Table 8. Clinical Practice Guidelines for Diagnosis and Colon Surveillance of Familial Adenomatous Polyposis (FAP) continued...
(Refer to the Issues With Informed Consent for MSI and IHC Tumor Testing section in the Psychosocial Issues in Hereditary Colon Cancer Syndromes section of this summary for information about educational strategies and issues related to informed consent for MSI and IHC testing.)
In instances where tumor is not available from individuals to test for MSI and/or MMR protein IHC, germline mutation analysis of MLH1, MSH2, and MSH6 may be considered. This approach is, however, time consuming and expensive. Strategies to screen for mutations using heteroduplex analysis-based techniques have been explored. These techniques are limited by the need to perform DNA sequencing as a subsequent step on all aberrant samples detected in screening. Additionally, such techniques frequently detect numerous variants of uncertain significance. They cannot, therefore, be recommended for routine clinical use at this time.
Genetic testing for germline mutations in MLH1, MSH2, MSH6, and PMS2 can help formulate appropriate intervention strategies for the affected mutation-positive individual and at-risk family members.
If a mutation is identified in an affected person, then testing for that same mutation could be offered to at-risk family members (referred to as predictive testing). Family members who test negative for the familial mutation are generally not at increased risk of CRC or other LS-associated malignancies and can follow surveillance recommendations applicable to the general population. Family members who carry the familial mutation should follow surveillance and management guidelines for LS. (Refer to the Interventions/LS section of this summary for more information.)
If no mutation is identified in the affected family member, then testing is considered uninformative for the individual and at-risk family members. This would not exclude an inherited susceptibility to colon cancer in the family, but rather could indicate that current gene testing technology is not sensitive enough to detect the mutation in the genes tested. The current sensitivity of testing is between 50% and 95%, depending on the methodology used. Mutation testing utilizing sequencing alone will not detect large genomic rearrangements in MSH2 or MLH1 that may be present in a significant number of LS probands.[287,288,289] An assessment of 365 probands with suspected LS showed 153 probands with germline mutations in MLH1 or MSH2, 12 of 67 (17.9%) and 39 of 86 (45.3%) of which were large genomic alterations in MLH1 and MSH2, respectively. Such mutations can be detected by MLPA or Southern blotting (MLPA has largely replaced Southern blotting).[291,292] MLPA analysis of MLH1, MSH2, and MSH6 is commercially available and should be performed in cases where no mutation is detected by sequence analysis.
Alternatively, the family could have a mutation in a yet-unidentified gene that causes LS or a predisposition to colon cancer. Another explanation for a negative mutation test is that, by chance, the individual tested in the family has developed colon cancer through a nongenetic mechanism (i.e., it is a sporadic case), while the other cases in the family are really due to a germline mutation. If this scenario is suspected, testing another affected individual is recommended. Finally, failure to detect a mutation could mean that the family truly is not at genetic risk despite a clinical presentation that suggests a genetic basis. If no mutation can be identified in an affected family member, testing should not be offered to at-risk members. They would remain at increased risk of CRC by virtue of their family history and should continue with recommended intensive screening. (Refer to the Interventions/LS section of this summary for more information.)