Hereditary GI Cancer Syndromes: Lynch Syndrome and Gene-Specific Risk

0:00Welcome to Board Pearls. This is episode two of five of the Hereditary GI Cancer Syndromes chapter, in the Special Populations and Acute or Supportive Care module. In this episode we cover Lynch syndrome: the MMR-gene-specific cancer risk that drives gene-specific surveillance intervals, the colorectal and endometrial and urothelial and gastric programs, aspirin chemoprevention, checkpoint inhibition in MSI-high disease, and the Lynch variants.

0:25The last episode built the tumor-based diagnostic framework. Now to the syndrome it most often surfaces. Lynch syndrome is autosomal dominant, caused by germline mutations in mismatch-repair genes MLH1, MSH2, MSH6, PMS2, or by three-prime EPCAM deletions that silence MSH2. MLH1 and MSH2 together account for over ninety percent of confirmed cases. The mechanism is loss of post-replicative spell-checking at microsatellite repeats. The unrepaired errors accumulate, drive a high mutational burden, and produce the microsatellite-unstable phenotype that defines the tumors. The same mutational burden explains why MSI-high tumors in both sporadic and Lynch settings respond to checkpoint inhibition: abundant neoantigens, strong T-cell response when the PD-one brake comes off.

1:20The classic Lynch colorectal cancer is right-sided, mucinous, often with tumor-infiltrating lymphocytes and Crohn-like peritumoral lymphoid aggregates. About half of cases present before age fifty, but a substantial minority of carriers, particularly MSH6 and PMS2 carriers, present with their first cancer in their fifties or sixties. That late-presenting subgroup is one of the reasons pedigree-based screening alone misses carriers, and it is also why MSH6 and PMS2 are the two genes most likely to be missed by Amsterdam-style criteria.

1:54Amsterdam two clinical criteria require three relatives with histologically verified Lynch-spectrum cancers across two or more generations, with at least one diagnosed before age fifty, FAP excluded, and pathology confirmed. The Lynch-spectrum cancers in those criteria are colorectal, endometrial, upper urinary tract, and small bowel. Revised Bethesda criteria identify probands for tumor MSI or IHC testing on slightly looser grounds. Both criteria sets sit downstream of universal tumor IHC in modern practice.

2:27Now to the part of Lynch that everything else flows from: lifetime cancer risk varies sharply by gene, and gene-specific risk is the clinical anchor for tailoring surveillance intervals. Each interval is calibrated to the dwell time of detectable preinvasive disease in that organ, not to a uniform schedule. MLH1, MSH2, and EPCAM carriers carry colorectal lifetime risks in the forty-six to sixty-one percent range with mean diagnosis around age forty-four. MSH6 carriers run colorectal risk in the ten to forty-four percent range with mean age in the forties or later. PMS2 carriers run colorectal risk in the eight to twenty percent range with mean age in the sixties.

3:11The non-obvious anchor on the endometrial side is one of the most testable single rules in this entire chapter. MSH6 is the only Lynch gene where uterine cancer outranks colorectal cancer as the dominant lifetime risk. MLH1, MSH2, and EPCAM carriers carry endometrial risks of thirty-four to fifty-four percent. MSH6 carriers carry endometrial risks of sixteen to forty-nine percent, but their colorectal risk is lower than the endometrial number, which flips the gynecologic surveillance to a heavier weight in the program. Risk-reducing hysterectomy is discussed earlier in MSH6 carriers as a consequence. PMS2 is the most attenuated phenotype overall, with substantial colorectal risk emerging only in carriers with a strong family history.

3:57The colorectal program runs colonoscopy every one to two years starting age twenty to twenty-five, or five years before the youngest affected family member, whichever is earlier. The short interval is set by dwell time. Lynch adenomas progress to cancer faster than sporadic adenomas, and a three-year interval is too long to catch the curve. When a Lynch carrier does develop colorectal cancer, particularly an MLH1 or MSH2 carrier, the operation is extended colectomy with ileorectal anastomosis rather than segmental resection in most cases. The reason is metachronous cancer risk in the residual colon is high, and a segmental resection leaves the rest of the at-risk field in place. The patient is left with a rectum that gets annual surveillance thereafter.

