Genetic Etiology for Alcohol-Induced Cardiac Toxicity

Background Alcoholic cardiomyopathy (ACM) is defined by a dilated and impaired left ventricle due to chronic excess alcohol consumption. It is largely unknown which factors determine cardiac toxicity on exposure to alcohol. Objectives This study sought to evaluate the role of variation in cardiomyopathy-associated genes in the pathophysiology of ACM, and to examine the effects of alcohol intake and genotype on dilated cardiomyopathy (DCM) severity. Methods The authors characterized 141 ACM cases, 716 DCM cases, and 445 healthy volunteers. The authors compared the prevalence of rare, protein-altering variants in 9 genes associated with inherited DCM. They evaluated the effect of genotype and alcohol consumption on phenotype in DCM. Results Variants in well-characterized DCM-causing genes were more prevalent in patients with ACM than control subjects (13.5% vs. 2.9%; p = 1.2 ×10−5), but similar between patients with ACM and DCM (19.4%; p = 0.12) and with a predominant burden of titin truncating variants (TTNtv) (9.9%). Separately, we identified an interaction between TTN genotype and excess alcohol consumption in a cohort of DCM patients not meeting ACM criteria. On multivariate analysis, DCM patients with a TTNtv who consumed excess alcohol had an 8.7% absolute reduction in ejection fraction (95% confidence interval: −2.3% to −15.1%; p < 0.007) compared with those without TTNtv and excess alcohol consumption. The presence of TTNtv did not predict phenotype, outcome, or functional recovery on treatment in ACM patients. Conclusions TTNtv represent a prevalent genetic predisposition for ACM, and are also associated with a worse left ventricular ejection fraction in DCM patients who consume alcohol above recommended levels. Familial evaluation and genetic testing should be considered in patients presenting with ACM.

The pathophysiology of ACM is not fully understood, and the relationship between the degree of alcohol exposure and severity of endorgan damage is not simple (2,6). In particular, not all individuals with high alcohol intake develop ACM, and this heterogeneity in response indicates differences in underlying susceptibility, likely both genetic and environmental. However, studies of heritable contributors to ACM are currently limited (7,8). Prognosis in ACM is poor, but is considered more favorable than DCM generally, with recovery observed in up to one-third of cases, especially when alcohol intake is reduced (3). Current management of ACM individuals includes cessation of alcohol exposure, standard heart failure medications, and prevention of sudden cardiac death (2,6,9). As the genetic contribution to ACM is currently unknown, familial evaluation is not part of ACM management.
DCM is estimated to affect up to 1 in 250 individuals (10) and has a significant genetic contribution, with truncation variants in the gene encoding titin (TTNtv), a key sarcomeric protein, representing the predominant genetic cause, seen in 10% to 20% of cases (11)(12)(13). It is also recognized that up to 1% of the general population carry a TTNtv, presenting a significant challenge to interpretation (12). Genetic and/or environmental factors likely underlie the variable penetrance and expressivity. In line with this, recent evidence has shown that over 10% of patients with peripartum cardiomyopathy carry a TTNtv, suggesting that in some patients, the DCM phenotype results from a combination of pregnancy with a genetic predisposing background (14).
Moreover, we have recently demonstrated that TTNtv found in the general population are not phenotypically silent (15); although the population prevalence of TTNtv exceeds the prevalence of DCM, careful phenotyping reveals differences in cardiac volumes in subjects with and without TTNtv. Using a rat model, we found essentially normal resting cardiac function, but subclinical metabolic abnormalities in TTNtv carriers and impaired cardiac physiology under conditions of cardiac stress (15). Together, these data suggest that TTNtv may predispose to cardiomyopathy, with environmental factors modulating penetrance and expressivity.
Here, we sought to evaluate genetic determinants in the pathophysiology of ACM by characterizing genetic variation in known DCM-causing genes in a large ACM cohort. We sequenced 141 individuals with ACM and compared these with healthy volunteers

METHODS
The study conformed to the ethical principles of the   Table 1).
In the case of titin, analysis was further restricted to truncating variants in exons constitutively expressed in the heart as described (12). Although the Illumina TruSight Cardio sequencing kit captures 61 genes purportedly associated with DCM (full gene list and variants detected are given in Online Table 7), we decided to be conservative and pre-specified a focused analysis on 9 genes with the most robust  Computing, Vienna, Austria). All data and code required to reproduce these analyses are available online (24).

RESULTS
GENETIC CONTRIBUTION TO ACM. To investigate the potential genetic contribution of cardiomyopathy   Table 2). TTNtv accounted for the majority of variants detected in ACM cases (9.9%) and were found with a frequency similar to that seen in DCM (12.0%; p ¼ 0.64), and significantly higher than in control subjects (0.7%; p ¼ 4.4 Â 10 À7 ). In line with studies in DCM (10-12,15), TTNtv found in ACM were in exons constitutively expressed in the heart and distributed across the gene (Online Figure 1) Table 3), and no detectable difference in event-free survival between the 2 groups ( Figure 1).

ALCOHOL AS A PHENOTYPIC MODIFIER IN DCM.
Having established a genetic contribution to ACM in a proportion of cases, primarily driven by TTNtv, we These individuals were more likely to be male, and in univariate analyses had modestly reduced LVEF An illustration of the utility of genetic management in ACM is shown in Figure 3, where familial evaluation identified several individuals with DCM, and molecular genetic testing enabled informed genetic counseling including reproductive advice. This reveals the importance of recognizing genetic disease and familial assessment, although future work will be needed to more fully understand the risk associated with genetic variants found in the absence of overt familial disease, to balance the costs and benefits associated with genetic testing and clinical surveillance, and to allow for fully informed genetic counseling.
We further identified a direct interaction between TTNtv and alcohol consumption in the context of of >1.9 million U.K. individuals showed that 8.4% recorded drinking above the recommended safe levels (28). If this accurately reflects the proportion of the population with above-recommended alcohol intake, then we see a significantly higher exposure in our nonalcoholic DCM population (111 of 716 ¼ 15.5%, p binomial ¼ 5 Â 10 À10 ). Together, these data suggest that alcohol alone, as well as in combination with  (Bottom) Family 1016: combined effect of excessive alcohol consumption and genetic background. The proband (arrow) was diagnosed with ACM at age 44 years and was identified as carrying a TTNtv variant (TTN c.64453C>T; p.R21485X). One brother and 1 sister with prolonged heavy alcohol consumption (red asterisk) and TTNtv also show ACM. Two family members with TTNtv but no regular alcohol intake, and 2 individuals with prolonged heavy alcohol consumption but without TTNtv, did not show cardiac involvement. Standard pedigree notation is used: squares and circles indicate male and female subjects, respectively, a strike-through indicates a Red asterisks indicate cases with documented prolonged heavy alcohol consumption. CTx ¼ cardiac transplant; DCM ¼ dilated cardiomyopathy; MI ¼ myocardial infarction; SCA ¼ sudden cardiac arrest; VHD ¼ valvular heart disease; other abbreviations as in Figure 1.
genetic factors, may account for a substantial proportion of disease risk.