HSCs for muscular dystrophy: a paradigm shift, back

Comment on Dell'Agnola et al, page 4311

Dell'Agnola and colleagues report that hematopoietic stem cells lack myogenic potential in a large animal model of muscular dystrophy, contrasting results in the murine model, and their report has important implications for the field of stem cell plasticity.

Evidence that adult-derived stem cells are capable of differentiation toward a variety of specific tissue fates has resulted in a major paradigm shift in the field of stem cell biology, and hematopoietic stem cells (HSCs) have been central to this shift. A growing number of studies have purported a contribution by HSCs toward ostensibly distinct tissues and suggest potential exploitation for the treatment of diseases ranging from diabetes to Parkinson disease.

Duchenne muscular dystrophy is a devas-tating X-linked form of muscular dystrophy characterized by severe and inexorable skeletal muscle degeneration that results in loss of ambulation at a very early age and death by the second decade. The identification of caus-ative mutations in the dystrophin gene on the X chromosome led to the development of animal models for its study, and the restoration of dystrophin expression within muscle fibers after HSC transplantation in the mdx mouse model was among the first observations to suggest the potential of HSCs for the treatment of disorders beyond the blood.^1,2 

Dell'Agnola and colleagues have now tested this potential in a large-animal model of muscular dystrophy, and their results are a sobering contrast to those of the rodent model. In this work, the investigators performed allogeneic bone marrow transplantation in 7 dogs with X-linked muscular dystrophy using nonaffected, dog leukocyte antigen (DLA)-matched littermate donors. Indeed, the canine model developed by the group has been instrumental in the design of nonmyeloablative allogeneic bone marrow transplantation regimens and has proved highly predictive of the results obtained in humans.

Using microsatellite marker polymorphism analyses of blood and marrow specimens, complete or near-complete donor hematopoietic chimerism was documented after conditioning and transplantation in all 7 dogs. Clinical improvement was not, however, observed. In order to assess a subclinical contribution by HSCs, muscle biopsies were obtained and dystrophin expression assessed prior to and 3 months following transplantation. To promote the mobilization of donor HSCs into the circulation, granulocyte colony-stimulating factor (G-CSF) was administered to 4 recipients following the 3-month biopsy; the same area was assessed 3 months later. Both dystrophin mRNA and protein levels were compared with baseline, and consistent with the clinical results, no changes were noted over the course of the study. Dystrophin was, however, easily detectable among muscle fibers after muscle grafting from the original hematopoietic stem cell donor performed as a positive control. Finally, clonal muscle cell cultures were established and analyzed for evidence of a donor contribution. Of the 251 clones analyzed, none were exclusively of the donor genotype, and a partial contribution by donor to a small percentage of the clonal muscle cell cultures suggests fusion or contamination by donor cells as 2 possible explanations.

Although these results are disappointing, the study is meritorious in that it was rigorously performed in a highly relevant large-animal model and serves to emphasize the value of such preclinical models. Certainly, these results would have been more disappointing were they obtained in a clinical trial initiated without large-animal preclinical testing. Thus, the paradigm shift that followed the demonstration of dystrophin expression in the murine mdx model has, at least for now, shifted back; yet whether HSCs can be coaxed toward a myogenic fate by further manipulation in vitro or in vivo will no doubt be the subject of future studies.