2010 Tenovus Medal Lecture

2006 Symposium
Professor Jessica Tyler, University of Colorado

BSc Biochemistry, University of Sheffield (1990)
PhD in Virology (Roger Everett’s group), University of Glasgow (1994)
Postdoctoral studies (James T. Kadonaga’s group) UCSD, USA (1994-2000)
Faculty at University of Colorado, Denver, USA (2000-onwards)

Regulation of genomic processes by chromatin assembly and disassembly

The packaging of the eukaryotic genome into chromatin is essential for normal growth, development, and differentiation. The fundamental repeating unit of chromatin is the nucleosome, comprising DNA wrapped around histone proteins H3, H4, H2A, and H2B. It is well established that histones are removed from the DNA prior to DNA replication and that the newly-replicated DNA is repackaged into chromatin. However, the machinery, mechanisms and cellular impact of chromatin assembly and disassembly were unknown. Furthermore, exactly how chromatin structures are accurately reassembled in order to reinstate the epigenetic information carried by the chromatin is still unclear.‌

My discovery (as a postdoc) of the ubiquitous histone chaperone Anti-Silencing Function 1 (Asf1), that together with Chromatin Assembly Factor 1 (CAF-1) deposits histones onto newly-replicated DNA in vitro, provided the entry point to investigate these critically important issues. Our studies have utilized a combination of yeast molecular genetics, tissue culture studies, biochemical, biophysical and structural approaches.

Until recently, chromatin disassembly and reassembly were believed to only occur during DNA replication. Our hypothesis was that all genomic processes that utilize the DNA, including transcription, DNA repair, and recombination, would necessitate chromatin disassembly and reassembly. In agreement with the high degree of conservation of chromatin structure and genomic processes among eukaryotes, we have shown that human Asf1 can functionally replace budding yeast Asf1. Using yeast, we discovered that Asf1 mediates the disassembly of chromatin during transcription. Furthermore, we discovered that the disassembly and assembly of chromatin at promoter regions is essential for transcriptional regulation in vivo. Contrary to the previous dogma for transcriptional activator function, we showed that the sole role of at least some activators in vivo is to stimulate the loss of nucleosomes from promoters. We have also established that chromatin assembly and disassembly occur during DNA repair; Asf1 and CAF-1 are critical for genomic stability and viability following double-strand DNA repair due to their roles in assembling chromatin around DNA lesions. Our studies have also revealed that chromatin assembly after DNA repair, not repair of the DNA per se, is the event that signals to the DNA damage cell cycle checkpoint that repair is complete.

Mechanistically, it was widely assumed that histones H3/H4 always exist as a heterotetramer and are deposited onto DNA in this form. However, we unequivocally proved using biophysical methods and x-ray crystallography that the Asf1-H3/H4 complex comprises one molecule of Asf1 bound to an H3/H4 heterodimer. Indeed, Asf1 physically blocks formation of the H3/H4 heterotetramer. As such, the current proposed mechanisms of chromatin assembly and disassembly now need to be reevaluated.

More recently, we have expanded our studies of chromatin dynamics towards a better understanding of human disease, including aging and cancer. Asf1 is required for the acetylation of histone H3 on lysine 56 – a histone mark that loosens the intrinsic structure of the nucleosome. We have recently discovered a new mechanism of aging in yeast that is due to this epigenetic mark and have exploited our finding to achieve life-span extension. To date, the vast majority of studies on the acetylation of histone H3 on lysine 56 have been limited to yeast. We have recently discovered that this epigenetic mark also occurs in humans and we have identified the enzymes that make and remove this acetylation mark in humans and Drosophila. Moreover, we have found a striking correlation between acetylation of histone H3 K56 and cancer. Consequently, studies of chromatin assembly and disassembly are fundamentally important for understanding all the activities of the genome, genomic instability, aging and cancer.