Maternal low-level somatic mosaicism of Cys155Tyr of F9 in severe hemophilia B

Hemophilia B is an X-linked bleeding disorder caused by deficient coagulation factor IX from a mutation in the F9 gene. Here, we report a family with two brothers having severe hemophilia B inherited from a mother with low-level somatic mosaicism of a F9 mutation. The proband was a 2-year-old boy with severe hemophilia B from a hemizygous mutation of F9, c.464G>A (p.Cys155Tyr). He was the first child and was considered a sporadic case based on the lack of family history of bleeding diathesis. His mother was tested for carrier status and was determined to be homozygous for wild-type genotypes (noncarrier). Subsequently, however, his brother was born and also had severe hemophilia B from Cys155Tyr. This prompted us to review the chromatogram of the mother, which revealed a small peak corresponding to the mutant genotype. On suspicion of somatic low-level mosaicism in the mother, we further performed allele-specific PCR and thymine and adenine cloning, and confirmed the presence of the mutant allele in the mother. To our knowledge, this is the first case of maternal somatic mosaicism for a cytosine-phosphate-guanine transition mutation in hemophilia B. The acknowledgment of somatic mosaicism and further molecular investigation are important in sporadic hemophilia B to deliver informative genetic counseling and risk assessment.