Unveiling the molecular profile of adenosquamous gallbladder carcinoma: characterization of a Caucasian cohort

Aims: Gallbladder carcinoma (GBC) is a rare and highly aggressive malignancy associated with poor clinical outcomes. While most GBCs are adenocarcinomas, only a small subset shows squamous differentiation. Tumours containing more than 25% squamous component are classified as gallbladder adenosquamous carcinomas (GBASC), a rare histological subtype characterized by a significantly worse prognosis than conventional adenocarcinomas. Due to their rarity, the molecular landscape and biological basis underlying the aggressive behaviour of GBASCs remain poorly understood. Methods and results: In this study, 25 retrospectively collected GBASC cases were comprehensively characterized at both immunohistochemical and molecular levels. Predictive biomarkers including programmed death-ligand 1 (PD-L1), mismatch repair proteins (MMR), human epidermal growth factor receptor 2 (HER2) and claudin 18.2 (CLDN18.2) were evaluated. Nearly all tumours (96%) showed PD-L1 positivity with a combined positive score (CPS) greater than 1. CLDN18.2 expression was identified in 28% of cases, although its confinement to the glandular component may limit its therapeutic applicability. All tumours were mismatch repair proficient (MMRp), and none demonstrated HER2 overexpression, in contrast to a subset of gallbladder adenocarcinomas. Molecular profiling revealed recurrent alterations in TP53 (64%), KRAS (16%) and CDKN2A (20%), partially overlapping with the genomic profile of gallbladder adenocarcinomas. However, GBASCs showed a significant enrichment of alterations involving the PI3K pathway, particularly PIK3CA (44%) and PTEN (24%) mutations, representing their most distinctive molecular feature. Conclusions: Overall, these findings support the recognition of GBASC as a distinct molecular entity potentially requiring dedicated therapeutic strategies.