METTL3 suppresses pancreatic ductal adenocarcinoma progression through activating endogenous dsRNA-induced anti-tumor immunity.

Purpose: Although immunotherapy improves clinical outcomes in several types of malignancies, as an immunologically 'cold' tumor, pancreatic ductal adenocarcinoma (PDAC) is arrantly resistant to immunotherapy. However, the role of N6-methyladenosine (mA) modification in the immune microenvironment of PDAC is still poorly understood.

Methods: The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) datasets were used to identify differentially expressed mA related enzymes. The biological role and mechanism of METTL3 in PDAC growth and metastasis were determined in vitro and in vivo. RNA-sequencing and bioinformatics analysis were used to identify signaling pathways involved in METTL3. Western blot, mA dot blot assays, co-immunoprecipitation, immunofluorescence, and flow cytometry were used to explore the molecular mechanism.

Results: Here, we demonstrate that METTL3, the key regulator of mA modification, is downregulated in PDAC, and negatively correlates with PDAC malignant features. Elevated METTL3 suppresses PDAC growth and overcomes resistance to immune checkpoint blockade. Mechanistically, METTL3 promotes the accumulation of endogenous double-stranded RNA (dsRNA) through protecting mA-transcripts from further Adenosine-to-inosine (A-to-I) editing. The dsRNA stress activates RIG-I-like receptors (RLRs) to enhance anti-tumor immunity, finally suppressing PDAC progression.

Conclusion: Our findings indicate that tumor cell-intrinsic mA modification participates in the regulation of tumor immune landscape. Adjusting the mA level may be an effective strategy to overcome the resistance to immunotherapy and increase responsiveness to immunotherapy in PDAC.