4:43Endometrial cancer is the most common extracolonic Lynch malignancy and is frequently the sentinel cancer in female carriers, which is the empirical anchor for universal endometrial IHC. Surveillance is annual or biennial Pipelle endometrial biopsy plus transvaginal ultrasound starting at age thirty to thirty-five, supplemented by serum CA one twenty-five in postmenopausal carriers. Risk-reducing total hysterectomy with bilateral salpingo-oophorectomy is offered after childbearing. A landmark study demonstrated that no carrier who underwent prophylactic surgery developed endometrial or ovarian cancer over follow-up, while a substantial proportion under surveillance did. The implication is that endometrial sampling and transvaginal ultrasound exist as a bridge to surgery rather than as a permanent substitute. The carrier who completes childbearing in her late thirties and accepts the surgery exits the gynecologic surveillance burden entirely.

5:39Upper urothelial tract cancer of the renal pelvis and ureter, distinct from bladder cancer, carries a four to twelve percent lifetime risk concentrated in MSH2 carriers. The reason the upper urothelium is selectively hit is mechanistic. The high-turnover urothelium of the renal pelvis and ureter behaves like colonic crypts under MMR deficiency, and the proliferative rate of that epithelium is what drives the cancer risk. Annual urinalysis with cytology starts at age thirty to thirty-five, with positive cytology triggering CT urogram and ureteroscopy. The MSH2 carrier with painless gross hematuria is the testable vignette.

6:19Gastric cancer surveillance is highest-yield in MLH1 and MSH2 carriers with Asian ancestry or family history of gastric cancer. Baseline EGD with random gastric biopsies starts at age thirty to forty, closer to forty in MSH6 carriers, with surveillance every two to four years. H pylori testing and eradication is part of the program. Lynch gastric cancers arise on a background of chronic atrophic gastritis with intestinal metaplasia. H pylori is the dominant amplifier of that inflammatory background. Eradicating the bug removes the inflammatory accelerant on a field that already carries a germline driver. Small bowel adenocarcinoma risk is elevated in relative terms but rare in absolute terms, so capsule endoscopy or CT enterography is reserved for symptomatic carriers rather than offered as routine surveillance.

7:12Pancreatic surveillance in Lynch is gated by family history rather than by the gene alone. Absolute lifetime pancreatic cancer risk in Lynch is roughly six percent, which sits below the threshold at which surveillance benefit outweighs harms. Only carriers with at least one first-degree or second-degree relative with pancreatic cancer enroll in annual EUS or pancreatic-protocol MRI starting at age fifty. This is the same absolute-risk-threshold logic that gates BRCA, ATM, and PALB2 carriers without family history out of pancreatic surveillance. The carriers who pass the threshold do so by adding a family-history risk modifier to the germline mutation. Prostate screening with DRE and PSA starts at age forty. Dermatology examination every one to two years addresses the sebaceous neoplasms of the Muir-Torre variant.

8:05Aspirin chemoprevention is now standard in Lynch carriers, and the underlying trial is testable, so it is worth getting right. The intention-to-treat analysis at the original endpoint did not show a colorectal cancer benefit, and that result was a source of confusion for years, but long-term follow-up at ten and twenty years then demonstrated a substantial reduction in colorectal cancer incidence, with the effect emerging only beyond five years, and the per-protocol benefit among carriers who actually took at least two years of aspirin was larger than the intention-to-treat estimate. The mechanistic interpretation is that aspirin's chemopreventive effect operates on early-adenoma biology and takes years to show up in cancer endpoints. The optimal dose is still under investigation, with lower doses being tested head-to-head against the original higher dose, so current practice accepts daily aspirin across a wide dose range and the conversation with the carrier is about choosing a tolerable dose rather than insisting on the highest one.

