| Summary: | Hereditary haemorrhagic telangiectasia (HHT) is a vascular dysplasia inherited as an autosomal dominant trait, due to a single heterozygous loss-of-function variant, usually in <i>ACVRL1</i> (encoding activin receptor-like kinase 1 [ALK1]), <i>ENG</i> (encoding endoglin [CD105]), or <i>SMAD4</i>. In a consecutive single-centre series of 37 positive clinical genetic tests performed in 2021–2023, a skewed distribution pattern was noted, with 30 of 32 variants reported only once, but <i>ACVRL1</i> c.1231C>T (p.Arg411Trp) identified as the disease-causal gene in five different HHT families. In the same centre’s non-overlapping 1992–2020 series where 110/134 (82.1%) HHT-causal variants were reported only once, <i>ACVRL1</i> c.1231C>T (p.Arg411Trp) was identified in nine further families. In a 14-country, four-continent HHT Mutation Database where 181/250 (72.4%) HHT-causal variants were reported only once, <i>ACVRL1</i> c.1231C>T (p.Arg411Trp) was reported by 12 different laboratories, the adjacent <i>ACVRL1</i> c.1232G>A (p.Arg411Gln) by 14, and <i>ACVRL1</i> c.1120C>T (p.Arg374Trp) by 18. Unlike the majority of HHT-causal <i>ACVRL1</i> variants, these encode ALK1 protein that reaches the endothelial cell surface but fails to signal. Six variants of this type were present in the three series and were reported 6.8–25.5 (mean 8.9) times more frequently than the other <i>ACVRL1</i> missense variants (all <i>p</i>-values < 0.0039). Noting lower rates of myocardial infarction reported in HHT, we explore potential mechanisms, including a selective paradigm relevant to ALK1′s role in the initiating event of atherosclerosis, where a plausible dominant negative effect of these specific variants can be proposed. In conclusion, there is an ~9-fold excess of kinase-inactive, cell surface-expressed <i>ACVRL1</i>/ALK1 pathogenic missense variants in HHT. The findings support further examination of differential clinical and cellular phenotypes by HHT causal gene molecular subtypes.
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