Introduction
============

Alcohol consumption is a major contributor to the global burden of disease[@R1]^,^[@R2] and is a major risk factor for cancer.[@R3]^--^[@R6] Of all alcohol-related cancers, oesophageal has the highest alcohol-attributable fraction[@R6] -- i.e. the highest proportion of these cancers would be prevented if no alcohol were consumed.[@R6]^--^[@R8] The global burden of disease (GBD) study estimates that in 2010 alcohol-attributable oesophageal cancer resulted in 76 700 deaths and 1 825 000 disability adjusted life years (DALYs) lost, globally.[@R9]

A large portion of oesophageal cancers attributable to alcohol consumption occur in Asian countries -- 52.2% (40 000) of all alcohol-attributable oesophageal cancer deaths and 51.8% (945 000) of all alcohol-attributable oesophageal cancer DALYs. The alcohol-attributable portions for countries in this region have been calculated based on global meta-analyses.[@R10]^,^[@R11] However, this assumes that the alcohol-attributable risk for oesophageal cancer is the same in all regions. Preliminary evidence, on the other hand, shows that the risk for this cancer is different for people of Asian origin, because of genetic polymorphisms -- most importantly the aldehyde dehydrogenase 2 (*ALDH2*) and alcohol dehydrogenase 1B (*ADH1B*) polymorphisms.[@R12]^--^[@R15] Thus, the real risk and burden of alcohol-attributable oesophageal cancer in Asia may have been underestimated.

In Japan in 2010, oesophageal cancer was among the top 20 causes of years of life lost (11 deaths and 181 DALYs per 100 000 people per year).[@R9] We did a systematic review and meta-analyses of studies conducted in the Japanese population to estimate the alcohol-attributable burden of oesophageal cancer. We then compared these estimates to the GBD 2010 estimates.[@R1] We also estimated risk functions according to *ALDH2* subsets and investigated potential interactions between *ALDH2* and *ADH1B* polymorphisms.

Methods
=======

Data search and selection
-------------------------

We followed the Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines.[@R16] We used the latest editions of the International Agency for Research on Cancer (IARC) monographs on alcohol[@R3]^,^[@R4] to identify potentially eligible studies. Additionally, we searched PubMed for publications published before 2014. We did two searches using the following search terms; Search 1: "cancer or neoplasm or carcinom\*" and "*ALDH2* or *ADH1B* or *ADH2* or *ADH3* or *ADH1C* or dehydrogenase\*" and "alcohol or ethanol"; Search 2: "alcohol or ethanol" and "cohort" and "cancer" and "japan" and "review" and "mortality". Inclusion criteria for analyses investigating the relationship between alcohol consumption, *ALDH2*, and oesophageal cancer were: (i) prospective or historical cohort or case-control study design; (ii) a measure of risk and its corresponding measure of variability was reported or there were sufficient data for us to calculate these; (iii) oesophageal cancer was reported as a separate outcome; (iv) data on total alcohol intake for at least two exposure categories among current drinkers, or estimates for *ALDH2* variants by alcohol intake were reported; (v) risk estimates were at least age-adjusted; and (vi) the study was conducted in Japan after 1980. In addition, we searched reference lists of identified articles for additional articles. No active filters or language restrictions were applied. We excluded measures of pure drinking frequency and qualitative characteristics -- such as social or problem drinker. Oesophageal cancer cases (*International Classification of Diseases* \[ICD\] version 9: 150, ICD-10: C15) were defined as newly diagnosed at the first visit to a specialized clinic, through cancer registries or cause of death on death certificates.

Most quality scores for primary studies are tailored for meta-analyses of randomized trials of interventions[@R17]^--^[@R19] and many criteria for such scores do not apply to epidemiological studies examined in this study. Additionally, quality score use in meta-analyses remains controversial.[@R19]^,^[@R20] As a result, we included quality components in the inclusion and exclusion criteria of the systematic search and separate meta-analyses -- such as study design and alcohol measurement -- and conducted subgroup analyses based on study design and genetic polymorphisms.

