We have shown that people who have inherited a mutation in the gene MUTYH from only one parent (monoallelic mutation carriers) are at increased risk of CRC but only by a small amount, about 15%. The novelty of this aspect of the analysis is that it includes the second largest casecontrol study to date, which was not included in the most recent meta-analysis. The statistical significance of the association is provided by our novel meta-analysis of studies using cases ascertained because they have a family history of the disease, which is more powerful in terms of sample size than unselected case–control studies for addressing this issue. This analysis allow us to reject the null hypothesis that the carrier frequency is the same for cases and controls (P = 0.02), as the cases with a family history of CRC will be enriched for monoallelic mutation carriers if monoallelic mutations increase the risk of CRC. The magnitude of the association is provided by the meta-analysis of standard case–control studies (OR = 1.15). The observed association is unlikely to be due to biallelic mutation carriers, as biallelic mutations were tested for both cases and controls by each study and we excluded them from our analysis.
The meta-analysis of studies in which cases were ascertained irrespective of a family history of CRC (unselected case–control studies) comprised a total pooled sample of 21,369 CRC cases and 14,639 controls, which is the largest study to date. We estimated the strength of association between monoallelic MUTYH
mutations and CRC after adjusting for possible sources of heterogeneity including carrier frequency of controls, control ascertainment, study site and the year of publication, but the association was not changed. The estimate of the effect size for this analysis was similar to that of the recent meta-analysis of Lubbe et al. [20
] (OR = 1.14). However, with the use of the familial cases we have been able to conclude that there was an statistically significant evidence that monoallelic carriers are at greater risk than non-carriers (P
= 0.02), whereas the study of Lubbe et al. which did not utilize familial cases reported a P
value of 0.12.
We did not find statistical significant evidence for an association between specific variants of MUTYH
mutation and CRC risk separately for Y179C and G396D, but had little power to detect even moderate associations given the decrease in sample size from stratification by mutation type. Among previous case–control studies, the study of Clearly et al. [19
] was the only one that screened for the nine most frequent variants of MUTYH
mutations in cases and controls. Mutation testing in all other studies was restricted to the most common mutations Y179C and G396D in Caucasian populations, or only one or two additional variants. A comprehensive mutation screening approach might be required to observe a significant association with CRC risk, if present.
Houlston and Peto [27
] have pointed out that standard case–control designs, in which cases are ascertained irrespective of a family history of the disease, have limited power to identify alleles with small associations with risk if the carrier frequency of the deleterious allele in population is low (e.g. MUTYH
mutations). Antoniou and Easton [26
] showed that the sample size required to detect a disease allele association was substantially reduced if the cases were selected because they had a family history of the disease. Therefore case–control studies using familial cases hold promise as more powerful to examine the question of the existence of an association between monoallelic MUTYH
mutations and CRC risk, despite their typically low frequency in the population. Our analyses here demonstrate the importance of study design using cases selected for a family history to detect evidence for a role of genetic variants on disease risk.
Two family studies based on the relatives of monoallelic MUTYH
carriers diagnosed with CRC did observe a significant association between monoallelic MUTYH
mutation and CRC risk. A kin-cohort study [25
] of the relatives of MUTYH
mutations carriers observed that monoallelic mutation carriers who had a relative diagnosed with CRC were at threefold risk of CRC (Hazard Ratio 2.9; 95% CI 1.2–7.0; P
= 0.02) in addition to that expected due to their family history. A retrospective cohort analysis [42
] of obligate carriers of monoallelic mutations in MUTYH
, being the parents of biallelic carriers, estimated monoallelic carriers had twice the CRC risk of general population (Standardized Incidence Ratio 2.12; 95% CI 1.30–3.28; P
< 0.01). It is important to note that the estimates of association from these studies (two to threefold increased risk) are relevant to monoallelic carriers who have a relative with CRC, whereas the result we have reported here (1.15-fold increased risk) is relevant to monoallelic carriers from the general population (irrespective of family history of CRC).
We have included all previously published unselected and familial case–control study in our meta-analyses. To assist readers evaluate the quality of and differences between different study designs we have provided extracted relevant details in . Meta-regression models might be able to resolve some of the inconsistencies across studies, within the limitations of power, but considerable residual heterogeneity for unmeasured modifiers of risk might remain.
When combined, all previous case–control studies of monoallelic MUTYH mutations and CRC suggest that the risk of CRC is increased for carriers, but only to a small degree, on average. Given the rarity of monoallelic mutations they account for only a trivial proportion of CRC. Therefore the clinical significance of increased risk of CRC for monoallelic mutation carriers is minimal. This study demonstrates the value of using familial cases for detecting rare, “low penetrance” cancer susceptibility alleles.