Human hematopoietic progenitor cell-directed gene therapy in a murine xenotransplantation model of mucopolysaccharidosis type VII

Mucopolysaccharidosis type VII (MPSVII) is a lysosomal storage disease (LSD) caused by a deficiency in the enzyme beta-glucuronidase (GUSB). Without GUSB activity, glycosaminoglycans progressively accumulate in tissues throughout the body, leading to a range of devastating clinical manifestations. Several LSDs including MPSVII have shown a positive response to bone marrow transplantation, suggesting that gene therapy directed at hematopoietic progenitor cells (HPCs) would be a viable therapeutic approach. Clinical experience has emphasized the importance of optimizing HPC gene transfer protocols directly on human cells using an in vivo assay. Xenotransplantation of human HPCs into mice like the Non-Obese Diabetic/Severe Combined Immunodeficient (NOD/SCID) strain has allowed such optimization to be performed. In order to evaluate the therapeutic efficacy of MPSVII gene therapy directed at human HPCs, a novel mouse model was generated by backcrossing the murine MPSVII mutation onto the NOD/SCID xenotransplantation strain. Transplantation of these NOD/SCID/MPSVII mice with unmanipulated CD34+ HPCs from healthy GUSB+ human donors led to high levels of long-term donor-derived lymphoid and myeloid engraftment in the bone marrow, spleen, and blood. Engrafted human cells were also identified by virtue of their GUSB activity in non-hematopoietic tissues including the liver, kidney, lung, heart, and brain. Importantly, a reduction of pathologic storage material was found in several of these tissues, thereby establishing a baseline to measure the efficacy of a subsequent gene therapy experiment. For this translational study, human CD34+ HPCs were isolated from the mobilized peripheral blood of an MPSVII patient and transduced with an HIV-based lentiviral vector encoding a functional human GUSB cDNA. Twelve weeks following transplantation of these cells to NOD/SCID/MPSVII recipients, a substantial fraction of the donor-derived human lymphoid and myeloid engraftment highly expressed GUSB. Positive cells were detected throughout the recipient mice and therapeutic effects were observed in several tissues. In conclusion, through use of NOD/SCID/MPSVII mice it was determined that a gene transfer protocol was able to correct primitive human MPSVII HPCs with enough efficiency to yield therapeutic effects similar to that of healthy donor cell transplantation. These preclinical results may facilitate the organization of potential trials for diseased patients.