regeneration restoring expression of dystrophin and colonized the satellite cell niche.⁴¹ Human pericytes are enriched or exhausted in some myopathies^45,46 and, interestingly, they seem to be more efficient when injected by the systemic route compared with direct intramuscular injection, making pericytes a promising therapeutic tool for cell therapy.⁴¹ On the basis of all these results a first phase I/II clinical trial (EudraCT No. 2011-000176-33), aiming at assessing safety and efficacy of intraarterial delivery of HLA-identical allogeneic mesoangioblasts, has been completed for patients with DMD. Recently published results of this trial have shown that intraarterial transplantation of donor mesoangioblasts is relatively safe (with one adverse event which was not clearly demonstrated as correlated to the infusion of cells), with limited clinical benefit although transient functional stabilization was observed.⁴⁷

CD133⁺ cells

Human blood- and skeletal muscle-derived CD133⁺ cells have been shown to possess myogenic capacities in vitro and after implantation in vivo. By intramuscular and systemic delivery in immunodeficient mice, they showed ability to contribute to muscle regeneration, including colonization of the satellite cell niche.^48

–51 Blood- and muscle-derived CD133⁺ cells isolated from patients with DMD have also been genetically corrected and injected into immunodeficient dystrophic mice, where they were able to give rise to dystrophin-positive fibers.⁴⁹ All these characteristics (systemic delivery, ability to be genetically modified, and capacity to colonize the satellite cell niche) make human CD133⁺ cells another candidate for cell therapy. In 2007 a double-blinded phase I clinical trial, testing the safety of autologous muscle-derived CD133⁺ cells in patients with DMD, was completed.⁵² Results showed an increase in capillary vascularization in four of five injected muscles and no side effects, but no evidence of effective integration in muscle fibers was noted.

ALDH⁺ cells

Aldehyde dehydrogenase 1A1 (ALDH) is a detoxifying enzyme involved in the metabolism of aldehydes and retinoic acid. ALDH activity is one hallmark of human bone marrow, umbilical cord blood, and peripheral blood primitive progenitors presenting high reconstitution capacities in vivo. An ALDH⁺ cell population has been identified within human skeletal muscles.⁵³ In vivo, after intramuscular injection, ALDH⁺CD34^– cells proliferated and robustly contributed to muscle regeneration, including integrating the pool of satellite cells. If systemic delivery of these cells is possible, their proliferative capacity will represent a considerable advantage in terms of colonization of the host muscle and participation in regeneration.

MuStem cells

A population of muscle-derived stem cells, called MuStem cells, was isolated from golden retriever muscular dystrophy (GRMD) dogs (the clinically relevant large-animal model for DMD), using a preplating approach.⁵⁴ Intraarterial injection of canine MuStem cells into immunosuppressed GRMD dogs resulted in the formation of new regenerated muscle fibers and the restoration of dystrophin expression. MuStem cells also colonized the satellite cell niche and induced partial remodeling of muscle tissue and persistent clinical stabilization of transplanted GRMD dogs. A study on the global gene expression profile of these MuStem cells, 4 months after transplantation, showed an upregulation of genes enhancing muscle regeneration; this could play an essential role in the stabilization of dystrophic muscle.⁵⁵ Human MuStem cells have now been identified (J. Lorant and K. Rouger, personal communication). Although of different origins (i.e., skeletal muscle for MuStem cells and muscle blood vessels for mesoangioblasts) and isolated by different methods, common phenotypic properties, such as multilineage potential and homing to damaged muscle when injected systemically (see Table 1), may suggest some similarities between these two types of myogenic cells. Further studies to characterize MuStem cells in terms of heterogeneity and regenerative potential are needed before they can be considered for therapeutic application.

