FACTOR-X AND ITS DEFICIENCY STATES

Factor X is one of the vitamin-K-dependent serine proteases. As a resu lt of its position at the convergence of the intrinsic and extrinsic p athways of the clotting cascade, it plays a crucial role in blood coag ulation. Factor X interacts with components of both pathways of coagul ation, leading to its activation and the formation of the prothrombina se complex. The gene for factor X has been cloned and sequenced and ma ps to the long arm of chromosome 13, approximately 2.8 kb downstream o f the factor VII gene. Each of the exons of factor X encodes a specifi c functional domain within the protein. In terms of its gene structure and amino acid sequence, factor X shows significant homology with oth er vitamin-K-dependent clotting factors, suggesting an origin in some common ancestral protein. Factor X deficiency is one of the rarest of the inherited coagulation disorders. Such deficiencies are inherited i n an autosomal recessive manner and are characterized by a variable bl eeding tendency. In its homozygous form, factor X deficiency has an es timated prevalence of 1:500 000 but in its heterozygous form it has an estimated frequency of similar to 1:500 although affected individuals are often clinically asymptomatic. Acquired deficiencies of factor X are uncommon and in isolation are seen most frequently in patients wit h amyloidosis and in association with upper respiratory tract infectio ns. Treatment of the deficiency state involves factor X replacement wi th either fresh frozen plasma or prothrombin complex concentrates. How ever, the latter may be associated with an increased risk of thrombosi s.