Data extraction
---------------

From all relevant articles we extracted: authors' names, year of publication, country, calendar year(s) of baseline examination, follow-up period, setting, assessment of oesophageal cancer diagnosis, range of age at baseline, sex, number of observed oesophageal cancer cases among participants by alcohol exposure category, number of total participants by alcohol exposure category, adjustment for potential confounders and effect size with its standard error. We used the most fully adjusted effect size reported and selected estimates where lifetime abstainers were used as the risk reference group when those were available. Assessment of full-text articles with uncertain eligibility and data abstraction were conducted independently by two authors who discussed differences until consensus was reached. When there was not enough information presented in the article, we contacted the corresponding author.

We converted alcohol intake into grams of pure alcohol per day (g/day) using the midpoints (mean) of reported categories in the studies. For open-ended categories of alcohol intake, we added three-fourths of the previous category's range to the lower bound of the open-ended categories. We used reported conversion factors in the studies when standard drinks were the unit of measurement. Hazard ratios and odds ratios were assumed to be equivalent to relative risks (RR). We used fractional polynomials[@R21] to derive the best fitting function for average alcohol consumption in g/day using the pool-first approach described by Orsini et al.[@R22] Linear and first-degree models were estimated using the following range of powers: −2, −1, 0, 1, 2, 3.[@R21] Significant increases in deviance were determined by likelihood ratio tests with one degree of freedom.

Data analyses
-------------

We conducted several meta-analyses and used the most comprehensive data available separately for each analysis when multiple reports from the same cohort were published. For studies providing data on two or more alcohol intake categories among current drinkers, we pooled data from (i) cohort studies; (ii) case-control studies; (iii) case-control studies that provided stratified data by *ALDH2* variants. We conducted sensitivity analyses on the interaction between variants of *ADH1B* within the genetic variants of *ALDH2.* In analyses investigating *ALDH2* variants, studies were pooled separately for the active variant (*ALDH2\*1/\*1*) and inactive variants (*ALDH2\*1/\*2 and ALDH2\*2/\*2*). No cohort studies provided *ALDH2* genotype data. Where possible, we avoided *ALDH2\*2/\*2* variants because of the low number of cases. No systematic information on the distribution of *ALDH2* variants by drinking level was available and we therefore used the distribution of drinking by *ALDH2* variants among controls in case-control studies to estimate this distribution at the population level. Finally, studies were pooled using DerSimonian-Laird random-effect models to allow for between-study heterogeneity.[@R23] Variation in the effect size other than chance because of heterogeneity between studies was quantified using the *I^2^* statistic.[@R24] We conducted meta-regression analyses to identify study characteristics that influenced the association between alcohol consumption and oesophageal cancer risk. Because of few available studies, we were only able to investigate study design in such meta-regression analyses. Examination of potential publication bias using Egger's regression-based test[@R25] was planned, but was not done because of the few studies included. All meta-analyses were conducted on the natural log scale in Stata statistical software, version 12.1 (StataCorp. LP, College Station, United States of America) and *P* \< 0.05 (two-sided) was considered statistically significant.

We estimated deaths and DALYs lost from oesophageal cancer attributable to alcohol consumption in Japan applying a standardized alcohol-attributable fraction method[@R26] using the statistical software package R, version 3.0.3 (R Foundation for Statistical Computing, Vienna, Austria). These deaths and DALYs were estimated by comparing the risk difference of oesophageal cancer under current conditions compared to the risk of oesophageal cancer under the theoretical-minimum-risk exposure scenario where no one has consumed alcohol.[@R1]^,^[@R7] These calculations combine information on the prevalence of alcohol consumption adjusted for per capita consumption and RRs for oesophageal cancer. Lifetime abstainers were used as the reference group and compared to former drinkers and current drinkers -- by average daily alcohol consumption. Data on alcohol drinking status were obtained from the 2010 GBD study,[@R1] where data on drinking status were based on data from large population surveys. Data on per capita consumption were from the Global Information System on Alcohol and Health.[@R27] Calculations for Japan were based on RRs from this study and RRs for GBD estimates for Japan were based on Corrao et al.[@R11]

