The packet definition attributes the condition to pathogenic variants in MSH2, a mismatch repair gene located on chromosome 2. The packet does not supply a ClinGen gene-disease validity classification for MSH2. According to GeneReviews, Lynch syndrome in most individuals is caused by a heterozygous germline pathogenic variant in a mismatch repair gene and is inherited in an autosomal dominant manner, and the packet's inheritance field lists autosomal dominant. GeneReviews states that the majority of individuals diagnosed with Lynch syndrome inherited a pathogenic variant from a parent who may or may not have had cancer. A de novo pathogenic variant is described as a possible explanation when a variant is not detected in either parent, with the de novo rate described as unknown but estimated to be extremely low. GeneReviews also notes that no instances of inheritance from a parent with germline mosaicism have been reported to date.

GeneReviews states that penetrance of colorectal and extracolonic cancers is less than 100%, so some individuals with a cancer-predisposing pathogenic variant may never develop cancer. A parent with such a variant may not have had cancer because of incomplete penetrance, variable age of cancer development, risk reduction from screening or prophylactic surgery, or early death. The wide spread in individual cancer risk described in the chapter, attributed to genetic and/or environmental modifiers, reflects variable expression among carriers. GeneReviews further notes accumulating evidence that high body mass, cigarette smoking, type 2 diabetes, and high cholesterol increase colorectal cancer risk in Lynch syndrome.

According to GeneReviews, no consensus clinical diagnostic criteria for Lynch syndrome have been published. The chapter describes suspected diagnosis in a proband with a tumor of the Lynch syndrome spectrum together with one of the following tumor-tissue findings: microsatellite instability (MSI) high on testing; loss of expression of one or more mismatch repair proteins on immunohistochemistry; next-generation sequencing of tumor tissue showing MSI; or identification of a pathogenic variant in a mismatch repair gene in tumor tissue.

The chapter also describes suspected diagnosis in a person with colorectal or endometrial cancer plus additional features such as diagnosis before age 50 years, synchronous or metachronous Lynch syndrome-related cancers, MSI-high histology, or affected first-degree relatives. Other listed settings are a family member with a confirmed diagnosis and a probability of at least 5% of carrying a pathogenic variant on risk assessment models (PREMM5, MMRPredict, MMRPro). GeneReviews notes that molecular genetic testing ideally begins with a person who has had a Lynch syndrome-related cancer, although in some families no such individual may be alive or willing to be tested.

Population screening approaches described include testing all colorectal and endometrial cancers with MSI or immunohistochemistry, described as a cost-effective way of identifying individuals who would be offered germline genetic testing. The chapter's differential diagnosis tabulates other hereditary cancer syndromes with increased colorectal cancer risk, including familial adenomatous polyposis, attenuated familial adenomatous polyposis, and polymerase proofreading-associated polyposis. Newborn-screening information is not certified in this packet.

No FDA-approved treatment records are present in this packet, and the packet's foundational-therapy field is not populated. The management content comes from GeneReviews, which tabulates evaluations following initial diagnosis, treatment of manifestations, and surveillance by organ system. For colorectal cancer, the chapter lists colonoscopy with removal of precancerous polyps, noting that colonoscopy is favored over flexible sigmoidoscopy because of the predominance of proximal colon cancers. For endometrial and ovarian cancers, the chapter lists education of females about symptoms. For gastric and duodenal cancers, upper endoscopy is listed with biopsy evaluation for H pylori infection. Capsule endoscopy or small bowel enterography is listed for symptomatic persons, urine cytology for those with a family history of urothelial cancer, and pancreatic screening with endoscopic ultrasound or MRI/MRCP for those with a family history of pancreatic cancer. Genetic counseling by genetics professionals is also listed. For other Lynch syndrome-associated cancers, the chapter lists general population screening guidelines.

For treatment of manifestations, GeneReviews lists complete endoscopic polypectomy with follow-up colonoscopy for colon adenomas, segmental or extended colonic resection for colorectal cancer depending on factors such as age, proctectomy or total proctocolectomy for rectal adenocarcinoma, and management of other tumors as in the general population. GeneReviews also states that treatments supporting the anti-tumor immune response, such as immune checkpoint blockade, showed great success in MSI-high tumors.

According to GeneReviews, colorectal cancers with MSI tend to have a better prognosis in a stage-wise comparison than microsatellite-stable tumors, potentially reflecting active anti-tumor immune responses. The chapter describes the risk of recurrent colorectal cancer as increased in Lynch syndrome. The packet does not supply survival figures or life expectancy. GeneReviews notes that screening and removal of precancerous polyps and prophylactic surgery may prevent colon or endometrial cancer in some at-risk relatives.

The packet records active clinical trial listings for Lynch syndrome, all sponsored by San Raffaele University. NCT06708429, "Lynch Syndrome X-Talk of Enteral Mucosa With Immune System," is recruiting (2023-06-01 to 2034-06-01). NCT05704010, "Videocapsule Endoscopy in Lynch Syndrome," is a recruiting study with phase listed as not applicable (2018-11-01 to 2029-12-31). NCT07450612, "Liquid Biopsy and Machine Learning for Early Colorectal Cancer, Adenomas, Lynch Cancers, and Residual Disease Detection," is recruiting (2024-01-01 to 2031-08-15). A trial title is not evidence of an effect. Patient organizations are not certified in this packet. GeneReviews also describes therapies under investigation: two studies compared colonoscopy imaging approaches for surveillance, in which chromoendoscopy better identified flat or diminutive adenomas but was associated with longer withdrawal time, and epidemiologic studies associating oral contraceptive use with reduced endometrial cancer risk, with no prospective trials evaluating that impact to date.