Structure and function of factor IX: defects in haemophilia B.

The genetics of haemophilia B and the structure-function relationships of factor IX interactions with cofactors and substrates have been reviewed. Emphasis has been placed on contributions to our understanding made by analysis of variants. Amino acid substitutions at or near the site of activation lead to inactive factor IX or to factor IX species with decreased clotting activity. Release of the activation peptide is necessary for optimal interaction of factor IX with its cofactors and substrates. Abnormalities in the calcium binding region, whether Gla independent or dependent, also decrease clotting activity. The defects in haemophilia Bm variants somehow affect factor VII-tissue factor interactions with factor X. Other mutations may affect the factor IX heavy chain, probably at or near the active site. Amino acid substitutions may cause conformational changes in factor IX that interfere with other interactions such as with antithrombin III and factor VIII. Recombinant DNA techniques have been employed to analyse normal and abnormal factor IX genes. DNA sequence analysis of factor IX cDNA clones revealed the primary structure of the mature protein and a predicted leader peptide. Knowledge of the primary sequence of factor IX allowed identification of the specific defect in the factor IX Chapel Hill variant. Analysis of normal factor IX genomic clones has determined that the 35 kb gene is composed of eight coding exons and seven intervening sequences. Sequence analysis of the CRM+ variants will identify mutations disrupting the normal interactions of factor IX. Southern analysis of CRM- variants has revealed gross factor IX gene deletions in some cases. Such deletions have been employed for carrier deletion in some families. Restriction fragment length polymorphisms in the factor IX gene have also proven useful for carrier identification. Manipulations of the cloned factor IX gene to make specific mutations in vitro and improvements in the technology for expression of deliberately modified genes will further elucidate the relationships between factor IX structure and function.