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Short Communication |
1 Radiation Epidemiology Branch, 2 Hormonal and Reproductive Epidemiology Branch, 3 Biostatistics Branch, 4 Occupational and Environmental Epidemiology Branch, 5 Epidemiology and Biostatistics Program, and 6 Genetic Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, NIH, Department of Health and Human Services, Bethesda, Maryland
Requests for reprints: Parveen Bhatti, Radiation Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, NIH, Department of Health and Human Services, 6120 Executive Boulevard, EPS 7091, MSC 7238, Bethesda, MD 20892-7238. Phone: 301-496-7950; Fax: 301-402-0207. E-mail: bhattip{at}mail.nih.gov
| Abstract |
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0.70 or
1.40, levels that might be notably different. Among the various respondent group comparisons, haplotype and short tandem repeat frequencies were not significantly different by willingness to participate. We observed little evidence to suggest that genotype differences underlie response characteristics in molecular epidemiologic studies, but a greater variety of genes should be examined, including those related to behavioral traits potentially associated with willingness to participate. To the extent possible, investigators should evaluate their own genetic data for bias in response categories. | Introduction |
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Although genetic variation with "true" nonresponse (i.e., those who did not provide genetic material) is impossible to address, genetic studies with recruitment waves provide a unique opportunity to investigate genetic frequency differences by participation. We examined frequencies of single nucleotide polymorphism genotypes, haplotypes, and short tandem repeat alleles by response status in control subjects from three studies with different recruitment designs allowing comparisons of early, late, and never questionnaire responders, one or more participation incentives, and blood or buccal DNA donation.
| Materials and Methods |
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Study B participants consisted of non-Hispanic Caucasian controls (516 males and 466 females) recruited for a case-control study of non-Hodgkin's lymphoma, from within four areas of the Surveillance, Epidemiology, and End Results cancer registry of the National Cancer Institute (12). Of these, 554 controls chose to provide a blood sample for genetic analyses, whereas 209 controls who did not provide blood samples did provide saliva (buccal) samples; 741 of these subjects responded to biospecimen donation at the time of study questionnaire administration (regular responders), whereas 22 subjects who initially refused to provide blood or buccal cell samples provided buccal cells after a final mail query at the end of the study (late responders).
Study C participants were controls that provided blood samples (232 females and 958 males) from a case-control study of lung cancer from the Lombardy region of Italy. Two-hundred and fifty-two controls (less incentivized group) were recruited by mail and telephone follow-up; the invitation was accompanied by cash or gas coupons and by a letter endorsing the study signed by the subjects' family physician. Nine-hundred and thirty-eight controls (highly incentivized group) were recruited using a letter of invitation, accompanied by a direct call by the subjects' family physician, a letter from the mayor of the participating cities supporting the research, and gas coupons to the subjects and family physicians; a toll-free number through which potential participants could obtain information about the study was also established and television advertisements were made.
Laboratory Methods
Study A participants were genotyped for 36 single nucleotide polymorphisms in DNA repair and growth factor genes (13). All samples from studies B and C were analyzed at the Core Genotyping Facility of the National Cancer Institute (http://cgf.nci.nih.gov/home.cfm). Study B participants were genotyped for 103 single nucleotide polymorphisms in genes involved in immune, oxidative stress, metabolism, cell cycle, and DNA repair pathways. For short tandem repeat analysis in all three studies, samples were quantified using PicoGreen and reverse transcription-PCR analysis and profiled using the Applied Biosystems Identifiler kit. Fifteen short tandem repeat loci were analyzed in studies A and C, and nine were analyzed in study B.
