Abstract

Epigenetic mechanisms, processes that alter gene expression without changes in DNA sequence, precisely regulate gene expression by dynamic remodeling of chromatin. Eukaryotic DNA is wrapped around a histone octamer (H3/H4 heterotetramer and two H2A/H2B dimers) to form the nucleosome, the fundamental building block of chromatin. Histone proteins are subject to covalent modifications that modulate the local structure of chromatin and influence gene expression by regulating the accessibility of gene loci to transcriptional machinery. The dynamic remodeling of chromatin through histone modifications by nuclear proteins including histone acetyltransferases, deacetylases, and methyltransferases, as well as by demethylases, modulate spatiotemporal gene expression during cartilage development, homeostasis and disease.

Disruptor of telomeric silencing 1-like (DOT1L) is the main histone-modifying enzyme that catalyzes the methylation on lysine-79 of histone 3 (H3K79). The human protein DOT1L contains 1537 aminoacids, with its N-terminal part responsible for the methyltransferase activity. The remaining C-terminal part is involved in physical interactions with many transcription relevant proteins. The general function of DOT1L is to methylate H3K79 as a member of a large protein complex, which can influence the transcriptional state. DOT1L's histone methyltransferase activity has been linked to active transcription, and plays a role in many biological processes including the DNA damage response, the cell cycle, embryonic development and cell reprogramming. In cancer biology, DOT1L is involved in MLL-rearranged leukemia and in other tumors. We reported a strong genetic association between polymorphisms in the human DOT1L gene and hip cartilage thickness as well as osteoarthritis (OA). Variations in the DOT1L gene are also associated with human height. Furthermore, we earlier demonstrated that silencing of Dot1L inhibited chondrogenic differentiation of murine progenitor cells. Interestingly, DOT1L-associated H3K79 methylation has been linked to the Wingless-type (Wnt) cascade. Gain and loss of function studies for Wnt signaling mediator beta-catenin or Wnt antagonists such as Frizzled related protein have demonstrated that tight regulation of this pathway is key to joint health. This could specifically identify DOT1L as an attractive target for joint diseases, more particularly for OA. DOT1L is an enzyme and, thus, can be pharmacologically modulated.

We therefore hypothesized that DOT1L plays a major role in cartilage homeostasis by tightly regulating gene expression patterns of key signaling pathways, such as the Wnt cascade. New insights into the transcriptional mechanisms may contribute to the development of new epigenetics-based strategies for the treatment of joint diseases, adding a new way to modulate fundamental signaling pathways in joint disease and repair.