Athn 10 - Novel Variants in Persons with Rare Coagulation Disorders across Hemophilia Treatment Centers in the United States

The incidence of ultra-rare bleeding coagulation disorders is unknown but is critically important to understand the need for specialized health care services and linkage of individuals to treatment. However, the ability to identify and characterize these disorders may be difficult based upon the absence of specialized testing and the presence of more than one condition, which may impact bleeding phenotype. To better understand the nature and frequency of ultra-rare bleeding coagulation disorders in the US, the American Thrombosis and Hemostasis Network (ATHN) initiated ATHN 10, a collaborative effort within its network of 140 ATHN-affiliated hemophilia treatment centers (HTCs), to perform whole gene sequencing and collection of detailed phenotypic data, including bleeding scores on enrolled patients with one of 31 identified ultra-rare bleeding coagulation disorders. Thus far, 1,225 individuals with an established ultra-rare bleeding coagulation disorder have been sequenced for their expected factor deficiency in a panel that includes their targeted gene along with 29 other clotting factor and selected platelet protein genes to determine the presence of novel variants and possibility of more than one disorder impacting the bleeding phenotype. Over 20,720 whole gene sequences have been analyzed in ATHN 10 with 176 novel variants being identified. In 31% of individuals, an additional mutation was found in a second gene not suspected in the original diagnosis. Some of the secondary variants, resulted in a decreased production of the second affected factor, which possibly influenced the bleeding phenotype more than expected for identified deficiency alone. These secondary deficiencies, or modifying variants, were surprisingly common and found in participants across 8 regions in the US Hemophilia Treatment Center Network, with unique clusters identified. Although most of the 176 novel variants, not previously described in the literature, were identified in single individuals, there were 6 novel variants on 5 different genes that were found in multiple individuals. The variants F10: R445H, ITGB3: 615-1G>C, and F7: G406D were found in individuals at different HTCs. Variants F7: G432S, FGB: 719-9_719-2delCATTTGCA, and FGG: T80fs*7 were found in three multigenerational family studies at the same HTC. By leveraging the infrastructure at ATHN-affiliated HTCs and the laboratory services at the Center for Inherited Blood Disorders (CIBD)/Hematology Advanced Diagnostic Laboratory (CAP/CLIA Certified), the ATHN 10 project reduced the barriers associated with genetic testing and has expanded knowledge of the incidence of new pathogenic variants at reduced cost, allowing HTCs to provide more specific care and genetic counseling. In addition, whole gene sequencing has confirmed the role of certain genetic hot spots in clotting factor genes as well as the role of splice site variants in determining factor activity levels and bleeding phenotypic variance. This information not only helps patients and physicians, but also provides researchers with valuable resources to further our understanding of the pathophysiology of factor function and deficiencies.