In an abnormal
fibrinogen (
fibrinogen Naples) associated with congenital
thrombophilia we have identified a single base substitution (G----A) in the B beta chain gene that results in an amino acid substitution of
alanine by
threonine at position 68 in the B beta chain of
fibrinogen. The propositus and two siblings were found to be homozygous for the mutation, whereas the parents and another sibling were found to be heterozygous. Individuals homozygous for the defect had a severe history of both arterial and
venous thrombosis; heterozygous individuals had no clinical symptoms. The three homozygotes had a prolonged
thrombin clotting time in plasma, whereas the heterozygotes had a normal
thrombin clotting time.
Fibrinopeptide A and B (FpA and FpB) release from purified
fibrinogen by human
alpha-thrombin was delayed in both the homozygous propositus and a heterozygous family member. Release of FpA from the normal and abnormal
amino-terminal disulfide knot (NDSK) corresponded to that found with the intact
fibrinogens, indicating a decreased interaction of
thrombin with the NDSK part of
fibrinogen Naples. Binding studies showed that
fibrin from homozygous abnormal
fibrinogen bound less than 10% of active site inhibited
alpha-thrombin as compared with normal
fibrin, while
fibrin formed from heterozygous abnormal
fibrinogen bound approximately 50% of
alpha-thrombin. These results suggest that the mutation of B beta Ala 68----Thr affects the binding of
alpha-thrombin to
fibrin, and that defective binding results in a decreased release of FpA and FpB in both homozygous and heterozygous
abnormal fibrinogens.