Permanent phenotypic correction of Haemophilia B in immunocompetent mice by prenatal gene therapy

Haemophilia B arises from mutations in the factor IX gene. Its treatment in humans, by recombinant protein substitution, is expensive thus limiting its application to intermittent treatment in bleeding episodes and prophylaxis during surgery; development of inhibitory antibodies is an associated hazard. This study demonstrates permanent therapeutic correction of this disease without development of immune reactions by introduction of HIV-based lentiviral vector encoding the human factor IX protein into the fetal circulation of immunocompetent haemophiliac and normal outbred mice. Plasma factor IX persists at therapeutic concentrations in all treated mice to date (ten months). Functional normalisation of the blood clotting has been achieved in two haemophiliac mice and significant correction in a third. In comparison, administration of an adenoviral vector encoding human factor IX resulted in transient and diminishing factor IX expression which was nearly undetectable by 8 months. Following lentiviral administration, no anti-factor IX antibodies were detected by ELISA and no cellular immunity was detected by immunohistochemical staining of macrophages, neutrophils, CD4-positive or CD8-positive cells. There was no elevation of serum liver enzymes or evidence of vector spread to the maternal circulation. No vector spread to sperm was detected by TaqMan analysis. This is the first demonstration of complete phenotypic correction of a severe genetic disease by in utero gene therapy with no evidence of toxicity.