Arteriovenous malformation from a patient with JP-HHT harbours two second-hit somatic DNA alterations in <i>SMAD4</i>.

Abstract

Hereditary haemorrhagic telangiectasia (HHT) is an inherited disorder of vascular malformations. It is caused by inherited loss-of-function mutations in one of three genes, <i>ENG</i>, <i>ACVRL1</i> or <i>SMAD4</i>. We recently showed that HHT-associated vascular malformations from liver, lung, brain and skin develop via a two-hit genetic mechanism resulting from biallelic loss-of-function mutations in either <i>ENG</i> or <i>ACVRL1</i>. Second-hit somatic mutations in <i>SMAD4</i> have not been reported in HHT-associated vascular malformations. Here, we investigate a large, aggressively growing craniofacial arteriovenous malformation (AVM) from an individual with juvenile polyposis-HHT caused by a germline mutation in <i>SMAD4</i>. We sequenced DNA from the AVM using a targeted gene sequencing panel to at least 1000X to identify somatic mutations that might contribute to the development of the AVM. We analysed whole genome SNP genotyping data using the algorithm Mosaic Chromosomal Alterations (MoChA) to identify somatic loss of heterozygosity. We confirmed the germline mutation in <i>SMAD4</i> (c.1610A>T, p.Asp537Val) and identified a second-hit somatic mutation also in <i>SMAD4</i> (c.350dup, p.Tyr117*) that occurred in <i>trans</i> relative to the germline mutation. We also identified somatic loss of heterozygosity on the q arm of chromosome 18, including <i>SMAD4</i>. Additionally, we confirmed that the loss of heterozygosity causes loss of the wild-type allele. Thus, we identified two independent somatic alterations in <i>SMAD4</i> causing biallelic loss of <i>SMAD4</i> function in the AVM tissue. We identified biallelic loss of function of <i>SMAD4</i> in a craniofacial AVM, evidence that <i>SMAD4</i> also follows the two-hit mutation mechanism of HHT-associated vascular malformation pathogenesis.