In this large population-based study, we systematically investigated the association of tumor MSI status with basic demographics and family history variables. Two novel and clinically relevant observations emerged. The first observation illustrates the major contribution of Lynch syndrome to colorectal cancer cases presenting with a family history of colorectal cancer; after subtracting MSI-H Amsterdam families, the proportions of MSI-H, MSI-L, and MSS cancers did not vary significantly with the numbers of affected family members diagnosed with colorectal cancer. Prior studies have not examined risks in relatives after subtracting out the syndromic groups. Second, we observed that MSI-L cancers represent a distinct tumor subtype because patients with MSI-L cancers exhibit certain epidemiologic characteristics different from both MSI-H and MSS tumors. Specifically, the distribution of MSI-L colorectal cancers by tumor location and sex differs from both MSI-H and MSS cases. The relationships with family history and age are also quite distinct for patients with MSI-L and MSI-H cancers.
Assessment of clinicopathologic features of colorectal cancer cases stratified by tumor MSI status showed that MSI-H tumors were more proximally located, and were more common among female cases than males. We observed that MSI-H tumors, but not MSI-L tumors, were significantly associated with colorectal cancer cases meeting the clinical diagnostic Amsterdam criteria and Bethesda Guidelines, when compared with MSS tumors. Furthermore, only MSI-H tumors showed a significant association with a diagnosis of colorectal cancer or endometrial cancer, or both, among first-degree relatives of the colorectal cancer cases (). Taken together, MSI-H tumors show a preferential association with familial colorectal cancer.
We observed that the proportion of MSI-H tumors varied by age in a U-shaped pattern; the younger cases undoubtedly reflect germline involvement of the DNA MMR genes, whereas the late-onset MSI-H cases arise nearly always through methylation of the promoter of MLH1
). As a surrogate for having germline testing and tumor methylation testing on all registry patients, the combination of MSI-H tumors plus Amsterdam pedigree criteria was used to define a subset that is highly likely to possess DNA germline MMR mutations. This definition has been shown to be quite specific for the presence of DNA MMR germline mutations although the converse is not true. It is widely acknowledged that some Lynch syndrome cases will be missed by use of the conventional Amsterdam criteria (30
), that is, they lack high sensitivity due to several possible reasons, which include incomplete penetrance, variable expressivity, small families, nonpaternity, adoption, and loss of contact with family members.
As expected, in determining the association of tumor MSI status with clinical and family history characteristics, we observed a robust association between the MSI-H status of colorectal cancer probands and the Amsterdam Criteria and the Bethesda Guidelines. MSI-H tumor status was significantly associated with familial cancer predisposition, with the proportion of MSI-H cases increasing as the number of first-degree relatives diagnosed with such cancers increased. Of interest, however, when these putative Lynch syndrome probands were excluded from the analysis, there was no similar association between MSI-H group and family history. This observation may suggest that there is not another common highly penetrant single-gene hereditary disorder contained in the MSS, MSI-L, and MSI-H groups, once the Lynch syndrome families have been removed, or alternatively, the single-gene syndromes are equally represented in these three remaining groups.
Among Bethesda criteria –negative families, about 11% colorectal cancers were MSI-H. These are likely to be sporadic cases. Possible factors contributing to MSI-H status in these patients are epigenetic silencing mechanisms, including MLH1 promoter hypermethylation, and/or other mechanisms of somatic MMR inactivation, including mutations or allelic deletions.
This case-case study was not designed to assign or to be capable of assigning absolute risks for colorectal cancer among relatives, but rather to compare the proportions of MSS, MSI-L, and MSI-H across groups of patients defined by family history. Seeing the excess familial risk peel away from the MSI-H group with subtraction of the putative Lynch syndrome cases may raise the question of whether all familial risk in colorectal cancer studies is driven by the syndromic causes, of which Lynch syndrome is by far the most common. The design of this study does not permit analysis of this possibility because it could only assess differences in risk across MSI subgroups. Pinsky modeled familial clustering of colorectal cancer populations, incorporating what was then known about the inheritance, prevalence, and penetrance of HNPCC, and found the calculated risks closely matched the risks reported in the literature, raising the possibility that all the familial risk in colorectal cancer was driven by HNPCC (44
). Given the relative rarity of syndromic causes among all colorectal cancers, (e.g., 2.2% of all colorectal cancer have Lynch syndrome; ref. 30
), the increase in familial risks reported in the large population-based studies is expected to persist but a case-control study that incorporates tumor phenotyping will be required to address this point.
A second line of information that emerged from this study was characterization of MSI-L probands and their families. The very existence of a MSI-L subgroup has been controversial. Might not all cancers have some MSI if one searched through enough markers? Or perhaps these were MSI-H tumors that had not yet evolved to a high level of MSI? In previous studies, MSI-L cases have generally been categorized with the MSS cases; clinically, however, it has been less clear if this lumping was justified, as family history characteristics had not been carefully evaluated in a larger series as we have done in this study. It was therefore interesting to note that the clinicopathologic characteristics of this group did not follow the pattern of either the MSS group or the MSI-H group. MSI-L tumors have a significant predilection to proximal location compared with MSS tumors, but gender distribution and family history association are similar to the MSS tumors (Supplemental Table S2
). We looked carefully to determine if there was any association with endometrial cancer or other cancers in general category with MSI-L tumors but no association was detected.
Our study has several strengths. Results from this large, multicentered study have general applicability across different populations. We used a rigorous definition of family history to examine its association with colorectal cancers stratified by MSI-H, MSI-L, and MSS subtypes. The weaknesses of this study are the case-case design and a lack of germline MMR mutation data which would have helped to increase the specificity of our designation of Lynch Syndrome. Also, tumor location information is missing for a small number of colorectal cancers in our study.
In summary, this study made two observations. First, nearly all of the observed excess familial risk of colorectal cancer in MSI-H tumors is driven by the Lynch syndrome. When those families are subtracted from the analysis, even using as blunt an instrument as MSI-plus-Amsterdam Criteria, the familial aggregation is indistinguishable from all other colorectal cancer subtypes. This suggests there will not be a common, highly penetrant, single-gene predisposition syndrome that accounts for the late-onset MSI-H cases that have MLH1 promoter methylation. A case-control study design will be required to assess the actual colorectal cancer risks among relatives of non–Lynch Syndrome cases. Second, we observed that MSI-L tumors may be biologically different from MSS or MSI-H tumors. MSI-L tumors are more proximal than MSS tumors, but the gender distribution of the MSI-L group of colorectal cancers was quite similar to MSS tumors and they do not seem to be MSI-H tumors that have not yet accumulated enough microsatellite alterations to be detected. The contribution of MSI-L tumors does not vary across different age groups, unlike both MSI-H and MSS tumors. Further analyses are encouraged to further define characteristics such as response to therapy and prognosis among these different tumor subtypes.