Results
=======

After removal of duplicates, we evaluated 1333 records for inclusion in our study. Based on titles and abstracts, we excluded 1174 articles and screened 159 in full-text articles ([Fig. 1](#F1){ref-type="fig"}). After excluding duplicate reports of the same cohorts, we analysed 11 case-control studies[@R28]^--^[@R38] and 3 cohort studies.[@R39]^--^[@R41] Eight case-control and cohort studies[@R32]^--^[@R36]^,^[@R39]^--^[@R41] reported estimates for at least two alcohol intake categories in comparison to non-drinkers. These studies were used for a nonlinear dose--response analysis of oesophageal cancer risk, including stratified analyses by *ALDH2* variants. Four case-control studies[@R28]^--^[@R31] provided indirect evidence for only one alcohol intake category [Table 1](#T1){ref-type="table"} (available from: <http://www.who.int/bulletin/volumes/93/5/14-142141>). In total five studies had data on *ALDH2* and *ADH1B* variants stratified or adjusted by level of alcohol consumption.[@R29]^,^[@R31]^,^[@R36]^--^[@R38]

![Flowchart for the selection of studies on alcohol consumption and oesophageal cancer in Japan](BLT.14.142141-F1){#F1}

###### Summary of studies assessing the relationship between alcohol and oesophageal cancer in Japan

  -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Study and year                    Study design (follow-up)      Setting                                                                                                                                                                       Study period, age and sex                 No. of cases and controls                                                                     Case and control identification                                                                                            Alcohol assessment                                                                                                                                                                                                                            Adjustment
  --------------------------------- ----------------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ----------------------------------------- --------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Yokoyama et al., 1998[@R28]       Case-control                  National Institute on Alcoholism, Kurihama National Hospital                                                                                                                  1987--1997, ≥ 40 years, men               Cases: 87, whereof *ALDH2\*1/\*1*: 41, *ALDH2\*1/\*2* and *ALDH2\*2/\*2*: 46\                 Cases: SCC histologically diagnosed at alcohol treatment entry or before onset of alcoholism\                              Alcohol dependence (DSM-III), mean alcohol intake 123 g/day                                                                                                                                                                                   Age at admission to alcohol treatment, daily alcohol consumption, number of cigarettes
                                                                                                                                                                                                                                                                                          Controls: 487                                                                                 Controls: cancer-free alcoholics                                                                                                                                                                                                                                                                                                                                         

  Takezaki et al., 2000[@R35]       Case-control                  Aichi Cancer Centre                                                                                                                                                           1988--1997, 40--79 years, men             Cases: 284\                                                                                   Cases: first out-visit outpatients diagnosed with primary cancer of the oesophagus (ICD-9: 150, ICD-10: C15)\              Drinking levels: never or occasionally, former drinkers, current drinkers \< 1.5 drinks/day, ≥ 1.5 drinks/day. One drink = 1 *go* (Japanese sake with 27 mL ethanol)                                                                          Age, year and season of visit, smoking (never, former, current, \< 30 and ≥ 30 pack-years), consumption of raw vegetables
                                                                                                                                                                                                                                                                                          Controls: 11 384 (former alcohol drinkers were excluded in the analysis)                      Controls: first-visit outpatients confirmed to be cancer-free (including no history of cancer assessed by questionnaire)                                                                                                                                                                                                                                                 

  Yokoyama et al., 2001[@R29]^,a^   Case-control                  National Institute on Alcoholism, Kurihama National Hospital                                                                                                                  1993--2000, ≥ 40 years, men               Cases: 112, whereof *ALDH2\*1/\*1*: 50, *ALDH2\*1/\*2* and *ALDH2\*2/\*2*: 62\                Cases: SCC histologically diagnosed at alcohol treatment entry or before onset of alcoholism\                              Alcohol dependence (DSM-III), mean alcohol intake 123 g/day                                                                                                                                                                                   Age at admission to alcohol treatment, daily alcohol consumption, number of cigarettes
                                                                                                                                                                                                                                                                                          Controls: 526                                                                                 Controls: cancer-free alcoholics                                                                                                                                                                                                                                                                                                                                         

