Successful therapeutic effect in a mouse model of erythropoietic protoporphyria by partial genetic correction and fluorescence-based selection of hematopoietic cells

Erythropoietic protoporphyria is characterized clinically by skin photosens itivity and biochemically by a ferrochelatase deficiency resulting in an ex cessive accumulation of photoreactive protoporphyrin in erythrocytes, plasm a and other organs. The availability of the Fech(m1Pas)/Fech(m1Pas) murine model allowed us to test a gene therapy protocol to correct the porphyric p henotype. Gene therapy was performed by ex vivo transfer of human ferrochel atase cDNA with a retroviral vector to deficient hematopoietic cells, follo wed by re-injection of the transduced cells with or without selection in th e porphyric mouse. Genetically corrected cells were separated by FAGS from deficient ones by the absence of fluorescence when illuminated under ultrav iolet light. Five months after transplantation, the number of fluorescent e rythrocytes decreased from 61% (EPP mice) to 19% for EPP mice engrafted wit h low fluorescent selected BM cells. Absence of skin photosensitivity was o bserved in mice with less than 20% of fluorescent RBC. A partial phenotypic correction was found for animals with 20 to 40% of fluorescent RBC. In con clusion, a partial correction of bone marrow cells is sufficient to reverse the porphyric phenotype and restore normal hematopoiesis. This selection s ystem represents a rapid and efficient procedure and an excellent alternati ve to the use of potentially harmful gene markers in retroviral vectors.