Tumour-Suppressive Nuclear Functions of Oncogenic Nucleotide Synthesis Enzymes

Nucleotide synthesis enzymes (NSEs), including IMPDH2, GMPS, and CTPS1, are traditionally characterised as cytoplasmic proteins that support tumour growth by supplying nucleotides for DNA and RNA synthesis. However, emerging evidence from our lab and others has revealed a surprising and transformative non-canonical function for these enzymes in their stress-induced nuclear localisation to bind chromatin and modify gene expression programs that suppress tumour progression. This unexpected role links cellular metabolism with nuclear gene regulation, potentially opening new avenues for cancer therapy. Our unpublished data have shown that nuclear localisation of IMPDH2 suppresses tumour cell proliferation, migration, and in vivo melanoma growth. We identified a key regulatory mechanism whereby p53 activation induces dephosphorylation of IMPDH2 at serine 122, potentially unmasking a cryptic nuclear localisation signal and promoting nuclear import via KPNA3. In the nucleus, IMPDH2 downregulates E2F-dependent transcription and activates tumour suppressor pathways. Intriguingly, we also found that other NSEs share similar CDK phosphorylation sites and predicted NLS motifs, suggesting a conserved, previously unrecognised tumour suppressive function across this enzyme family. We hypothesise that NSEs act as nuclear tumour suppressors via p53-dependent, CDK2-regulated nuclear shuttling. To test this, our project will: (1) Define the molecular mechanism of IMPDH2 nuclear import following p53 activation. (2) Characterise the tumour suppressor functions of nuclear IMPDH2 across solid cancers and assess the impact of cancer-associated IMPDH2 mutations; and (3) extend these findings to determine which other NSEs also shuttle to the nucleus and suppress tumour growth via similar mechanisms. This work represents a paradigm shift in cancer biology from viewing NSEs solely as cytoplasmic metabolic oncoproteins to recognising their nuclear, tumour-suppressive roles. By targeting this dual functionality, we aim to develop novel therapeutic strategies that both inhibit the cytoplasmic tumour- promoting the activity of NSEs and promoting their nuclear tumour suppressive potential.