PRMT1 is a known contributor to breast cancer through methylation of histone and non-histone substrates, but PRMT1-mediated transcriptional coactivation by deposition of H4R3me2a remains understudied. As a result, potential contributions to malignant gene expression via H4R3me2a deposition remain largely unknown. To confront this issue, we made efforts to optimise bio-orthogonal profiling by utilising a PRMT1-Y29F/M38G-Pob-SAM pairing to label histone H4 with a surrogate alkynyl moiety. Although further optimisation is required, data gathered so far suggest that this technology may prove a viable option for genome-wide analysis of H4R3me2a in breast cancer, enabling global comparisons of this epigenetic mark with non-transformed mammary epithelial cells.
In addition, the biochemical consequences of PRMT1 interaction with histone code reader SPIN1 were characterised, after SPIN1 was identified as a breast cancer-enriched interactor of PRMT1 using SILAC quantitative proteomics. SPIN1 was found to positively regulate PRMT1 transcription, making it challenging to validate PRMT1-dependent methylation of SPIN1 in vitro. In contrast, recombinant PRMT1 methylated immunoprecipitated SPIN1, possibly at R117 within its P loop, suggesting a role for R117 in modulating phosphate binding. Supporting this, PRMT1-mediated SPIN1 methylation in a cell-free context was enhanced in phosphate-free conditions. Finally, expression of SPIN1-R117K promoted S-phase cell cycle accumulation, implying that R117 methylation is biologically significant. Ultimately, these findings may expose a SPIN1-PRMT1 axis that can be therapeutically targeted in breast cancer.