Genetic Regulation of Factor VIII Inhibitor Development - PROJECT SUMMARY Regular infusion of replacement factor VIII (FVIII) is the gold standard of care to prevent bleeding in most patients with hemophilia A (HA). Because HA patients have absent or altered FVIII, normal FVIII constitutes a foreign antigen. Accordingly, HA patients treated with FVIII can develop anti-FVIII antibodies that can neutralize the pro- coagulant activity of FVIII and/or shorten its circulatory lifespan. Antibodies against FVIII can thus render FVIII treatment ineffective, making patients prone to bleeding as well as increasing their risk of morbidity and mortality. However, only a subset of patients with HA become immunized to FVIII. The exact mechanisms driving immunity to FVIII remain poorly understood, though there is evidence suggesting that multiple B cell pathways of antibody production may be involved. What regulates whether a patient will form anti-FVIII antibodies and the pathway by which these antibodies will develop remains unclear. In the current application, we demonstrate that immunity to FVIII is regulated by recipient genetics in a tractable mouse model, providing a sensitive system to elucidate genetic regulators of the immune response to FVIII. The current application proposes to use a specific outbred population of mice (Diversity Outbred (DO) mice) that was generated by multi-generational breeding of 8 inbred parental strains. As DO mice are highly variant and have undergone extensive recombination, the use of DO mice greatly increases the likelihood of identifying key genetic traits regulating the immune response to FVIII. Quantitative trait loci (QTL) analysis will be carried out using several important phenotypes found within the B cell and antibody response to FVIII. In addition, as the genomics platform used can distinguish contributions from each of the 8 inbred parental strains used to generate the DO mice, and the complete genomic sequence of each strain is known, the resulting study will be able to narrow down variants in coding regions, regulatory elements, or areas predicted to affect transcript splicing. The capacity of this approach to identify genetic risk factors is shown in preliminary data through which the same team of investigators on the current application successfully mapped genes associated with immune responses to a distinct antigen (other than FVIII) but for which there are also genetic regulators of antibody responses in mice. In aggregate, identification of genes regulating the B cell response and ability to generate an antibody response to FVIII using this approach would both provide specific targets for the development of novel therapeutics to prevent an immune response to FVIII, as well as having the immediate benefit of providing a clinical test to help predict, a priori, which patients are more likely to make anti-FVIII antibodies and/or benefit from immune tolerance induction. Lastly, FVIII produced following gene therapy has the potential to induce the formation of antibodies against FVIII that can limit (or eliminate) the benefits of gene therapy. Thus, the proposed study has relevance not only to maintaining FVIII as a lifesaving therapy as part of current treatments for HA, but also to the developing field of gene therapy that promises to be a cure for HA and for which anti-FVIII antibodies can be a serious impediment.
