The ultimate goal of screening for HNPCC is to identify all cases of Lynch syndrome or other hereditary defects present in a population. In view of limited resources, identification of individuals who would benefit most from genetic testing is essential. To most efficiently identify germline mutations in patients identified through family history and clinical criteria to be at risk for HNPCC, a stepwise diagnostic procedure is recommended (Figure 19-4). The utility of this tiered approach has been demonstrated in studies that have successfully screened an unselected
Identify patients appropriate for testing using clinical criteria (see Table 24-1)
MSI and IHC analyses on tumor tissue from affected family member
MSI-L or MSS
Germline testing of MMR gene indicated by IHC analysis
Mutation screen and Southern blot
No alteration identified
Stop testing (does not rule out other hereditary causes of CRC)
Stop testing J
Mutation screen and Southern blot
No alteration identified
\ Sequence change of undetermined significance
Test at-risk family members f Stop testing
Test at-risk family members
Figure 19-4. Flowchart for recommended HNPCC diagnostic procedure.
population with CRC to identify those who might benefit from genetic testing.16,17
Utilizing this approach, a combination of MSI and IHC analysis is first performed on tumor tissue as an initial screen for individuals at increased risk to have germline MMR defects. Since MSI is found in virtually all tumors derived from individuals with Lynch syndrome, it is generally unproductive to search for germline mutations in MMR genes in patients whose tumors do not demonstrate a mutator phenotype. Depending on the prior risk of the patient being tested, a large fraction of cases may not have evidence of defective MMR. This is especially the case for individuals that have a moderate risk for having HNPCC. Data from our Molecular Diagnostic Laboratory, which includes both moderate-risk (young age of onset only, one additional family member with CRC only, etc.) and high-risk (Amsterdam criteria) referral cases, has demonstrated that up to 70% of cases do not have evidence of defective MMR. Direct sequencing of several MMR genes in such cases would generally not be productive. The combination of MSI and IHC testing is a relatively inexpensive prescreen that can eliminate unnecessary and expensive germline testing when performed as the first step in a testing protocol.
If the tumor demonstrates an MSI-H phenotype, then IHC provides important information about the specific MMR gene that is most likely mutated. The subset of patients identified then would be considered for germline testing with appropriate genetic counseling. Germline testing of the appropriate MMR gene (identified by IHC) by mutation screening or direct sequencing in conjunction with Southern blot (or other assays capable of detecting germline deletions or other genomic rearrangements) would follow in those patients who provide informed consent. In this case, only the relevant gene is analyzed for a germline mutation, again minimizing the need to test several MMR genes. This approach is designed to be the most cost-effective and judicious use of resources at the present time. However, as technological advancements continue, this testing algorithm may be amended after further studies have proven the effectiveness of other approaches.
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