Intraductal Papillary Squamous Neoplasm (IPSN) of the Pancreas: Histological and Molecular Characterization of a Novel and Distinct Intraductal Cancer Precursor.

We report 6 intraductal papillary squamous neoplasms (IPSNs) of the pancreas, a rare but distinctive tumor whose biological features remain largely unknown. Five cases were investigated using an integrated approach combining histomorphological evaluation, immunohistochemistry, and multiregional molecular profiling through whole-exome DNA sequencing and whole-transcriptome RNA sequencing. Only targeted DNA sequencing was available on a sixth recently diagnosed case. Histologically, the intraductal lesions were characterized by large, confluent papillae with fibrovascular cores lined by multilayered epithelial cells with diffuse squamous differentiation. All cases harbored a concomitant invasive carcinoma. The associated invasive carcinomas consistently included a pancreatic tubular/ductal adenocarcinoma; in 5 cases, a poorly differentiated squamous cell carcinoma was also present, the proportion/features of which met the diagnostic criteria of adenosquamous carcinoma in 2 of them. Genomic analyses revealed that IPSNs and their matched invasive carcinomas shared the majority of somatic alterations, supporting a shared clonal origin for the 2 components. Activating KRAS mutations and biallelic inactivation of CDKN2A were detected in all cases. Recurrent mutations involved members of the SWI/SNF chromatin-remodeling complex and KMT2D. Additionally, FGFR1 and MYC amplifications were identified in 2 distinct cases (1 case each). Molecular alterations restricted to the invasive component involved mediators of the transforming growth factor-β signaling pathway. Transcriptomic profiling demonstrated a basal-like expression pattern in all IPSNs and squamous cell carcinomas, although in 2 cases, the matched pancreatic tubular/ductal adenocarcinoma shifted toward a classical transcriptomic subtype. In conclusion, through integrated histological assessment and multiregional molecular sequencing, we demonstrate that IPSN represents a bona fide precursor of invasive pancreatic cancer, a new addition to the intraductal neoplasms category. This study challenges the current paradigm that pancreatic squamous epithelium plays no role in the initiation of pancreatic carcinogenesis, providing the first evidence of its involvement in early tumorigenic processes and yielding immediate implications for pancreatic tumor classification and biological understanding.