  Matsuo et al., 2001[@R30]         Case-control                  Aichi Cancer Centre                                                                                                                                                           1984--2000, 40--76 years, women and men   Cases: 102, whereof *ALDH2\*1/\*1*: 35, *ALDH2\*1/\*2*: 66, *ALDH2\*2/\*2*: 1\                Cases: first diagnosis for oesophageal cancer\                                                                             Drinking status (2 categories): \> 3 *go* (Japanese sake with 75 mL pure alcohol)/day ≥ 5 times per week, and all others (non-drinkers and drinkers with ≤ 3 *go* per day and \< 5 times per week)                                            Age, smoking (never, former, current, \< 30 and ≥ 30 pack-years), consumption of raw vegetables
                                                                                                                                                                                                                                                                                          Controls: 241                                                                                 Controls: first-visit outpatients confirmed by gastroscopy to have no oesophagus or stomach cancer                                                                                                                                                                                                                                                                       

  Yokoyama et al., 2002[@R36]^,a^   Case-control                  National Cancer Centre Hospital, National cancer Centre Hospital East, Kawasaki Municipal Hospital, National Osaka Hospital                                                   2000--2001, 40--79 years, men             Cases: 220 SCC, whereof *ALDH2\*1/\*1*: 60, *ALDH2\*1\*2*: 160\                               Cases: SCC newly diagnosed by histology within 3 years before registration in study\                                       Drinking levels: non- or rare drinkers, former drinkers, current drinkers 1--8.9 U/week, 9--17.9 U/week, ≥ 18 U/week. U = unit of alcohol (1 serving of sake, 22 g pure alcohol/U)                                                            Age, frequency of drinking strong alcoholic beverages, smoking (pack years), intake frequency of green-yellow vegetables, intake frequency of fruits
                                                                                                                                                                                                                                                                                          Controls: 590                                                                                 Controls: cancer-free men who visited two Tokyo clinics for annual health check-up                                                                                                                                                                                                                                                                                       

  Yokoyama et al., 2003[@R32]       Case-control                  National Cancer Centre Hospital, National cancer Centre Hospital East, Kawasaki Municipal Hospital, National Osaka Hospital                                                   2000--2001, 40--79 years, men             Cases: 220 SCC\                                                                               Cases: SCC newly diagnosed by histology within 3 years before registration in study\                                       Drinking levels: non- or rare drinkers, former drinkers, current drinkers 1--8.9 U/week, 9--17.9 U/week, ≥ 18 U/week. U = unit of alcohol (1 serving of sake, 22 g pure alcohol/U)                                                            Age, frequency of drinking strong alcoholic beverages, smoking (pack years), intake frequency of green-yellow vegetables, intake frequency of fruits
                                                                                                                                                                                                                                                                                          Controls: 598 (former alcohol drinkers were excluded in the analysis)                         Controls: cancer-free men who visited two Tokyo clinics for annual health check-ups                                                                                                                                                                                                                                                                                      

  Nakaya et al., 2005[@R40]         Cohort (7 years follow-up)    Miyagi II                                                                                                                                                                     1990--1997, 40--64 years, men             Cases: 48 among 19 607 participants (former alcohol drinkers were excluded in the analysis)   Cases were identified via record linkage to cancer registry                                                                Drinking levels: five categories based on drinking frequency and amount per occasion: never, former-drinkers, current drinkers \< 22.8 g pure alcohol/day, 22.8--45.5 g/day, and ≥ 45.6 g/day                                                 Age, smoking (never, former, current 1--19 cigarettes per day, 20--29 per day, 30 or more per day), education, daily consumption of orange and other fruit juice, spinach, carrot or pumpkin, and tomato

  Yokoyama et al., 2006[@R33]       Case-control                  National Cancer Centre Hospital, National cancer centre Hospital East, Kawasaki Municipal Hospital, National Osaka Hospital                                                   2000--2004, 40--79 years, women           Cases: 43 SCC, whereof *ALDH2\*1/\*1*: 25, *ALDH2\*1/\*2*: 18\                                Cases: SCC newly diagnosed by histology within 3 years before registration in study\                                       Drinking levels: non- or rare drinkers, former drinkers, current drinkers 1--8.9 U/week, 9--17.9 U/week, ≥ 18 U/week. U = unit of alcohol (1 serving of sake, 22 g pure alcohol/U)                                                            Age, smoking (pack years), intake frequency of green-yellow vegetables, intake frequency of fruits, preference for hot food or drinks
                                                                                                                                                                                                                                                                                          Controls: 365                                                                                 Controls: cancer-free women who visited two Tokyo clinics for annual health check-up                                                                                                                                                                                                                                                                                     

