Blocking tri-methylguanosine synthase 1 (TGS1) stops anchorage-independent growth of canine sarcomas

Tri methylguanosine synthase 1 (TGS1) is the enzyme that hyper methylates the hallmark 7-methyl-guanosine cap (m7G-cap) appended to the transcription start site of RNAs. The m7G-cap and the eIF4E-cap binding protein guide canonical cap-dependent translation of mRNAs, whereas hyper methylated cap, m2,2,7G-cap (TMG) lacks adequate eIF4E affinity and licenses entry into a different translation initiation pathway. The potential role for TGS1 and TMG-capped mRNA in neoplastic growth is unknown. Canine sarcoma has high translational value to the human disease. Cumulative downregulation of protein synthesis in osteosarcoma OSCA-40 was achieved cooperatively by siTGS1 and Torin-1. Torin-1 inhibited the proliferation of three canine sarcoma explants in a reversible manner that was eliminated by siRNA-downregulation of TGS1. TGS1 failure prevented the anchorage-independent growth of osteo- and hemangio-sarcomas and curtailed sarcoma recovery from mTOR inhibition. RNA immunoprecipitation studies identified TMG-capped mRNAs encoding TGS1, DHX9 and JUND. TMG-tgs1 transcripts were downregulated by leptomycin B and TGS1 failure was compensated by eIF4E mRNP-dependent tgs1 mRNA translation affected by mTOR. The evidence documents TMG-capped mRNAs are hallmarks of the investigated neoplasms and synergy between TGS1 specialized translation and canonical translation is involved in sarcoma recovery from mTOR inhibition. Therapeutic targeting of TGS1 activity in cancer is ripe for future exploration.