image: Mechanistic model for G9a-H3K9me2 in preserving centromere integrity. G9a-mediated H3K9me2 at centromeres prevents the invasion of H3K9me3 into core centromeric domains, which ensures proper centromeric transcription, R-loop formation, and RPA loading, eventually facilitating the activation of the ATR-Chk1-Aurora B cascade and faithful chromosome segregation.
Credit: ©Science China Press
The maintenance of centromere integrity is essential for proper chromosome segregation. Human centromeric chromatin is organized with a central region marked by nucleosomes containing Centromere protein A (CENP-A), a histone H3 variant, flanked by compact pericentromeric regions on both sides. Deposition of di- and tri-methylated histone H3 on lysine 9 (H3K9me2 and H3K9me3, respectively) at pericentromeric heterochromatin is vital for maintaining proper centromere structure and function. In contrast to the established function of the SUV39H-mediated H3K9me3 in creating repressive heterochromatin, the role of G9a-mediated H3K9me2 at (peri)centromeres remains poorly understood.
Recently, a joint study from Wuhan University and the Children’s Hospital of Chongqing Medical University investigated the role of G9a-mediated H3K9me2 in maintaining centromere integrity and function. The researchers found that G9a/GLP, the methyltransferase complex for H3K9me2, localizes to centromeres to promote local H3K9 di-methylation. They demonstrated that this methylation is important for proper chromosome segregation. Consequently, G9a knockdown prolongs mitotic duration and increases errors in chromosome segregation. It turns out that G9a-H3K9me2 acts through facilitating the activation of centromeric Aurora B, a key kinase for correcting erroneous kinetochore-microtubule attachments.
Furthermore, the study revealed that G9a-mediated H3K9me2 helps to contain the pericentromeric repressive mark H3K9me3, preventing its encroachment or mislocalization into core centromeric domains. When G9a is depleted, H3K9me3 invades core centromeres, diminishing the local recruitment of RNA Polymerase II, which compromises local transcription and R-loop formation. Additionally, the absence of G9a leads to reduced or misincorporated CENP-A, a key epigenetic marker of centromeres. Based on these observations, the authors concluded that G9a-H3K9me2 is important for maintaining appropriate transcription and the chromatin landscape at centromeres.
Mechanistically, they found that G9a depletion hinders the loading of RPA, which binds the single-strand DNA in R-loops, at centromeres. Reduced RPA further compromises the activation of the ATR-Chk1-Aurora B signaling cascade, ultimately leading to chromosome missegregations and genome instability. The study also links G9a somatic mutations to improper chromosome segregation, offering insights into how these clinical mutations may drive tumorigenesis.
In summary, this research highlights G9a-mediated H3K9me2 as a novel mechanism for preserving centromere integrity and ensuring faithful chromosome segregation. It extends our understanding of its role in maintaining genome stability. It also sheds light on the potential therapeutic prospects of G9a and its inhibitors in clinical settings.
Journal
Science Bulletin