COL18A1 genotypic associations with endostatin levels and clinical features in pulmonary arterial hypertension: a quantitative trait association study

To the Editor:

Endostatin (ES) is a circulating peptide derived from collagen XVIII alpha 1 (COL18A1) known to inhibit angiogenesis [1, 2]. Decreased angiogenesis is a feature of pulmonary arterial hypertension (PAH) in animal models [3] and human subjects [4]. Our group has reported strong associations between circulating ES levels and haemodynamics and survival in PAH [5–7]. We have also reported that a missense variant in COL18A1, which encodes ES, confers lower ES and longer survival, suggesting that variation within the gene contributes to circulating levels [5]. In the current study, we assessed COL18A1 variant associations with clinical phenotypes and outcomes, including COL18A1 associations with circulating ES levels, in a large, multicentre PAH cohort in which we previously investigated ES as a prognostic biomarker [6].

This study was approved by the Johns Hopkins University Institutional Review Board. Serum samples contributed to the National Heart, Lung, and Blood Institute-sponsored PAH Biobank underwent single nucleotide polymorphism (SNP) genotyping using the Omni5-4 BeadChip (Illumina) and whole exome sequencing (WES) through the Regeneron Genetics Center [8]. An electrochemiluminescence assay was developed to quantitate ES. Sample collection and processing methods have been previously published [6, 9, 10].

ES measurements were regressed on genotypes of COL18A1 variants to perform a multivariable protein quantitative trait locus (pQTL) analysis. Linear regression models adjusted for age and sex were restricted to subjects of European ancestry (EA) or African ancestry (AA). To determine whether ES-associated SNPs also affected regulation of COL18A1 gene expression, pQTLs were queried in a publicly available expression quantitative trait locus (eQTL) database of whole-blood RNA samples [11]. Associations with clinical phenotypes and survival were modelled using multivariable linear and Cox regressions. Minor allele frequencies (MAFs) for COL18A1 variants were compared with the Genome Aggregation Database (https://gnomad.broadinstitute.org). Linkage disequilibrium (LD) across the COL18A1 region was assessed using Dʹ [12]. A p-value of <0.05 was considered nominally significant. An LD-adjusted correction for multiple testing was applied for QTL analyses equalling 0.0016 in EA and 0.0013 in AA subjects. Statistical tests were performed using Stata version 15.1 (StataCorp., College Station, TX, USA), SAS version 9.4 (SAS Institute, Cary, NC, USA) and PLINK version 1.9 (http://pngu.mgh.harvard.edu/purcell/plink) [13].

The cohort consisted of 2017 subjects with median age 53 years, of whom 80% were female and 82% were EA subjects. Full clinical characteristics of this cohort have been published previously [6]. Briefly, subjects had prevalent disease (median duration at sample collection 48 months, interquartile range 14–92 months) and moderately severe PAH at enrolment, with mean±sd pulmonary artery pressure 50±15 mmHg, pulmonary vascular resistance 10±6 Wood units, and 45% with New York Heart Association functional class III or IV symptoms. Most subjects had idiopathic PAH (n=870) or connective tissue disease-associated PAH (n=623). From the Omni5 SNP array, 100 COL18A1 variants in 1400 EA subjects and 126 COL18A1 variants in 209 AA subjects passed quality control (Hardy–Weinberg equilibrium (HWE) >0.001, MAF >0.05 and genotype missing rate <5%), with 91 variants present in both EA and AA subjects. In multivariable pQTL analysis, 26 cis-acting SNPs were associated with ES levels in EA individuals, and eight were associated with ES levels in AA individuals. There were no pQTLs in common for EA and AA subjects. 23 of 26 pQTLs in EA and five of eight pQTLs in AA individuals were associated with differences in COL18A1 gene expression. In EA subjects, two Omni5 SNPs demonstrated associations with cardiac index: the T allele was associated with a 0.11 L·min⁻¹·m⁻² lower cardiac index for rs7281138 (p=0.043), and a 0.12 L·min⁻¹·m⁻² lower cardiac index for rs2838917 (p=0.028). QTL data and genotype–phenotype associations are shown in table 1.

Of 102 COL18A1 WES variants, 22 had a frequency of ≥5%; none deviated from HWE. Six SNPs overlapped between exonic SNPs and Omni5 SNPs. Three of the 16 unique exonic variants in EA and one in AA subjects were associated with differences in serum ES. All four exonic pQTLs identified were also associated with significant differences in COL18A1 gene expression in eQTL analysis. In EA subjects, two exome variants demonstrated associations with survival: the A allele was associated with 23% lower mortality for rs7499 (hazard ratio 0.77, 95% CI 0.62–0.96; p=0.018), and the A allele was associated with 24% lower mortality for rs1050351 (hazard ratio 0.76, 95% CI 0.61–0.95; p=0.015). Five exonic variants with chromosomal positions in close proximity were associated with longer 6-min walk distance, and an additional three exonic variants, also in close proximity, were associated with higher cardiac index (table 1). There were no observed differences in MAFs of COL18A1 variants compared to available controls.

Our QTL results suggest circulating ES levels are partially genetically influenced by variants in and around COL18A1. The eQTL results suggest some variation in ES abundance may be due to variations in mRNA expression. Most known QTLs are associated with changes in mRNA expression, with downstream effects on ribosome occupancy and protein abundance [14]. Thus, eQTLs often have smaller effect sizes on protein levels than on gene expression [14, 15], consistent with our results. We found some signal for genetically influenced phenotypic variation, although none of the phenotypically associated variants were ES-associated pQTLs, and all but one (rs7499 in the 3ʹ untranslated region) were synonymous variants. Interestingly, rs7499 has been associated with significantly reduced risk of hepatocellular carcinomas in patients with hepatitis B infection [16], suggesting some biological significance of this variant in humans.

In contrast to our 2015 report [5], we did not find an association between rs12483377 and ES levels or outcomes. This discrepancy may be due to the smaller sample size in the first study. Genotype at rs12483377 was not associated with survival in two large PAH genome-wide association studies (GWAS) later published [17], although these GWAS excluded patients with connective tissue disease and may have investigated genetically different cohorts.

This study has several limitations. We are limited by the cohort size available for a rare disease; consequently, some of our results are of nominal significance, with a higher likelihood of observation due to chance alone. The QTL results are based on associations in whole blood, as mRNA or protein expression data from human tissues most relevant to disease are not available. The genetic and clinical associations with ES are based on a single time point for each subject. Furthermore, Omni5 genotyping and WES leaves many genetic variants uncharacterised. Therefore, the identified genotype–phenotype associations may not be causal themselves, but rather in LD with true, unidentified QTLs.

Aside from reports on BMPR2 (bone morphogenetic protein receptor type 2), our study is one of only a few [17, 18] that offer insights into genetic influences on disease severity and heterogeneity in PAH, a strength of our work. Heritable modifiers of phenotype have not been well-established in PAH, and most genetic studies have focused on identifying loci contributing to disease susceptibility, rather than disease severity or prognosis.

In conclusion, these results suggest that PAH disease heterogeneity is influenced in part by genetic variation around the COL18A1 gene. ES levels have been linked to variation in PAH severity and outcomes, and our results suggest that ES levels may be genetically influenced. Understanding influences on transcription and translation of genes implicated in disease can clarify therapeutic targeting strategies. Future work on ES/COL18A1 is needed to better understand genetic and cellular mechanisms underlying PAH pathobiology.