Lynch syndrome is caused by inherited disruption of the DNA mismatch repair pathway. This pathway is a quality control system that recognizes and corrects small errors that occur naturally when cells copy their DNA. When one copy of a mismatch repair component is altered from birth, cells become vulnerable to accumulating uncorrected DNA changes, which over time can drive the development of cancer. A specific molecular signature, high-frequency microsatellite instability, is detectable in tumors that arise through this mechanism. The condition is inherited in an autosomal dominant pattern. Each child of an affected individual has a 50 percent chance of inheriting the predisposition, although penetrance is incomplete, meaning some individuals who carry the predisposition may never develop cancer. The size and location of the underlying alteration can influence which organs are most at risk, and some structural variants confer a more colon restricted risk profile while others confer broader extracolonic cancer risk.

There are no consensus clinical diagnostic criteria for Lynch syndrome based on signs and symptoms alone, so the workup combines family history, tumor tissue testing, and germline genetic testing. A diagnosis is suspected when a person develops a cancer in the Lynch syndrome spectrum, particularly at a younger age, when there is a notable family history of related cancers, or when tumor tissue testing flags features consistent with mismatch repair deficiency. Tumor tissue testing typically includes microsatellite instability analysis and immunohistochemistry, which checks for loss of expression of the mismatch repair proteins. Universal tumor screening at the time of certain cancer diagnoses, such as colorectal or endometrial cancer, is increasingly used to identify individuals who should proceed to germline testing. Confirmation requires germline genetic testing, which examines DNA from a blood or saliva sample to identify a heritable alteration in a mismatch repair component. Symptoms of Lynch syndrome can overlap with other hereditary and sporadic cancer presentations. Genetic testing is required to confirm the diagnosis and distinguish it from conditions with similar presentations. Once a diagnosis is established, predictive testing of at-risk relatives allows tailored screening for family members.

There is no therapy that corrects the underlying inherited predisposition, so management focuses on early cancer detection, cancer prevention, and individualized treatment of any cancer that develops. The cornerstone of care is structured surveillance. Colonoscopy at regular intervals beginning in young adulthood, with removal of any precancerous polyps, has been shown to substantially reduce colorectal cancer mortality. For females, education about endometrial and ovarian cancer symptoms, gynecologic examination, and consideration of endometrial sampling and transvaginal ultrasound are typical components of monitoring. Additional surveillance may include upper gastrointestinal endoscopy, urinalysis or urine cytology, and dermatologic examination, with the specific schedule personalized based on personal and family history. Risk reducing surgery, such as hysterectomy with bilateral salpingo oophorectomy after childbearing is complete, is sometimes considered for females and is best discussed with a multidisciplinary team. Chemoprevention with daily aspirin has been studied as a strategy to reduce colorectal cancer risk in Lynch syndrome and may be considered in selected individuals. When cancer does develop, treatment follows site specific oncology principles, including surgery, local therapies, systemic therapies, and supportive care, with planning guided by stage, tumor characteristics, and the multidisciplinary team. Mismatch repair deficient tumors are notable for often responding to immune checkpoint inhibitor therapy, and several immune based agents are approved for use in mismatch repair deficient or microsatellite instability high cancers. Genetic counseling is an essential element of care. Regular surveillance is an essential part of long-term care, helping to identify potential complications before they become serious. Patients should discuss treatment options with their healthcare team to determine which therapies may be appropriate for their specific situation.

Outlook in Lynch syndrome depends heavily on whether and when cancer develops, which cancers occur, and the stage at which they are detected. Lifetime cancer risks are substantially elevated compared with the general population, with the highest risks for colorectal cancer and endometrial cancer, but penetrance is not complete and a meaningful subset of individuals never develops cancer. With consistent surveillance, many cancers can be prevented or detected at an early, more treatable stage, which has been associated with substantial reductions in cancer-related mortality. Outcomes for cancers that do arise are influenced by stage at diagnosis, tumor biology, and access to multidisciplinary care, and mismatch repair deficient tumors often retain favorable response characteristics to certain modern therapies. Lifelong monitoring is required, and outcomes are generally most favorable when individuals are engaged in a coordinated surveillance plan and when at-risk relatives are also identified and offered testing.

Active research in Lynch syndrome spans cancer prevention, early detection, and improved treatment of associated cancers. Several clinical studies are underway, exploring areas such as new surveillance strategies, chemoprevention, immunoprevention vaccines, novel imaging or stool-based detection methods, and immune-based therapies for mismatch repair deficient cancers. Investigators are also studying biomarkers that better predict individual cancer risk and response to therapy. Individuals interested in clinical trials can search ClinicalTrials.gov or consult their care team about eligibility.