HER2, HER3, and Mismatch Repair Protein Expression in Stage IV Small Bowel Adenocarcinoma: Results From a Multicenter Series

Small bowel adenocarcinoma (SBA) is a rare and aggressive malignancy. One-third of SBA cases are diagnosed at an advanced stage, often with limited treatment options. Recent clinical trials have underscored the importance of testing predictive biomarkers for therapy response in advanced solid tumors, particularly with respect to antibody-drug conjugates and immunotherapy. However, few studies have investigated the expression of mismatch repair (MMR) proteins, HER2, and HER3 in metastatic SBA. In this study, the immunohistochemical expression of MMR proteins, HER2 (using gastric cancer scoring criteria), and HER3 was assessed in a multicentric series of 26 stage IV SBAs. In 7 cases, tissues from both the primary tumor and paired distant metastasis were tested for all biomarkers, and any discordance in biomarker expression (ie, spatial heterogeneity) was recorded. For comparison, a cohort of 47 stage III SBA patients was also evaluated. MMR deficiency (MMRd), HER2 positivity (score 3+ or 2+ with gene amplification), and HER3 positivity were identified in 11.5%, 7.7%, and 40% of stage IV SBAs, respectively, with limited overlap among the biomarkers. One stage IV SBA showed heterogeneous MMR status, with a distinct subclone lacking MSH6 expression. When stage III SBAs were compared with stage IV SBAs, MMRd was more frequent (31.9%) among stage III SBAs. In 1 case (14%), the primary tumor was MMR-proficient, HER2-negative, and HER3-negative, whereas the metachronous distant metastasis was MMR-deficient, HER2-negative, and HER3-positive. The percentage of MMRd and HER2 positivity observed in stage IV SBAs highlights the need to assess HER2 and MMR status in these rare tumors. Our findings also suggest that evaluating the response of SBAs to anti-HER3 therapies should be considered in future clinical trials. Additionally, evidence of spatial heterogeneity in biomarker expression emphasizes the importance of assessing biomarkers in metastatic tissue to identify actionable alterations that may not be present in the primary tumor.