Universal colorectal and endometrial tumor screening is recommended for all individuals with newly-diagnosed disease as a cost-effective approach to help identify those with Lynch syndrome. This process was recommended by the Evaluation of Genomic Applications in Practice and Prevention (EGAPP) panel for all colorectal cancers in 2007. There are multiple benefits to identifying as many individuals with Lynch syndrome as possible. Individuals found to have Lynch syndrome may need a different treatment plan for their cancer, they will have an increased risk for secondary cancers and their relatives could benefit from genetic testing.
Universal screening can be performed either by pathology – utilizing immunohistochemistry (IHC) staining for the mismatch repair (MMR) proteins - or by microsatellite instability (MSI) testing. These screening methods can identify 90-95% of individuals with Lynch syndrome. While 12-20% of tumors will demonstrate abnormal IHC and/or MSI, additional reflex testing can further clarify whether the abnormality is sporadic or due to a germline MMR mutation.
What is mismatch repair deficiency (dMMR)?
dMMR describes cells that lack expression or functionality of one or more of the MMR proteins (MLH1, MSH2, MSH6 or PMS2).
Detecting dMMR with IHC
Staining tumor tissue for the MMR proteins can reveal dMMR. Among sporadic colon cancer cases, ~20% will exhibit loss of 1-2 MMR proteins while 83% of Lynch syndrome cases will exhibit dMMR. Screening by IHC generally reveals loss of MLH1 and PMS2 in ~15% of all cases, loss of MSH2 and MSH6 in ~3% of cases, loss of MSH6 alone in ~1% of cases, and loss of MSH2 alone in ~1% of cases. Staining patterns for tumors exhibiting dMMR can help guide genetic testing decisions, decreasing the cost of testing by analyzing only one or two genes instead of four. The sensitivity of IHC to detect dMMR in individuals with Lynch syndrome is 92%.
Detecting dMMR with MSI testing
By testing the DNA extracted from both tumor and normal tissue, a laboratory can determine whether a tumor exhibits microsatellite stability (MSS) or instability (MSI). Microsatellites are stretches of DNA with a repetitive sequence of nucleotides (e.g. CGCGCGCG or AAAAA) that are more susceptible to acquiring errors when the MMR function is impaired. Therefore, tumors arising from cells with defective MMR gene functionality exhibit an inconsistent number of microsatellite repeats when compared to normal tissue. This is termed MSI.
MSI testing may be conducted by either using a panel of microsatellite markers or by using next generation sequencing (NGS) technology. Panel testing markers vary across laboratories, with the most common panel incorporating five markers (BAT25, BAT26, D2S123, D5S246, and D17S250). Tumors are considered MSI-High if two or more of the five markers show instability (or >30% for other panels). Tumors are considered MSI-low if one of the five markers show instability (or <30% for other panels). Tumors are considered MSI-Stable if zero of the five markers show instability (or 0% for other panels).
Among sporadic colon cancer cases, ~15% will exhibit MSI while 77-89% of Lynch syndrome colon cancer cases will exhibit MSI. Among endometrial cancers, 20-30% will exhibit MSI, though the majority are due to MLH1 promoter methylation.
Causes of dMMR
- If MLH1 and PMS2 are absent by IHC, ~80% will be explained by acquired hypermethylation of the MLH1 promoter (with ~68% of acquired methylation due to V600E mutation in the BRAF gene). The rest of the MLH1/PMS2 absent cases (~20%) will be due to Lynch syndrome.
- If MSH2 and MSH6 are absent by IHC, a germline mutation in MSH2 or MSH6 is the most likely cause.
- If MSH6 is absent by IHC, a germline mutation in MSH6 is the most likely cause.
- If PMS2 is absent by IHC, a germline mutation in PMS2 is the most likely cause.
- Among cases of abnormal IHC or MSI that are not explained by either acquired MLH1 promoter methylation or a germline mutation in one of the MMR genes, acquired somatic mutations in the MMR genes explain the abnormal IHC or MSI ~70% of the time. As such, genetic counselors are recommended to offer patients paired germline and tumor testing when assessing for the cause of dMMR.
Notably, the sensitivity of universal screening for Lynch syndrome using either IHC or MSI testing is not 100%. Review of personal and family history information for red flags that raise suspicion for hereditary cancer predisposition syndromes is still recommended for patients with colorectal or endometrial tumors with normal IHC and/or MSS.
If you have a patient with colorectal or endometrial cancer who has questions about universal screening for Lynch syndrome, these two fact sheets are written in patient-friendly language and may be helpful:
