Induction of Immune Tolerance to Hexosaminidase M Using Rapamycin and Prednisone in the scAAV9-HEXM Gene Therapy Treatment of Sandhoff Disease
Sandhoff disease (SD) is a neurodegenerative disorder caused by the toxic accumulation of GM2 gangliosides in the brain. This is due to a deficient enzyme, β-hexosaminidase A (HEXA), involved in the catabolism of GM2 gangliosides. A recently constructed isoenzyme analogous to HEXA, called Hexosaminidase M (HEXM), can efficiently catabolize GM2 gangliosides, while its gene can be compactly packaged into the self-complementary adeno-associated viral vector serotype 9 (scAAV9). Previous gene transfer studies have shown a significant survival benefit of scAAV9-HEXM in the hexb(-/-) knockout SD murine model. However, subsequent analyses revealed that scAAV9-HEXM has the potential to provoke an immune response against the expressed human HEXM product. This undesirable immune response can prove to be an obstacle for the long-term treatment efficacy, especially in individuals who lack the native HEXA protein (e.g., in infantile forms of the diseases). We hypothesized that suppressing the immune system will allow the body to tolerate the scAAV9-HEXM treatment and the newly-produced HEXM protein. Rapamycin (R) and prednisone (P) are immunosuppressants with a regulatory T-cell sparing effect, and both are used clinically to suppress cytotoxic immune responses. In this study, the administration of R and/or P in conjunction with scAAV9-HEXM was studied in SD mice. It was observed that administering both R and P long-term with scAAV9-HEXM demonstrated a significant reduction in antibody and IFN-g T cell responses to HEXM. which persisted after R&P administration ceased. This successful use of the R&P and scAAV9-HEXM resulted in significantly higher hexosaminidase enzyme activity levels in serum, reduced GM2 accumulation in the brain, and an increase in biodistribution of the vector in the liver. These mice also showed an extended lifespan beyond that achieved by the gene therapy in the absence of immunosuppression, with mice living for up to 7 weeks longer. The outcomes of this study may provide a proof-of-concept toleration immunosuppression regimen for long-term efficacy of not only the scAAV9-HEXM treatment, but also other gene therapy studies using AAV.