Human Bladder Tumors With 2-Hit Mutations of Tumor Suppressor Gene TSC1 and Decreased Expression of p27

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Abstract
Because loss of chromosome 9 is known to be the most common finding in human bladder tumors, we studied the mutation of the tumor suppressor gene TSC1 (chromosome 9q34) in bladder tumors. Since another tumor suppressor gene, TSC2 (chromosome 16p13.3), is reported to interact with TSC1 in the pathway that modulates tumor suppression, we assessed loss of heterozygosity (LOH) at 16p13.3. Furthermore, we also examined the expression of p27 because the TSC1 product is reported to influence the level of p27. Microsatellite markers were used to evaluate LOH at 9q34 or 16p13.3. Mutations of TSC1 were screened by single strand conformation polymorphism analysis and verified by direct sequencing. The expression of p27 was examined by reverse transcriptase-polymerase chain reaction and immunohistochemical examination. We identified LOH at 9q34 in 12 of 37 bladder tumors (32.4%) but no LOH at 16p13.3 was observed. Furthermore, on single strand conformational polymorphism analysis we identified tumor specific mutations of TSC1 in 4 cases, of which all had LOH at 9q34, demonstrating the 2-hit mutations of TSC1. The expression of p27 was suppressed in all 4 cases with the 2-hit mutations of TSC1. Unexpectedly p27 suppression was detected at the transcription level, although its mechanism is unknown. Our data suggest that the TSC1 mutation possibly has a causative role in the initiation or progression of some bladder tumors and this process is possibly related to the functional loss of p27.