Table 8. Clinical Practice Guidelines for Diagnosis and Colon Surveillance of Familial Adenomatous Polyposis (FAP) continued...
In contrast to the scenario of MSI associated with loss of expression of MSH2, MSH6 or PMS2, absence of MLH1 expression is not specific to LS. Most instances of absence of MLH1 expression are caused by the sporadic hypermethylation of the MLH1 promoter. Therefore, absent MLH1 expression is less specific for LS than absence of the other MMR proteins. In addition, rare instances of inherited germline MLH1 methylation have added further complexity to the interpretation of MSI associated with absence of MLH1 expression. (Refer to the Microsatellite instability [MSI] section for more information about germline MLH1 hypermethylation.)
The prevalence of MSH2 mutations in individuals or families with LS has varied across studies. MSH2 mutations were reported in 38% to 54% of LS families in studies including large cancer registries, cohorts of early-onset CRC (<55 years), and registries around the world.[217,254]
The lifetime risk of colon cancer associated with MSH2 mutations is estimated to be between 48% and 68%.[215,220,314] In a case series of LS patients, those carrying germline MSH2 mutations (49 individuals, 45% females) had a lifetime (cutoff of age 60 years) risk of extracolonic cancers of 48% compared with 11% for MLH1 carriers (56 individuals, 50% females). In addition, the same group reported a significantly higher prevalence of poorly differentiated CRCs (44% for MSH2 carriers vs. 14% for MLH1 carriers; P = .002) and Crohn-like reaction (49% for MSH2 carriers vs. 27% for MLH1 carriers; P = .049). Another study reported no significant differences between the prevalence of colorectal and extracolonic cancers in 22 families with germline MLH1 mutations and in 12 families with germline MSH2 mutations.
Multiple groups have reported that MSH2 and MSH6 carriers have a greater chance of presenting with endometrial cancers before CRCs than do MLH1 carriers.[3,246,317] The average age at diagnosis of endometrial cancers differed with genotype in two studies: age 41 years for MSH2 , age 49 years for MLH1, and age 55 years for MSH6 carriers.[318,319]
Practices and pitfalls in testing