Gene therapy for canine alpha-L-iduronidase deficiency: In utero adoptive transfer of genetically corrected hematopoietic progenitors results in engraftment but not amelioration of disease

Canine alpha-L-iduronidase (iduronidase) deficiency is a model of the human lysosomal storage disorder mucopolysaccharidosis type I (MPS I), We used t his canine model to evaluate the therapeutic potential of hematopoietic ste m cell (HSC) gene therapy for enzyme deficiencies. In previous studies, idu ronidase-deficient dogs infused with autologous marrow cells genetically mo dified to express iduronidase had long-term engraftment with provirally mar ked cells, but there was no evidence of proviral iduronidase expression or clinical improvement. The presence of humoral and cellular immune responses against iduronidase apparently abrogated the therapeutic potential of HSC gene therapy in these experiments, To evaluate HSC gene therapy for canine MPS I in the absence of a confounding immune response, we have now performe d in utero adoptive transfer of iduronidase-transduced MPS I marrow cells i nto preimmune fetal pups, In three separate experiments, 17 midgestation fe tal pups were injected with 0.5-1.5 x 10(7) normal or MPS I allogeneic long -term marrow culture (LTMC) cells transduced with neo(r)- or iduronidase-co ntaining retroviral vectors. Nine normal and three MPS I pups survived the neonatal period and demonstrated engraftment of provirally marked progenito rs at levels of up to 12% for up to 12 months. However, the proportion of p rovirally marked circulating leukocytes was similar to 1%. Neither iduronid ase enzyme nor proviral-specific transcripts were detected in blood or marr ow leukocytes of any MPS I dog. Humoral immune responses to iduronidase wer e not detected in neonates, even after "boosting" with autologous iduronida se-transduced LTMC cells. All MPS I dogs died at 8-11 months of age from co mplications of MPS I disease with no evidence of amelioration of MPS I dise ase. Our results suggest that iduronidase-transduced primitive hematopoieti c progenitors can engraft in fetal recipients, contribute to hematopoiesis, and induce immunologic nonresponsiveness to iduronidase in MPS I dogs, How ever, the therapeutic potential of HSC gene transfer in this model of iduro nidase deficiency appears to be limited by poor maintenance of proviral idu ronidase gene expression and relatively low levels of genetically corrected circulating leukocytes.