Although breast cancer (BrCa) may be detected at an early stage, there is a shortage of markers that predict tumor aggressiveness and a lack of targeted therapies. Histone chaperone FACT, expressed in a limited number of normal cells, is overexpressed in different types of cancer, including BrCa. Recently, we found that FACT expression in BrCa correlates with markers of aggressive BrCa, which prompted us to explore the consequences of FACT inhibition in BrCa cells with varying levels of FACT.FACT inhibition using a small molecule or shRNA caused reduced growth and viability of all BrCa cells tested. Phenotypic changes were more severe in high- FACT cells (death or growth arrest) than in low-FACT cells (decreased proliferation). Though inhibition had no effect on the rate of general transcription, expression of individual genes was changed in a cell-specific manner. Initially distinct transcriptional profiles of BrCa cells became similar upon equalizing FACT expression. In high-FACT cells, FACT supports expression of genes involved in the regulation of cell cycle, DNA replication, maintenance of an undifferentiated cell state and regulated by the activity of several proto-oncogenes. In low-FACT cells, the presence of FACT reduces expression of genes encoding enzymes of steroid metabolism that are characteristic of differentiated mammary epithelia.Thus, we propose that FACT is both a marker and a target of aggressive BrCa cells, whose inhibition results in the death of BrCa or convertion of them to a less aggressive subtype.

Major advances in the chromatin and epigenetics fields have uncovered the importance of core histones, histone variants and their post-translational modifications (PTMs) in modulating chromatin structure. However, an acutely understudied related feature of chromatin structure is the role of linker histone H1. Previous assumptions of the functional redundancy of the 11 nonallelic H1 variants are contrasted by their strong evolutionary conservation, variability in their potential PTMs, and increased reports of their disparate functions, sub-nuclear localizations and unique expression patterns in different cell types. The commonly accepted notion that histone H1 functions solely in chromatin compaction and transcription repression is now being challenged by work from multiple groups. These studies highlight histone H1 variants as underappreciated facets of chromatin dynamics that function independently in various chromatin-based processes. In this review, we present notable findings involving the individual somatic H1 variants of which there are seven, underscoring their particular contributions to distinctly significant chromatin-related processes.

Expression of histone chaperone FACT is increased in tumors and associated with poor prognosis. We investigated why aggressive tumor cells need FACT using a model where FACT could be turned off and confirmed that while FACT is not essential for non-tumor cells, cells become dependent on FACT following oncogene-induced transformation. We compared the phenotypic and transcriptional changes induced by FACT loss and excluded a direct role for FACT in the transcription of genes essential for the viability of transformed cells. Moreover, we established that in immortalized and transformed cells, FACT has a weak negative effect on gene expression. At the same time, we observed a positive correlation between FACT enrichment and the rate of transcription, which was consistent with previous reports. To explain these puzzling observations, we hypothesized that FACT does not facilitate transcription elongation in transformed cells, but prevents nucleosome loss associated with transcription. Indeed, we observed destabilization of chromatin in immortalized and transformed cells upon FACT loss. Furthermore, transformed cells had less stable chromatin than nontransformed cells, which made them vulnerable to FACT loss. However, the mechanisms of cell death upon chromatin destabilization needs to be established. Our data suggest that malignant transformation is accompanied by general chromatin destabilization, and FACT prevents irredeemable chromatin loss.