  Ozasa et al., 2007[@R41]          Cohort (not reported)         JACC                                                                                                                                                                          1988--1990, 40--79 years, men             Cases: 117 among 42 578 participants (former alcohol drinkers were excluded)                  Death certificates (ICD-10: C15)                                                                                           Drinking levels: non- or rare drinkers, former drinkers, current drinkers \< 54 mL pure alcohol/day, 54--80 mL/day, ≥ 81 mL/day                                                                                                               Age, study area

  Cui et al., 2009[@R31]^,a^        Case-control                  Biobank Japan                                                                                                                                                                 2003--2008, 35--85 years, men and women   Cases: 1 066, whereof *ALDH2\*1/\*2*: 735, *ALDH2\*1/\*1* and *ALDH2\*2/\*2*: 331\            Cases: histologically diagnosed SCC\                                                                                       Drinking status: none/rare (0--96.5 g pure alcohol/week), and other drinkers (≥ 96.5 g/week)                                                                                                                                                  Age, gender (analyses among heavy alcohol consumers, \> 96.5 g/week)
                                                                                                                                                                                                                                                                                          Controls: 2 761                                                                               Controls: volunteers or registered in Biobank for diseases other than cancer                                                                                                                                                                                                                                                                                             

  Ishiguro et al., 2009[@R39]       Cohort (14 years follow-up)   JPHC I+II                                                                                                                                                                     1990 and 1993, 40--59 years, men          Cases: 215 SCC among 60 876 participants                                                      Active patient notification from hospital and linkage to Cancer Registry (ICD-0--3: C15.0--15.9)                           Drinking levels: non-drinkers, less than weekly drinking (frequency only), 1--149 g pure alcohol /week, 150--299 g/week, ≥ 300 g/week                                                                                                         Age, study area, body mass index, preference for spicy food and drinks, smoking status (never, past, current), flushing response

  Oze et al., 2010[@R34]            Case-control                  Aichi Cancer Centre                                                                                                                                                           2001--2005, ≥ 18 years, men and women     Cases: 260, whereof *ALDH2\*1/\*1*: 67, *ALDH2\*1/\*2* and *ALDH2\*2/\*2*: 198\               Cases: first out-visit outpatients diagnosed with primary cancer of the oesophagus (ICD-10: C15)\                          Drinking levels: never, moderate (≤ 4 days/week), high-moderate (≥ 5 days/week and \< 46 g pure alcohol/occasion), heavy (≥ 5 days/week and ≥ 46 g/occasion)                                                                                  Frequency matched by age group (\< 40, 40--49, 50--59, 60--69, ≥ 70 years) and sex. Adjustment for cumulative smoking, facial flushing, fruit and vegetable intake, frequent intake of hot beverages and body mass index
                                                                                                                                                                                                                                                                                          Controls: 487                                                                                 Controls: First-visit outpatients confirmed to be cancer-free (including no history of cancer assessed by questionnaire)                                                                                                                                                                                                                                                 

  Yang et al., 2005[@R37]^,a^       Case-control                  Aichi Cancer Centre                                                                                                                                                           2000--2004, 18--79 years, men and women   Cases: 165, whereof *ALDH2\*1/\*1*: 38, *ALDH2\*1/\*2* and *ALDH2\*2/\*2*: 127\               Cases: histologically diagnosed with primary cancer of the oesophagus (159 SCC, 6 adenocarcinomas)\                        Drinking levels: non-drinker, non-heavy drinkers (\< 5 drinking days/week and \< 50 g pure alcohol/occasion) and heavy drinkers (drinking ≥ 5 days/week and ≥ 50 g pure alcohol/occasion) were adjusted for in regression model as reported   Age, sex, smoking, drinking
                                                                                                                                                                                                                                                                                          Controls: 495                                                                                 Controls: first-visit outpatients confirmed to be cancer-free (including no history of cancer assessed by questionnaire)                                                                                                                                                                                                                                                 

