Background

Post-translational modifications (PTMs) of histone proteins are key mechanisms regulating gene expression. Specifically, lysine residues on histone H3 undergo modifications such as methylation and acetylation, and their combinations influence cell fate. However, elucidating the function of individual lysine residues in mammalian cells has been challenging. The mammalian genome contains multiple copies of the histone H3 gene, making it difficult to selectively manipulate specific residues using conventional gene editing or enzyme inhibitors. Base editing also has limitations in the types of amino acids that can be substituted, leaving the systematic decoding of the histone code as a difficult task.

Key Findings

In this study published in Nature Genetics, Daniel Price, Grigory Zemlyanskiy, and colleagues developed a CRISPR prime editing platform optimized for large-scale histone substitution. The core of the system is a TPMB cell line, created by introducing a doxycycline-inducible PEmax-MLH1dn system into mouse embryonic stem cells (mESCs) to simultaneously control mismatch repair inhibition and prime editing.

Using this system, they substituted each of the 13 lysine residues of H3 with arginine (K→R) and evaluated the residue-specific essentiality. The results showed a stark difference in cellular response. Five residues – H3K4, H3K9, H3K14, H3K18, and H3K79 – exhibited strong negative selection. H3K4R achieved only 2 out of 36 clones with 11-12% editing efficiency, and H3K18R did not exceed 50%. In contrast, H3K23, H3K36, H3K37, H3K64, H3K115, and H3K122 showed 100% editing efficiency in some clones, indicating that they are not essential for cell proliferation.

Combinatorial substitution experiments are also noteworthy. The simultaneous introduction of H3K27R and H3K36R, which had minimal individual effects, significantly impaired the colony-forming ability of stem cells. RNA sequencing revealed that this double mutant exhibited an epistatic pattern, with decreased expression of pluripotency factors (Klf4, Chd4, etc.) and increased expression of differentiation-inducing factors (Fgfr2, Dkk, etc.). Another finding is the histone variant compensation mechanism. When K18 is substituted in canonical H3, cells upregulate H3.3 expression to compensate, but this compensation pathway is blocked when H3.3K18R is co-introduced.

Significance and Prospects

This study is the first to systematically map the function of histone H3 lysine residues in mammalian cells. It directly confirmed that the essentiality of H3K4 and H3K9, previously known in yeast and Drosophila, is conserved in mammals, and it revealed a species-specific difference in H3K18, which allows alanine substitution in Drosophila but is essential in mammals. Analysis of approximately 12,000 tumor samples revealed that a significant number of essential residues (H3K4, H3K14, H3K18, H3K27, H3K36) harbor cancer driver mutations, supporting a direct link between epigenetic dysregulation and tumorigenesis.

Meanwhile, a loss of copy number in histone gene cluster 1 was observed in 4-34% of clones during the prime editing process, and some clones showed deletions of approximately 20-35 kb. The researchers suggest that non-disrupted clones can be selected through appropriate screening, but continuous monitoring is necessary in high-throughput experiments. This platform is expandable to other histone variants, ubiquitin, rRNA, and other repetitive sequence elements, and is expected to become a versatile tool for epigenetic research in mammals.