The histone chaperone DAXX maintains the structural organization of heterochromatin domains.
BACKGROUND
The death domain-associated protein (DAXX) collaborates with accessory proteins to deposit the histone variant H3.3 into mouse telomeric and pericentromeric repeat DNA. Pericentromeric repeats are the main genetic contributor to spatially discrete, compact, constitutive heterochromatic structures called chromocentres. Chromocentres are enriched in the H3K9me3 histone modification and serve as integral, functionally important components of nuclear organization. To date, the role of DAXX as an H3.3-specific histone chaperone has been investigated primarily using biochemical approaches which provide genome-wide views on cell populations and information on changes in local chromatin structures. However, the global chromatin and subnuclear reorganization events that coincide with these changes remain to be investigated.


RESULTS
Using electron spectroscopic imagine (ESI), a specialized form of energy-filtered transmission electron microscopy that allows us to visualize chromatin domains in situ with high contrast and spatial resolution, we show that in the absence of DAXX, H3K9me3-enriched domains are structurally altered and become uncoupled from major satellite DNA. In addition, the structural integrity of nucleoli and the organization of ribosomal DNA (rDNA) are disrupted. Moreover, the absence of DAXX leads to chromatin that is more sensitive, on a global level, to micrococcal nuclease digestion.


CONCLUSIONS
We identify a novel role of DAXX as a major regulator of subnuclear organization through the maintenance of the global heterochromatin structural landscape. As well, we show, for the first time, that the loss of a histone chaperone can have severe consequences for global nuclear organization.
Program in Genetics and Genome Biology, The Hospital for Sick Children, Toronto, ON M5G 0A4 Canada ; Department of Biochemistry, The University of Toronto, Toronto, ON M5S 1A8 Canada.
Lindsy M
LM
Rapkin
Program in Genetics and Genome Biology, The Hospital for Sick Children, Toronto, ON M5G 0A4 Canada.
Kashif
K
Ahmed
Program in Genetics and Genome Biology, The Hospital for Sick Children, Toronto, ON M5G 0A4 Canada.
Stanimir
S
Dulev
Program in Genetics and Genome Biology, The Hospital for Sick Children, Toronto, ON M5G 0A4 Canada.
Ren
R
Li
Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-Ku, Yokohama 226-8501 Japan.
Hiroshi
H
Kimura
Department of Anatomy and Cell Biology, University of Florida College of Medicine, and University of Florida Cancer Center, Gainesville, FL 32610 USA.
Alexander M
AM
Ishov
Program in Genetics and Genome Biology, The Hospital for Sick Children, Toronto, ON M5G 0A4 Canada ; Department of Biochemistry, The University of Toronto, Toronto, ON M5S 1A8 Canada.
David P
DP
Bazett-Jones
Chromocentres; DAXX; Electron spectroscopic imaging; Heterochromatin; Nuclear organization