Two factor IX mutations in the family of an isolated haemophilia B patient: direct carrier diagnosis by amplification mismatch detection (AMD)

Abstract

Rapid identification of gene defects allows definite carrier and prenatal diagnosis in virtually every family with haemophilia B. We report a study of the family of an isolated patient. Analysis of all the essential regions of the patient's factor IX gene (promoter, exons, transcript processing signals) revealed two mutations: one C----T transition at residue 17762 and another at residue 30890. The former created a translation stop at codon 116, and the latter caused substitution of His 257 by Tyr. The translation stop is an obvious detrimental mutation, while the His 257----Tyr substitution has uncertain functional consequences. From analysis of other family members, it was found that the first mutation had occurred at grandpaternal gametogenesis, in keeping with the negative family history, while the second was of more remote origin and did not reduce the maternal grandfather's factor IX coagulant and antigen level. This neutral mutation (His 257----Tyr) pinpoints a poorly conserved amino acid in factor IX and related serine proteases. Its coexistence with a detrimental mutation stresses the need to examine all essential regions of a gene before attempting to interpret the functional consequences of its sequence changes.

Two mutations in the same low-density lipoprotein receptor allele act in synergy to reduce receptor function in heterozygous familial hypercholesterolemia

“…Pathogenicity should not be ascribed to missense mutations including small in-frame deletions, unless gene expression in cells has been studied. Missense mutations have been described in several genes (Brody et al, 1992;Montandon et al, 1990) including the LDL receptor (Jensen et al, 1994b) that do not cause disease. Our data also demonstrate that expression experiments do not necessarily reveal pathogenicity.…”

Polymorphic variation within “conserved” sequences at the 3′ end of the human RDS gene which results in amino acid substitutions

“…Indeed a missense mutation has been observed in Factor IX (H257Y) which does not cause hemophilia B in a male (Montandon et al, 1990). …”

The contribution of DNA analysis to carrier detection and prenatal diagnosis of hemophilia A and B

“…Direct detection of mutations will be valuable especially for female relatives of isolated hemophilia patients, the more so, since knowledge about the occurrence of germline mosaicism has further limited the usefulness of RFLP analysis in these families. However, in all cases of DNA-based diagnosis, it should be determined whether the alteration in the DNA sequence is indeed the causative mutation in a particular family [6,52]. This implies that with advances in DNA technology, studies at the protein level remain essential, not only for scientific but also for practical purposes.…”