Cells May Rest, but Not Histones

    Even between rounds of cell division, during the so-called resting state, a cell stays busy, unpacking and repacking portions of the genome, accumulating epigenetic marks, and turning specific genes on and off. So, it stands to reason that all this activity would degrade histone proteins, the genome’s packing material, and that histone proteins would be in need of constant replacement. Yet histone replacement is poorly understood, at least in the “resting” cell, where the day-to-day routine unfolds between bursts of DNA replication.

    To hear the hum of histone turnover without the clamor of replication, scientists at the Babraham Institute and MRC Clinical Sciences Centre listened in on developing mouse egg cells, oocytes. Developing oocytes provide a system in which the packing and unpacking of DNA is relatively easy to study. Oocytes do not divide, and so there is no DNA replication. Also, in oocytes, genomes are highly active. As they mature and ready themselves for fertilization, oocytes turn genes on and off throughout the genome, which simultaneously undergoes epigenetic modification.

    The scientists found that by deleting the gene for a histone replacement protein, they could dampen the hum of histone turnover. Moreover, using single-cell analysis, the scientists evaluated how interfering with histone turnover affected egg cell development, DNA integrity, and the accumulation of DNA methylation.

    The results of this work appeared November 5 in the journal Molecular Cell, in an article entitled, “Continuous Histone Replacement by Hira Is Essential for Normal Transcriptional Regulation and De Novo DNA Methylation during Mouse Oogenesis.” This article describes how the scientists deleted the H3.3 chaperone Hira in developing mouse oocytes, and how they assessed the importance of continuous H3.3/H4 deposition in sustaining chromatin dynamics.

    Essentially, the scientists found that disturbing H3.3/H4 deposition alters chromatin structure, resulting in increased DNase I sensitivity, the accumulation of DNA damage, and a severe fertility phenotype.

    “On the molecular level, abnormal chromatin structure leads to a dramatic decrease in the dynamic range of gene expression, the appearance of spurious transcripts, and inefficient de novo DNA methylation,” wrote the authors. “Our study thus unequivocally shows the importance of continuous histone replacement and chromatin homeostasis for transcriptional regulation and normal developmental progression in a non-replicative system in vivo.”

    “Oocytes lacking the Hira histone chaperone showed severe developmental defects which often led to cell death,” said Gavin Kelsey, Ph.D., one of the authors of the Molecular Cell paper and a research group leader in the Babraham Institute’s epigenetics program. “The whole system is disrupted, eggs accumulate DNA damage and the altered chromatin means that genes cannot be efficiently silenced or activated. But we also uncovered an intricate relationship between the different epigenetic systems operating in the oocyte, where failure to ensure normal histone levels severely compromised deposition of methylation on the underlying DNA.”

    The research addresses the importance of histone turnover in maintaining genomic fidelity and adds to our understanding about the mechanisms in place to protect the integrity of the genome as it is remodeled and reshaped. Studying this in the context of the developing oocytes provides new insights into our dynamic genome, unclouded by the complications of DNA replication, and also reveals how important maintaining chromatin dynamics is to the integrity of our gametes.