Hemophilia B

Haemophilia B, a congenital haemorrhagic disease caused by mutations in coagulation factor IX gene (F9), is considered an appropriate target for genome editing technology. Here, we describe treatment strategies for haemophilia B mice using the clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 system. Administration of adeno-associated virus (AAV) 8 vector harbouring Staphylococcus aureus Cas9 (SaCas9) and single guide RNA (sgRNA) to wild-type adult mice induced a double-strand break (DSB) at the target site of F9 in hepatocytes, sufficiently developing haemophilia B. Mutation-specific gene editing by simultaneous induction of homology-directed repair (HDR) sufficiently increased FIX levels to correct the disease phenotype. Insertion of F9 cDNA into the intron more efficiently restored haemostasis via both processes of non-homologous end-joining (NHEJ) and HDR following DSB. Notably, these therapies also cured neonate mice with haemophilia, which cannot be achieved with conventional gene therapy with AAV vector. Ongoing haemophilia therapy targeting the antithrombin gene with antisense oligonucleotide could be replaced by SaCas9/sgRNA-expressing AAV8 vector. Our results suggest that CRISPR/Cas9-mediated genome editing using an AAV8 vector provides a flexible approach to induce DSB at target genes in hepatocytes and could be a good strategy for haemophilia gene therapy.

Gene therapy has the potential to revolutionise treatment for patients with haemophilia and is close to entering clinical practice. While factor concentrates have improved outcomes, individuals still face a lifetime of injections, pain, progressive joint damage, the potential for inhibitor development and impaired quality of life. Recently published studies in adeno-associated viral (AAV) vector-mediated gene therapy have demonstrated improvement in endogenous factor levels over sustained periods, significant reduction in annualised bleed rates, lower exogenous factor usage and thus far a positive safety profile. In making the shared decision to proceed with gene therapy for haemophilia, physicians should make it clear that research is ongoing and that there are remaining evidence gaps, such as long-term safety profiles and duration of treatment effect. The eligibility criteria for gene therapy trials mean that key patient groups may be excluded, eg children/adolescents, those with liver or kidney dysfunction and those with a prior history of factor inhibitors or pre-existing neutralising AAV antibodies. Gene therapy offers a life-changing opportunity for patients to reduce their bleeding risk while also reducing or abrogating the need for exogenous factor administration. Given the expanding evidence base, both physicians and patients will need sources of clear and reliable information to be able to discuss and judge the risks and benefits of treatment.

BackgroundAdeno-associated virus (AAV)-mediated gene therapy has emerged as a promising treatment for hemophilia B. Data on safety and durability from 13 years of follow-up in a cohort of patients who had been successfully treated with scAAV2/8-LP1-hFIXco gene therapy are now available.MethodsTen men with severe hemophilia B received a single intravenous infusion of the scAAV2/8-LP1-hFIXco vector in one of three dose groups (low-dose: 2×1011 vector genomes [vg] per kilogram of body weight [in two participants]; intermediate-dose: 6×1011 vg per kilogram [in two]; or high-dose: 2×1012 vg per kilogram [in six]). Efficacy outcomes included factor IX activity, the annualized bleeding rate, and factor IX concentrate use. Safety assessments included clinical events, liver function, and imaging.ResultsParticipants were followed for a median of 13.0 years (range, 11.1 to 13.8). Factor IX activity remained stable across the dose cohorts, with mean factor IX levels of 1.7 IU per deciliter in the low-dose group, 2.3 IU per deciliter in the intermediate-dose group, and 4.8 IU per deciliter in the high-dose group. Seven of the 10 participants did not receive prophylaxis. The median annualized bleeding rate decreased from 14.0 episodes (interquartile range, 12.0 to 21.5) to 1.5 episodes (interquartile range, 0.7 to 2.2), which represented a reduction by a factor of 9.7. Use of factor IX concentrate decreased by a factor of 12.4 (interquartile range, 2.2 to 27.1). A total of 15 vector-related adverse events occurred, primarily transient elevations in aminotransferase levels. Factor IX inhibitor, thrombosis, or chronic liver injury did not develop in any participant. Two cancers were identified but were deemed by the investigators, together with an expert multidisciplinary team, as being unrelated to the vector. A liver biopsy that was conducted in 1 participant 10 years after the infusion revealed transcriptionally active transgene expression in hepatocytes without fibrosis or dysplasia. Levels of neutralizing antibodies to AAV8 remained high throughout follow-up, thus indicating potential barriers to readministration of the vector.ConclusionsA single administration of scAAV2/8-LP1-hFIXco gene therapy resulted in durable factor IX expression, sustained clinical benefit, and no late-onset safety concerns over a period of 13 years. These data support the long-term efficacy and safety of AAV gene therapy for severe hemophilia B. (Funded by the U.K. Medical Research Council and others; ClinicalTrials.gov number, NCT00979238; EudraCT number, 2005-005711-17.).

Hemophilia is an inherited disorder characterized by impaired blood clotting caused by mutations in the genes responsible for producing coagulation factor (F) VIII (hemophilia A, HA) or FIX (hemophilia B, HB). Current treatment primarily relies on replacement therapy, involving frequent and costly infusions of FVIII or FIX concentrates. While effective, these treatments come with the risk of developing neutralizing antibodies (inhibitors) against the infused factor. In recent years, non-factor replacement therapies have emerged as innovative treatment options, offering enhanced efficacy especially for patients with inhibitors. Despite their advantages, these approaches still fall short of providing a definitive, long-term cure. Since hemophilia is a monogenic disease, it presents an excellent opportunity for cell and gene therapy approaches aimed at achieving durable treatment and potentially a cure. Over the past three decades, remarkable advancements have been made in hemophilia gene therapy, culminating in the approval of Valoctocogene roxaparvovec (ROCTAVIAN, AAV-FVIII) and Etranacogene dezaparvovec (HEMGENIX, AAV-FIX) for patients with severe HA and HB, respectively. Nevertheless, gene therapy poses questions regarding its long-term efficacy and safety. This review synthesizes findings from clinical trials, addresses persistent challenges in hemophilia gene therapy, and underscores the biological constraints and limitations inherent to viral vector-based approaches.