Results
From the database of 2545 index cases and relatives, 356 women with a pathogenic mutation in MLH1, MSH2 or MSH6 were identified. In addition, a further 73 obligate carriers (due to their position in the pedigree in relation to relatives testing positive for a mutation) were identified. Therefore, in total, there were 157 MLH1, 219 MSH2 and 53 MSH6 mutation carriers and positive obligates. Mutation status was unknown for 206 MLH1, 262 MSH2 and 31 MSH6 female FDRs.
Initial Analysis
The cumulative incidence of breast cancer in mutation carriers and positive obligates was calculated, censoring at age 70 years. The Kaplan–Meier graph is shown in figure 1A. Of the MLH1 mutation carriers, 15/157 (two bilateral) developed breast cancer, equating to a cumulative risk to 70 years of 17.4%. Only 5/219 MSH2 carriers developed breast cancer giving a lower cumulative lifetime risk of 4.9%. The difference was significant (log rank p=0.010). In the 53 MSH6 carriers, three developed breast cancer; due to the small numbers, no further detailed analysis was performed for MSH6. Cumulative incidence in FDRs of unknown mutation status was 15.8% to age 70 years for MLH1 (14/206 women (two bilateral)) and 11.8% for MSH2 (12/262). Combining both mutation carriers and FDRs of unknown mutation status gave cumulative risks to 70 years of 16.5% and 8.1% for MSH1 and MSH2 carriers, respectively (log rank for difference p=0.035). No breast cancer case tested negative for an MLH1 family mutation (three isolated cases tested positive) but the only woman with an isolated breast cancer (no other Lynch syndrome cancer) tested negative for their MSH2 family mutation.
(Enlarge Image)
Figure 1.
Cumulative risk of breast cancer for MLH1 and MSH2 for (A) proven mutation carriers and (B) proven mutation carriers and first-degree relatives with assumed mutation status.
Second Analysis (To Establish Penetrance)
The cumulative risk for MSH2 carriers was similar to population risk; therefore, untested FDRs with breast cancer were allocated carrier status on a 1:1 basis sequentially by age.
The cumulative risk for MLH1 carriers was twice the population risk. Mutation carriers had a risk of 2.22-fold the current upper estimate for population risk. For FDRs, 50% will not carry the mutation and will be at population risk. In order to generate a risk of 15.8% to 70 years in the mutation carriers a higher risk would be required. If we took a sample of 2000 women, 1000 would not carry the mutation and generate 75–80 cases of breast cancer by 70 years. In order to arrive at a 15.8% cumulative risk for both carriers and non-carriers combined (316 in 2000) a further 236–241 cases would have to occur in the mutation carriers, equivalent to 23.6%–24.1% or a relative risk of 2.95–3.21. Therefore untested FDRs with breast cancer were conservatively allocated carrier status on a 2:1 basis sequentially by age.
Following allocation of carrier status to the FDR untested group, an additional 230 (104 MLH1 and 126 MSH2) patients were entered into the analysis, giving, in total, 261 MLH1 carriers and 345 MSH2 carriers. Overall, the cumulative risk of breast cancer at age 70 years for mutation carriers was 13.2%. The revised Kaplan–Meier curve is shown in figure 1B, comparing the cumulative risks of MLH1 and MSH2. The risk to age 70 years for MLH1 was 18.6%, while that for MSH2 was 11.2%. The difference between MLH1 and MSH2 carriers was statistically significant (p=0.014). There were very few MSH6 carriers for any statistical inference to be made in this group. The life tables and risk per decade with 95% CIs are shown in Table 2 . The index cases were included in the analysis. Five of 15 MLH1 untested FDRs with breast cancer had other Lynch syndrome cancers compared with only 1/16 for MSH2.
Prospective Analysis
Finally we carried out an assessment of breast cancer incidence in female MLH1 carriers from date of family ascertainment. In 1437 woman-years of follow-up (mean=7.6, median=6.8), six invasive breast cancers (five in MLH1 proven carriers) occurred in 112 mutation positive MLH1 carriers and 77 FDRs when 2.14 were expected (OR 2.81 95% CI 1.26 to 6.24). Confining the analysis to the 112 mutation carriers, there were five breast cancers in 861 woman-years of follow-up with only 1.55 expected (OR 3.23 95% CI 1.34 to 7.75). Finally, as most of the FDRs with unknown status would not carry the family mutation, we carried out a further analysis as previously described to allocate FDRs to mutation positive status. This reduced the total number of assumed carriers to 145 and the years of follow-up to 1120 with 1.76 cancers expected giving an OR of 3.41 (95% CI 1.53 to 7.59).