First case report of hemophilia B Leyden in Japan.

Hemophilia B Leyden is a unique subtype of hemophilia B, characterized by increasing factor IX activity (FIX:C) after puberty and a lower normal range of FIX:C throughout adulthood. However, to date, no Japanese case has been reported. Here, we report a case of hemophilia B Leyden in a 22-year-old male. He suffered from subgaleal hematoma, and was subsequently diagnosed with hemophilia B (FIX:C 0.2%) in the neonatal period. Both his parents are Japanese. There was no history of hemophilia in his family. FIX:C gradually increased with age (8% at age = 1; 14% at age = 7; 19% at age = 12; 32% at age = 18). FIX:C is within the range 30–40% in recent several years. He once required administration of FIX concentrate against traumatic tongue bleeding at 7 years of age. Genotyping analysis of FIX was performed after informed consent at 21 years of age, and a point mutation (c.–35G>A) was detected. This mutation has been reported previously as the Leyden mutation. Although it has been reported that hemophilia B Leyden is seen in 1.9% of patients with hemophilia B, the present case is the first report of hemophilia B Leyden from Japan.