Clinical Trial: Example of a Phase I/iia Study of Autologous Myoblast Cell Transplantation for Opmd

Intramuscular injections of myoblasts—despite their reduced proliferative capacity and migration—always lead to localized tissue repair at the site of injection.²⁵ OPMD appears as an ideal situation, where myoblast transplantation is applicable. Indeed, OPMD is a late-onset autosomal dominant, slow-progressing inherited dystrophy caused by an abnormal trinucleotide repeat expansion in the PABPN1 gene,^56,57 where a small group of specific muscles (eyelid and pharyngeal) are primarily affected, leading to both ptosis and dysphagia. Affected cricopharyngeal muscle is characterized by exacerbated fibrosis and atrophy.⁵⁸ Surgical correction for eyelid and pharyngeal muscle weakness are to date the only therapy to increase the quality of life of patients with OPMD. However, surgical myotomy (to fight dysphagia) provides in most cases only a transient benefit, with a secondary progressive recurrence of dysphagia a few years later.⁵⁹ We demonstrated that myoblasts isolated from clinically unaffected muscles of patients with OPMD had a normal proliferation rate and differentiation capacity,⁶⁰ suggesting that autologous transplantation of myoblasts isolated from clinically unaffected muscles and implanted into affected pharyngeal muscles may improve their motility and restore some muscle function. A preclinical study in dogs confirmed the feasibility of such a protocol.⁶⁰ A phase I/IIa clinical study (ClinicalTrials.gov NCT00773227) was then launched in 2004 with 12 patients with OPMD to assess the feasibility and toxicity of autologous myoblast transplantation in pharyngeal muscles after myotomy. The results of the trial demonstrated the safety and good tolerance of the procedure with no adverse side effects. A cell dose-dependent functional improvement in swallowing was observed in this safety study.⁶¹ This study is now being extended to 12 new patients, including patients who have not undergone myotomy, to decipher the respective contributions of myoblast injection and myotomy.

Cell Therapy: Future Perspectives

Improving the therapeutic potential of myoblast cell therapy

It is now confirmed that culture conditions modify the transplantation efficiency of muscle progenitors: mouse myoblasts are less efficient than the satellite cells from which they are derived⁶² or even less so than the initial satellite cells in their niche⁶³; and this is probably also true for human cells. Although freshly isolated satellite cells present a high regenerative potential, they will probably not be a therapeutic option because of their limited numbers. Identifying the modifications arising during the expansion of myoblasts would allow the identification of optimized conditions to improve the regenerative potential of amplified myoblasts. For example, it has been shown that inhibition of the p38 signaling pathway during human myoblast expansion in vitro greatly enhances their engraftment in vivo.⁶⁴

Regarding the niche, the importance of the physiological rigidity of substrates used in vitro on the control of stem cell fate has been documented.⁶⁵ Gilbert and colleagues showed that in vitro substrate elasticity is a potent regulator of the fate that mouse satellite cells will acquire after transplantation in vivo: culturing cells on bioengineered hydrogel substrates that mimic muscle tissue elasticity (12 kPa) modulates their stem cell fate, enhances their survival, prevents early differentiation, and promotes self-renewal in vitro. More importantly, such conditions of culture result in the increased engraftment ability of these cells when injected in vivo into regenerating muscles.⁶⁶ Yennek and colleagues, using micropatterns coated with ECM, proposed that the geometric micropattern has a predominant effect on the fate of the dividing cells.⁶⁷ All these approaches have been tested on myoblasts, but substrate rigidity is known to modify the fate of other stem cells.⁶⁵ Although these approaches cannot yet be applied in clinical conditions for various reasons (cost, complexity, scale-up), the involved parameters, from the modulation of signaling to optimization of the substrates, will have to be taken into account for the optimization of large-scale cell cultures for future therapeutic use of stem cells in clinical trials.

