July 28, 2008

Studies Bode Well for LGMD2D Gene Therapy Trial

Experiments conducted in mice suggest gene therapy for type 2D limb-girdle muscular dystrophy (LGMD2D) has the potential for safety and efficacy in humans with this serious muscle disease, says a new MDA-supported study.

Long-time MDA grantee, neurologist and clinical gene therapy specialist Jerry Mendell of Ohio State University and Nationwide Children’s Hospital in Columbus, Ohio, coordinated the research team, which included investigators from those institutions and from Harvard Medical School in Boston.

The investigators, who published their findings July 22 in Neurology, transferred human genes for the alpha-sarcoglycan protein, which is deficient in the muscles of people with LGMD2D, into mice bred not to produce this muscle protein.

In contrast to a study published in 2002 that suggested overproduction of alpha-sarcoglycan could have toxic effects in muscle tissue, this new study found intramuscular injections of the alpha-sarcoglycan gene to be safe and effective in the mice.

(The earlier study, conducted by Eric Hoffman at Children’s National Medical Center in Washington and colleagues, was published in the Sept. 1, 2002, issue of Human Gene Therapy.)

In the current study, the researchers inserted alpha-sarcoglycan genes into transporters made from modified type 1 adeno-associated viruses (AAV1s) and attached different types of “promoters,” molecular switches that cause cells to synthesize proteins from genetic instructions.

They injected these constructs into a leg muscle of 4- to 5-week-old mice that have a disease resembling human LGMD2D because they lack alpha-sarcoglycan.

With two of the promoters, MCK and truncated MCK, the researchers saw sustained production of alpha-sarcoglycan protein in 60 percent to 70 percent of the fibers in the injected area six and then 12 weeks after the injections. The other two promoters were less effective with respect to high-level, sustained protein production.

The MCK promoters are designed to switch on protein production only in muscle tissue, preventing alpha-sarcoglycan synthesis in tissues where it wouldn’t normally be found and could be hazardous.

Mendell noted the difference in their results compared to those in the 2002 study could be related to the type of AAV used (the earlier study used AAV2), the type of promoter used (the 2002 study used CMV) or the method used to purify the modified viruses.

“These data set the stage for a phase 1 clinical gene therapy trial for this severe LGMD2D,” the authors note. In fact, Mendell said, two patients had already been injected by mid-July.

The authors also note the mouse studies can only go so far in answering questions about safety and efficacy for gene therapy in humans and that better understanding of these outcomes await the results of the clinical trial.

Factors that bode well for the success of LGMD2D gene therapy are that the entire alpha-sarcoglycan gene can fit into a small AAV transporter (unlike large genes, such as the one for the muscle protein dystrophin); and most LGMD2D patients are deficient in but don’t entirely lack alpha-sarcoglycan protein, making a harmful immune response to the protein unlikely.

For details about the LGMD2D gene therapy clinical trial, see “Gene Transfer in Alpha-Sarcoglycan-Deficient LGMD”.

For details about a gene therapy clinical trial in Duchenne muscular dystrophy (DMD), see “Transfer of Dystrophin Genes”.