Background:

Small cell bladder cancer (SCBC) is a rare histologic subtype with insufficient genomic characterization. Patients with limited stage (LS) SCBC have a poor prognosis, and no biomarker exists to optimize treatment selection. We sought to identify genomic aberrations in patients with LS-SCBC using a comprehensive next generation sequencing (NGS) platform. Mutations in the PTEN/AKT pathway are important in urothelial tumor biology but have an undefined role in SCBC.

Methods:

23 LS-SCBC cases were identified. NGS was performed on diagnostic transurethral bladder tumor resection or cystectomy specimens containing SCBC. Detected variants were filtered by in silico algorithms predicting for a deleterious impact on protein function. Variant allele frequencies (VAF) greater than 2% were permitted in this analysis. Variants in the PTEN gene were assessed for association with relapse-free survival (RFS) and overall survival (OS) using Kaplan-Meier techniques and Cox proportional hazards models.

Results:

Median follow up for the cohort was 4.02 years. 14/23 (60.9%) patients have died. Six unique deleterious PTEN mutations were observed in 9/23 (39.1%) patients. p.W274C was the most common PTEN variant and was detected in 5 (21.7%) patients. Three variants were detected at > 10% VAF. All 9 patients with a deleterious PTEN variant died. The presence of deleterious PTEN variants [HR = 4.68 [(1.54, 14.27), p = 0.003]] predicted for inferior OS. In the 19 patients with known relapse history, 6/7 (85.7%) with and 3/12 (25%) without any deleterious PTEN mutation relapsed. The presence of deleterious PTEN variants [HR = 9.41 [(2.32, 38.23), p < 0.001]] also predicted for inferior RFS.

Conclusions:

Pathogenic variants in tumor suppressor PTEN were associated with inferior RFS and OS in this pilot cohort of patients with LS-SCBC, suggesting that disruption of PTEN function may be a critical genomic event underlying the progression of small cell bladder cancer. Our findings also support prior reports that pathogenic gene variants detected at low allele frequencies may be clinically important.