Novel mechanisms of MITF regulation identified in a mouse suppressor screen

MITF, a basic-Helix-Loop-Helix Zipper (bHLHZip) transcription factor, plays vital roles in melanocyte development and functions as an oncogene. We performed a genetic screen for suppressors of the Mitf-associated pigmentation phenotype in mice and identified an intragenic Mitf mutation that terminates MITF at the K316 SUMOylation site, leading to loss of the C-end intrinsically disordered region (IDR). The resulting protein is more nuclear but less stable than wild-type MITF and retains DNA-binding ability. Interestingly, as a dimer, it can translocate wild-type and mutant MITF partners into the nucleus, improving its own stability thus ensuring nuclear MITF supply. To further understand the effect of the suppressor mutation (MITF-sl), we performed CUT&RUN experiments to determine how MITF-sl affects the genome-wide occupancy of MITF. MITF-sl had a higher number of peaks than MITF-WT, with 10,636 peaks (p<0.01) exhibiting statistically significant differences (p < 0.01) between MITF-WT and MITF-sl. Gene ontology analysis indicates that the peaks significantly different between MITF-WT and MITF-sl are associated with genes involved in axonogenesis, axon development, cell growth, and positive regulation of MAPK cascade biological pathways. Our results suggest that the 316-419 domain is critical for selective genome occupancy and transcriptional activation of MITF. Overall design: Cleavage Under Targets and Release Using Nuclease CUT&RUN experiments were performed as previously described (Skene and Henikoff, 2017; Zhu et al., 2019) with minor modifications. A375P cells were induced to express FLAG-tagged MITF mutant fusion proteins (MITF-WT and MITF-sl) for 48 hours.