PR-Set7-dependent methylation of histone H4 Lys 20 functions in repression of gene expression and is essential for mitosis

Abstract

The histone methyl transferase PR-Set7 mediates histone H4 Lys 20 methylation, a mark of constitutive and facultative heterochromatin. We isolated a null mutation in Drosophila PR-Set7 that suppresses position effect variegation, indicating that PR-Set7 indeed functions in silencing general gene expression. In PR-Set7 larval leg and eye discs, the number of cells is lower than normal, and the DNA content in these cells is significantly increased. These data show that PR-Set7-dependent methylation is essential for the process of mitosis. The methylation mark is highly stable and is maintained even in the absence of PR-Set7 protein.Supplemental material is available at http://www.genesdev.org.

Catalytic Function of the PR-Set7 Histone H4 Lysine 20 Monomethyltransferase Is Essential for Mitotic Entry and Genomic Stability

“…Following mitosis, PR-Set7 and monomethylated H4K20 levels dramatically decrease coincident with G 1 -associated chromatin decondensation. This paradigm is consistent with recent reports in the unicellular parasite, Apicomplexa, and Drosophila where the cell cycle profiles of H4K20 monomethylation are identical; the lack of PR-Set7 in flies results in abnormally high amounts of DNA and chromatin decondensation (33,46,55). Consistent with our findings, fly neuroblasts lacking PR-Set7 tended to accumulate in prophase and prometaphase and those that entered mitosis displayed lagging chromosomes, strongly suggesting that these cells were deficient for chromatin condensation.…”

“…Regardless, the aberrant centrosome amplification could result in chromosome segregation defects. This is consistent with findings that fly imaginal discs lacking PR-Set7 have a delayed cell cycle and the cells have a significant increase in DNA content, strongly suggesting aneuploidy (33). Therefore, mammalian cells with inappropriate doses of PR-Set7 and monomethylated H4K20 that escape the G 2 arrest and survive may result in daughter cells prone to a cancer phenotype.…”

“…However, we found that the PR-Set7 RNAi cells were capable of completing S phase and, in a p53-independent manner, arrested at prometaphase, when PR-Set7 and monomethylated H4K20 levels peak. Consistent with recent findings indicating that these factors are important for chromatin compaction, the PR-Set7 RNAi cells displayed enlarged nuclei containing decondensed chromosomes (33,34). In addition, we observed significant increases in DNA damage and aberrant centrosome amplification in cells lacking PR-Set7, two key events associated with oncogenic transformation.…”

Histone H4 lysine 20 monomethylation promotes transcriptional repression by L3MBTL1

“…The cell cycle-regulated chromatin association of L3MBTL1 mimics the emergence of the H4K20me1 mark (but not its peak), and the significant decrease in L3MBTL1 levels that precede the onset of mitosis is likely critical for proper cell cycle progression. Notably, the absence of PR-SET7 (and the H4K20me1 mark) also results in defects in cell cycle progression (Karachentsev et al, 2005;Jorgensen et al, 2007;Tardat et al, 2007). The absence of L3MBTL1 in the premitotic fraction 9 (Figure 3b) when H4K20me1 is abundant suggests that additional post-translational modifications of histones, or of L3MBTL1, may target the protein for premitotic destruction.…”

“…Initial reports highlighted the importance of H4K20me1 in 'facultative' heterochromatin and Xinactivation (Fang et al, 2002;Nishioka et al, 2002;Kohlmaier et al, 2004;Karachentsev et al, 2005;Martens et al, 2005;Sims et al, 2006). Recently, this exclusivity has been questioned by studies that suggest a role in transcriptional competence and elongation (Talasz et al, 2005;Vakoc et al, 2006;Barski et al, 2007;Pesavento et al, 2008).…”