Clinicopathologic and molecular characteristics of neuroendocrine carcinomas of the gallbladder.

Gallbladder neuroendocrine carcinomas (GB-NECs) are a rare subtype of malignant gallbladder cancer (GBC). The genetic and molecular characteristics of GB-NECs are rarely reported. This study aims to assess the frequency of microsatellite instability (MSI) in GB-NECs and characterize their clinicopathologic and molecular features in comparison with gallbladder adenocarcinomas (GB-ADCs). Data from six patients with primary GB-NECs and 13 with GB-ADCs were collected and reevaluated. MSI assay, immunohistochemistry for mismatch repair proteins (MLH1, MSH2, MSH6, and PMS2), comprehensive genomic profiling (CGP) via next-generation sequencing (NGS), and evaluation of tumor mutation burden (TMB) were conducted on these samples. The six GB-NEC cases were all female, with a mean age of 62.0±9.2 years. Of these, two cases were diagnosed as large cell neuroendocrine carcinomas (LCNECs), while the remaining four were small cell neuroendocrine carcinomas (SCNECs). Microsatellite states observed in both GB-NECs and GB-ADCs were consistently microsatellite stable (MSS). Notably, <i>TP53</i> (100%, 6/6) and <i>RB1</i> (100%, 6/6) exhibited the highest mutation frequency in GB-NECs, followed by <i>SMAD4</i> (50%, 3/6), <i>GNAS</i> (50%, 3/6), and <i>RICTOR</i> (33%, 2/6), with <i>RB1, GNAS,</i> and <i>RICTOR</i> specifically present in GB-NECs. Immunohistochemical (IHC) assays of p53 and Rb in the six GB-NECs were highly consistent with genetic mutations detected by targeted NGS. Moreover, no statistical difference was observed in TMB between GB-NECs and GB-ADCs (<i>p</i>=0.864). Although overall survival in GB-NEC patients tended to be worse than in GB-ADC patients, this difference did not reach statistical significance (<i>p</i>=0.119). This study has identified the microsatellite states and molecular mutation features of GB-NECs, suggesting that co-mutations in <i>TP53</i> and <i>RB1</i> may signify a neuroendocrine inclination in GB-NECs. The IHC assay provides an effective complement to targeted NGS for determining the functional status of p53 and Rb in clinical practice.