Degrading mutant IDH1 employing a PROTAC-based approach impairs STAT3 activation

Heterozygous mutations in IDH1 (isocitrate dehydrogenase 1) are found in most grade II and III brain tumors. A slew of mutant IDH1 inhibitors were identified soon after the discovery of IDH1 mutations in brain tumors. But recent reports show that mutant IDH1 inhibitors reverse therapeutic vulnerabilities and activate the oncogenic transcription factor STAT3 in mutant IDH1-expressing cells. Thus, inhibiting mutant IDH1 using mutant IDH1-specific inhibitors can result in drug resistance. Therefore, to block mutant IDH1, it is imperative to identify alternative modes of therapy. In these lines, recent findings show that PROteolysis TArgeting Chimera (PROTAC) molecules can be designed to degrade target proteins in cancer cells. However, it is unknown whether degrading mutant IDH1 leads to STAT3 activation. Therefore, in this study, we asked if degrading mutant IDH1 by employing a PROTAC-based approach leads to STAT3 activation. To answer the question, we adopted the dTAG system, where we fused FKBP12^F36V to mutant IDH1 proteins and used the FKBP12^F36V-specific PROTAC, dTAG-13, to degrade mutant IDH1-FKBP12^F36V. We assessed STAT3 activation in dTAG-13-treated cells expressing mutant IDH1-FKBP12^F36V. We found that fusing FKBP12^F36V-HA to mutant IDH1 phenocopies mutant IDH1 with similar expression levels, enzyme activity, and cellular localization. We observed that dTAG-13 degrades mutant IDH1-FKBP12^F36V-HA in a dose- and time-responsive manner. Unlike inhibiting, degrading mutant IDH1-FKBP12^F36V-HA did not lead to pSTAT3-Y705 activation. We conclude that degrading mutant IDH1 by employing a PROTAC-based approach impairs STAT3 activation. Based on these observations, we suggest that mutant IDH1-specific PROTACs can be developed to degrade mutant IDH1 in gliomas.