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Etiology Program, Cancer Research Center of Hawaii, University of Hawaii, Honolulu, Hawaii 96813
| Abstract |
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| Introduction |
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Although much is known about the lifestyle risk factors for CRC, the molecular genetic basis for this inherited susceptibility remains largely undefined. Nevertheless, it is possible to use family history of the disease as a proxy for a genetically determined predisposition. The aims of the present study were: (a) to identify specific lifestyle risk factors with a particularly strong effect on the CRC risk of individuals at increased familial risk for the disease; and (b) to assess the individual and joint effects of family history and a summary measure of lifestyle on CRC risk. We used the data from a large population-based case-control study of CRC to explore interactions between various lifestyle factors and family history of the disease among first degree relatives. We previously reported that, consistent with past data, such a history was associated with a 2.5-fold (95% CI, 1.83.4) increase in CRC risk in this study (2) .
| Methods |
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Controls were randomly selected among the participants in an ongoing health survey conducted by the Hawaii State Department of Health in a 2% random sample of state households. Because this survey is mandated by law, the refusal rate is low. One control was matched to each case on sex, race, and age (controls age at interview within ± 2.5 years of the cases age at diagnosis). The overall participation rate for the controls was 71%. Reasons for nonparticipation included refusal (18%), inability to locate (5%), serious illness (3%), and death (3%).
Interviews were conducted in person, most often at the subjects homes, by trained interviewers who could not be blinded to the case-control status of the subjects but who were unaware of the specific hypotheses under study. The questionnaire took an average of 2 h to administer and included: (a) detailed demographic information, such as the race of each grandparent; (b) a quantitative food frequency questionnaire; (c) a lifetime history of tobacco and alcohol use; (d) a history of recreational sports activities since age 18; (e) a lifetime history of occupational physical activity; (f) a personal history of various relevant medical conditions; (g) a family history of cancer, including CRC; and (h) information on height and weight.
The food frequency questionnaire used in this study (6) had been validated previously in our population (7) . Frequencies and amounts consumed were sought for more than 282 food items. The reference period for the diet questionnaire was the 3-year period before the onset of symptoms for the cases and the 3-year period before interview for the controls. The particular foods included in the questionnaire were chosen based on 3-day food records collected from randomly selected individuals of the five main ethnic groups in Hawaii. The questionnaire was designed to capture the complete diet, including total caloric intake. Colored photographs of most food items, showing three different portion sizes, as well as measuring cups and spoons, were used during the interview to facilitate quantification of intakes. Subjects were also questioned about the brand and dose of any vitamin and mineral supplements taken for a minimum of 6 months during the reference period. The food composition data were primarily based on the United States Department of Agricultures nutrient database (8) and were supplemented with data from other research and commercial publications (9, 10, 11) . Nutrient content information for supplements was obtained from the manufacturers.
For all of the analyses, family history of CRC was defined as reporting at least one first-degree relative (parent or sibling) with the disease. Nutrient intakes were adjusted for calories by the method of residuals (12) . The main effects of family history and other risk factors, such as daily nutrient and food intakes, and the interactions with family history were modeled using conditional logistic regression (13) with adjustment for relevant covariates. Interaction is defined here to be any significant deviation from the joint effect predicted from the logistic regression (i.e., multiplicative interaction) and was tested by a likelihood ratio test comparing the model with interaction terms and a model containing only the main effects; P < 0.10 was used for statistical significance because of the low power of this test for interaction. Strata were defined to be the age- and race-matched case-control pairs. Separate models were created for men and women. First, the interactions with lifestyle variables were modeled by five dummy variables representing the upper two tertiles of exposure for those with no family history and all of the tertiles for those with family history. The group in the lowest tertile of exposure and with no family history was taken as the reference group. Tertile cutpoints were sex-specific and based on data from the controls. ORs and 95% CIs were computed for the six risk groups by exponentiating the ß coefficients and their CIs, assuming normality.
