Nuclear function of IMPDH2 in melanocyte differentiation

IMPDH2, an oncogene in various cancers, is a key rate-limiting enzyme in nucleotide synthesis, maintaining guanine-based nucleotides for nucleic acid synthesis, protein synthesis via ribosome function, and molecular signalling through G-proteins. IMPDH2 is upregulated in melanoma cells compared to melanocytes, and high IMPDH2 expression correlates with high EZH2 expression and with poor survival in human melanoma. Notably, depletion of guanine-based nucleotides via IMPDH2 inhibition with mycophenolate (MPA) induces cellular differentiation. IMPDH2 is mostly nuclear in normal human melanocytes, in which EZH2 is absent, and in melanoma cells is mostly cytosolic, wherein we found it interacted with cytosolic EZH2. However, when EZH2 is suppressed, IMPDH2 nuclear expression increases and it binds to pigmentation gene promoters, including Oca2 and Tyr and induces pol II binding/ pigmentation gene expression in melanoma cells. In contrast, IMPDH2 silencing reversed siEZH2-induced pigment gene expression. We thus hypothesize that EZH2-IMPDH2 interaction facilitates IMPDH2 cytoplasmic shuttling, and that cytosolic IMPDH2 acts as an GTP producing enzyme to positively regulate malignant cell states. On the other hand, in the absence of EZH2 interaction, nuclear IMPDH2 acts as a transcriptional activator to induce melanocytic differentiation. Thus, we aim to test the nuclear function of IMPDH2 in melanocyte differentiation: Aim 1: Identify IMPDH2-bound gene promoters by ChIP-seq, Aim 2: Identify IMPDH2 target genes by RNAseq, Aim 3: Identify DNA binding and transactivation domains of IMPDH2 by testing the effects of different truncation mutants of IMPDH2 on melanocyte differentiation gene expressions.