Fibrotic environment

If elasticity plays a key role in satellite cell fate ex vivo, it should also be taken into account in vivo: transplanting cells in a stiffer environment may modify their behavior. Studies of muscle fiber rigidity in the mdx mouse showed that dystrophic fibers are more rigid than wild-type fibers, probably because of the absence of dystrophin⁶⁸ and/or increased muscular fibrosis. During fibrosis invasion, functional muscle tissue is progressively replaced by nonfunctional connective tissue. Fibrosis is a complex process present in a large variety of MDs, and characterized by excessive accumulation and modifications of ECM components: we have shown that advanced fibrosis is correlated with modified composition of glycosaminoglycans of muscle ECM in DMD.⁶⁹ Moreover, it has been shown that in a fibrotic environment, specialized cells such as satellite cells acquire plasticity toward a fibrogenic fate via a transforming growth factor (TGF)-β-mediated pathway, decreasing their normal regenerative functions,^70,71 and potentially resulting in a further increase in tissue fibrosis. In addition, a subpopulation of pericytes has been identified as a major source of profibrotic cells during acute muscle tissue injury and in aged skeletal muscle.^72,73 In mice it has been shown that muscle-derived fibroadipogenic progenitors (FAPs) rapidly proliferate in response to acute muscle damage, suggesting that they may play an active role during muscle regeneration^74,75 and/or in the instigation of the fibrosis associated with ECM deposition in the dystrophic mdx mouse model.⁷⁶ If the presence of FAPs is confirmed in humans, it will be essential to decipher their influence on regeneration and their potential cross-talk with satellite cells, and to characterize their role in fibrogenesis to ameliorate muscle cell therapy. Indeed, muscular fibrosis hampers the success of gene or cell therapy at advanced stages of the disease,⁷⁹ and pre-antifibrotic treatment using modified tendon fibroblasts (expressing angiogenic factors and metalloproteinase) that can restore microcirculation and reduce connective tissue deposition in aged dystrophic mice could ameliorate muscle tissues in patients at advanced stages of the disease.⁷⁷ Although it is not yet known how deregulation of ECM components (proteins, glycoproteins, glycosaminoglycans, etc.) contributes to worsening of the disease and participates in the formation of fibrosis, improving the quality of receiver muscle by antifibrotic therapies before grafting will certainly be essential to optimize the fate of implanted cells in cell therapy.

Human embryonic and induced pluripotent stem cells: future candidate cells?

Much energy has been expended to increase the myogenic commitment of human pluripotent stem cells: embryonic stem (hES) cells⁷⁸ and induced pluripotent stem (iPS) cells.^79,80 hES cells are isolated from the early embryo (5–6 days) and have the potential to differentiate into all tissues of the three germ layers. Generation and use of hES cells in therapy pose ethical issues, related to the destruction of human embryos, but also safety issues because hES cells can lead to the formation of teratomas if they are not fully differentiated.⁸¹ Although myogenic hES cells capable of muscle engraftment have been reported (for more details see Loperfido et al.⁸²), most of these strategies used genetic manipulation of hES cells (lentiviral vector integration), introducing additional risks related to the immunogenicity of viral vector proteins and the integration of the transgene into the human genome. Moreover, hES cell-based therapy will have to be a heterologous approach, and the immune privilege of hES cells is today debated⁸³ because they seem to increase their immunogenicity as they progress toward differentiation.⁸⁴

The generation of iPS cells poses fewer ethical issues than with hES cells because iPS cells are typically generated from adult somatic cells by reprogramming them with a defined and limited set of transcription factors (Oct3/4, Sox2, c-Myc, Klf4). Described first in mice,^79,80 this approach is now applied to human somatic cells.⁸⁵ Several studies have shown that combinations of compounds,⁸⁶ recombinant proteins fused to cell-penetrating peptide,⁸⁷ expression plasmids⁸⁸ or microRNA,⁸⁹ or even altering intercellular communication via Notch signaling⁹⁰ could partly substitute for or entirely eliminate the use of integrative viral vectors, resulting in the production of safer iPS cells. Like for hES cells, myogenic differentiation of hiPS cells can be induced by genetic modifications, further complicating safety issues for the possible use of hiPS cells in human trials (see Loperfido et al.⁸² for a comprehensive review). hiPS cells have also been generated from the fibroblasts of patients with DMD and BMD (Becker muscular dystrophy).⁹¹ Interestingly, mesoangioblast-like cells can be derived from dystrophic iPS cells⁴⁵ and thus potentially corrected for autologous transplantation, with all the advantages that mesoangioblasts present, such as the possibility to be delivered systemically. Finally, the engagement of murine and human iPS cells in myogenesis by treatment with factors has been reported, avoiding the use of any transgenesis.⁹² Although many problems, including safety, need to be solved before these cells can be used in clinical trials, these results open a new field of investigation for future potential candidates for cell therapy.

