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.

Congenital factor XIII (FXIII) deficiency is a rare autosomal recessive bleeding disorder mainly caused by mutations in the F13A1 gene on 6p25.1, which lead to defective A subunit of FXIII. We herein describe two unrelated Korean patients with congenital FXIII deficiency. Proband 1 (a 30-year-old man) and Proband 2 (a 10-year-old girl) presented with severe bleeding episodes (huge intramuscular hematoma and acute intracerebral hemorrhage). Coagulation screening tests for bleeding diathesis were normal, but the FXIII activity was undetectable on urea clot lysis assay. The molecular genetic analysis of F13A1 revealed two mutations in the patients: Proband 1 was homozygous for a previously reported mutation c.1984C>T (p.Arg662) and Proband 2 was compound heterozygous for c.1029T>A (p.His343Gln) and c.1984C>T (p.Arg662). His343Gln was a novel missense mutation occurring in the core domain of the FXIII A subunit. This is the first report of genetically confirmed FXIII deficiency in Korea, with novel and recurrent F13A1 mutations.