Adenosquamous carcinoma of the colon with an aberrant MLH1 immunostaining pattern: a diagnostic pitfall in mismatch repair assessment

Background Adenosquamous carcinoma (ASC) is a rare histological subtype of colorectal cancer (CRC) associated with a poor prognosis. Some studies suggest a distinct molecular profile, including a possibly higher frequency of mismatch repair deficiency (dMMR), PD-L1 overexpression, and KRAS mutations, although data on dMMR remain conflicting. With recent advances in CRC therapeutics, molecular characterization of rare histological subtypes has become increasingly important. Here, we report a case of colorectal ASC with dMMR and an aberrant clone dependent MLH1 immunohistochemical staining pattern, and we review the literature on this rare neoplasm. Case presentation We report the case of a 64-year-old woman who presented with a three-month history of worsening abdominal pain and recent-onset hematochezia. Abdominal CT revealed a large mass involving the right colon. She underwent a total pelvic exenteration. Histopathology showed a high-grade, poorly differentiated tumor with mixed glandular and squamous components, staged as pT4b pN1a. Immunohistochemistry confirmed both glandular and squamous differentiation. Mismatch repair protein analysis showed loss of PMS2 expression and a granular, dot-like MLH1 staining pattern—interpreted as loss of expression and consistent with a known clone-dependent staining pattern—while MSH2 and MSH6 were retained, suggestive of a dMMR phenotype. The patient received mFLOX chemotherapy but died four months postoperatively due to disease-related complications. Discussion This case illustrates the clinicopathological features of ASC, alongside a dMMR immunoprofile that may carry therapeutic relevance given the efficacy of immune checkpoint inhibitors in MSI-high colorectal tumors. The findings also underscore the importance of correctly recognizing the VENTANA M1 clone-specific MLH1 aberrant staining pattern, as misinterpretation could falsely suggest isolated PMS2 loss and trigger unnecessary germline testing for Lynch syndrome. Given the rarity of ASC and its limited molecular characterization, accurate MMR status assessment remains critical for guiding treatment decisions and avoiding diagnostic pitfalls.