Gardner syndrome

Gardner syndrome results from germline mutations in the APC (adenomatous polyposis coli) tumor suppressor gene, following the classic two-hit hypothesis of tumorigenesis. It is inherited in an autosomal dominant pattern and is considered a phenotypic variant of FAP rather than a distinct genetic entity, since both arise from APC mutations.

Key clinical features include: - Hundreds to thousands of colonic adenomatous polyps (indistinguishable from classic FAP), typically appearing in the second decade of life - Osteomas, particularly of the skull and mandible - Epidermoid (sebaceous) cysts of the skin - Desmoid tumors (aggressive fibromatoses), often intra-abdominal, sometimes triggered by prior surgery - Congenital hypertrophy of retinal pigment epithelium (CHRPE) - Supernumerary teeth and dental abnormalities - Increased risk of thyroid carcinoma (particularly papillary, cribriform-morular variant) and other extracolonic malignancies

Pathophysiology: APC normally functions as a negative regulator of the Wnt signaling pathway by promoting degradation of beta-catenin. Loss of APC function leads to beta-catenin accumulation, translocation to the nucleus, and unchecked activation of genes promoting cell proliferation, driving adenoma formation throughout the colon.

Clinical significance: Without intervention, colorectal cancer develops in virtually 100% of patients by age 40-50, since each polyp has malignant potential and patients have hundreds to thousands of polyps. Management involves prophylactic total colectomy (often with ileal pouch-anal anastomosis) once polyps are identified, typically in the teenage years. Regular surveillance for extracolonic manifestations (thyroid ultrasound, abdominal imaging for desmoids) is also necessary. Genetic counseling and testing for family members is essential given the autosomal dominant inheritance pattern.

Differentiation from other APC-related syndromes: Turcot syndrome (FAP + CNS tumors, particularly medulloblastoma) also involves APC mutations, distinguishing it from Gardner syndrome by the type of extraintestinal tumor present.