Ovarian cancer risk, ALDH2 polymorphism and alcohol drinking: Asian data from the Ovarian Cancer Association Consortium

The aldehyde dehydrogenase 2 (ALDH2) polymorphism rs671 (Glu504Lys) causes ALDH2 inactivation and adverse acetaldehyde exposure among Asians, but little is known of the association between alcohol consumption and rs671 and ovarian cancer (OvCa) in Asians. We conducted a pooled analysis of Asian ancestry participants in the Ovarian Cancer Association Consortium. We included seven case‐control studies and one cohort study comprising 460 invasive OvCa cases, 37 borderline mucinous OvCa and 1274 controls of Asian descent with information on recent alcohol consumption. Pooled odds ratios (OR) with 95% confidence intervals (CI) for OvCa risk associated with alcohol consumption, rs671 and their interaction were estimated using logistic regression models adjusted for potential confounders. No significant association was observed for daily alcohol intake with invasive OvCa (OR comparing any consumption to none = 0.83; 95% CI = 0.58‐1.18) or with individual histotypes. A significant decreased risk was seen for carriers of one or both Lys alleles of rs671 for invasive mucinous OvCa (OR = 0.44; 95% CI = 0.20‐0.97) and for invasive and borderline mucinous tumors combined (OR = 0.48; 95% CI = 0.26‐0.89). No significant interaction was observed between alcohol consumption and rs671 genotypes. In conclusion, self‐reported alcohol consumption at the quantities estimated was not associated with OvCa risk among Asians. Because the rs671 Lys allele causes ALDH2 inactivation leading to increased acetaldehyde exposure, the observed inverse genetic association with mucinous ovarian cancer is inferred to mean that alcohol intake may be a risk factor for this histotype. This association will require replication in a larger sample.

The aldehyde dehydrogenase 2 (ALDH2) polymorphism rs671 (Glu504Lys) causes ALDH2 inactivation and adverse acetaldehyde exposure among Asians, but little is known of the association between alcohol consumption and rs671 and ovarian cancer (OvCa) in Asians. We conducted a pooled analysis of Asian ancestry participants in the Ovarian Cancer Association Consortium. We included seven case-control studies and one cohort study comprising 460 invasive OvCa cases, 37 borderline mucinous OvCa and 1274 controls of Asian descent with information on recent alcohol consumption. Pooled odds ratios (OR) with 95% confidence intervals (CI) for OvCa risk associated with alcohol consumption, rs671 and their interaction were estimated using logistic regression models adjusted for potential confounders. No significant association was observed for daily alcohol intake with invasive OvCa (OR comparing any consumption to none = 0.83; 95% CI = 0.58-1.18) or with individual histotypes. A significant decreased risk was seen for carriers of one or both Lys alleles of rs671 for invasive mucinous OvCa (OR = 0.44; 95% CI = 0.20-0.97) and for invasive and borderline mucinous tumors combined (OR = 0.48; 95% CI = 0.26-0.89). No significant interaction was observed between alcohol consumption and rs671 genotypes. In conclusion, self-reported alcohol consumption at the quantities estimated was not associated with OvCa risk among Asians. Because the rs671 Lys allele causes ALDH2 inactivation leading to increased acetaldehyde exposure, the observed inverse genetic association with mucinous ovarian cancer is inferred to mean that alcohol intake may be a risk factor for this histotype. This association will require replication in a larger sample. Approximately 239 000 females developed a new ovarian cancer in 2012 and 152 000 women died globally of the disease. 1 Despite its high incidence and mortality, the etiology is not fully understood; however, established epidemiological risk factors for ovarian cancer include age, parity, oral contraceptive use, tubal ligation, and inherited germline mutations in BRCA1 and BRCA2. 2,3 Alcohol consumption is one of the possible modifiable risk factors for ovarian cancer. Several studies have investigated the association between alcohol drinking and ovarian cancer risk and reported inconsistent results. [4][5][6][7][8][9][10][11][12][13][14] To resolve this inconsistency, pooled analyses have been conducted. 5,[15][16][17] These studies failed to show a clear association between alcohol drinking and ovarian cancer risk overall; however, some showed a different trend in associations with alcohol by histological subtypes, 16,17 suggesting different biological etiologies according to histology. 18 Generally, a differential distribution pattern of the histological subtypes of epithelial ovarian cancer has been observed across ethnicities and countries. 19 Among Asian women, the prevalence of serous adenocarcinoma is relatively low, whereas that of clear cell adenocarcinoma is higher, compared with ovarian cancers among women of European descent. Furthermore, Asian women are likely to have different genetic and sociocultural backgrounds, which includes less alcohol consumption, 20 lower prevalence of hormone therapy use 21 and a different distribution of the aldehyde dehydrogenase 2 (ALDH2) polymorphism Glu504Lys (rs671). 22 The rs671 polymorphism in ALDH2 is more prevalent in East-Asian populations (minor allele frequency [MAF] in HapMap-JPT = 0.24, and 0.15 in HapMap-HCB) 22 and absent among Europeans (MAF HapMap-CEU = 0). The Lys allele of rs671 is strongly associated with inactivation of ALDH2, 23,24 which results in prolonged exposure to the intermediate metabolite acetaldehyde, a potential carcinogen in various organs. [25][26][27][28][29][30] To our knowledge, there are no studies exploring the association between rs671 in ALDH2 and ovarian cancer risk, particularly among Asian women.
To investigate whether there is an association between alcohol drinking, the rs671 polymorphism in ALDH2 and ovarian cancer risk, we conducted a pooled analysis of data from women of Asian ancestry participating in the Ovarian Cancer Association Consortium (OCAC).