Next, a stepwise logistic regression was performed to determine which interactions with family history were independently important. Family history and all of the main effects found to be related to risk in previous analyses (calories, beef, eggs, calcium, NSP from vegetables, ethanol, pack-years, Quetelet index 5 years ago, and lifetime recreational physical activity) were forced into the model, whereas other main effects (other variables in Table 1
and polyunsaturated fat) and all of the possible interaction terms between family history and lifestyle variables competed for inclusion. Because no threshold effects were observed for the lifestyle exposures, they were entered as continuous variables to improve the power for detecting associations. For males, interactions between family history and two lifestyle variables were significant. For females, no interactions entered the model. We denote x as the indicator variable for family history, z as the first lifestyle variable interacting with family history, y as the second such variable, and w1 wm as the other lifestyle risk factors. Then, the final model for men can be represented as:
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(a) the magnitude of interactions in the male model was investigated by representing z and y by dummy variables indicating tertile membership (whereas all of the other lifestyle variables were continuous) and jointly computing the OR for each combination of family history and lifestyle exposure for these variables (Table 3)
. Interaction with family history in the male model was tested for significance by a likelihood ratio test that compared the model with interaction terms and a model containing only the main effects; the P for this four-degrees-of-freedom test is given in Table 3
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the lowest tertile for adverse risk factors or
the highest tertile for protective risk factors), that covariate is set to zero. | Results |
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Table 2
presents the adjusted ORs and 95% CIs for the two-way interactions of family history and several lifestyle variables found to be associated with CRC in previous analyses of this study (5
, 20
, 21)
. Significant interactions (P
0.1) with family history were observed for: (a) P:S ratio and intakes of beef and chicken without skin in males; (b) processed meat, total calcium, and methionine intakes in females; and (c) saturated fat in both sexes. There was no association with beef intake in females irrespective of family history. A particularly strong association was found with ethanol intake in men reporting a family history, with an OR for the upper tertile of 5.7 (95% CI, 2.413.2), although the P for interaction was not significant.
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65) or subsite of the large bowel (right colon, left colon, rectum) did not suggest that these interactions were stronger for a particular age group or subsite, although the power was limited for these subset analyses. However, consistent with the stronger overall association of family history with CRC in Japanese compared with Caucasians in this study (2)
, the interactions with meat and alcohol were stronger for Japanese men (P = 0.16) than for Caucasian men (P =0.52).
Finally, overall ORs were computed from the final adjusted model (with all of the covariates entered continuously) for an increment of all of the lifestyle variables equal to the intertertile range of exposure in subjects with and without family history (Table 4)
. For men with a healthy lifestyle (i.e., with each environmental exposure variable set to the low-risk tertile), family history was not associated with risk. In contrast, the combination of an unhealthy lifestyle (i.e., when each variable was set to the high-risk tertile) and a positive family history was particularly detrimental with an OR of 11.7 (95% CI, 5.823.9), compared with no family history and a healthy lifestyle. In women, the main effects of family history and lifestyle were both statistically significant and of comparable magnitude (there was no interaction term in the female model).
Because lifestyle is modifiable, we computed the AR for lifestyle in the final models, in which all of the lifestyle variables were compared with the "healthy" tertile values. Because all of the lifestyle exposures were required to be at the lower-risk tertile, our definition of a healthy lifestyle is somewhat extreme and is only meant to be illustrative of the contribution of lifestyle to risk. For men, the AR for lifestyle was 74.1% among those with no family history and 90.6% among those with a family history. For women, the AR for lifestyle was 70.5% and did not vary by family history. As expected, the ARs for beef, ethanol, and both factors combined varied greatly depending on family history in men (1.1, 19.0, and 19.8%, respectively, for men without family history; and 51.1, 44.7, and 74.2%, respectively, for men with a positive family history). For women, the ARs for these dietary variables were 4.4, 5.3, and 9.5%, respectively. No single variable for men without family history or for women had an AR greater than 21%.
| Discussion |
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Most past studies have focused on individuals with no obvious familial predisposition and have identified a number of potentially modifiable risk factors for colon cancer, including (a) high fat, high red meat, and high ethanol intakes; (b) low fiber and low vegetable intakes; (c) smoking; (d) physical inactivity; and (e) obesity (22) . The present study in Hawaii confirms these associations (5 , 20 , 21) but also suggests that lowering exposure to these lifestyle risk factors may achieve a major reduction in the incidence of CRC (a 70% reduction if exposures are all decreased to the lower-risk tertile level). Because no single risk factor had a particularly high AR, a comprehensive approach to lifestyle modification seems most promising as a general recommendation. In contrast, for the group of men with a family history, our data predicted that an equally strong risk reduction might be obtained by focusing only on lowering beef and ethanol intakes, a change that may be more easily achieved.
We are aware of only one previous report on lifestyle risk factors for CRC in individuals with a family history of the disease. A hospital-based case-control study by Fernandez et al.(23) in northern Italy assessed risk factors for CRC among individuals with a first-degree family history of the disease and found similar risk factors as for individuals without a family history. However, the strengths of these associations were not compared between familial and nonfamilial presentations of the disease.
To date, the numbers of individuals who have been unequivocally characterized as genetically susceptible (e.g., because they carry an APC mutation or a defective DNA mismatch repair gene) are too limited to allow the investigation of possible interactions of genetic predisposition and environmental risk factors. Therefore, we have used family history as a proxy for genetic susceptibility in this study. Admittedly, family history is an imperfect surrogate for genetic predisposition. Familial clustering may not always be observed (e.g., because of small families, late age of onset, or limited penetrance) or may merely result from shared environmental exposures. Although lifestyle is expected to correlate somewhat among adult relatives, there are only limited and inconsistent data documenting the extent of this correlation (24 , 25) . Because the present data did not allow us to separate the contributions of genetic susceptibility and lifestyle to familial aggregation, they should be considered as exploratory. Nevertheless, there is other evidence for a role of environmental risk factors in the development of CRC in genetically predisposed individuals.
