Erythropoietic porphyrias: animal models and update in gene-based therapies.

The inherited porphyrias are inborn errors of haem biosynthesis, each resulting from the deficient activity of a specific enzyme of the haem biosynthetic pathway. Porphyrias are divided into erythropoietic and hepatic according to the predominant porphyrin-accumulating tissue. Three different erythropoietic porphyrias (EP) have been described: erythropoietic protoporphyria (EPP, MIM 177000) the most frequent, congenital erythropoietic porphyria (CEP, MIM 263700), and the very rare hepatoerythropoietic porphyria (HEP, MIM 176100). Bone marrow transplantation is considered as the only curative treatment for severe cases of erythropoietic porphyria (especially CEP), if donors are available. Some EPP patients who undergo liver failure may require hepatic transplantation. Murine models of EPP and CEP have been developed and mimic most of the human disease features. These models allow a better understanding of the pathophysiological mechanisms involved in EP as well as the development of new therapeutic strategies. The restoration of deficient enzymatic activity in the bone marrow compartment following gene therapy has been extensively studied. Murine oncoretroviral, and recently, lentiviral vectors have been successfully used to transduce hematopoietic stem cells, allowing full metabolic and phenotypic correction of both EPP and CEP mice. In CEP, a selective survival advantage of corrected cells was demonstrated in mice, reinforcing the arguments for a gene therapy approach in the human disease. These successful results form the basis for gene therapy clinical trials in severe forms of erythropoietic porphyrias.