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

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. 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. Overall, these findings support the recognition of GBASC as a distinct molecular entity potentially requiring dedicated therapeutic strategies.