Original ArticleThe histone H2A isoform Hist2h2ac is a novel regulator of proliferation and epithelial-mesenchymal transition in mammary epithelial and in breast cancer cells

- Hist2h2ac is highly expressed in undifferentiated mammary epithelial cells. Its transcription is activated by the growth factors.
- Hist2h2ac is necessary to allow growth factor-dependent Zeb-1 expression and E-cadherin downregulation.
- Cells counterbalance Hist2h2ac silencing by upregulating EGFR and ERBB2 and activating ERK 1/2 but still cannot proliferate.
- HIST2H2AC is mutated or upregulated in over 16% breast cancers analysed.

Proliferation and differentiation are controlled through chromatin remodelling. Therefore, there is an enormous biological significance and clinical value in understanding how specific signalling pathways are affected by histone replacement in the nucleosome. In this work, mass spectrometry was used to screen HC11 mammary epithelial cells for changes in histone levels throughout cell differentiation. The canonical histone isoform Histone H2A type 2-C (Hist2h2ac) was found only in undifferentiated/proliferating cells. Hist2h2ac mRNA was induced by EGF, specifically in the CD24+/CD29hi/DC44hi cell subpopulation. Hist2h2ac mRNA was increased by MEK1/2 or PI3-K activation in HC11 and EpH4 mammary epithelial cells, and in MC4-L2 and T47-D breast cancer cells. Hist2h2ac silencing inhibited EGF-induced Zeb-1 expression and E-cadherin down-regulation, and this effect was reverted by Hist2h2ac re-expression. Notably, silencing of Hist2h2ac increased EGFR, ERBB2, and ERK1/2 activation but did not allow EGF-induced proliferation. HIST2H2AC was expressed in all breast cancer molecular subtypes and found altered in 17% breast cancers, being 16.8% of the cases related to HIST2H2AC gene amplification and/or mRNA upregulation. In summary, this is the first study that identifies a canonical histone isoform -Hist2h2ac-downstream of the EGFR pathway, regulating oncogenic signalling and thereby contributing to deregulation of target genes.