Results from an MDA-supported, phase 1 study of gene therapy for the type 2D form of limb-girdle muscular dystrophy (LGMD2D) show sustained protein production from the transferred genes in two out of three trial participants six months after injection of the genes into a foot muscle.
The new results follow an announcement last year showing protein production from transferred genes in the first three participants in this trial at either six weeks or three months after the gene transfer.
The gene delivered in this trial was for the muscle protein known as alpha-sarcoglyan, which is deficient in people with LGMD2D.
A cautionary note is that one of the three participants whose foot muscle was biopsied at six months post-injection showed evidence of an immune response against the viral vehicle used to deliver the gene. The vehicle is derived from a type 1 adeno-associated virus (AAV1).
Plans for a trial of whole-limb delivery of alpha-sarcoglycan genes in LGMD2D patients are moving forward.
About the new findings
Neurologist Jerry Mendell at Nationwide Children's Hospital in Columbus, Ohio, coordinated the study team, which published its findings online Oct. 28, 2010, in Annals of Neurology. Mendell, who received MDA support for this study, directs the MDA clinic and the Center for Gene Therapy at Nationwide.
The findings include results of muscle biopsy examinations in three LGMD2D patients who received injections of alpha-sarcoglycan genes encased in AAV delivery vehicles into a foot muscle and a saline injection into the corresponding muscle in the other foot.
Six months after the injections, two of the three trial participants showed sustained production of alpha-sarcoglycan protein in the injected foot muscle. However, one participant did not.
A previous paper, published online in April 2009 in Annals of Neurology, reported good alpha-sarcoglycan protein production in the injected foot muscle in the first three trial participants in this phase 1 trial, whose biopsies were taken at six weeks (two people) or three months (one person) post-injection. (See LGMD gene therapy trial first to show promise beyond safety alone.)
In the 2009 report, no adverse events were seen, and alpha-sarcoglycan protein production was four to five times greater in the gene-injected foot muscle compared to the saline-injected foot muscle.
At that time, the researchers noted that the alpha-sarcoglycan protein apparently had inserted itself into the proper place in the muscle-fiber membrane; that associated proteins had properly attached themselves to it; and that muscle-fiber size had increased in the person whose biopsy was taken at three months after treatment.
In these first three trial participants, the researchers noted some evidence of an immune response against the viral delivery vehicle in one subject, but they did not consider it significant.
In the new paper, the investigators report on three additional trial participants, each of whom underwent a foot-muscle biopsy at six months post-injection, allowing the investigators to assess whether alpha-sarcoglycan production could be sustained over that length of time. (Each of the total of six trial participants underwent only one muscle biopsy, during which the entire small foot muscle was removed. Therefore, it would not have been possible for the researchers to compare the same person's foot muscle at different time points.)
In two of the participants whose biopsies were taken at six months, the research team saw sustained production of alpha-sarcoglycan, with the protein reaching normal levels in the treated foot muscle; restoration of the proteins that normally attach to alpha-sarcoglycan in the muscle-fiber membrane; and an increase in muscle-fiber size.
In the third person whose foot muscle was examined at six months, they saw no difference between the gene-injected and the saline-injected foot muscles.
This person, but not the other two, showed evidence of two types of immune response (cellular and antibody-related) to the AAV1 delivery vehicle. Blood tests showed this person had pre-existing immunity to AAV, perhaps from an earlier viral infection.
The researchers say this type of testing could be used to screen future trial participants so that those who may benefit the most from the gene therapy can be selected for a trial.
No serious adverse events were seen in any of the three subjects in the second part of this phase 1 trial, although two developed sore throats three weeks and nine months after gene transfer. (There's no evidence that the sore throat had any relationship to the treatment.)
Differences from Duchenne gene therapy results
In marked contrast to the recent report of an immune response to a newly synthesized dystrophin protein in a gene therapy trial in Duchenne muscular dystrophy (DMD) (see Immune Response Must Be Considered in DMD Gene Therapy) the researchers did not see any evidence that participants in the LGMD2D trial mounted an immune response against the newly synthesized alpha-sarcoglycan protein.
In the DMD trial, they did see an immune response against the dystrophin protein being produced from the transferred dystrophin genes but saw no evidence of an immune response against the AAV delivery vehicle.
The next step in LGMD2D gene therapy is to deliver the alpha-sarcoglycan gene to an entire limb, via the bloodstream. Mendell has an MDA grant to pursue this type of gene therapy for this disease.
Meaning for people with LGMD2D
The researchers say they do not feel the immune response seen in one patient to the AAV1 delivery vehicle is concerning enough to stop progress on whole-limb delivery of alpha-sarcoglycan genes, although it will likely modify the criteria they use to select participants for this higher-dose gene therapy trial.
"The overall favorable findings in this clinical trial lay the foundation for further gene therapy steps that can be taken for LGMD2D patients," the researchers write in the October 2010 paper. They say the study "opens the door for potential safe and effective delivery of the sarcoglycan gene through the circulation."