Cindy Boer

Epigenomics in bone and cartilage disease | 39 1.2 cells (MSCs). Pluripotent MSCs can also differentiate into other cell types, such as ad- ipocytes and myocytes. The differentiation of MSCs toward the osteoblastic lineage is induced by the master transcription factors RUNX2 and osterix and is stimulated by ligands of the Wnt and BMP pathways[31]. As it happens in other tissues, epigenetic factors play critical roles in determining the fate of MSCs. Specifically, genes that are characteristic of the osteoblast‐osteocyte lineage (such as alkaline phosphatase, scle- rostin, RANKL, osteoprotegerin , etc.) tend to undergo demethylation and de‐repression during the differentiation of MSCs[32-34]. In line with this concept, the demethylating agents 5‐azacytidine and 5‐deoxy‐azacytidine improve the osteogenic differentiation of MSCs[35, 36]. The ability of MSCs to proliferate and differentiate may decrease with aging, a phenomenon that may be explained, at least partially, by changes in the methyl- ation and hydroxymethylation of DNA[37-39]. The differentiation of osteoclast precursors is associated with marked changes in their DNA methylation signature, with hypermethylation and hypomethylation oc- curring in a variety of gene categories. In this process, PU.1 may play an important role, by recruiting DNMT3B to hypermethylated promoters, and TET2, which converts 5‐ methylcytosine to 5‐hydroxymethylcytosine, to genes that become demethylated[40]. Another DNA methyltransferase, DNMT3A, is essential to methylate and repress anti‐ osteoclastogenic genes, thus allowing osteoclast differentiation to continue[41]. On the other hand, by influencing the expression of RANKL and OPG in cells of the osteoblast‐ osteocyte lineage, DNA methylation indirectly contributes to regulating osteoclasto- genesis[33]. Although cartilage does not undergo a remodeling process as bone does, epigenetic mechanisms also contribute to regulation of chondrogenesis and cartilage maintenance. DNA methylation modulates the expression of genes involved in cartilage homeostasis, such as GDF5, SOX9 , and MMP13 [42]. A number of studies have explored the relation between genome-wide methyl- ation patterns and skeletal disease status in humans. These so‐called epigenome-wide association studies (EWAS) have a hypothesis‐free approach that have the potential to find novel genes and/or pathways involved in skeletal disease. Because epigenetic marks are tissue specific, it makes sense to perform these studies in the tissue of inter- est. For osteoporosis, this would be bone, but for osteoarthritis, the tissue of interest is less straightforward because there are many tissues involved, including cartilage, bone, and synovial tissue. In addition, epigenetic marks in the circulation (blood) could be indicative of systemic mechanisms playing a role in disease and could also be easily accessible biomarkers for the disease. An overview of published studies in the field of osteoarthritis and osteoporosis is given in Table 1.