Pathogenic Variant Frequencies in Hereditary Haemorrhagic Telangiectasia Support Clinical Evidence of Protection from Myocardial Infarction

Abstract

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 ACVRL1 (encoding activin receptor-like kinase 1 [ALK1]), ENG (encoding endoglin [CD105]), or SMAD4. 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 ACVRL1 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, ACVRL1 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, ACVRL1 c.1231C>T (p.Arg411Trp) was reported by 12 different laboratories, the adjacent ACVRL1 c.1232G>A (p.Arg411Gln) by 14, and ACVRL1 c.1120C>T (p.Arg374Trp) by 18. Unlike the majority of HHT-causal ACVRL1 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 ACVRL1 missense variants (all p-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 ACVRL1/ALK1 pathogenic missense variants in HHT. The findings support further examination of differential clinical and cellular phenotypes by HHT causal gene molecular subtypes.

Keywords: atherosclerosis; dominant negative activity; heart attack; low-density lipoprotein (LDL) transcytosis; missense variant.

Figure 1

Relevant molecular considerations for the differing types of HHT causal pathogenic variants. A simple categorization of major conceptual classes of loss-of-function variants, distinguishing defective signalling variants from other classes that generate proteins that do reach the cell surface, or no protein: premature termination codons (PTCs) where aberrant protein generated varies [75], trafficking defect due to impaired protein folding [79,80,81], and whole gene deletions. Created using BioRender (Toronto, ON, Canada), Student Plan licence publication agreement number FO25IXFDHY, 24 June 2023).

Figure 2

Series 1: Positive HHT gene tests between 2021 and 2023. (A) Overall number of times a variant was reported. (B) Sequence traces in one of the five patients with ACVRL1 c.1231C>T (p.Arg411Trp). The upper bar indicates the reference sequence for 7 nucleotides spanning the variant, where the heterozygous call is highlighted by the blue/red horizontal bars. The 33 numbered rows show individual next-generation sequencing reads where variants are highlighted by letter. At Chr12:51,916,218,308, the genomic site corresponding to ACVRL1 c.1231C, 308 (53%) of reads were the wild type C sequence (blue) and 275 (47%) were the variant sequence (red, indicated by an individual red ‘T’ on each sequence trace).

Figure 3

Series 2. Positive HHT gene tests in 501 patients with HHT reviewed between 1992 and 2020. (A) Overall number of times a variant was reported. (B) Major HHT manifestations across the population including nosebleeds (epistaxis), arteriovenous malformations (AVMs), and gastrointestinal (GI) bleeding. Note there is a strong referral bias due to a separate national pulmonary AVM referral service. Additionally, individuals referred with known or suspected HHT undergo routine screening for pulmonary AVMs, whereas imaging of the liver and brain for asymptomatic screens are not routinely performed. (C) First-recorded haemoglobin (Hb) and (D) first-recorded oxygen saturation (SaO₂) in the erect posture.

Figure 4

Series 3 ACVRL1 pathogenic reports on the HHT Mutation Database, itemised by number of different international laboratories reporting the variant [41,79,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117]. Tabulated text data were downloaded in 2018 as described in [71], and were categorized before generating these graphs. (A) Overall number of times any ACVRL1 pathogenic variant was reported. (B) Overall number of times any missense ACVRL1 pathogenic variant was reported.

Figure 5

ACVRL1 missense variants in three essentially independent series, plotted as percentage of total ACVRL1 missense variants per series. Individual variants are numbered from 5′ to 3′ and detailed in Supplementary Table S1, annotated by NM_000020.3 [88]. (A) Series 1: Hammersmith/Imperial database, 2021–2023. (B) Series 2: Hammersmith/Imperial database, 1992–2020. (C) Series 3: HHT Mutation Database, 2018, as reported in Reference [71]. The three most common variants are highlighted in red (ACVRL1 c.1120C>T, p.(Arg374Trp), blue (ACVRL1 c.1231C>T, p.(Arg411Trp), and purple (ACVRL1 c.1232G>A, p.(Arg411Gln). Also highlighted in green are the other cell surface-expressed, BMP9-binding but kinase-dead variants in the series (ACVRL1 c.1039G>C, p.(Arg347Pro); ACVRL1 c.1121G>A, p.(Arg374Trp); and ACVRL1 c.1450C>T, p.(Arg484Trp)).

Figure 6

ACVRL1 missense variants comparing the 6 encoding kinase-inactive, cell surface-expressed ALK1 protein compared to 131 other ACVRL1 pathogenic/likely pathogenic missense variants reported in the 3 series as detailed in Supplementary Table S1. (A) Mean number of reports of individual variants per category (encoding cell surface-expressed, kinase-inactive protein [N = 6] or other missense variant [N = 131]) in each of the 3 series. Mean and standard error of mean reports per variant displayed. (B) Number of reports for each of the 6 kinase-inactive variants per series, normalised to the mean number of reports for the other ACVRL1 variants in that series. Individual variant values, mean, and standard error of mean are displayed. For Series 1 and 2, kinase-inactive variants that were not identified have a value of zero (compare Figure 5). A red dotted line at 1 indicates where the expected value would be if, for a kinase-inactive variant present in that series, there was no difference from the other ACVRL1 variants in that series.

Figure 7

Relevant clinical considerations. Reported heart attacks (‘myocardial infarctions’) for 6047 individuals in hereditary haemorrhagic telangiectasia families comprising 2827 with HHT and 3011 without HHT. Data originally published in 2016 [63] were from 349 men and 670 women with HHT who also reported HHT transmission and cardiac events in 1978 parents and 3050 grandparents. Mean and standard error of the mean displayed; p-values calculated by Dunn’s test post Mann–Whitney (****, p < 0.0001). Reprinted/adapted with permission from Ref. [63]. Copyright year 2016, copyright owner’s name: Claire Shovlin.