279. Efficient and Safe Lentiviral Vector-Mediated Hematopoietic Stem Cell Gene Therapy in MNGIE Mice

Abstract

Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) patients are deficient in thymidine phosphorylase(TP) resulting in systemic thymidine(Thd) and deoxyuridine(dUrd) accumulation affecting mtDNA replication and causing mitochondrial dysfunction. Common symptoms are gastrointestinal dysmotility, progressive ophthalmoplegia and leukoencephalopathy. Allogenic hematopoietic stem cell (HSC) transplantation has been shown to reduce disease symptoms, but is not well tolerated due to the inherent toxicity of the procedure. Therefore, syngeneic ex vivo lentiviral vector HSC gene therapy overexpressing the native cDNA or the codon optimized (TPco) sequence driven by the phosphoglycerate kinase (PGK) or spleen focus forming virus (SFFV) promoters in Tp−/-Upp−/- double knockout mice, a model for MNGIE disease, was investigated. At 1 month post transplantation after sublethal total body irradiation, very low TP activity was detected in blood of control wild type mice (0.07±0.03nmoles/h/mg), but enzyme activities in PGK treated mice were at least 90-fold higher (PGK-TP = 150±4 and PGK-TPco = 96±4 nmoles/h/mg), and in SFFV recipient mice 400-fold higher (450±5 nmoles/h/mg). Consequently, a significant reduction of plasma and urine Thd and dUrd levels was observed. Long-term follow up (14 months) showed on average 1.2-fold wild type TP activity levels increase in LV-PGK-TP and LV-PGK-TPco and 36-fold in SFFV-TPco treated mice. This was sufficient for sustained reduction of plasma and urine nucleoside levels, which was achieved at 76.5±8.2% donor chimerism levels with low LV vector copy numbers (1.0±1.1VCN/donor cell). The LV integration profile in bone marrow cells of primary recipients was analyzed; LVs displayed the expected tendency to integrate within highly expressed genes and the integration pattern did not differ from that of other SIN-LV vectors in other disease models (primary immune deficiencies and lysosomal enzyme storage disorders). Overall, stem cell gene therapy provided stable TP expression and long-term biochemical correction in MNGIE mice without genotoxicity or apparent phenotoxicity, which will be further evaluated for somatic and neurological phenotype correction and optimized to develop a clinical protocol to treat MNGIE patients.