Merry christmas for patients with hemophilia B.

Hemophilia B (also known as Christmas disease) is due to deficiency of coagulation factor IX (FIX). In this issue of the Journal, Nathwani et al. report the first unequivocal evidence of successful gene therapy for hemophilia B — a major advance in this field.1 This success for hemophilia may translate into gene therapy for other blood protein deficiencies. Hemophilia is due to deficiency in a coagulation factor and results in a bleeding disorder that often involves joints and muscles. The most common types are hemophilia A and B, which are due to deficiencies of factor VIII and FIX, respectively, and show X-linked inheritance. The first written account of hemophilia was in the 2nd century in the Babylonian Talmud, when Rabbi Judah decreed, “If she circumcised her first child and he died, and a second one also died, she must not circumcise her third child.”2 The first reported case of hemophilia due to FIX deficiency was in 1952 and was called “Christmas disease” after the patient, a 10-yearold boy named Stephen Christmas.3 Queen Victoria of the United Kingdom (1819–1901) was the most famous carrier of the hemophilia B gene. Although the protein and the gene were not known during her lifetime or the lifetimes of her affected male progeny (members of the royal families of the United Kingdom, Spain, Germany, and Russia), recent analysis of polymerase-chain-reaction assays of bones exhumed from the graves of Czar Nicholas II and his family showed that the FIX gene was mutated in his son Alexei Romanov, who had hemophilia.4 At least nine sons, grandsons, or great-grandsons of Queen Victoria were affected with hemophilia B, and the average age at death was 24 years. FIX concentrates were first used in the late 1960s to treat patients with hemophilia B, and their routine use for bleeding episodes increased the median lifespan to 63 years.5 Although enthusiasm for protein therapy was temporarily dampened by the HIV epidemic in the early 1980s, improved methods for producing FIX have increased its safety. Recently, implementation of prophylactic rather than on-demand treatment has reduced the risk of crippling joint disease.6 With the success of protein therapy, why would gene therapy be needed? In the United States and other developed countries, annual costs for a single adult patient of clotting factors for hemophilia are approximately $150,000 for on-demand therapy and $300,000 for prophylaxis,6 which could incur a lifetime cost of over $20 million. In developing countries, prophylactic and frequent on-demand therapy is not affordable, and patients still have chronic joint disease and die young.7 Nathwani et al. report the truly remarkable finding that a single intravenous injection of an adenovirus-associated virus (AAV) vector that expresses FIX can successfully treat patients with hemophilia B for more than a year.1 AAV is a small (4.8 kb), nonpathogenic, single-stranded DNA virus from the parvovirus family. The vector was generated by replacing the coding sequence for the cap and rep genes of the virus with a liver-specific promoter and the FIX coding sequence. The vector was packaged in cells that express cap and rep from a different piece of DNA that does not enter viral particles, thus generating a replication-incompetent vector that cannot propagate after gene transfer. Preclinical studies had shown that AAV vectors could be expressed from liver in large animals for at least