Gene Therapy in Haemophilia

Haemophilia A (factor VIII {FVIII} deficiency) and haemophilia B (factor IX {FIX} deficiency) are rare X-linked bleeding disorders occurring at an incidence of 1:5,000 and 1:25,000 males throughout the world. Treatment of these conditions by replacement therapy with plasma-derived FVIII or FIX concentrates has been established in the developed world since the mid 1970s, but has brought with it the transmission of blood-borne viruses such as HIV and hepatitis B and C. Although today viral inactivation procedures have virtually eliminated such transmission, concerns over possible new viruses remain. Also in much of the world, blood transfusion services are unlikely to ever be able to meet the demand for blood product required to treat haemophilia effectively. The identification and cloning of the FVIII and FIX genes and their expression in mammalian cells in culture some 10 years ago has resulted today in the availability of recombinant FVIII and FIX, although such products are expensive and have had little impact on the lack of treatment available to over 80% of the world’s haemophiliacs. However, these studies have opened up the possibility of gene therapy for haemophilia and the dream of a “cure”. For the reasons outlined below, many gene therapy studies have focused upon haemophilia B and FIX. Gene therapy for haemophilia is achieved when the FVIII or FIX gene, once inserted into cells or tissues in the body of a haemophiliac, expresses the missing factor which is then secreted from the cell into the blood in quantities sufficient to correct the bleeding diathesis. However as a first approach it would also be worthwhile if plasma levels of the factor could be achieved such as to convert a severe case into a mild one by a modest 5–10% increase in plasma level. In order to achieve gene therapy, the gene and appropriate expression sequences must be inserted into normal cells, either ex vivo followed by transplantation back into the body or in vivo. This is performed either by transfection (by cell membrane disruption) or transduction (controlled insertion) of the DNA in a vector. Vectors are usually viral based although non-viral systems are increasingly being studied. The properties of vectors are summarised in Table 1 and are discussed below.