  Tanaka et al., 2010[@R38]^a^      Case-control                  Juntendo University Hospital, National Cancer Center Hospital, Kurume University Hospital, Saitama Cancer Center, Kagoshima University Hospital, Kyushu University Hospital   2000--2008, 35--85 years, men and women   Cases: 742, whereof *ALDH2\*1/\*1*: 194, *ALDH2\*1/\*2* and *ALDH2\*2/\*2*: 548\              Cases: pathologically newly diagnosed SCC\                                                                                 Drinking levels: Non-drinker and ever drinkers                                                                                                                                                                                                Sex, age, study area
                                                                                                                                                                                                                                                                                          Controls: 820                                                                                 Controls: healthy controls without cancer history recruited from Kyushu University and Hospital and related hospitals                                                                                                                                                                                                                                                    
  -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

ADH1B: alcohol dehydrogenase 1B; ALDH2: aldehyde dehydrogenase 2; DSM: diagnostic statistical manual; ICD: International Classification of Diseases; JACC: Japan collaborative cohort study; JPHC: Japan public health centre-based prospective study; SCC: squamous cell carcinoma of the oesophagus.

^a^ Included in the sensitivity analysis on interaction between *ALDH2* and *ADH1B* on oesophageal cancer risk in Japan.

Note: *ALDH2\*2/\*2* was excluded in our analyses where possible because of the low number of cases.

As shown in [Fig. 2](#F2){ref-type="fig"}, the risk for oesophageal cancer identified in cohort studies from Japan[@R39]^--^[@R41] was higher compared with the most recent GBD estimate (RR: 11.71; 95% confidence interval, CI: 2.67--51.32 and RR: 3.59; 95% CI: 3.34--3.87, respectively at 100 g/day of pure alcohol intake). The risk identified in case-control studies[@R32]^--^[@R35] (RR: 11.88; 95% CI: 4.41--31.99 at 50 g/day of pure alcohol intake; RR: 33.11; 95% CI: 8.15--134.43 at 100 g/day of pure alcohol intake) was much higher than the Japanese cohort studies or GBD estimates. In a meta-regression, the difference between case-control studies and cohort studies was significant (*P* = 0.014). We observed moderate heterogeneity among cohort studies (*I^2^* = 60%, *P* = 0.082), and high heterogeneity among case-control studies (*I^2^* = 89%, *P* \< 0.001).

![Risk curves for alcohol consumption and oesophageal cancer risk based on Japanese studies or the Global Burden of Disease 2010 study](BLT.14.142141-F2){#F2}

The risk curves by *ALDH2* variants in Japan are displayed in [Fig. 3](#F3){ref-type="fig"}. Three case-control studies[@R33]^,^[@R34]^,^[@R36] provided dose--response data for an investigation of *ALDH2* polymorphisms in reference to non-drinkers: *ALDH2\*1/\*2* (372 cases) and *ALDH2\*1/\*1* (151 cases). Inactive variants of ALDH*2* enzyme showed markedly higher risks with increasing alcohol consumption. The RR compared to non-drinkers was 36.15 (95% CI: 10.34--126.40) at 50 g/day of pure alcohol and 159 (95% CI: 27.2--938.2) at 100 g/day of pure alcohol intake among people carrying the *ALDH2\*1/\*2* variant. In comparison, the RR among those carrying the *ALDH2\*1/\*1* variant was 2.99 (95% CI: 1.75--5.12) at 50 g/day of pure alcohol intake and 8.94 (95% CI: 3.05--26.23) at 100 g/day of pure alcohol. Based on two studies that included people with alcohol dependence (median 120 g/day of pure alcohol intake), people with the inactive variant of *ALDH2* had an RR of 13.00 (95% CI: 8.99--18.80) compared to those with the active variant.[@R28]^,^[@R29] We interpolated this difference in risk in the curve for *ALDH2\*1/\*2* in [Fig. 3](#F3){ref-type="fig"}, and held the risk increase among people with this *ALDH2* variant constant beyond 100 g/day of pure alcohol intake because there were insufficient data to reliably estimate this risk function. Once case-control studies were stratified by *ALDH2* variant, there was little or no heterogeneity (*ALDH2\*1/\*1*, *I^2^* = 0%, *P* = 0.78; *ALDH2\*1/\*2*, *I^2^* = 44%, *P* = 0.17). Another two studies,[@R30]^,^[@R31] although they did not provide data in reference to non-drinkers, were in close agreement with our reported risk functions.

