Recycling of parental histones preserves the epigenetic landscape during embryonic development

Safeguarding the epigenome through the cell cycle: a multitasking game

Sustaining cell identity and function across cell division is germane to human development, healthspan, and cancer avoidance. This relies significantly on propagation of chromatin organization between cell generations, as chromatin presents a barrier to cell fate and cell state conversions. Inheritance of chromatin states across the many cell divisions required for development and tissue homeostasis represents a major challenge, especially because chromatin is disrupted to allow passage of the DNA replication fork to synthesize the two daughter strands. This process also leads to a twofold dilution of epigenetic information in histones, which needs to be accurately restored for faithful propagation of chromatin states across cell divisions. Recent research has identified distinct multilayered mechanisms acting to propagate epigenetic information to daughter strands. Here, we summarize key principles of how epigenetic information in parental histones is transferred across DNA replication and how new histones robustly acquire the same information postreplication, representing a core component of epigenetic cell memory.

Reduced histone gene copy number disrupts Drosophila Polycomb function

The chromatin of animal cells contains two types of histones: canonical histones that are expressed during S phase of the cell cycle to package the newly replicated genome, and variant histones with specialized functions that are expressed throughout the cell cycle and in non-proliferating cells. Determining whether and how canonical and variant histones cooperate to regulate genome function is integral to understanding how chromatin-based processes affect normal and pathological development. Here, we demonstrate that variant histone H3.3 is essential for Drosophila development only when canonical histone gene copy number is reduced, suggesting that coordination between canonical H3.2 and variant H3.3 expression is necessary to provide sufficient H3 protein for normal genome function. To identify genes that depend upon, or are involved in, this coordinate regulation we screened for heterozygous chromosome 3 deficiencies that impair development of flies bearing reduced H3.2 and H3.3 gene copy number. We identified two regions of chromosome 3 that conferred this phenotype, one of which contains the Polycomb gene, which is necessary for establishing domains of facultative chromatin that repress master regulator genes during development. We further found that reduction in Polycomb dosage decreases viability of animals with no H3.3 gene copies. Moreover, heterozygous Polycomb mutations result in de-repression of the Polycomb target gene Ubx and cause ectopic sex combs when either canonical or variant H3 gene copy number is reduced. We conclude that Polycomb-mediated facultative heterochromatin function is compromised when canonical and variant H3 gene copy number falls below a critical threshold.

Reduced histone gene copy number disrupts Drosophila Polycomb function

The chromatin of animal cells contains two types of histones: canonical histones that are expressed during S phase of the cell cycle to package the newly replicated genome, and variant histones with specialized functions that are expressed throughout the cell cycle and in non-proliferating cells. Determining whether and how canonical and variant histones cooperate to regulate genome function is integral to understanding how chromatin-based processes affect normal and pathological development. Here, we demonstrate that variant histone H3.3 is essential for Drosophila development only when canonical histone gene copy number is reduced, suggesting that coordination between canonical H3.2 and variant H3.3 expression is necessary to provide sufficient H3 protein for normal genome function. To identify genes that depend upon, or are involved in, this coordinate regulation we screened for heterozygous chromosome 3 deficiencies that impair development of flies bearing reduced H3.2 and H3.3 gene copy number. We identified two regions of chromosome 3 that conferred this phenotype, one of which contains the Polycomb gene, which is necessary for establishing domains of facultative chromatin that repress master regulator genes during development. We further found that reduction in Polycomb dosage decreases viability of animals with no H3.3 gene copies. Moreover, heterozygous Polycomb mutations result in de-repression of the Polycomb target gene Ubx and cause ectopic sex combs when either canonical or variant H3 gene copy number is also reduced. We conclude that Polycomb-mediated facultative heterochromatin function is compromised when canonical and variant H3 gene copy number falls below a critical threshold.