GENE-TRANSFER OF THE UROPORPHYRINOGEN-III SYNTHASE CDNA INTO HEMATOPOIETIC PROGENITOR CELLS IN VIEW OF A FUTURE GENE-THERAPY IN CONGENITAL ERYTHROPOIETIC PORPHYRIA

Congenital erythropoietic porphyria (CEP) is an inherited metabolic di sorder characterized by an overproduction and accumulation of porphyri ns in bone marrow. This autosomal recessive disease results from a def iciency of uroporphyrinogen III synthase (UROIIIS), the fourth enzyme of the haem biosynthetic pathway. It is phenotypically heterogeneous: patients with mild disease have cutaneous involvement, while more seve rely affected patients are transfusion dependent. The cloning of UROII IS cDNA and genomic DNA has allowed the molecular characterization of the genetic defect in a number of families. To date, 22 different muta tions have been characterized. Allogeneic bone marrow transplantation is the only curative treatment available for the severe, transfusion-d ependent, cases. When bone marrow transplantation cannot be performed owing to the absence of a suitable donor, the autografting of genetica lly modified cells is an appealing alternative. The best approach to s omatic gene therapy in this disease involves the use of recombinant re troviral vectors to transduce cells ex vivo, followed by autologous tr ansplantation of the genetically modified cells. We investigated retro viral transfer in deficient human fibroblasts, immortalized lymphoblas ts as well as bone marrow cells, and obtained a complete restoration o f the enzymatic activity and full metabolic correction. Using K562 cel ls, an erythroleukaemic cell line, the expression of the transgene rem ained stable during 3 months and during erythroid differentiation of t he cells. Finally, a 1.6- to 1.9-fold increase in enzyme activity comp ared to the endogenous level was found in normal CD34(+) cells, a popu lation of heterogeneous cells known to contain the progenitor/stem cel ls for long-term expression. The future availability of a mouse model of the disease will permit ex vivo gene therapy experiments on the ent ire animal.