We have shown that CHD8 negatively regulates p53 function by recruiting histone H1 to the promoters of p53 target genes. Formation of the p53–CHD8–histone H1 complex requires expression of CHD8 and stabilization of p53 by genotoxic stress. CHD8 is preferentially expressed in embryonic tissues and in cancer cell lines that are thought to reflect the embryonic state. Loss of CHD8 induced hyperactivation of p53, resulting in apoptosis, which was prevented by depletion of p53. The biochemical and genetic evidence provided by our study may explain how p53 function is regulated by histone H1.
Histone modification is a fundamental mechanism for epigenetic control of gene expression. The role of core histones in gene regulation has been studied extensively, but that of the linker histone H1 has remained unclear. In vitro
studies suggest that linker histone molecules influence chromatin structure, nucleosome mobility and gene regulation25– 32
. Histone H1 is not essential for growth or cell division in several unicellular eukaryotes33–36
, but ablation of three of eight genes that encode isoforms of histone H1 in mouse ES cells, resulting in a 50% reduction in histone H1 content, led to embryonic death of the mutant mice23,37
. Such downregulation of histone H1 elicited marked changes in chromatin structure, including a global reduction in nucleosome spacing, local reduction in chromatin compaction and changes in the modification of core histones. Despite these changes, microarray analysis of the mutant ES cells revealed that expression of only a few genes was altered23
. These observations suggest that histone H1 participates in the regulation of specific genes that are essential for survival.
In response to genotoxic stress, mammalian cells activate a complex network of proteins, a key element of which is p53. Post-translational modification of p53 results in transcriptional activation of its target genes1,6,7
. Regulation of p53 by checkpoint signalling was originally thought to be mediated almost exclusively at the post-translational level1,6,8
. More recently, p53 has also been found to be regulated at the transcriptional38
levels. We have now uncovered another mode of p53 regulation mediated by CHD8-dependent recruitment of histone H1 to the promoters of its target genes.
Cells proliferate extensively with short G1
phases during early embryogenesis. The DNA replication checkpoint may therefore be readily activated at this time, with the consequent risk of inducing p53-dependent apoptosis. We propose that the biological role of CHD8 is to suppress unwanted apoptosis during early embryogenesis (). Consistent with this notion, we found that the amounts of CHD8 mRNA and protein are higher during the early and mid phases of mouse embryonic development than in newborns. Genetic ablation of CHD8 results in extensive p53-dependent apoptosis in mouse embryos at a stage when Chd8
is expressed at high levels in wild-type mice. On the other hand, apoptosis is necessary for organogenesis, which occurs mainly during mid to late embryogenesis, when the level of Chd8
expression decreases. CHD8 may therefore regulate a threshold for apoptosis induction in a developmental-stage-specific manner: The threshold for apoptosis triggered by p53 is high during early embryogenesis, whereas it is lower after the mid-stage. In cancer cell lines, the level of CHD8 is relatively high, suggesting that cancer cells generally show an undifferentiated phenotype and reflect embryonic stages of development, or that cells expressing CHD8 may have a selective advantage in terms of cell growth or acquisition of immortality as a result of the suppressing p53 function. Thus we suggest that p53 function is, at least in part, suppressed by CHD8 in such cancer cell lines. Furthermore, haploinsufficiency at the CHD8
locus was recently implicated in the pathogenesis of a human developmental anomaly40
, providing further genetic evidence in support of a crucial role for CHD8 in development.