MOLECULAR GENETICS OF HAEMOPHILIA
Authors: Giannelli B F 
Published: August 2018

Haemophilia B, an X-linked recessive disease with an incidence of 1/30,000 newborn males, is due to defects in the gene for coagulation factor IX, which is on the long am of the X chromosome at band Xq27.1. This gene consists of approximately 34 Kb and contains 8 exons which specify a mRtfc of 2803 residues coding for a protein of 415 aa preceded by a prepro signal peptide of 46 aa. Coripanson of the functional domains of the factor IX protein with the exon structure of the gene supports the exon/protein domain hypothesis of gene evolution. The factor IX gene seems to be formed by a number of functionally and evolutionally independent modules. The signal peptide and the gla (γcarboxy-glutamic) region encoded in the first three exons are homologous to those of factor X, protein C and prothrombin. Thevfourth and fifth exons which code for the connecting peptide are homologous to one another and to the epidermal growth factor, a module that has been used in the construction of a great variety of proteins including different members of the coagulation and fibrinolytic pathways. The sixth exon encodes the activation peptide region, while the catalytic region of factor IX is coded by the seventh and eighth exon. This is at variance with other serine protease genes that have different exons for the segments containing the cardinal ami no-acids of the active centre (histidine, aspartic acid and serine).

Natural selection acts against detrimental mutations of the factor IX gene and at each generation a proportion of haemophilia B genes is eliminated, as a significant number of patients does not reproduce. There appears to be no selective advantage to the heterozygote and therefore haemophilia B is maintained in the population by new mutations. Consequently, a significant proportion of patients should be born to non-carrier mothers, and unrelated patients should carry different gene defects, as recently verified by detailed analysis of individual haemophilia B genes.

The defects of factor IX described so far comprise both point mutations and gene deletions. The latter affect either part or the whole of the gene and are often associated with the development of antibodies against therapeutically adninistered factor IX (the inhibitor complication). Since gene deletions may result in the complete absenceof factor IX synthesis or in the production of an extremely abnormal product, it has been suggested that mutationspreventing the synthesis of a factor IX gene product capable of inducing immune tolerance to normal factor IX is important in predisposing to the inhibitor complication.

Among the point mutations described so far, those affecting the signal peptide are of particular interest. Substitutions of the arginine at positions -4 and -1 cause failure of propeptide cleavage. Thus they indicate that the propeptide consists of 18 aa an(lthat lts excision is necessary for factor IX function. It appears also that the propeptide contains a signal for γcarboxylation which has been conserved during the evolution of different γcarboxylated proteins.

In spite of coagulant treatment, haemophilia B is a serious disease and one for which genetic counselling is required. Paramount for this is the detection of carriers and the diagnosis ofaffected male fetuses. DNA probes derived from the cloned factor IX gene have been used for this purpose. Carrier and first or second trimester prenatal diagnoses have been done using factors IX gene markers to follow the transmission of haemophilia B genes. Six sequence variations causing restriction fragment length polymorphisms (RFLP) in the factor IX gene have been detected and used as markers for such indirect diagnoses The efficiency of the above markers is reduced by linkage disequilibrium but, nevertheless, they offer definite carrier and nremtal diagnoses in 75-80% of the relatives of familial cases of haemophilia B.

The indirect detection of gene defects is of modest help in the counselling of individuals from the families of isolated patients, but new methods for the direct detection of gene mutations promise better results in such families and also the attainment of % diagnostic success in relatives of familial cases.

Finally the successful expression of recombinant factor IX genes in tissue culture and transgenic mammals raises hopes of therapeutic advances.