![Risk curves for alcohol consumption and oesophageal cancer risk based on aldehyde dehydrogenase 2 polymorphisms, Japan](BLT.14.142141-F3){#F3}

With regard to differences in risk curves by study design, [Table 2](#T2){ref-type="table"} shows that among case-control studies with multiple alcohol intake categories, 72% (350/483) of oesophageal cancer cases among drinkers occurred in 32% (313/980) of the drinking population, namely individuals with the genetic variant *ALDH2\*1/\*2*. When the risk curves from case-control studies ([Fig. 3](#F3){ref-type="fig"}) were combined (weighted by their distribution of alcohol consumption by *ALDH2* variants at the population level) the risk functions from case-control and cohort studies almost entirely overlapped ([Fig. 4](#F4){ref-type="fig"}). Combining adjusted case-control and cohort studies, at 100 g/day pure alcohol intake, the risk in Japan was markedly elevated (RR: 11.65, 95% CI: 4.16--32.62) compared to GBD estimates (RR: 3.55, 95% CI: 3.30--3.82) ([Fig. 5](#F5){ref-type="fig"}).

###### Distribution of alcohol consumption and the *ALDH2* polymorphism in individuals with oesophageal cancer and study controls, Japan

  Polymorphism                   Alcohol consumption, No*.* of individuals (%)                                   
  ------------------------------ ----------------------------------------------- ---------- ---------- --------- -----------
  **Controls**                                                                                                   
  *ALDH2\*1/\*2*                 336 (52)                                        234 (36)   51 (8)     28 (4)    649 (100)
  *ALDH2\*1/\*1*                 145 (18)                                        350 (43)   231 (28)   86 (11)   812 (100)
  **Oesophageal cancer cases**                                                                                   
  *ALDH2\*1/\*2*                 22 (6)                                          94 (25)    180 (48)   76 (20)   372 (100)
  *ALDH2\*1/\*1*                 18 (12)                                         35 (23)    61 (40)    37 (25)   151 (100)

ALDH2: aldehyde dehydrogenase 2.

Note: The *ALDH2\*1/\*1* corresponds to an active enzyme and *ALDH2\*1/\*2* corresponds to an inactive enzyme.

Data sources: Yokoyama et al.[@R36] Yokoyama et al.[@R33] Oze et al.[@R34]

![Risk curves for alcohol consumption and oesophageal cancer risk adjusted for aldehyde dehydrogenase 2 polymorphisms, Japan](BLT.14.142141-F4){#F4}

![Risk curves for alcohol consumption and oesophageal cancer risk based on Japanese studies adjusted for aldehyde dehydrogenase 2 polymorphisms and the Global Burden of Disease 2010 study](BLT.14.142141-F5){#F5}

To investigate the interaction between *ALDH2* and *ADH1B* gene variants, we performed a sensitivity analysis using five of the 11 identified case-control studies.[@R29]^,^[@R31]^,^[@R36]^--^[@R38] Regardless of *ALDH2* variant, the pooled RRs were higher for Japanese with the slow-acting *ADH1B\*1/\*1* variant than for Japanese with the fast-acting *ADH1B\*1/\*2* or *\*2/\*2* variants. The RR was 3.99 (95% CI: 2.41--6.61; I^2^ = 81%, *P* \< 0.001) and 2.40 (95% CI: 1.92--3.16; I^2^ = 1%, *P* = 0.40) for individuals with an inactive and active ALDH2, respectively ([Fig. 6](#F6){ref-type="fig"} and [Fig. 7](#F7){ref-type="fig"}).

![Relationship between alcohol dehydrogenase 1B polymorphisms and oesophageal cancer risk in Japanese with inactive aldehyde dehydrogenase 2](BLT.14.142141-F6){#F6}

![Relationship between alcohol dehydrogenase 1B polymorphisms and oesophageal cancer risk in Japanese people with active aldehyde dehydrogenase 2](BLT.14.142141-F7){#F7}

Using our calculated risk relations for alcohol-attributable oesophageal cancer results in almost twofold higher estimates for deaths (5279) and DALYs lost (102 988) compared with the current GBD estimates (2749 and 53 826, respectively; [Table 3](#T3){ref-type="table"}). These results are irrespective of whether the estimates were based on cohort studies or on case-control studies, in each case adjusted for population prevalence of genotypes.

