FTO-Dependent N6-Methyladenosine Modifications Inhibit Ovarian Cancer Stem Cell Self-Renewal by Blocking cAMP Signaling

N-6-Methyladenosine (m(6)A) is the most abundant modification of mammalian mRNAs. RNA methylation fine tunes RNA stability and translation, altering cell fate. The fat mass- and obesity-associated protein (FTO) is an m(6)A demethylase with oncogenic properties in leukemia. Here, we show that FTO expression is suppressed in ovarian tumors and cancer stem cells (CSC). FTO inhibited the self-renewal of ovarian CSC and suppressed tumorigenesis in vivo, both of which required FTO demethylase activity. Integrative RNA sequencing and m(6)A mapping analysis revealed significant transcriptomic changes associated with FTO overexpression and m(6)A loss involving stem cell signaling, RNA transcription, and mRNA splicing pathways. By reducing m(6)A levels at the 3'UTR and the mRNA stability of two phosphodiesterase genes (PDE1C and PDE4B), FTO augmented second messenger 3', 5'-cyclic adenosine monophosphate (cAMP) signaling and suppressed stemness features of ovarian cancer cells. Our results reveal a previously unappreciated tumor suppressor function of FTO in ovarian CSC mediated through inhibition of cAMP signaling. Significance: A new tumor suppressor function of the RNA demethylase FTO implicates m(6)A RNA modifications in the regulation of cyclic AMP signaling involved in stemness and tumor initiation.