Identification and Characterization of Two Novel Oncogenic mTOR Mutations

Mammalian target of rapamycin (mTOR) signaling is often aberrantly activated, particularly when genetically activated, in human cancers. mTOR inhibitors targeting the activated mTOR signaling are highly promising anti-cancer drugs. Knowing the activating genetic change in mTOR can help guide the use of mTOR inhibitors for cancer treatment. This study was conducted to identify and characterize novel oncogenic mTOR mutations that can potentially be therapeutic targets in human cancer. We sequenced 30 exons of the mTOR gene in 12 thyroid cancer cell lines, 3 melanoma cell lines, 20 anaplastic thyroid cancer (ATC) tumors, and 23 melanoma tumors and functionally characterized the identified novel mTOR mutations in vitro and in vivo. We identified a novel point mutation A1256G in ATC cell line and G7076A in melanoma tumor in exon 9 and exon 51 of the mTOR gene, respectively. Overexpression of the corresponding mTOR mutants H419R and G2359E created through induced mutagenesis showed dramatically elevated protein kinase activities associated with the activation of mTOR/p70S6K signaling in HEK293T cells. Stable expression of the two mTOR mutants in NIH3T3 cells strongly activated the mTOR/p70S6K signaling pathway and induced morphologic transformation, cell focus formation, anchorage-independent cell growth, and invasion. Inoculation of these mutant-expressing cells in athymic nude mice induced rapid tumor development, showing their driving oncogenicity. We also demonstrated that transfection with the novel mutants conferred cells high sensitivities to the mTOR inhibitor temsirolimus. We speculate that human cancers harboring these mTOR mutations, such as ATC and melanoma, may be effectively treated with inhibitors targeting mTOR.