Mice without macroH2A histone variants.

MacroH2A core histone variants have a unique structure that includes a C-terminal nonhistone domain. They are highly conserved in vertebrates and are thought to regulate gene expression. However, the nature of genes regulated by macroH2As and their biological significance remain unclear.

Genome-wide distribution of macroH2A1 histone variants in mouse liver chromatin.

Studies of macroH2A histone variants indicate that they have a role in regulating gene expression. To identify direct targets of the macroH2A1 variants, we produced a genome-wide map of the distribution of macroH2A1 nucleosomes in mouse liver chromatin using high-throughput DNA sequencing. Although macroH2A1 nucleosomes are widely distributed across the genome, their local concentration varies over a range of 100-fold or more.

The Sgp3 locus derived from the 129 strain is responsible for enhanced endogenous retroviral expression in macroH2A1-deficient mice.

The endogenous retroviral envelope glycoprotein, gp70, implicated in murine lupus nephritis is secreted by hepatocytes, and its expression is largely regulated by the Sgp3 (serum gp70 production 3) locus derived from lupus-prone mice. Because of the localization of the macroH2A1 gene encoding macroH2A histone variants within the Sgp3 interval and of an up-regulated transcription of endogenous retroviral sequences in macroH2A1-deficient C57BL/6 (B6) mice, we investigated whether macroH2A1 is a candidate gene for Sgp3. macroH2A1-deficient B6 mice carrying the 129-derived Sgp3 locus, which was co-transferred with the 129 macroH2A1 mutant gene, displayed increased levels of serum gp70 and hepatic retroviral gp70 RNAs comparable to those of B6.

A phosphorylated subpopulation of the histone variant macroH2A1 is excluded from the inactive X chromosome and enriched during mitosis.

Histone variants play an important role in numerous biological processes through changes in nucleosome structure and stability and possibly through mechanisms influenced by posttranslational modifications unique to a histone variant. The family of histone H2A variants includes members such as H2A.Z, the DNA damage-associated H2A.

macroH2A1-dependent silencing of endogenous murine leukemia viruses.

We show that macroH2A1 histone variants are important for repressing the expression of endogenous murine leukemia viruses (MLVs) in mouse liver. Intact MLV proviruses and proviruses with deletions in env were nearly silent in normal mouse liver and showed substantial derepression in macroH2A1 knockout liver. In contrast, MLV proviruses with a deletion in the 5' end of pro-pol were expressed in normal liver and showed relatively low levels of derepression in knockout liver.

Developmental changes in histone macroH2A1-mediated gene regulation.

macroH2A histone variants have been implicated to function in gene silencing by several studies, including ones showing a preferential association of macroH2A on the inactive X chromosome. To examine macroH2A function in vivo, we knocked out macroH2A1. macroH2A1 knockout mice are viable and fertile.

macroH2A1 histone variants are depleted on active genes but concentrated on the inactive X chromosome.

Using a novel thiol affinity chromatography approach to purify macroH2A1-containing chromatin fragments, we examined the distribution of macroH2A1 histone variants in mouse liver chromatin. We found that macroH2A1 was depleted on the transcribed regions of active genes. This depletion was observed on all of the 20 active genes that we probed, with only one site showing a small amount of enrichment.

Reconstitution of nucleosomes with histone macroH2A1.2.

MacroH2A histones have an unusual hybrid structure, consisting of an N-terminal domain that is approximately 65% identical to a full-length histone H2A and a large C-terminal nonhistone domain. To develop an in vitro approach for investigating the effects of macroH2A proteins on chromatin structure and function, we reconstituted nucleosomes with recombinant macroH2A1.2, substituting for conventional H2A.