###### Estimated mortality and burden of disease for alcohol-attributable oesophageal cancer in Japan 2010

  Estimate                                 Women           Men           Total               
  ---------------------------------------- ------- ------- ----- ------- -------- -- ------- ---------
  GBD 2010                                 202     3 089         2 547   50 737      2 749   53 826
  Japanese cohort studies                  346     5 498         4 925   97 284      5 271   102 782
  Japanese adjusted case-control studies   346     5 514         4 933   97 474      5 279   102 988

DALYs: disability-adjusted life-years; GBD: Global Burden of Disease.

Discussion
==========

For Japan, we estimated a twofold higher mortality and burden of disease using risk functions derived from Japanese populations compared with the 2010 GBD estimates, which are based on global risk functions. We obtained separate estimates based on independent methods -- either Japanese cohort data or adjusted Japanese case-control studies -- and these estimates were comparable. This strengthens our conclusion that the current GBD method underestimates Japanese oesophageal cancer outcomes. Since we took into consideration genetic polymorphisms commonly observed in the Japanese population, we would predict a similar degree of underestimation for alcohol-attributable oral, pharynx and larynx cancers in Japan.[@R34]^,^[@R42]^,^[@R43] Furthermore, the burden of other alcohol-attributable cancers where acetaldehyde plays an important role might be underestimated.[@R3]^,^[@R4]

We found that the slow-acting *ADH1B* variant also increased the risk for oesophageal cancer, regardless of *ALDH2* variant. However, the slow-acting variant is only present in 6% of the Japanese population,[@R44] whereas 90% of Caucasians carry the variant.[@R45] As we had restricted all our analyses to Japanese individuals, the potential protective effect of the fast-acting *ADH1B\*1/\*2* or *\*2/\*2* variants has been already included. Similarly, risk estimates from cohort studies should not be affected by the differential risk curves for *ALDH2* and *ADH1B* variants and their combinations if the prevalence of each combination of polymorphisms is reflected in the sample.

The study has some limitations. First, any systematic review or meta-analysis is only as good as the literature it is based on. Although case-control studies initially showed high heterogeneity as measured by *I^2^* values indicating potential bias, there was little heterogeneity after these studies were stratified by *ALDH2* variants. Second, while the procedures to estimate alcohol-attributable fractions are standard,[@R26]^,^[@R27]^,^[@R46] subsequent adjustments to survey results may bias consumption in either direction.[@R47]^--^[@R49] However, as the same method of triangulation of surveys and per capita estimation was applied to GBD and to national estimates,[@R1] the comparison between these estimates should be valid. Finally, the estimates of attributable risk and burden of disease for heavy alcohol intake are based on few studies and thus may be biased.[@R50] By including risk estimates of people with alcohol dependence, we attempted to minimize this bias.

While there may be some biases in our quantitative estimates of alcohol-attributable burden for oesophageal cancer, they still show that global estimates underestimate the burden in Japan. This will likely be true for GBD estimates for China and the Republic of Korea as well, where a considerable proportion of the population also carry the inactive *ALDH2* allele, (34% and 29%, respectively).[@R44] In populations with a high proportion of these polymorphisms, studies based on global dose response data are likely to underestimate many alcohol-attributable cancers.

Efforts should be made to estimate country-specific risks for diseases affected by genetic polymorphisms, especially in countries with higher proportions of such polymorphisms. The current standard of applying global risk functions to local exposure data should be replaced by country-specific risk functions whenever possible. Country-specific risk functions should also be applied for other risk factors than alcohol.[@R1]^,^[@R51] This will allow for better estimation of the burdens caused by risk factors and consequently better informed policy measures.

SH has received grants from Lundbeck Japan during the conduct of the study; grants from the Japanese government, Lundbeck Japan, Suntory, grants and personal fees from Nippon Shinyaku, outside the submitted work. JR has received an unrestricted grant from Lundbeck pharmaceuticals for the study. The other authors declare no competing interests.
