Primary cutaneous lymphoma is a microsatellite stable tumor: An analysis of microsatellite instability

A deficiency in DNA mismatch repair (MMR) leads to microsatellite instability (MSI), which is associated with a favorable response to immune checkpoint inhibitors (ICIs), and the Promega MSI Analysis System is approved as a companion diagnostic tool for it. In this study, we investigated the MMR status in patients with primary cutaneous lymphoma (PCL) diagnosed at our hospital. MSI was found in 1 of the 29 patients (3.4%), an 87-year-old man diagnosed with subcutaneous panniculitis-like T-cell lymphoma. Only the NR-21 marker was present in both tumor and normal tissue, indicating that the MMR status was MSI-low, and he had a germline mutation of SLC7A8. Our study showed that most PCLs are microsatellite stable tumors. This study is a single-center small-sample investigation and requires validation in larger cohorts.