[Severe prekallikrein deficiency due to a compound heterozygosis in the KLKB1-gene].

A 14 year old boy was referred to us for a detailed coagulation study because a previously performed aPTT has been found prolonged. The boy had no history of bleeding symptoms and also the family history was negative for bleeding or thrombotic events. The aPTT in the patient was 96 s (reference range: 24-36 s), prothrombin time and thrombin time were both normal. As the cause for the prolonged aPTT we identified a severe prekallikrein deficiency (prekallikrein activity < 1%). The prekallikrein deficiency results from two mutations in the KLKB 1-gene: first, an insertion of 1 bp in codon 149 in exon 5 and, second, a base exchange Cys 548 (TGC) > Tyr (TAC) in exon 14. The boy inherited the first mutation from his father and the second from his mother. The mutation in the paternal allele was not described before the completion of our study. There are two brothers of the propositus, one with normal prekallikrein activity and no mutations in the KLKB1-gene, the other showed the same constellation as the propositus.