61. Successful Gene Therapy of a Mouse Model of Congenital Erythropoietic Porphyria with an Erythroid-Specific Lentiviral Vector

Abstract

Congenital erythropoietic porphyria (CEP) is a recessive autosomal disorder characterized by a deficiency in uroporphyrinogen III synthase (UROS), the fourth enzyme of the heme biosynthetic pathway. The severity of the disease, the lack of specific treatment except for allogenic bone marrow transplantation (BMT) and the knowledge of the molecular lesions are strong arguments towards gene therapy. We have recently developed a knock-in mouse model of CEP (Ged et al., Genomics, 2006) in order to evaluate the feasibility of gene therapy in haematopoietic stem cells (HSCs). This novel mouse model closely mimics the CEP disease in humans (porphyrins accumulation in urine, blood and spleen, erythrodontia, moderate photosensitivity, hepatosplenomegaly, and haemolytic anaemia). Here we develop a self-inactivating lentiviral vector containing human UROS cDNA driven by the human ankyrin-1/b-globin HS-40 chimeric erythroid promoter/enhancer. Murine HSCs from CEP donor mice were transduced by the erythroid-specific lentiviral vector and injected them into CEP recipient mice conditioned with high doses of busulfan. We observed a high transduction efficiency of HSCs (88 |[plusmn]| 14% PCR+ CFC, 20 weeks after BMT) resulting in an effective gene therapy of primary recipient CEP mice without any selectable system. A full restoration of enzymatic activity in BM, spleen and peripheral blood cells was obtained (2 fold increased UROS activity versus normal mice) resulting in skin photosensitivity correction and disappearance of splenomegaly. Furthermore, we observed a dramatic decrease of porphyrin accumulation in red blood cells (RBCs) and urines as well as a full correction of haemolytic anemia. In addition, a complete disappearance of fluorescent RBCs (fluorocytes) was observed by FACS. We are currently investigated the existence of a natural selective advantage of corrected cells in CEP mice. We demonstrate for the first time the high efficiency of HSCs gene therapy using a SIN erythroid-specific lentiviral vector in a murine model of CEP. This study forms the basis of gene therapy clinical trial for this severe congenital erythropoietic porphyria disease.