Association Between Titin Loss-of-Function Variants and Early-Onset Atrial Fibrillation

Abstract

Importance: Atrial fibrillation (AF) is the most common arrhythmia affecting 1% of the population. Young individuals with AF have a strong genetic association with the disease, but the mechanisms remain incompletely understood.

Objective: To perform large-scale whole-genome sequencing to identify genetic variants related to AF.

Design, setting, and participants: The National Heart, Lung, and Blood Institute's Trans-Omics for Precision Medicine Program includes longitudinal and cohort studies that underwent high-depth whole-genome sequencing between 2014 and 2017 in 18 526 individuals from the United States, Mexico, Puerto Rico, Costa Rica, Barbados, and Samoa. This case-control study included 2781 patients with early-onset AF from 9 studies and identified 4959 controls of European ancestry from the remaining participants. Results were replicated in the UK Biobank (346 546 participants) and the MyCode Study (42 782 participants).

Exposures: Loss-of-function (LOF) variants in genes at AF loci and common genetic variation across the whole genome.

Main outcomes and measures: Early-onset AF (defined as AF onset in persons <66 years of age). Due to multiple testing, the significance threshold for the rare variant analysis was P = 4.55 × 10-3.

Results: Among 2781 participants with early-onset AF (the case group), 72.1% were men, and the mean (SD) age of AF onset was 48.7 (10.2) years. Participants underwent whole-genome sequencing at a mean depth of 37.8 fold and mean genome coverage of 99.1%. At least 1 LOF variant in TTN, the gene encoding the sarcomeric protein titin, was present in 2.1% of case participants compared with 1.1% in control participants (odds ratio [OR], 1.76 [95% CI, 1.04-2.97]). The proportion of individuals with early-onset AF who carried a LOF variant in TTN increased with an earlier age of AF onset (P value for trend, 4.92 × 10-4), and 6.5% of individuals with AF onset prior to age 30 carried a TTN LOF variant (OR, 5.94 [95% CI, 2.64-13.35]; P = 1.65 × 10-5). The association between TTN LOF variants and AF was replicated in an independent study of 1582 patients with early-onset AF (cases) and 41 200 control participants (OR, 2.16 [95% CI, 1.19-3.92]; P = .01).

Conclusions and relevance: In a case-control study, there was a statistically significant association between an LOF variant in the TTN gene and early-onset AF, with the variant present in a small percentage of participants with early-onset AF (the case group). Further research is necessary to understand whether this is a causal relationship.

Figure 1.. Study overview with sample selection and analytic workflow.

The initial sample selection of cases and controls from the TOPMed program is indicated in the top three boxes (blue). Because early-onset AF cases were ascertained from European ancestry, genetically determined individuals of European ancestry were identified. The middle section in (red) illustrates the study populations used for common variant, rare variant, and TTN sensitivity analyses. Although common variants were tested across the entire genome, rare variants were examined in genes at AF candidate loci. Unrelated individuals were defined as their pairwise relationships that were greater than fourth degree relatives. Further post hoc analyses were performed to characterize an association identified in the rare variant analyses. The bottom section (green) illustrates the replication populations from the UK Biobank (common variant analyses) and the MyCode study (rare variant analyses). The grey boxes on right side describe the individuals removed during each step of quality control and filtering. Abbreviations: AF, atrial fibrillation; FHS, Framingham Heart Study; Het/Hom, heterozygous homozygous ratio; SNP, single nucleotide polymorphism; LVEF, left ventricular ejection fraction; HF, heart failure; CM, cardiomyopathy

Figure 2.. Common variants associated with early-onset atrial fibrillation.

Figure 2 shows results of genome wide association analysis results between early-onset atrial fibrillation status and common genetic variants with minor allele frequency ≥ 0.01. In total, 7,740 participants including 2,781 early onset AF cases and 4,959 controls were analyzed. Blue dots represent variants located in one of the 25 known atrial fibrillation associated loci in individuals of European ancestry. Six loci (KCNN3, PRRX1, PITX2, NEURL1, SOX5, and ZFHX3) reached genome wide significant (P value less than 5 × 10⁻⁸, dotted line) level. Red dots illustrate variants in the recently identified locus (NAV2). The gene names represent the gene in closest proximity to the most significant variant at each locus.

Figure 3.. Loss of function variants in TTN among early-onset atrial fibrillation cases and controls.

Loss-of-function (LOF) variants in early-onset atrial fibrillation cases (first row) and controls (second row) are plotted relative to their genomic position. If multiple variants are co-localized, the number of unique variants is indicated above. The participants in this figure arose from the TTN sensitivity analysis and included 2,047 early-onset atrial fibrillation cases and 2,166 controls. There were 40 LOF variants in TTN among the early-onset AF cases and 22 LOF variants in the controls. For consistency with prior reports, the TTN domains (Z-disk, I -band, A-band, M-band) are illustrated with red, blue, green, and purple colors, respectively. The region indicated in grey is a large, final exon present in one TTN transcript (Novex-3).

Figure 4.. Proportion of TTN loss of function variant carriers in early-onset atrial fibrillation stratified by age.

The percentage of TTN loss of function carriers is plotted versus age (years) category. The age categories for AF cases are not mutually exclusive, and are cumulative when moving from a younger to an older age. This figure represents a total of 4,163 unrelated participants from controls and early-onset atrial fibrillation patients without evidence of heart failure and a left ventricular ejection fraction ≥50%. Whiskers around each diamond show 95% exact binomial confidence intervals. Abbreviations: AF, Atrial fibrillation; LOF, Loss-of-function.