N7-methylguanosine tRNA modification promotes esophageal squamous cell carcinoma tumorigenesis via the RPTOR/ULK1/autophagy axis

The cancer cells selectively promote the translation of oncogenic mRNAs to facilitate cancer progression, while the molecular mechanisms remain unclear. The tRNA N7-methylguanosine (m7G) modification is not essential for yeast growth, but in mammals mis-regulations of tRNA m7G modification cause stem cell defect and developmental disorders. Here we found that tRNA m7G methyltransferase complex components METTL1 and WDR4 are elevated in esophageal squamous cell carcinoma (ESCC) tissues and associated with poor ESCC prognosis. Functionally, depletion of METTL1 or WDR4 suppresses proliferation, migration, and invasion of ESCC cells. In addition, forced expression of METTL1 or WDR4 promotes ESCC progression depending on the tRNA m7G methyltransferase activity. Mechanistically, METTL1 knockdown leads to reduced tRNA m7G modification and decreased expression of m7G-modified tRNAs. Depletion of METTL1 selectively reduces the translation of a subset of oncogenic transcripts, including the genes related to mTOR signaling and autophagy in m7G related codon dependent manner. Rescue assays revealed the essential function of RPTOR/ULK1/autophagy axis in METTL1 driven ESCC progression. Furthermore, ESCC initiation and progression models using Mettl1 conditional knockout and knockin mice uncovered the strong physiological function of Mettl1 in promoting in vivo ESCC tumorigenesis. Our study uncovered a new layer of translation regulation mechanism mediated by tRNA m7G modification, provided strong evidence to support the important physiological function of mis-regulated tRNA modification in cancer, and suggested that targeting METTL1 and its downstream signaling axis could be a promising strategy for ESCC treatment. Overall design: TRAC-Seq were developed to identify the tRNA m7G methylome in ESCC cell line. Polyribosome-bound-RNA-seq was used to study the differentially translated genes in the METTL1 knockdown and control cells. Ribo-seq was performed to study the differentially translated genes and ribosome processivity in the METTL1 knockdown and control cells. RPTOR-rescued polyribosome-bound-RNA-seq was used to distinguish the m7G-dependent and RPTOR-dependent mRNA translation in the K150 cells.