Epigenetic Mechanisms Mediating Cell State Transitions in Chondrocytes

Inhibition of KDM6B prevents osteoarthritis by blocking growth plate-like H3K27me3 loss in bivalent genes

Osteoarthritis (OA) is the most prevalent joint disorder occurring with articular cartilage degradation. It includes a switch from an articular to a growth plate chondrocyte phenotype. Here, we investigated the histone modification profiles and found significant H3K27me3 loss in OA, which led to disease-associated gene expression. Surprisingly, these genes were occupied by both H3K27me3 and H3K4me3 in normal chondrocytes, showing a poised bivalent state. Furthermore, we observed the derepression of similar bivalent genes in growth plate chondrocytes. Finally, a KDM6B inhibitor GSK-J4 prevented the H3K27me3 loss and cartilage damage in the rat OA model. Our results reveal an inherited bivalent epigenetic signature on developmental genes that makes articular chondrocytes prone to hypertrophy and contributes to a promising epigenetic therapy for OA.

Growth disorders caused by variants in epigenetic regulators: progress and prospects

Epigenetic modifications play an important role in regulation of transcription and gene expression. The molecular machinery governing epigenetic modifications, also known as epigenetic regulators, include non-coding RNA, chromatin remodelers, and enzymes or proteins responsible for binding, reading, writing and erasing DNA and histone modifications. Recent advancement in human genetics and high throughput sequencing technology have allowed the identification of causative variants, many of which are epigenetic regulators, for a wide variety of childhood growth disorders that include skeletal dysplasias, idiopathic short stature, and generalized overgrowth syndromes. In this review, we highlight the connection between epigenetic modifications, genetic variants in epigenetic regulators and childhood growth disorders being established over the past decade, discuss their insights into skeletal biology, and the potential of epidrugs as a new type of therapeutic intervention.

Mechanical memory stored through epigenetic remodeling reduces cell therapeutic potential

Understanding how cells remember previous mechanical environments to influence their fate, or mechanical memory, informs the design of biomaterials and therapies in medicine. Current regeneration therapies, such as cartilage regeneration procedures, require 2D cell expansion processes to achieve large cell populations critical for the repair of damaged tissues. However, the limit of mechanical priming for cartilage regeneration procedures before inducing long-term mechanical memory following expansion processes is unknown, and mechanisms defining how physical environments influence the therapeutic potential of cells remain poorly understood. Here, we identify a threshold to mechanical priming separating reversible and irreversible effects of mechanical memory. After 16 population doublings in 2D culture, expression levels of tissue-identifying genes in primary cartilage cells (chondrocytes) are not recovered when transferred to 3D hydrogels, while expression levels of these genes were recovered for cells only expanded for eight population doublings. Additionally, we show that the loss and recovery of the chondrocyte phenotype correlates with a change in chromatin architecture, as shown by structural remodeling of the trimethylation of H3K9. Efforts to disrupt the chromatin architecture by suppressing or increasing levels of H3K9me3 reveal that only with increased levels of H3K9me3 did the chromatin architecture of the native chondrocyte phenotype partially return, along with increased levels of chondrogenic gene expression. These results further support the connection between the chondrocyte phenotype and chromatin architecture, and also reveal the therapeutic potential of inhibitors of epigenetic modifiers as disruptors of mechanical memory when large numbers of phenotypically suitable cells are required for regeneration procedures.