Epigenetic mechanisms in bone.

Epigenetics refers to the study of mechanisms able to influence gene expression in a stable and potentially heritable manner without altering the DNA sequence. These mechanisms include posttranslational histone modifications, miRNA-mediated post-transcriptional regulation and DNA methylation. The accumulation of molecular errors over time resulting, at least partly, in the alteration of normal epigenetic patterns is being widely associated with aging. Epigenetic processes are also considered important mechanisms through which environmental and stochastic stressors promote numerous pathologies in humans. It is, therefore, reasonable to expect that several complex multi-factorial late-onset disorders, like osteoporosis and osteoarthritis, could have a strong epigenetic component. The focal point of all skeletal pathologies is the deregulation of bone remodeling, mediated by bone-forming osteoblasts and bone-resorbing osteoclasts. In order to keep both processes in balance, the activity, differentiation and apoptosis of both cell types have to be tightly regulated. In particular, the differentiation of osteoblasts and osteoclasts is accompanied by profound changes in gene expression. It has been shown that histone deacetylation and DNA methylation negatively regulate the expression of several genes associated with different stages of osteoblast differentiation; however, several miRNAs promote osteoblastogenesis. Furthermore, inactivating mutations in the miRNA coding regions could be associated with the pathogenesis of osteoporosis. The aim of this review is to highlight the role of epigenetic mechanisms in bone remodeling and bone homeostasis, so as to implicate their diagnostic and therapeutic potential in skeletal diseases.