Alveolar soft part sarcoma (ASPS) is a distinct type of soft tissue sarcoma holding a specific ASPL-TFE3 fusion transcript. Curative therapy is based on surgical removal, whereas lately, antiangiogenic targeted therapy regimens have proven effective. In ASPS, analysis of small series additionally display mTOR (mammalian target of rapamycin) pathway activity, thus making mTOR a possible additive target in ASPS, because it is in other tumor entities. Therefore, we systematically evaluated mTOR pathway activity in a large series of ASPS in comparison with soft tissue sarcomas of other differentiation (non-ASPS). Upstream and downstream factors of mTOR signaling and ancillary targets were analyzed in 103 cases (22 ASPS, 81 non-ASPS) by immunohistochemistry mostly using phospho-specific antibodies. TFE3 (transcription factor for immunoglobulin heavy-chain enhancer 3) translocation status was determined by FISH and RT-PCR. All ASPS were positive in TFE3 break-apart FISH and exhibited specific fusion products when RNA was available (type 1: 9x, type 2: 11x), whereas TFE3-immunoreactive non-ASPS did not. In ASPS, TFE3-, cMET-, pAKT T308- (all P < .0001), pp70S6K- (P = .002), and p4EBP1 (P = .087) expression levels were elevated, whereas pAKT S473 was decreased (P < .0001). In addition, ASPS exhibited higher TFE3-, cMET-, pAKT T308-, and pp70S6K- expression levels compared with TFE3-immunopositive non-ASPS sarcomas (all P < .001). We demonstrate elevated mTOR complex 1 (mTORC1) activity in ASPS independent of mTOR complex 2 (mTORC2) activation. mTORC1 activity seems to be related to the existence of ASPL-TFE3 fusion transcripts because TFE3-immunoreactive non-ASPS without ASPL-TFE3 fusion transcripts exhibit significantly lower mTORC1 activation status. Small molecule-based targeting of mTOR might therefore represent a potential mechanism in ASPS alone or in combination with contemporary upstream approaches.
Keywords: (p)ERK; (phosphorylated) extracellular signal-regulated kinases; 3-phosphoinositide–dependent protein kinase 1; ASPL-TFE3; ASPS; Alveolar soft part sarcoma; FFPE; FISH; IRS; LMS; LS; MEK; MiTF; PDK1; PI3K; RT-PCR; Raf; Rapamycin; Ras; SYS; TFE3; TSC1/TSC2; Targeted therapy; UPS; alveolar soft part sarcoma; cMET; fluorescence in situ hybridization; formalin-fixed and paraffin-embedded tissue; fusion protein of the translocation of the alveolar soft part sarcoma critical region-1 and the transcription factor for immunoglobulin heavy-chain enhancer 3 genes; gene encoding the hepatocyte growth factor receptor protein; immunoreactive score; leiomyosarcoma; liposarcoma; mTOR; mTORC1; mTORC2; mammalian target of rapamycin; mammalian target of rapamycin complex 1; mammalian target of rapamycin complex 2; microphthalmia transcription factor; mitogen-activated protein kinase kinase; p4EBP1; pAKT S473; pAKT T308; phosphatidylinositide 3-kinases; phosphorylated 4E-binding protein 1; phosphorylated p70-S6 kinase 1; pp70S6K; protein kinase B phosphorylated at serine 473; protein kinase B phosphorylated at threonine 308; rat fibrosarcoma protein family; rat sarcoma protein family; reverse transcription polymerase chain reaction; synovial sarcoma; transcription factor for immunoglobulin heavy-chain enhancer 3; tuberous sclerosis protein 1/tuberous sclerosis protein 2 complex; undifferentiated pleomorphic sarcoma.
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