With regard to the rare but highly penetrant inherited syndromes, a role for environmental factors in HNPCC would be consistent with the variation in tumor spectrum noted for this syndrome over time and between countries. A recent update on the original HNPCC family described by Warthin 100 years ago revealed that the excess of stomach and uterine cancers initially observed in this family had been replaced by a predominance of CRCs in recent generations (26) , following the same (presumably environmentally-induced) secular trend as in the general population. Similarly, in Japan, an area of high stomach cancer incidence, members of HNPCC families develop gastric cancer more often than in the United States (27) . Moreover, that environmental factors can alter the phenotypic expression of a familial form of CRC has been demonstrated in FAP patients by a small randomized intervention trial in which wheat fiber was shown to reduce the number of polyps (28) .
In this study, we did not seek to collect all of the data needed to reliably characterize patients with HNPCC or FAP. However, in an attempt to subset such cases based solely on first-degree family history of CRC, we reran our analysis excluding subjects with two or more affected siblings or parents. The strengths of the interactions between family history and beef and alcohol were not substantially changed (P = 0.04 compared with P = 0.03 in Table 3
), which suggests that highly penetrant genes may not play a predominant role in explaining our findings.
Another class of susceptibility genes, referred to as "low-penetrance genes," have been found to interact with environmental exposures to moderately increase or decrease CRC risk. These genes, because of their high frequency in the population, may play an important role in sporadic cases. Examples are (functional) polymorphisms in genes that are involved in the metabolic activation or detoxification of carcinogens or are part of the metabolic pathways of critical nutrients (29, 30, 31) . It is notable that the lifestyle factors found to interact with family history in the present databeef, which is the main type of red meat consumed in Hawaii, and ethanolare dietary factors that have been incriminated in studies of genetic polymorphisms and CRC. Individuals who are expected, based on genotype for metabolic genes (such as NAT2, CYP1A2, and CYP1A1), to be particularly apt at enzymatically activating carcinogens present in cooked red meats have been found to be at increased risk for CRC (29 , 30) . Alcohol consumption has also been shown to negate the protective effect against CRC of a common polymorphism in the gene coding for 5,10-methylenetetrahydrofolate reductase (MTHFR), a polymorphism associated with higher intracellular levels of a folate metabolite (5,10-methylenetetrahydrofolate) which is required for normal DNA synthesis (31) .
The different findings by gender in this study may be caused by chance or by lower exposure levels in females (especially for ethanol) or may reflect underlying biological differences. Gender discrepancies in risk factors for CRC have been noted in several past studies (reviewed in Ref. 5 ). It is possible that purely constitutive factors may play a greater role in the etiology of CRC in women than men.
In this study, the history of CRC among parents and siblings was based on self-report. Studies have shown that the accuracy of such recall is reasonably good (32, 33, 34) and, most importantly, nondifferential between CRC patients and controls (33 , 34) . Thus, it does not appear likely that misclassification on family history because of recall error could explain our results. Similarly, recall bias on dietary exposures may have occurred but is expected from past studies to be limited (35) . It is unclear how such a recall bias, and measurement error in general, could have spuriously created the observed interactions. Because of the exploratory nature of this study, a relatively large number of interactions were tested. As a result, some of the findings may have been due to chance and need replication in other datasets.
In summary, the present data suggest that family history increases the risk of sporadic CRC in men mainly through its interaction with lifestyle exposures, primarily a high beef and ethanol intake. They also suggest that the combination of a familial predisposition and an unhealthy lifestyle increases risk of CRC considerably. In the absence of a family history for the disease, an unhealthy lifestyle would also increase male CRC risk substantially. The study failed to detect such interactions in women. Estimation of population ARs suggested that a comprehensive reduction in exposure to lifestyle risk factors, and more specifically ethanol and beef for individuals with a familial predisposition for the disease, may have a large beneficial effect on CRC incidence.
| Acknowledgments |
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| Footnotes |
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1 Supported in part by Grant PO1-CA-33619 and Contract NO1-CN-05223 from the National Cancer Institute. ![]()
2 To whom requests for reprints should be addressed, at Etiology Program, Cancer Research Center of Hawaii, 1236 Lauhala Street, Suite 407, Honolulu, HI 96813. ![]()
3 The abbreviations used are: CRC, colorectal cancer; FAP, familial adenomatous polyposis; HNPCC, hereditary nonpolyposis colorectal cancer; OR, odds ratio; CI, confidence interval; NSP, nonstarch polysaccharides; AR, attributable risk; P:S, polyunsaturated fat:saturated fat; APC, adenomatous polyposis coli. ![]()
Received 7/23/98; revised 10/19/98; accepted 10/29/98.
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