Gastric neuroendocrine hyperplasia and dysplasia in a patient with Lynch Syndrome

Lynch syndrome (LS), previously known as hereditary non-polyposis colorectal cancer (HNPCC), is an autosomal dominant disorder that predisposes individuals to various malignancies, most commonly adenocarcinomas. While low-grade neuroendocrine tumors (NETs) associated with Lynch syndrome are rarely reported, gastric neuroendocrine tumors (g-NETs) account for less than 1% of all gastric tumors. This case report presents a rare occurrence of gastric neuroendocrine hyperplasia and dysplasia in a 68-year-old female diagnosed with Lynch syndrome. The patient, previously treated for well-differentiated adenocarcinoma of the colon, was found to have a polyp in the gastric fundus during routine surveillance. Histological analysis revealed adenocarcinoma alongside neuroendocrine cell proliferation, confirmed by synaptophysin immunostaining. This case highlights a novel association between Lynch syndrome and gastric neuroendocrine hyperplasia/dysplasia. This finding raises awareness among clinicians regarding Lynch syndrome in patients with low-grade NETs or neuroendocrine precursors and underscores the need for comprehensive surveillance and further research to understand the underlying mechanisms and clinical implications. It suggests that mutations in mismatch repair (MMR) deficiency may play a role in neuroendocrine cell proliferation, expanding the spectrum of Lynch syndrome-associated tumors, and pointing to new directions for studying the molecular pathways and pathogenesis of NETs.