HEREDITARY COLORECTAL CANCER

Hereditary Colorectal Cancer — from Lynch syndrome (where immunotherapy produces dramatic responses) to familial adenomatous polyposis (where prophylactic colectomy prevents cancer), genetic diagnosis determines the prevention and treatment strategy.

Whole genome sequencing evaluates all hereditary colorectal cancer genes — MLH1, MSH2, MSH6, PMS2, EPCAM (Lynch), APC (FAP), MUTYH, SMAD4, BMPR1A, STK11, POLD1, POLE — providing comprehensive assessment beyond single-syndrome testing.

MLH1
MSH2
MSH6
PMS2
APC
MUTYH
STK11
SMAD4

About this condition

Colorectal Cancer — Hereditary

Colorectal cancer (CRC) is the third most common cancer in the US (~153,000 new cases annually), with approximately 5-10% attributable to hereditary cancer syndromes. The most common hereditary CRC syndromes are Lynch syndrome (hereditary nonpolyposis CRC — MLH1, MSH2, MSH6, PMS2, EPCAM deletions; 50-80% lifetime CRC risk), familial adenomatous polyposis (FAP — APC; ~100% CRC risk without colectomy), attenuated FAP, and MUTYH-associated polyposis (MAP — biallelic MUTYH; 43-100% CRC risk). Rarer syndromes include Peutz-Jeghers (STK11), juvenile polyposis (SMAD4, BMPR1A), and polymerase proofreading-associated polyposis (POLD1, POLE).

Lynch syndrome is particularly important because of its therapeutic implications. Lynch-associated CRC is microsatellite-instable (MSI-high) due to defective DNA mismatch repair — and MSI-high tumors respond dramatically to immune checkpoint inhibitors. The KEYNOTE-177 trial demonstrated that first-line pembrolizumab (Keytruda) doubled progression-free survival in MSI-high/dMMR CRC compared to chemotherapy. The landmark study by Cercek et al. (2022) showed that dostarlimab produced 100% clinical complete response in dMMR rectal cancer — potentially eliminating the need for surgery and radiation. Germline Lynch diagnosis identifies patients eligible for these immunotherapy approaches.

Beyond treatment, hereditary CRC diagnosis transforms cancer screening. Lynch syndrome carriers begin colonoscopy at age 20-25 with 1-2 year intervals (vs. age 45 with 10-year intervals for average risk). FAP carriers require annual colonoscopy from age 10-12 with prophylactic colectomy recommended in the late teens or twenties. MUTYH carriers (biallelic) begin colonoscopy at age 25-30. These gene-specific screening protocols prevent CRC deaths by detecting precancerous polyps and early cancers — but they cannot be implemented without molecular diagnosis identifying the specific syndrome.

Dostarlimab produced 100% clinical complete response in dMMR rectal cancer in a landmark 2022 study — potentially eliminating surgery entirely. This remarkable result only applies to MMR-deficient tumors, most of which arise from Lynch syndrome germline mutations.

Gene locus: MLH1 (3p21.3), MSH2 (2p21), MSH6 (2p16.3), PMS2 (7p22.1), APC (5q22.2), MUTYH (1p34.1), STK11 (19p13.3), SMAD4 (18q21.2)

Lynch, FAP, and MUTYH each have different screening protocols, surgical thresholds, and treatment responses. This page serves as the comprehensive entry point for hereditary CRC genetic testing.

Immunotherapy response in Lynch CRC is dramatic — but requires knowing the tumor is MSI-high, which Lynch germline testing reveals

Pembrolizumab as first-line therapy for MSI-high CRC produces significantly better outcomes than chemotherapy. Germline Lynch testing simultaneously identifies the patient as a candidate for immunotherapy AND triggers cascade testing of family members — who may have Lynch syndrome and benefit from early colonoscopy screening that prevents CRC entirely. Without germline testing, the patient may receive only tumor MMR/MSI testing (if performed), missing the hereditary cancer counseling and family screening opportunity.

FAP requires prophylactic colectomy — but attenuated FAP and MUTYH have different surgical timing, and only molecular testing distinguishes them

Classic FAP (truncating APC variants) produces hundreds to thousands of polyps by age 20 and near-100% CRC risk without colectomy — prophylactic surgery is typically performed in the late teens. Attenuated FAP (specific APC variant locations) produces fewer polyps with later onset and may be managed with intensive surveillance rather than early colectomy. MUTYH-associated polyposis is autosomal recessive (not dominant like FAP), has different recurrence risk for family members, and responds to celecoxib chemoprevention. Only molecular genotyping distinguishes these clinically similar but differently managed conditions.

Hereditary Colorectal Cancer — from Lynch syndrome (where immunotherapy produces dramatic responses) to familial adenomatous polyposis (where prophylactic colectomy prevents cancer), genetic diagnosis determines the prevention and treatment strategy.

Colorectal Cancer — Hereditary

Lynch, FAP, and MUTYH each have different screening protocols, surgical thresholds, and treatment responses. This page serves as the comprehensive entry point for hereditary CRC genetic testing.

Immunotherapy response in Lynch CRC is dramatic — but requires knowing the tumor is MSI-high, which Lynch germline testing reveals

FAP requires prophylactic colectomy — but attenuated FAP and MUTYH have different surgical timing, and only molecular testing distinguishes them

Others in Hereditary Cancer

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