Hemophilia B: molecular pathogenesis and mutation analysis

Abstract

Hemophilia B is an X-chromosome-linked inherited bleeding disorder primarily affecting males, but those carrier females with reduced factor IX activity (FIX:C) levels may also experience some bleeding. Genetic analysis has been undertaken for hemophilia B since the mid-1980s, through linkage analysis to track inheritance of an affected allele, and to enable determination of the familial mutation. Mutation analysis using PCR and Sanger sequencing along with dosage analysis for detection of large deletions/duplications enables mutation detection in > 97% of patients with hemophilia B. The risk of the development of inhibitory antibodies, which are reported in ~ 2% of patients with hemophilia B, can be predicted, especially in patients with large deletions, and these individuals are also at risk of anaphylaxis, and nephrotic syndrome if they receive immune tolerance induction. Inhibitors also occur in patients with nonsense mutations, occasionally in patients with small insertions/deletions or splice mutations, and rarely in patients with missense mutations (p.Gln237Lys and p.Gln241His). Hemophilia B results from several different mechanisms, and those associated with hemophilia B Leyden, ribosome readthrough of nonsense mutations and apparently 'silent' changes that do not alter amino acid coding are explored. Large databases of genetic variants in healthy individuals and patients with a range of disorders, including hemophilia B, are yielding useful information on sequence variant frequency to help establish possible variant pathogenicity, and a growing range of algorithms are available to help predict pathogenicity for previously unreported variants.

Keywords: DNA Mutational Analysis; factor IX; genetic carrier detection; hemophilia B; prenatal diagnosis.

Figure 1

Factor IX mutation type distribution in patients with hemophilia B on the F9 mutation database. Three thousand six hundred and fifty‐six mutations (P.M. Rallapalli, personal communication) are broken down by type in the larger pie chart. Point mutations are broken down by type in the smaller pie chart. ND, mutation type not determined.

Figure 2

F9 gene dosage analysis using multiplex ligation‐dependent probe amplification (MLPA). MLPA probes include those for normalization across the genome (control) and for F7, F8, and F9. (A) Dosage in a female lacking a large deletion. (B) Dosage in a female with complete F9 deletion. (C) Dosage in a female with a partial F9 deletion of exons 7–8.