Loss of 4.1B Drives PRMT3-Mediated Regulation of GBM Brain Tumour Stem Cell Growth

Background: Protein arginine methyltransferase 3 (PRMT3), a type I family PRMT, regulates the activity of downstream substrates by catalyzing the asymmetric dimethylation of arginine residues. While PRMT3 activity has been reported to be deregulated in many cancers, including glioblastoma (GBM), the underlying signalling mechanisms that contribute to disease progression are largely unknown. Methods: We tested the efficacy of a PRMT3 chemical probe, SGC707, in a cohort of GBM patient-derived primary and recurrent brain tumour stem cell (BTSC) lines. RNA-sequencing, CRISPR-cas9 knockout, and inducible overexpression methods were used to investigate the molecular mechanisms regulated by the aberrant activity of PRMT3 in different BTSC lines. Results: We show that expression of the tumour suppressor protein 4.1B, a negative regulator of PRMT3, predicts the response of GBM BTSCs to the PRMT3 chemical probe, SGC707. Furthermore, PRMT3 modulates the stability and subcellular localization of the downstream effector, UHRF1, a member of the DNA methylation complex. These findings suggest that UHRF1 and DNMT1 may suppress the expression of 4.1B through the increased promoter methylation of EPB4.1L3. Intriguingly, the inducible overexpression of EPB4.1L3 in the BT248EPB4.1L3low BTSC line mimicked the effects of the pharmacologic and genetic inhibition of PRMT3. In contrast, knockout of EPB4.1L3 in BT143EPB4.1L3high cells reduced the interactions between PRMT3 and 4.1B proteins, resulting in increased sensitivity of knockout cells to SGC707 treatment. Conclusions: These findings show that 4.1B, PRMT3, and UHRF1/DNMT1 function together to promote BTSC growth. Thus, targeting PRMT3 or UHRF1/DNMT1, especially in tumours with low endogenous 4.1B protein, may have high therapeutic relevance.