Mutations in the mismatch repair (MMR) system predict the response to immune checkpoint inhibitors (ICIs) like colon or gastric cancer. However, the MMR system’s involvement in non-small cell lung cancer (NSCLC) remains unknown. Addressing this issue will improve clinical guidelines in the case of mutations in the main genes of the MMR system (MLH1, MSH2, MSH6, and PMS2). This work retrospectively assessed the role that these gene mutations play in the response to and survival of ICIs in NSCLC. Patients with NSCLC treated with nivolumab as the second-line treatment in the University Hospital of Salamanca were enrolled in this study. Survival and response analyses were performed according to groups of MMR system gene expression (MMR expression present or deficiency) and other subgroups, such as toxicity. There was a statistically significant relationship between the best response obtained and the expression of the MMR system (p = 0.045). The presence of toxicity grade ≥ 3 was associated with the deficiency expression of MMR (dMMR/MSI-H) group (p = 0.022; odds ratio = 10.167, 95% confidence interval (CI) 1.669–61.919). A trend towards greater survival and response to ICIs was observed in NSCLC and dMMR. Assessing the genes in the MMR system involved in NSCLC is key to obtaining personalized immunotherapy treatments.

Proliferating trichilemmal tumours (PTT) are defined by a benign squamous cell proliferation inside a trichilemmal cystic (TC) cavity. A possible explanation of this proliferative phenomenon within the cyst may be molecular alterations in genes associated to cell proliferation, which can be induced by ultraviolet radiation. Among other genes, alterations on TP53 and DNA mismatch repair proteins (MMR) may be involved in the cellular proliferation observed in PTT. Based on this assumption, but also taking into account the close relationship between the sebaceous ducts and the external root sheath where TC develop, a MMR, a p53 expression assessment and a TP53 study were performed in a series of 5 PTT cases, including a giant one. We failed to demonstrate a MMR disorder on studied PTT, but we agree with previous results suggesting increased p53 expression in these tumours, particularly in proliferative areas. TP53 alteration was confirmed with FISH technique, demonstrating TP53 deletion in most cells.