Whole F9 gene sequencing identified deep intronic variations in genetically unresolved hemophilia B patients

Background: The disease-causative variant remains unidentified in approximately 0.5% to 2% of hemophilia B patients using conventional genetic investigations, and F9 deep intronic variations could be responsible for these phenotypes.Objectives: This study aimed to characterize deep intronic variants in hemophilia B patients for whom genetic investigations failed.Methods: We performed whole F9 sequencing in 17 genetically unsolved hemophilia B patients. The pathogenic impact of the candidate variants identified was studied using both in silico analysis (MaxEntScan and spliceAI) and minigene assay.Results: In total, 9 candidate variants were identified in 15 patients; 7 were deep intronic substitutions and 2 corresponded to insertions of mobile elements. The most frequent variants found were c.278-1806A>C and the association of c.278-1244A>G and c.392-864T>G, identified in 4 and 6 unrelated individuals, respectively. In silico analysis predicted splicing impact for 4 substitutions (c.278-1806A>C, c.392-864T>G, c.724-2385G>T, c.723+4297T>A). Minigene assay showed a deleterious splicing impact for these 4 substitutions and also for the c.278-1786_278-1785insLINE. In the end, 5 variants were classified as likely pathogenic using the American College of Medical Genetics and Genomics guidelines, and 4 as of unknown significance. Thus, the hemophilia B-causing variant was identified in 13/17 (76%) families.Conclusion: We elucidated the causing defect in three-quarters of the families included in this study, and we reported new F9 deep intronic variants that can cause hemophilia B.

Automated summary

This study aimed to characterize deep intronic variants in hemophilia B patients for whom genetic investigations failed. Whole F9 sequencing was performed in 17 genetically unsolved hemophilia B patients. A total of 9 candidate variants were identified and their pathogenic impact was studied. Five variants were classified as likely pathogenic and four variants remained of unknown significance. The study successfully identified the hemophilia B-causing variant in 76% of the families included.