Hereditary GI Cancer Syndromes: Tumor Screening and Reflex Testing

0:00Welcome to Board Pearls. This is episode one of five of the Hereditary GI Cancer Syndromes chapter, in the Special Populations and Acute or Supportive Care module. In this episode we cover the diagnostic framework for hereditary GI cancer: universal MSI and MMR immunohistochemistry on colorectal cancer, the BRAF and MLH1 promoter methylation reflex that separates sporadic from Lynch, the multi-gene panel logic, and the multidisciplinary referral that cascade testing requires.

0:31The reason the framework is built around tumor-based screening rather than family history is empirical. Pedigree-based gatekeeping alone misses thirty to fifty percent of carriers. The classic family with three affected relatives across two generations exists, but the modern Lynch carrier is just as often a fifty-eight-year-old woman with a sporadic-looking right colon cancer and no notable family history. If you wait for the pedigree to declare itself, you miss her. So the standard of care on every newly diagnosed colorectal and endometrial cancer is universal mismatch-repair immunohistochemistry, four proteins stained: MLH1, MSH2, MSH6, and PMS2. Some institutions extend universal IHC to small bowel, urothelial, and gastric tumors. Microsatellite instability testing by PCR is the alternative and is concordant enough that institutions pick one as primary and run it on everyone.

1:30The yield on universal endometrial IHC is the part worth sitting with. Roughly five percent of newly diagnosed endometrial cancers harbor a Lynch carrier, and the female Lynch carrier whose sentinel cancer is endometrial rather than colorectal is the prototypical case the universal endometrial pathway captures. She would not have hit Bethesda or Amsterdam, because her colon is still healthy. The endometrium declared first, and the IHC on her hysterectomy specimen is what surfaces the syndrome.

2:04Now to the reflex pathway, which is the single most testable rule in this section. The four-protein stain comes back in one of a small number of patterns, and the pattern determines whether the next test is germline sequencing or a sporadic-cancer rule-out.

2:19Combined loss of MLH1 and PMS2 is the most common abnormal pattern. The temptation is to read that as Lynch and send for germline testing. That reading is wrong. Roughly seventy percent of MLH1 and PMS2-deficient colorectal cancers are sporadic, driven by a somatic BRAF V600E mutation and acquired MLH1 promoter hypermethylation in the tumor rather than by a germline mutation. The mechanism is that the colon acquires a sporadic mutation in BRAF, the same BRAF mutation that sits on the serrated pathway, and that mutation tends to come with epigenetic silencing of the MLH1 promoter. The tumor then loses MLH1 protein, drags PMS2 down with it because PMS2 is unstable without its MLH1 partner, and looks on IHC exactly like a Lynch tumor. But the germline is normal. The patient is not a carrier, and her relatives are not at risk.

3:14So the reflex sequence on combined MLH1 and PMS2 loss runs through two sporadic-cancer gates before germline sequencing. Test the tumor for the BRAF V600E mutation first. If BRAF is present, you have the sporadic answer, and you stop. If BRAF is absent, test the tumor for MLH1 promoter methylation. If methylation is present, you again have the sporadic answer, and you stop. If both are absent, the patient is presumed Lynch, and germline sequencing is the next test.

3:48The other IHC patterns do not get the BRAF and methylation gate. Isolated loss of MSH2, MSH6, or PMS2 has no analogous somatic mechanism. There is no common sporadic methylation event that silences MSH2 or MSH6. PMS2-only loss without MLH1 loss does not have a sporadic counterpart either. Any of those patterns reflexes directly to germline sequencing.

4:19The one wrinkle on MSH2-pattern loss is EPCAM. Combined loss of MSH2 and MSH6 is the EPCAM signature. The EPCAM gene sits immediately upstream of MSH2, and three-prime deletions in EPCAM silence the MSH2 promoter by read-through methylation. The IHC pattern is identical to a primary MSH2 mutation. EPCAM-driven Lynch behaves clinically like MSH2-driven Lynch, with the same multi-organ surveillance. The only reason the gene matters is that the germline test has to look at EPCAM as well as MSH2 to find the variant.

5:02That is the algorithm. Universal four-protein IHC on every new colorectal and endometrial cancer. MLH1 and PMS2 loss runs the BRAF V600E and MLH1 methylation reflex before germline. Anything else, including the MSH2 and MSH6 pair, reflexes straight to germline. The combined MSH2 and MSH6 pattern carries EPCAM in the differential at germline.

5:33Single-gene testing has been displaced by multi-gene panels because phenotypes overlap. A patient who clinically resembles one syndrome often carries a mutation in a different gene, and the panel captures that opportunistically. The panel is chosen by dominant clinical phenotype. A patient whose presentation is colorectal cancer gets a GI cancer panel. A polyposis patient gets a polyposis panel. A multi-organ pattern gets a comprehensive hereditary cancer panel. Pre-test counseling is required regardless, because the implications of positive, negative, and uncertain results have to be understood before the blood is drawn. Once a pathogenic variant is confirmed in the proband, cascade testing of first-degree relatives is the highest-yield downstream step. Each relative is fifty-fifty at risk. The surveillance program is the same regardless of who in the family carries the gene.

6:27The three-generation pedigree still has work to do. It notes tumor type, age at diagnosis, and pathology where available, and it drives pre-test probability and panel choice. Amsterdam two and revised Bethesda criteria still serve as risk-stratifiers even though universal tumor IHC has largely replaced them as the testing gate. They remain testable as recognition rules and as the scaffold for pedigree-driven referral when tumor tissue is not available.

6:59Multi-disciplinary referral closes the framework. A confirmed Lynch carrier needs gastroenterology, gynecology, urology, dermatology, and surgical oncology working from the same plan. An FAP carrier needs colorectal surgery and ophthalmology and a Spigelman-driven duodenal program. A CDH1 carrier needs surgical oncology and breast care from the moment the result returns. Genetic counseling is the infrastructure that holds the program together, and the typical real-world failure mode the boards test is the carrier whose colonoscopy stays on schedule but whose gynecologic surveillance lapses for three years.

7:37So the way to hold the diagnostic framework is this. It runs on tumor biology, not on pedigree, because universal IHC catches the carriers that family history misses. The protein-loss pattern decides the next step: MLH1 and PMS2 loss runs the BRAF V600E and MLH1 methylation reflex first, because most of those tumors are sporadic serrated-pathway cancers, and everything else, including the MSH2 and MSH6 pair with EPCAM in its differential, reflexes straight to germline. Multi-gene panels have displaced single-gene testing because phenotypes overlap, cascade testing of relatives is the highest-yield downstream step once a proband is confirmed, and multidisciplinary referral is what keeps the multi-organ surveillance from lapsing.

8:26The next episode takes the first and most tested syndrome that framework surfaces, Lynch syndrome, where lifetime cancer risk varies sharply by gene and gene-specific risk drives gene-specific surveillance intervals, with aspirin for chemoprevention and checkpoint inhibition for the MSI-high metastatic tumor.

8:46For the full chapter, the practice vignettes, and the topic-tagged question bank, head to board pearls dot com. You'll find the rest of the series on Apple Podcasts, Spotify, or wherever you listen to podcasts. That brings us to the end of episode one of five of chapter thirty six, and I'll see you in the next one.