Multiple sequence variants in hereditary hemorrhagic telangiectasia cases: Illustration of complexity in molecular diagnostic interpretation

Hereditary hemorrhagic telangiectasia is an autosomal dominant disease caused by mutations in the ACVRL1 and ENG genes characterized by arterio-venous malformations and telangiectases. Over 700 mutations have been described in these two genes, and missense mutations are common. We describe 10 cases in which more than one potentially pathogenic mutation was identified. We report that 8 novel missense mutations, as well as previously reported pathogenic missense mutations, were seen in combination with a second mutation, which raises questions with regards to their respective pathogenicity. Our data and discussion indicate the challenges of classifying missense mutations as pathogenic or benign and the value of co-segregation studies, as well as suggest that there may be hereditary hemorrhagic telangiectasia gene mutations that have only mild phenotypic effects. We present evidence to suggest that four missense mutations (ENG p.G331S, ENG p.L8P, ENG p.P452L and ACVRL1 p.C344R) are pathogenic, two novel mutations (ACVRL1 p.A311T and ENG p.S576G) are neutral, and two previously reported disease-causing mutations are benign or have suspected benign variants (ACVRL1 p.A482V and ENG p.V504M). We conclude that for the purpose of establishing a causative hereditary hemorrhagic telangiectasia mutation in a family proband, all exons and intron/exon borders of both genes should be sequenced and deletion/duplication analysis should be performed unless a mutation that is well-proven to be pathogenic is identified.