Terry Magnuson, PhD

Research Interests

Key words: Mammalian Genetics/Genomics/Development/Mouse Models of Human Disease

Epigenetics

PRC2’s histone H3 lysine-27 methylation activity plays a pivotal role in cellular homeostasis maintenance, cell lineage specification, and disease development through maintaining chromatin structure and transcriptional programs. Genome-wide H3K27 methylation is restored in daughter cells for cell identity maintenance during cell proliferation and are also transmitted into next generation through gametes for gene regulation in early embryogenesis. The epigenetic memory of H3K27me landscapes is determined by the temporospatial control of PRC2 recruitment and assembly on targeting chromatin loci. The interaction between PRC2 and chromatin is mediated through a complicated process involving repressive transcriptional states, CpG-rich DNA elements, chromatin-binding proteins, DNA modifications, histone modifications, and noncoding RNAs. This process is particularly important for mammalian spermatogenesis, which requires numerous epigenetic changes to accompany the transition from somatic, diploid precursors to mature, haploid gametes. Faithful execution of the meiotic program requires that the genome undergoes large-scale changes to histone and DNA modifications as well as to chromatin structure, all of which require the action of a large number of chromatin modifying pathways. Homologous recombination occurs during the first meiotic prophase. DNA double-strand breaks (DSBs) are induced, and repair at these breaks generates DNA recombination between homologous chromosomes. Many of the factors required for repair of stress-induced DNA damage in somatic cells function during male meiosis. In addition to their well-characterized roles in transcriptional regulation, chromatin-remodeling complexes also have roles in DNA repair. Because male germ cell development is characterized by DSBs and dynamic changes to gene expression patterns, including a transition from somatic to germ-cell-specific genes, global repression of transposon activity, and meiotic sex chromosome inactivation, it stands to reason that this process is particularly sensitive to the activity of several epigenetic regulators known to influence meiotic recombination. Experiments are addressing the mechanisms by which PRC1/2 and SWI/SNF subunits regulate epigenetic memory during spermatogenesis, as well as defining how associations between complexes and lncRNAs shape the male epigenome during meiosis.

Chromatin Remodeling

The eukaryotic genome is stored in the nucleus as chromatin, a dynamic structure of DNA and histone proteins. Chromatin contains a vast array of features that directs how, when, and where genomic DNA is made accessible by regulatory machinery. Nucleosomes are the core units of chromatin and are highly dynamic structures. Nucleosomes can be variable in histone protein composition, undergo a wide array of post-translational modifications, and are subject to changes in their localization at genomic sites. These variations in nucleosome biology instruct cells how to utilize epigenomic loci and the underlying DNA sequences. As a result, regulation of chromatin delivers precise instructions to cells leading to secondary processes that are physiologically critical. Thus, beyond serving as a DNA storage molecule, chromatin serves as a template that enables cells to leverage multivalent signals surrounding genomic sites as instructions to cellular machinery for biological programs. There are three categories of proteins and enzymes that modify chromatin by depositing, removing or recognizing post-translational modifications (PTMs) of the histone, categorized as writers, erasers, and readers, respectively. A fourth class of chromatin modifiers consists of ATP-dependent chromatin remodelers that regulate nucleosome positioning. These complexes are multi-subunit molecular machines that mobilize nucleosomes by breaking histone-DNA contacts using ATP hydrolysis. The SWI/SNF family of chromatin remodelers are the subject of this proposal. It was not until The Cancer Genome Atlas (TCGA) program began to sequence over 20,000 primary cancer and matched normal samples spanning 33 cancer types that it became clear that there are widespread mutations in SWI/SNF subunits in a variety of cancers. These data established SWI/SNF complexes as major factors in tumor biology. The work outlined here analyzes the regulation of assembly and genomic targeting of biochemically distinct forms of SWI/SNF chromatin remodeling complexes, and how these processes impact the functional and phenotypic diversity of remodelers. Experiments are addressing (i) changes to the composition and assembly of the SWI/SNF complex in liver and liver cancer cells, and (ii) the relevance of functional interactions between SWI/SNF complex subunits and RNAs.

Terry Magnuson in UNC Genetics News