Binding of histone H1e-c variants to CpG-rich DNA correlates with the inhibitory effect on enzymic DNA methylation

Abstract

Within the H1 histone family, only some fractions enriched in the H1e-c variants are effective in causing a marked inhibition, in vitro, of enzymic DNA methylation and, in gel retardation and Southwestern blot experiments, in binding double-stranded (ds) CpG-rich oligonucleotides. Both the 6-CpG ds-oligonucleotide and the DNA purified from chromatin fractions enriched in 'CpG islands' are good competitors for the binding of H1e-c to 6-meCpG ds-oligonucleotide. Because of their ability to bind any DNA sequence and to suppress the enzymic methylation in any sequence containing CpG dinucleotides, these particular H1 variants could play some role in maintaining linker DNA at low methylation levels and even in preserving the unmethylated state of the CpG-rich islands which characterize the promoter regions of housekeeping genes.

The Unmethylated State of CpG Islands in Mouse Fibroblasts Depends on the Poly(ADP-ribosyl)ation Process

“…A possible mechanism is that histone H1 in its covalently modified isoform could position itself on the CpG islands, for which it shows a greater affinity (6), and, when there, attract the long and branched polymers that inhibit methylation of doublestranded DNA (6), thus preventing DNA methyltransferase from having access to these DNA regions. Histone H1 could participate in this role through its genetic variant H1e, which is (i) the only one involved in inhibiting the in vitro DNA methylation process (4,5); (ii) the only one capable of binding CpG-rich DNA regions (4,5); (iii) the only one involved in chromatin condensation and, in its poly(ADP-ribosyl)ated isoform, chromatin decondensation (48). Using methyl-accepting ability assays, we have been able to demonstrate that the poly(ADP-ribosyl)ation of the histone H1e variant facilitates chromatin decondensation, even though this modification does not remove histone H1 from chromatin (48).…”

“…Our previous in vitro experiments, carried out with the aim of individuating chromatin proteins involved in determining and/or maintaining in some way the DNA methylation pattern, have shown that histone H1 (2, 3), through its variant H1e (4,5), is a chromatin protein that is able to greatly inhibit (Ͼ90%) methylation of double-stranded DNA. Moreover, gel retardation experiments have emphasized that H1e is the only variant able to bind CpG-rich sequences, both unmethylated CpG-rich double-stranded oligonucleotides and double-stranded DNA purified from chromatin fractions enriched in CpG islands.…”

The Importance of Aberrant DNA Methylation in Cancer

“…In contrast to "typical" loosly-bound histone H1, tightly-bound histone H1 has been shown to facilitate methylation of linker DNA (Santoro et al, 1993). The histone H1, which is able to bind CpG-rich DNA sequences and inhibit double-stranded DNA methylation, has later been identified as variant H1e (Santoro et al, 1995) that promotes chromatin condensation or, upon poly(ADP-ribosyl)ation (pARylation), chromatin decondensation (D'Erme et al, 1996). Appart from the importance of this histone variant and its pARylation in chromatin decondensation, which allows recruitment of transcription factors, the same group has demonstrated the mandatory role of pARylation in www.intechopen.com the maintenance of hypomethylated state of CpG islands in mouse fibroblasts (Zardo & Caifa, 1998).…”

Eukaryotic DNA methyltransferases – structure and function

“…Once the newly synthesised DNA has been packaged into nucleosomes, a roving methyltransferase could complete the methylation working in a distributive manner. Except for a period following replication, access to the DNA will be limited by nucleosomes and particularly by histone H1 (21). Thus transcriptionally inactive chromatin will almost certainly be completely inaccessible once the 30 nm solenoidal structure reforms.…”

“…Paradoxically, high levels of methyltransferase are present when little de novo methylation is occurring and the enzymic activity is much reduced at the time de novo methyl groups (35). This may simply be the result of afailure to methylate DNA following replication, either because the methyltransferase is not located in the replication foci or as a result of the action of a particular histone H1 variant that has been shown, in vitro, to associate with CG-rich DNA and to inhibit its methylation (21). Alternatively, demethylation may be an active process involving the removal of methylcytosine from the DNA by a repair mechanism replacing the methylcytosine with cytosine.…”