Multiple tumor suppressor pathways negatively regulate telomerase.

Telomerase expression is repressed in most somatic cells but is observed in stem cells and a high percentage of human cancers and has been hypothesized to contribute to tumorigenesis and maintenance of stem cell states. To explore telomerase regulation, we employed a general genetic screen to identify negative regulators of hTERT. We discovered three tumor suppressor/oncogene pathways involved in hTERT repression. One, the Mad1/c-Myc pathway, had been previously implicated in hTERT regulation. The second, SIP1, a transcriptional target of the TGF-beta pathway, mediates the TGF-beta regulated repression of hTERT. The third, the tumor suppressor Menin, is a direct repressor of hTERT. Depleting Menin immortalizes primary human fibroblasts and causes a transformation phenotype when coupled with expression of SV40 Large and Small T antigen and oncogenic ras. These studies suggest that multiple tumor suppressor/oncogene pathways coordinately repress hTERT expression and imply that telomerase is reactivated in human tumors through oncogenic mutations.