RNA sequencing identifies novel regulated IRE1-dependent decay targets that affect multiple myeloma survival and proliferation

IRE1 is an unfolded protein response (UPR) sensor with kinase and endonuclease activity. It plays a central role in the endoplasmic reticulum (ER) stress response through unconventional splicing of XBP1 mRNA and regulated IRE1-dependent decay (RIDD), which cleaves RNA at an XBP1-like consensus sequence (CUGCAG) accompanied by a stem-loop structure. MM cells are known to exhibit an elevated level of baseline ER stress, but RIDD activity has not been well studied in this disease. To investigate novel RIDD targets of possible relevance to the survival/proliferation of MM cells we combined in vitro cleavage assay with RNA sequencing. Bioinformatic analysis revealed hundreds of putative IRE1 substrates, 32 of which were chosen for validation. Looking into the secondary structure of IRE1 substrates, we found that the consensus sequences of IRF4, PRDM1, IKZF1, KLF13, NOTCH1, ATR, DICER, RICTOR, CDK12, FAM168B, and CENPF mRNAs were accompanied by a stem-loop structure essential for IRE1-mediated cleavage. We show that mRNA and protein levels corresponding to these targets were attenuated in an IRE1-dependent manner by treatment with ER-stress-inducing agents. Our results demonstrate for the first time that IRE1 is a key regulator of several proteins of importance in MM survival and proliferation. Total RNA obtained from 6 biological replicates of the H929 cell line was exposed to an in vitro cleavage assay in the presence (3 samples) or absence (3 samples) of recombinant IRE1 protein. We carried out RNA-sequencing from purified poly (A)-containing mRNAs.