Blocking mutant IDH1 phosphorylation triggers APC/C CDH1-dependent ubiquitination in mitotic cells

IDH1 mutation occurs early in glioma development; thus, mutant IDH1-specific inhibitors are being developed as glioma therapy. But, recent reports suggest that mutant IDH1 inhibitors treatments result in loss of therapeutic vulnerabilities and makes cells resistant to anticancer agents. To overcome resistance, the new paradigm in drug discovery is to develop molecules that can degrade oncogenes by harnessing cellular ubiquitination machinery. Therefore, it is imperative to identify strategies for degrading mutant IDH1 employing cellular ubiquitination machinery. To address this, we found that concerted action of the mitotic kinases Cdk1/Cyclin B1 and Plk1 increases mutant IDH1 enzyme activity. It is known that phosphorylation is linked to protein stability, phosphorylation of a short linear degron motif or phosphodegron can trigger target protein ubiquitination. By contrast, phosphorylation of constitutively active degron motifs can block target protein ubiquitination - phospho-inactivated degron. As phosphorylation can trigger or block ubiquitination, it is unknown if phosphorylation affects mutant IDH1 ubiquitination in mitosis. Therefore, in this study, we asked if phosphorylation of mutant IDH1 is linked to ubiquitination. To answer this question, we examined ubiquitination of phosphomutants and phosphomimetics of mutant IDH1 in mitosis. We found that blocking IDH1^R132H phosphorylation is linked to ubiquitination. We observed that APC/C CDH1 ubiquitinates IDH1^{R132H-T77A-S94A}. Further, we show that APC/C CDH1 ubiquitinates lysines 301 and 321 in C-terminal domain of IDH1^{R132H-T77A-S94A}. Thus, blocking mutant IDH1 phosphorylation triggers APC/C CDH1-dependent ubiquitination in mitotic cells. We suggest employing mitotic inhibitors that also block phosphorylation of mutant IDH1 can ubiquitinate mutant IDH1 in cancer cells.