TYMS overexpression is frequent and linked to grade progression in urothelial bladder cancer

Purpose: While elevated Thymidylate synthase (TYMS) levels are observed in many cancers, its specific role in bladder cancer remains unclear. This study aims to clarify its activity in a large tumor cohort. Methods: A tissue microarray (TMA) containing over 2,700 bladder tumors was analyzed using immunohistochemistry and fluorescence in-situ hybridization (FISH) to assess TYMS expression and gene amplification. Results: TMYS immunostaining was detectable in 83.1% of 1,799 analyzable bladder cancers. The fraction of cancers with moderate to strong TMYS positivity increased markedly from pTaG2 low (6.5%) to pTaG2 high grade (20.7%), and pTaG3 cancers (29.0%; p < 0.0001). There was also a significant increase of moderate to strong staining from pTa to advanced stage pT2-4 cancers (pTa 13.2% vs. pT2-4 32.9% p < 0.0001). In muscle-invasive cancers, the frequency of TMYS immunostaining increased with tumor grade (p = 0.0007), but there was no association between TYMS expression and patient prognosis (p = 0.4365). TYMS amplification was found in 3.1% of 1,775 analyzable bladder cancers. TYMS amplification increased from pTa (0.6%) to pT2-4 (3.7%; p < 0.0001), but in muscle invasive-cancers TYMS copy number alterations were unrelated to tumor phenotype and patient prognosis. Strong TYMS positivity was significantly associated with TYMS amplification (p = 0.0096) but only a subset (28.6%) of amplified cancers showed a strong TMYS staining and only 7.2% of cancers with strong TYMS expression had a TYMS amplification. Conclusion: TYMS overexpression plays a role in early bladder cancer development and grade progression, but its expression is largely unrelated to the disease course in muscle-invasive cancers. Gene amplification is not the primary driver of TYMS protein overexpression.