| Study population
We conducted this pooled analysis using seven case-control studies and one cohort study with information on alcohol consumption from the OCAC. We included 460 invasive ovarian cancer cases, 37 borderline mucinous tumors and 1274 controls. Other borderline tumors (n = 23) except mucinous were excluded from the analysis because, unlike other ovarian histotypes, mutational evidence suggests mucinous tumors progress along a multistep continuum from benign to borderline to invasive tumors. 31 Information from the eight studies is summarized in Table 1

| Genotyping methods
Genotyping was carried out as part of the Collaborative Oncological Gene-environment Study (COGS), 43  We then used a set of 37 000 unlinked markers to carry out principal components analysis within the Asian ancestral group to identify residual population substructure. 46 For the analyses of Asian subjects, we included five principal components as covariates.

| Alcohol assessment and covariate data collection
Harmonization of daily alcohol intake across OCAC studies was previously described. 16 Briefly, daily alcohol consumption was estimated using validated food frequency questionnaires (FFQ) in AUS, 47 DOV, 48 HAW, 49 NEC, 50  Key clinical, demographic and questionnaire data on study subjects (see below) were merged into a common dataset by the coordinating center and checked for consistency. To assess the joint effect of genotype and alcohol intake, we created four categories combining genotype with alcohol intake: non-Lys allele carriers and no alcohol intake as a reference group; non-Lys allele carriers and any alcohol intake; Lys allele carriers and no alcohol intake; and Lys allele carriers and any alcohol intake.
Even though all study participants were of Asian ancestry, heterogeneity among studies might affect the results. Therefore, we repeated all analyses using random effects meta-analyses to calculate summary study-specific estimates.
P-value <.05 was considered statistically significant. All analyses were carried out using STATA version 13.1 (Stata Corp., College Station, TX, USA). and where the black clusters segregated according to country of genetic origin. In Figure 1     There was no significant interaction with alcohol consumption with any of the associations (Table S1).

| RESULTS
We also carried out meta-analyses to calculate summary studyspecific estimates (Tables S2-S5). Overall, the results did not change substantially, but the mucinous tumor cases were too few to calculate a study-specific OR, and thus some studies were not included in the meta-analyses.

| DISCUSSION
In the present study, we did not observe significant associations between total alcohol intake and invasive ovarian cancer risk in Asian populations. We found that the Lys allele of rs671 was associated with a decreased risk of both invasive mucinous ovarian cancer and invasive plus borderline mucinous ovarian cancers, but not the other histotypes, although the test for heterogeneity was not significant. No significant interactions were observed between rs671 genotypes in ALDH2 and alcohol intake with risk of invasive ovarian cancer.
Results from epidemiological studies investigating the association between alcohol drinking and ovarian cancer risk among Caucasians are inconsistent, reporting either a null association, 7-12 a positive association, 13,14 or negative associations. [4][5][6] Alcohol has been hypothesized to induce carcinogenesis by increasing the circulating level of estrogens, 52 oxidative stress, acetaldehyde, or depletion of folate. 53 In contrast, alcohol is reported to have a protective potential against ovarian carcinogenesis by decreasing follicle stimulating hormone, luteinizing hormone and gonadotropin levels. Polyphenols contained in red wine were proposed to explain the inverse association observed between red wine and risk of ovarian cancer. 5,6,10,54 We did not observe any statistically significant associations between alcohol intake and ovarian cancer in the Asian participants in our study. The evidence to support a role of alcohol in ovarian cancer epidemiology in Asian populations is scarce and may warrant additional evaluation in larger studies.
The present analysis also examined ovarian cancer risk using the functional ALDH2 rs671 polymorphism. The Lys allele acts as dominant negative, because the variant form can suppress the activity of the Glu allele by the formation of heterotetramers. 24  cancer (OR = 0.48, P = .018). This implies that alcohol consumption may be associated with increased risk of mucinous ovarian cancer.
The strengths of this investigation include the analysis of individual-level data from a relatively large sample compared to previous studies, which allowed us to quantify risk associations of the ALDH2 polymorphism, detailed drinking status and ovarian cancer risk. Other strengths are the uniform genotyping procedures and quality-control measures adopted. We were also able to control for population stratification by first using LAMP analysis to identify Asian ancestral membership separate from other genetically similar groups, and then including 5 principal components as model covariates to control for residual genetic heterogeneity within the Asian membership.
The present study does have some weaknesses. The models for alcohol intake did not adjust for all potential confounders, because a substantial number of subjects from a single study (SWH) had missing values for several covariates. Further, the self-reported alcohol quantities were either too low or measured with error and may have obscured an association with ovarian cancer if it existed whereas the genetic models are not influenced by these limitations. Despite the common prevalence of the ALDH2 polymorphism among Asians, the small sample sizes for the histological type analysis precludes a conclusive interpretation of the results for Mendelian Randomization, which must await further study with a larger sample size. Finally, we did not adjust for multiple comparisons and a cautious interpretation of the histologically specific results is required.
In conclusion, we observed an inverse association between the Lys allele of rs671 in ALDH2 and mucinous ovarian cancer risk in an Asian population. Because the rs671 Lys allele causes ALDH2 inactivation leading to increased acetaldehyde exposure, the observed inverse genetic association with mucinous ovarian cancer is inferred to mean that alcohol intake may be a risk factor for this histotype.

CONF LICTS OF INTEREST
Authors declare no conflicts of interest for this article.