Gene Therapy May Require Dodging Immune System

by Quest Staff on March 1, 2007 - 2:34pm

QUEST Vol. 14, No. 2

An MDA-supported research team, testing gene transfer in dogs using adeno-associated viral (AAV) shells (vectors), noted an unwanted immune response, a factor they recommend be considered in human trials. The response occurred even when the vectors weren’t carrying any genes.

Until now, AAV transporters have been thought not to provoke much of an immune response.

The team, from the University of Washington and the Fred Hutchinson Cancer Research Center, both in Seattle, injected leg muscles of dogs with types 2 and 6 AAV shells, both empty and carrying various genes (though none relevant to muscular dystrophy). They found the dogs mounted a “robust” immune response sufficient to largely eliminate production of proteins from the transferred genes. The results were published online Dec. 15 in Human Gene Therapy.

In earlier studies involving mice, the immune system appears to tolerate transferred genes well, as long as the genes are active only in muscle cells, where they seem to largely escape immunologic detection.

LGMD studies

A French team reported in the January issue of Molecular Therapy that it had successfully restored structure and function to the muscles of alpha-sarcoglycan-deficient mice, a model for human type 2D limb-girdle muscular dystrophy (LGMD2D). The investigators injected alpha-sarcoglycan genes in AAV vectors into an artery, using a synthetic molecular switch (promoter) called C5-12 to make the genes active only in muscle cells.

Using a similar strategy, MDA-supported University of Washington-Seattle scientists successfully transferred dystrophin genes via a tail vein in dystrophin-deficient mice with a disease resembling Duchenne muscular dystrophy (DMD). This group, which published results in the February issue of Molecular Therapy, used AAV vectors with a synthetic, muscle-only promoter called MHCK7.

MDA grantee Dongsheng Duan at the University of Missouri-Columbia, and colleagues, published findings online Jan. 30 in Molecular Therapy showing that their “split gene” strategy is realistic for systemwide gene delivery. In this approach, large genes (like the one for dystrophin) are divided into two parts and inserted into separate vectors. The extra space gained by using two AAV shells will, they say, allow scientists to add additional regulatory switches inside each shell.

Implications for humans

The jury is still out on the implications for humans. An unwanted immune response to AAV vectors alone, regardless of their payload, may require suppression when the vectors are injected systemically, if people respond to gene therapy as the dogs did. If the human response is similar to that of the mice, confining gene activity to the targeted tissue (such as muscle) will likely be sufficient to avoid immunologic rejection.

Stephen Tapscott at the Hutchinson Center, a member of the dog study team, suggests that the different responses of the two animals may have to do with the similar genetics of lab-bred rodents, compared to greater genetic diversity in randomly bred dogs. Tapscott, a member of MDA’s Scientific Advisory Committee, believes medications to suppress the immune system may be needed in at least some human gene therapy trials.

“We are currently exploring immunosuppressive regimens,” he said.

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