9:05Pembrolizumab is first-line therapy for advanced MSI-high or mismatch-repair-deficient Lynch-associated cancers based on randomized trial evidence in colorectal cancer. The mechanism connects all the way back to the microsatellite instability that defines the syndrome. High mutational burden generates abundant neoantigens. Releasing the PD-one brake on T-cells produces a strong response in a way that does not happen in MSI-stable tumors. The Lynch carrier with metastatic colorectal cancer and an MSI-high tumor goes to checkpoint inhibition before chemotherapy.

9:37Three Lynch variants are testable as recognition vignettes, each tied to a specific extra-colorectal feature. Muir-Torre syndrome is Lynch with cutaneous sebaceous neoplasms, including sebaceous adenoma, sebaceoma, sebaceous carcinoma, or keratoacanthoma. The sebaceous tumors themselves are microsatellite-unstable on pathology. Identification of a sebaceous neoplasm in a Lynch-suggestive pedigree is itself an indication for germline testing. Turcot syndrome of the Lynch type is Lynch with high-grade glioma, either glioblastoma or astrocytoma, and is discriminated from FAP-type Turcot by the brain tumor histology. FAP-type Turcot has medulloblastoma. The two Turcots split on what kind of brain tumor sits with the colon. Constitutional Mismatch Repair Deficiency, also called CMMRD or BMMRD, is the autosomal recessive biallelic variant in which both alleles of MLH1, MSH2, MSH6, or PMS2 are mutated. Affected children develop café-au-lait macules in childhood that mimic neurofibromatosis type one, plus a devastating cancer phenotype: childhood colorectal cancer in over eighty percent, brain tumors in seventy percent by age nine, and hematologic malignancies. CMMRD surveillance begins in infancy. Brain MRI runs every six months from age two, CBC every six months from age one, colonoscopy annually from six, EGD plus capsule annually from eight, and pelvic and urinalysis surveillance from twenty.

11:15Family Colon Cancer Syndrome X is the last recognition item, and it is the answer when the family meets Amsterdam one criteria but the tumor MMR and MSI testing is normal. Microsatellite stable, intact MMR proteins on IHC, no detectable germline mutation. The relative colorectal cancer risk is approximately two point three, with no extracolonic risk and no Lynch-spectrum tumors at extra sites. Surveillance is colonoscopy every three to five years starting ten years before the youngest affected relative. The boards use Family Colon Cancer Syndrome X as the diagnosis when the pedigree looks like Lynch but the tumor biology will not support it.

11:56Pull Lynch together. The management runs on gene-specific cancer risk calibrated to organ-specific dwell time, with MSH6 the exception where uterine outranks colorectal. The colonoscopy interval is short because Lynch adenomas progress fast, and the cancer surgery is extended because the residual colon stays at risk. Endometrial surveillance is a bridge to risk-reducing hysterectomy, not a substitute for it, and urothelial risk concentrates in MSH2. Pancreatic surveillance is gated by family history because absolute risk in Lynch alone sits below the threshold. Aspirin reduces colorectal cancer with the effect emerging beyond five years, and the per-protocol benefit is larger than the intention-to-treat estimate. Checkpoint inhibition is first-line in MSI-high metastatic disease because the mutational burden the syndrome generates is exactly what immunotherapy needs. And the variants, Muir-Torre, the two Turcots, CMMRD, and Family Colon Cancer Syndrome X, each turn on one recognition feature.

12:59The next episode moves to the inherited polyp syndromes, beginning with familial adenomatous polyposis, its Spigelman-staged duodenal surveillance and the timing of colectomy, and the attenuated FAP and MUTYH-associated polyposis that sit alongside it.

13:15For 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 two of five of chapter thirty six, and I'll see you in the next one.