Mutation analysis of Swedish haemophilia B families - high frequency of unique mutations

Introduction: Haemophilia B is caused by a heterogeneous spectrum of mutations. Mutation characterization is important in genetic counselling, prenatal diagnosis and to predict risk of inhibitor development. Aims: To study the mutation spectrum, frequency of unique recurrent mutations, genotype-phenotype association and inhibitor development in a population-based study of the complete Swedish haemophilia B population. Methods: The study included, facilitated by centralized DNA diagnostics, the complete registered Swedish haemophilia B population (113 families: 47 severe, 22 moderate and 44 mild), each represented by a single patient. Mutation characterization was performed by conventional sequencing of all exons and haplotyping by genotyping of single nucleotide variants and microsatellites. Results: A mutation was found in every family: eight had large deletions, three had small deletions (<10 base pair) and 102 had single base pair substitutions (69 missense, 26 nonsense, four splice site and three promoter). Ten novel mutations were found and were predicted to be deleterious. Sixteen mutations (one total gene deletion, 14 substitutions and one acceptor splice site) were present in more than one family. Of the single nucleotide mutations (37/102), 36% arose at CpG sites. Haplotyping of families with identical mutations and present analyses showed that the frequency of unique mutations was at least 65%. Inhibitors developed in 9/47 (19%) patients with severe haemophilia B. Conclusion: The spectrum of haemophilia B mutations reveals at least 65% of the families carry a unique mutation, but with more inhibitor patients than reported internationally, probably as a result of many ` null' mutations.