Histone chaperones are diverse proteins which guide the assembly and disassembly of nucleosomes through preventing promiscuous and harmful interactions between highly charged histones with other cellular components. Histone chaperones also assure the maintenance of epigenetic states through correct incorporation of histone variants and modified histones. Hence, they are key to maintaining genome stability and cellular identity.
Mis- or overexpression of histone chaperones has been detected in many cancers. Thus, histone chaperones may be promising targets for new anticancer therapies. We are interested in dissecting molecular mechanisms and pathways of several oncogenic histone chaperones. Particularly, we study poorly understood and considered difficult to target bromodomain AAA+ ATPases. For this, we employ various approaches including genomics, biochemistry, genetics, CRISPR-Cas9 genome editing and microscopy. To understand the molecular functions of the chaperones, we perform experiments in a powerful model organism, fission yeast (Schizosaccharomyces pombe). In the future we plan to test the conservation of our findings in human cancer cells.
1.) Nuclear functions of bromodomain AAA+ ATPases
ATAD2 is an oncogene which is highly overexpressed in many different cancers. ATAD2 belongs to poorly studied family of AAA+ ATPases (ATPases Associated with diverse cellular Activities) with a C-terminal bromodomain. AAA+ ATPases are often attributed to regulation of cellular quality control, e.g. through protein degradation or large protein complex remodeling or repair. ATAD2 is a putative histone chaperone, it is involved in transcription and replication in human cells and heterochromatin boundary maintenance in budding yeast. A second ATAD2 related gene, ATAD2B, is expressed during neuronal differentiation and in cancer cells, however its functions are unknown. S. pombe genome also encodes two, highly conserved bromodomain AAA+ ATPases, Abo1(ATAD2) and Abo2 (ATAD2B). We are especially interested in elucidating what are the chromatin substrates of those ATPases, what are their enzymatic activities, how are they recruited to chromatin and what are the molecular pathways they are involved in (Fig. 1). Unravelling the precise molecular mechanisms of bromodomain AAA+ ATPases will aid to identify disease-specific mechanisms that can be targeted for therapy.
2.) Chromatin dynamics regulation by the FACT chaperone
Previous studies: The conserved and essential histone chaperone FACT (Facilitating Chromatin Transactions) is another chaperone often overexpressed in cancer. In the recent years, I have studied FACT focusing on its roles in active and silent parts of the genome. My studies shed light on the role of H2B monoubiquitination (H2Bub) on the FACT activity within active genes. Specifically, I showed that H2Bub fine-tunes FACT- histone interaction dynamics limiting FACT association to genic regions. Additionally, my research revealed that FACT is involved in compaction of subtelomeric structures (knobs) which are important for meiotic gene silencing (Murawska et al., 2020, Murawska and Ladurner, 2021) (Fig. 2). Recently, we identified FACT as a unique chaperone that promotes heterochromatin spreading along chromosomal arms (preprint on BioRvix), a process fundamental for maintenance of gene expression patterns and faithful inheritance of the epigenetic states.
We continue our studies on the FACT chaperone with a special focus on less understood, transcription-independent processes. We are particularly interested in addressing the following questions:
• What is the function of FACT at centromeres and in chromosome segregation?
• How does FACT cooperate with other chromatin remodelers and chaperones in replication and DNA repair?
• How is FACT stability and turnover regulated genome-wide?
Relevance and future perspectives
Since many histone chaperones and histone homeostasis pathways are mis-regulated in cancer, inhibition or destabilization of histone chaperones and/or their pathways may be an attractive strategy for specific elimination of cancer cells (Fig. 3). Thus, a comprehensive understanding of histone chaperone functions in health and disease will be fundamental in developing new histone-chaperone based therapies in the future.