Identification and characterization of RBM12 as a novel regulator of fetal hemoglobin expression.

The fetal-to-adult hemoglobin transition is clinically relevant as reactivation of fetal hemoglobin (HbF) significantly reduces morbidity and mortality associated with sickle cell disease (SCD) and β-thalassemia. Most studies on the developmental regulation of the globin genes, including genome-wide genetics screens, have focused on DNA binding proteins including BCL11A and ZBTB7A/LRF and their cofactors. Our understanding of RNA binding proteins (RBPs) in this process is much more limited. Two RBPs, LIN28B and IGF2BP1 are known post-transcriptional regulators of HbF production but a global view of RBPs is still lacking. Here, we carried out a CRISPR/Cas9-based screen targeting RBPs harboring RNA methyltransferase and/or RNA Recognition Motif (RRM) domains and identified RBM12 as a novel HbF suppressor. Depletion of RBM12 induced HbF expression and attenuated cell sickling in erythroid cells derived from SCD patients with minimal detrimental effects on cell maturation. Transcriptome and proteome profiling revealed that RBM12 functions independently of major known HbF regulators. Enhanced crosslinking and immunoprecipitation followed by high throughput sequencing (eCLIP-Seq) revealed strong preferential binding of RBM12 to 5'UTRs of transcripts, narrowing down the mechanism of RBM12 action. Notably, we pinpointed the first of five RRM domains as essential, and, in conjunction with a linker domain, as sufficient for RBM12-mediated HbF regulation. Our characterization of RBM12 as a negative regulator of HbF points to an additional regulatory layer of the fetal-to-adult hemoglobin switch and broadens the pool of potential therapeutic targets for SCD and β-thalassemia.