Table 8. Clinical Practice Guidelines for Diagnosis and Colon Surveillance of Familial Adenomatous Polyposis (FAP) continued...
Most series show a prevalence of germline MSH6 mutations in approximately 10% of LS families. However, the reported range (5%–52%) is large.[241,244,245,301,302,303,304] This wide variation is likely a result of small sample sizes, referral bias, and ascertainment bias.
The lifetime risk of colon cancer associated with MSH6 mutations is estimated to be between 12% and 22%.[213,215] It appears that the lifetime risk of CRC might be lower in MSH6 carriers than in MSH2 and MLH1 carriers. Initial studies have suggested that inactivating germline mutations of MSH6 might be more frequent in persons with a later average age at onset of CRC whose tumors exhibit a non-MSI-high phenotype.
One study reported on 146 MSH6 carriers (59 men and 87 women) from 20 families, all of whom had truncating mutations in MSH6. While the prevalence of CRCs by age 70 years was not significantly different between MSH6 and MLH1 or MSH2 carriers (P = .0854), the mean age at diagnosis for colorectal carcinoma in male MSH6 mutation carriers was 55 years (n = 21; range, 26–84 years) versus 43 years and 44 years in MLH1 and MSH2 mutation carriers, respectively. The prevalence of CRC was significantly lower in women with MSH6 germline mutations than in MLH1 or MSH2 carriers (P = .0049). The mean age at diagnosis for colorectal carcinoma in female MSH6 mutation carriers was 57 years (n = 15; range, 41–81 years) versus 43 years and 44 years in MLH1 and MSH2 mutation carriers, respectively.
In addition, endometrial cancer has been reported to be more common in MSH6 families. In the same study, the cumulative risk of uterine cancer was significantly higher in MSH6 mutation carriers (71%) than in MLH1 (27%) and MSH2 (40%) mutation carriers (P = .02). The mean age at diagnosis of endometrial carcinoma was 54 years in MSH6 mutation carriers (n = 29; range, 43–65 years) versus 48 years and 49 years in MLH1 and MSH2 mutation carriers, respectively. A group of researchers reported on ten MSH6 kindreds with LS in which 70% of females had been diagnosed with endometrial cancer compared with 31% and 29% in MLH1 and MSH2 carriers, respectively. One study found the prevalence of endometrial carcinoma to be 58% in 12 MSH6 families with a mean age at diagnosis of 57 years.
One group of researchers assembled the largest series of MSH6 mutation carrier families to estimate penetrance of cancers. A total of 113 families of MSH6 mutation carriers from five countries were ascertained through family cancer clinics and population-based cancer registries. The families contained an estimated 1,043 mutation carriers. By age 70 years, 22% (95% CI, 14%–32%) of male MSH6 mutation carriers developed CRC compared with 10% (95% CI, 5%–17%) of female MSH6 mutation carriers. By age 80 years, 44% (95% CI, 28%–62%) of male MSH6 mutation carriers were diagnosed with CRC, compared with 20% (95% CI, 11%–35%) of female MSH6 mutation carriers. For all MSH6 mutation carriers, the increased risk of CRC, relative to that of the general population, across all age groups was statistically significantly elevated (HR, 7.6; 95% CI, 5.4–10.8; P < .001). By ages 70 years and 80 years, 26% (95% CI, 18%–36%) and 44% (95% CI, 30%–58%), respectively, of women would be diagnosed with endometrial cancer. Female MSH6 mutation carriers had an endometrial cancer risk that was about 25 times higher than women in the general population (HR, 25.5; 95% CI, 16.8–38.7; P < .001).