Up to now, no universal cell candidate that could be maintained and amplified in vitro and delivered to patients without immune rejection has been described, and the promises raised by the myogenic potential of hiPS cells still suggest personalized medical treatment involving the generation of hiPS cells for each patient. Although costs for curing life-threatening diseases such as DMD should be analyzed in view of their devastating evolution, patient-tailored approaches may be variably accessible depending on the country and national health care organization, representing another kind of ethical concern. Independently of the cell candidate chosen, one solution could be the development of haplobanks matching an as-large-as-possible majority of the population, as suggested for iPS cells by Wilmut and colleagues in 2015.⁹³ These haplobanks, however, will have to be specific for genetically distinct populations; for example, an ideal haplobank for the Caucasians will be different from that for the Japanese population.

Conclusion

The first demonstration of cell therapy for muscle dystrophy in murine models raised some hope of a clinical application at a time when gene therapy faced many problems (size of the transgene for dystrophin, immune reaction, etc.). However, clinical trials for DMD using heterologous myoblasts generated some frustration with the repeated absence of clinical benefit for the patients. Targeting most of the musculature including diaphragm, as required for DMD, turned out to be a real challenge with myoblasts. The nature of the fibrotic receiving tissue created hurdles potentially blocking the success of cell therapy. However, the clinical success of autologous myoblast therapy for OPMD pharyngeal muscles, where a functional improvement of pharyngeal function was observed, provides a proof of concept that cell therapy is still an open clinical approach for defined targets, provided that the therapeutic strategy is adapted to the clinical situation. Several lines of research for the future will concern (1) the cell type, including systemic delivery of myogenic stem cells to reach a larger amount of tissue; (2) pretreatment of the targeted tissue to diminish fibrosis and enhance delivery of the therapeutic agent (this is also true for other approaches, including gene therapy); and (3) the combination of strategies, for example, combined gene and cell therapy to deliver corrected autologous cells, or combined cell types that may together enhance repair and/or revascularization (Fig. 1).

Figure 1.

Cell therapy for muscular dystrophies. Future directions of cell therapy include improving the critical step of cell amplification (green square), systemic delivery of myogenic candidate cells (pink square), and targeting muscular fibrosis in order to improve muscle quality before therapy (yellow square). (Figure created using Servier Medical Art [www.servier.com]). Color images available online at www.liebertpub.com/hum

Author Contributions

E.N., A.B., C.T., G.S.B.-B., and V.M. contributed equally to the writing of the manuscript and generation of the figure.

Footnotes

Acknowledgments

This laboratory has been supported by grants from the AFM (Association Française contre les Myopathies, OPMD Network Research Programs 15123 and 17110), MYOAGE (contract HEALTH-F2-2009-223576) from the 7th FP, the ANR Genopath-INAFIB, Fondation de l'Avenir (project ET1-622), INSERM, CNRS, and Université Pierre et Marie Curie.

Author Disclosure

No competing financial interests exist.

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Cellular Therapies for Muscular Dystrophies: Frustrations and Clinical Successes

Abstract

Adult Skeletal Muscle: An Ideal Target for Cell Therapy

Cell-Mediated Strategies for Muscle Diseases

Human Cell Therapy Candidates: Preclinical and Clinical Studies

Satellite cells and myoblasts

Other myogenic stem cells

Mesangioblasts/pericytes

CD133+ cells

ALDH+ cells

MuStem cells

Clinical Trial: Example of a Phase I/iia Study of Autologous Myoblast Cell Transplantation for Opmd

Cell Therapy: Future Perspectives

Improving the therapeutic potential of myoblast cell therapy

Fibrotic environment

Human embryonic and induced pluripotent stem cells: future candidate cells?

Conclusion

Author Contributions

Footnotes

Acknowledgments

Author Disclosure

References

CD133⁺ cells

ALDH⁺ cells