Lynch syndrome 1 results from pathogenic variants in MSH2 on chromosome 2. MSH2 encodes a key component of the post-replication DNA mismatch repair complex. When a somatic cell loses both functional copies of MSH2 through the combination of a germline pathogenic variant and a somatic second-hit mutation, mismatch repair failure allows replication errors to accumulate, producing microsatellite instability and driving malignant transformation across susceptible epithelial tissues. Lynch syndrome 1 follows an autosomal dominant inheritance pattern. Penetrance of cancer manifestations associated with MSH2 variants is incomplete; not all individuals who inherit a pathogenic MSH2 variant develop cancer.

No consensus clinical diagnostic criteria for Lynch syndrome have been published. Clinical suspicion for Lynch syndrome 1 arises in individuals with a tumor of the Lynch syndrome spectrum - including colorectal, endometrial, ovarian, stomach, small bowel, urinary tract (urothelial), biliary tract, prostate, brain (typically glioblastoma), skin (sebaceous adenomas, sebaceous carcinomas, or keratoacanthomas), or pancreatic cancer - in conjunction with specific molecular tumor findings or clinical and family history features. Tumor tissue testing approaches documented in certified GeneReviews data include: microsatellite instability (MSI) testing demonstrating MSI-high status; immunohistochemistry (IHC) demonstrating loss of MMR protein expression; next-generation sequencing of tumor tissue identifying MSI; or identification of a pathogenic variant in an MMR gene in tumor tissue. Additional clinical triggers for suspicion include colorectal or endometrial cancer diagnosed before age 50, synchronous or metachronous Lynch syndrome-spectrum tumors in the same individual, or colorectal cancer in a first-degree relative with Lynch syndrome. Germline molecular genetic testing of MSH2 confirms diagnosis in an index case and enables cascade predictive testing for at-risk family members.

According to certified GeneReviews data, management of Lynch syndrome 1 manifestations is directed at cancer diagnoses as they arise. For colorectal adenomas, complete endoscopic polypectomy with subsequent colonoscopic follow-up is described. For colorectal cancer, segmental or extended colonic resection is indicated, with the extent determined by clinical scenario factors including patient age; proctectomy or total proctocolectomy is indicated for rectal adenocarcinoma. Other tumor types arising in Lynch syndrome 1 - including endometrial, ovarian, urinary tract, gastric, and other extracolonic cancers - are addressed through standard-of-care management approaches applied in the general oncology setting for those specific cancer types. No disease-modifying pharmacologic agents targeting Lynch syndrome 1 itself are identified in this packet.

Penetrance of Lynch syndrome 1-associated cancers is documented at less than 100%; some MSH2 pathogenic variant carriers will not develop cancer during their lifetime. Among those who do develop Lynch syndrome-related malignancies, prognosis reflects cancer type, stage at detection, histologic characteristics, and available treatment, consistent with prognosis for those cancer types in the general oncology population. The certified GeneReviews chapter documents associations between elevated body mass, cigarette smoking, type 2 diabetes, and high cholesterol and increased colorectal cancer risk in Lynch syndrome, with these associations operating in a direction and magnitude similar to their effects in the general population.

Three clinical trials for Lynch syndrome 1 are recorded as actively recruiting in this packet. NCT06708429 ("Lynch Syndrome X-Talk of Enteral Mucosa With Immune System"; sponsor: San Raffaele University; status: recruiting; start: June 2023; projected completion: June 2034) investigates immune and mucosal biology in the Lynch syndrome context. NCT05704010 ("Videocapsule Endoscopy in Lynch Syndrome"; sponsor: San Raffaele University; phase: NA; status: recruiting; start: November 2018; projected completion: December 2029) evaluates videocapsule endoscopy as a surveillance approach. NCT07450612 ("Liquid Biopsy and Machine Learning for Early Colorectal Cancer, Adenomas, Lynch Cancers, and Residual Disease Detection"; sponsor: San Raffaele University; status: recruiting; start: January 2024; projected completion: August 2031) investigates liquid biopsy-based early cancer detection. The classified research literature encompasses 120 publications; reviews and meta-analyses constitute the dominant publication type (31 reviews), with biomarker research, gene therapy publications, and publications linked to recent trials also represented.