If parent cells and their daughter cells are to share a stable identity, parent cells must divide—and replicate their DNA—while ensuring that their histones are distributed properly to their daughter cells. But how, exactly, are the histones passed along? The question is of interest to researchers because cells of any particular lineage need to maintain a stable identity from generation to generation if they are to resist the changes associated with aging and diseases like cancer.
In a recent study, the question of histone inheritance was addressed by scientists based at the University of Copenhagen. These scientists, led by Professors Anja Groth and Geneviève Thon, discovered that, during replication, the fission yeast Mrc1 (CLASPIN in humans) is crucial to the symmetric inheritance of parental histones.
Mrc1 is part of a group of proteins called the fork protection complex (FPC), which is part of the replisome, a molecular machine that coordinates the replication of DNA, a process that includes the unwinding of double-stranded DNA into two single strands, as well as the synthesis of complementary DNA for each of the single strands. Both of these strands—the leading and the lagging strands—are allocated parental histones.
According to Groth, Thon, and colleagues, Mrc1 toggles histones between the lagging and leading strand recycling pathways to ensure epigenetic transmission to both daughter cells. Mrc1 does this by binding histones alone and with another protein called Mcm2.
These findings were presented by the scientists in the journal Cell, in an article titled, “The fork protection complex promotes parental histone recycling and epigenetic memory.”
“Mrc1 binds H3-H4 tetramers and operates as a central coordinator of symmetric parental histone inheritance,” the article’s authors wrote. “Mrc1 mutants in a key connector domain disrupted segregation of parental histones to the lagging strand comparable to Mcm2 histone-binding mutants. Both mutants showed clonal and asymmetric loss of H3K9me-mediated gene silencing.
“AlphaFold predicted co-chaperoning of H3-H4 tetramers by Mrc1 and Mcm2, with the Mrc1 connector domain bridging histone and Mcm2 binding. Biochemical and functional analysis validated this model and revealed a duality in Mrc1 function: disabling histone binding in the connector domain disrupted lagging-strand recycling while another histone-binding mutation impaired leading strand recycling.”
Further experimental analyses revealed that in fact Mrc1 juggles histones in multiple ways: the connector domain directs histones to the lagging strand as described above, while another histone-binding region is required for recycling histones onto the leading strand.
“Previously a role of Mrc1 in transmission of parental histones was not known,” said Sebastian Charlton, PhD,who participated in the study as a researcher in the Thon laboratory and is currently affiliated with the Novo Nordisk Foundation Center for Protein Research (CPR). “Now we have shown that Mrc1 is required for efficient histone recycling to both daughter strands. It appears that Mrc1 toggles histones between both the lagging and leading DNA strands, in part by intra-replisome co-chaperoning with Mcm2. This ensures both daughter cells inherit the correct epigenetic marks, which is essential for preserving gene expression patterns during cell division.”
Charlton is co-lead author of the current study, along with Valentin Flury, PhD, who is also affiliated with the CPR. “I don’t think we can estimate the full potential of our discovery yet,” Flury remarked, “but we have revealed a very fundamental mechanism that maintains cell identity which, if it can be manipulated, could have significant implications for future medical research.”
Maintaining chromatin landscapes across many cell generations is vital for developing and maintaining the multiple cell types in multicellular organisms. Losing cellular identity is an underlying cause of many diseases such as cancer, and of aging where evidence suggests that the chromatin landscape deteriorates over time. Flury noted that the research team had a “Eureka moment” when they mutated the histone binding sites in the mammalian homolog Claspin and observed a defect in parental histone transmission as with Mrc1 mutants in fission yeast cells.
