Microsatellite Stable Colorectal Tumours in Patients with Lynch Syndrome: A Case Report and Systematic Review Analysing Clinical Features and Implications for Immunotherapy.

Purpose: Lynch syndrome is an autosomal dominant genetic disorder associated with early-onset colorectal cancer (CRC), endometrial cancer and other malignancies. This condition is defined by deficient DNA mismatch repair and high microsatellite instability (dMMR/MSI-high), exhibiting a substantial response to immunotherapy. However, microsatellite-stable (MSS) tumours may infrequently occur in individuals with Lynch syndrome. Our aim was to evaluate the efficacy of immunotherapy in patients with Lynch Syndrome and dMMR/MSS colorectal cancer.

Methods: A systematic review of the literature in medical databases, major related conferences and relevant oncology journals was conducted to identify the available evidence. Medical records from the Medical and Clinical Oncology Department of the University Hospital of Ioannina were also reviewed.

Results: Four cases of MSS colorectal cancer associated with Lynch syndrome and MSH6 germline mutation were identified. Three of these four patients were treated with immune checkpoint inhibitors. Two patients with metastatic disease experienced disease progression, but one patient who received neoadjuvant immunotherapy achieved a partial response. All four patients were diagnosed with colorectal cancer in ages younger than 52 (16-51 years old).

Conclusion: MSS CRC tumours in patients with Lynch syndrome is an infrequent phenomenon and under-represented in the literature. The limited efficacy of immune checkpoint inhibitors is highlighted in this rare subset of patients.