Statistical Analysis
We only considered those single nucleotide polymorphisms with a minor allele frequency of
5% for analyses; 15 single nucleotide polymorphisms from study A and 16 single nucleotide polymorphisms from study B were too infrequent for inclusion. We reconstructed haplotypes for APEX, BACH1, BRCA2, TGFß1, XRCC1, and ZNF350 for study A and for IL10 and LTA/TNF for study B (separately for each comparison group) using the PHASE software package (14). Haplotypes were not reconstructed for regular versus late responder analyses in study B because of the small number of late responders. We analyzed single nucleotide polymorphism and haplotype frequencies among categories of study recruitment using contingency table analyses in SAS release 8.02 (SAS Institute, Inc., Cary, NC); in addition to
2 analyses and odds ratios comparing the frequency of single nucleotide polymorphism carriers and noncarriers between the various comparison groups, single nucleotide polymorphism genotypes (homozygous wild type, heterozygous, homozygous variant) were analyzed among participation categories using the Mantel-Haenszel test for trend. We noted odds ratios that were
0.70 or
1.40 because this magnitude is approximately symmetrical around 1.0, and values outside this range could conceivably impact genotype-disease associations. For short tandem repeat analysis, we used SAS release 8.02 to calculate short tandem repeat genotype means and ranges at each locus for the various comparison groups. In addition, using Arlequin version 2000 (15), we estimated the standardized fixation index or FST (ratio of the number of different alleles observed between two individuals in two different samples compared with the number of different alleles observed between two individuals in the same sample; ref. 16). FST provides a single measure of genetic differentiation when multiallelic loci are being considered, such as short tandem repeats. All tests for significance were two-sided with
set at 0.05.
| Results |
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0.70 or
1.40, respectively. The TGFß1 P25R variant differed significantly (trend test, P = 0.03) among the questionnaire response groups; one statistically significant trend was expected by chance. Haplotype frequencies for the various genes were not found to be statistically significantly different (
2 test; not shown).
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0.70 or
1.40. Haplotype frequencies were not found to be significantly different between the biospecimen groups (interleukin-10, P = 0.25; LTA/TNF, P = 0.45). Among the respondent groups, six single nucleotide polymorphism frequencies were significantly different: IL1A A114S, IL1A 12G>A, IL4R 28120T>C, NQO1 P187S, TYMS 157C>T, and MGMT L84F (not shown); eight statistically significant differences were expected by chance.
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| Discussion |
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Because polymorphism frequencies in nonresponders are unknown, investigators have assumed that participation in genetic studies was unrelated to genotype. This may not be true when variants in genes related to behavioral characteristics are under investigation or if a variant may be related to family disease history; willingness to participate has been associated with family history of the particular disease under study (9). In our analyses, there were a few statistically significant differences by participation status; whereas the number of such observations was consistent with expectation, there were no statistically significant differences consistent within and between studies. We also found no evidence of differences, beyond those expected by chance, between subjects opting to provide mouthwash samples for genetic analysis instead of blood samples.
As with confounding, a statistically significant association between a genetic variant and response is not necessary for bias to occur; a sufficient relationship must simply exist in the data (3). Thus, we have identified those single nucleotide polymorphisms in Table 1 and Fig. 1 with response differentials (odds ratios
0.70 or
1.40) that may result in substantial bias under certain circumstances; although not examined in these series of analyses, for biased odds ratios to occur in case-control studies, response differentials must themselves be different between cases and controls. The mathematics of participation bias has been described elsewhere (22).
The present analysis had several strengths in that multiple studies with polymorphisms in common permitted exploration and confirmation of study specific findings and each study provided data on plausible surrogates for nonresponse, such as reaction to incentives. Study A, in particular, provided a rare opportunity to assess genetic profiles of questionnaire nonresponders. A limitation was that the polymorphisms we examined were already available in these three studies; they were selected based on a priori disease associations, not as candidate variants in genes potentially related to willingness to participate.
Despite the apparent conundrum of assessing genetic characteristics of "true" nonresponders, we show there are opportunities to approach the question of response bias in molecular epidemiologic studies. Our findings, while reassuring, cannot exclude that differences by response exist in other genes. The potential for bias due to the "genetics of response" should continue to be evaluated, when possible, within the wider molecular epidemiologic research community.
| Acknowledgments |
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| Footnotes |
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Received 6/21/05; revised 8/ 3/05; accepted 8/12/05.
| References |
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