Bladder tumour-derived somatic TSC1 missense mutations cause loss of function via distinct mechanisms

Area of Science:

• Oncology
• Molecular Biology
• Genetics

Background:

• Over 50% of bladder transitional cell carcinomas exhibit loss of heterozygosity at the TSC1 locus (9q34).
• TSC1 mutations are found in 14.5% of bladder tumors, including missense, nonsense, splicing, and deletion types.

Purpose of the Study:

• To investigate the functional impact of six somatic TSC1 missense mutations identified in bladder cancer.
• To determine the mechanisms by which these missense mutations lead to loss of TSC1 function.

Main Methods:

• Analysis of mRNA and protein levels for TSC1.
• Assessment of pre-mRNA splicing errors.
• Evaluation of exogenous TSC1 protein stability.
• Immunohistochemistry to confirm TSC1 expression in tumors.
• Investigation of TSC1 protein localization.

Main Results:

• Five of six somatic TSC1 missense mutations resulted in loss of function via distinct mechanisms.
• Two mutations caused pre-mRNA splicing errors leading to premature protein truncation.
• Three mutations significantly reduced TSC1 protein stability.
• One mutation impaired TSC1 protein localization independently of TSC2 and mTOR.
• Tumors with splicing errors or reduced protein stability showed absent or significantly reduced TSC1 expression.

Conclusions:

• Somatic TSC1 missense mutations in bladder cancer lead to loss of function through diverse molecular defects.
• These mutations, particularly those affecting splicing and protein stability, contribute significantly to bladder tumorigenesis.
• While TSC1 missense mutations are rare in germline TSC disease, they represent a notable subset of somatic loss-of-function mutations in bladder cancer.