Vitamin K-Dependent Proteins in Skeletal Development and Disease

Vitamin K and Vitamin K-dependent proteins (VKDPs) are best known for their pivotal role in blood coagulation. Of the 14 VKPDs identified in humans to date, 6 play also important roles in skeletal biology and disease. Thus, osteocalcin, also termed bone Gla-protein, is the most abundant non-collagenous protein in bone. Matrix Gla protein and Ucma/GRP on the other hand are highly abundant in cartilage. Furthermore, periostin, protein S, and growth arrest specific 6 protein (GAS 6) are expressed in skeletal tissues. The roles for these VKDPs are diverse but include the control of calcification and turnover of bone and cartilage. Vitamin K plays an important role in osteoporosis and serum osteocalcin levels are recognized as a promising marker for osteoporosis. On the other hand, matrix Gla protein and Ucma/GRP are associated with osteoarthritis. This review focuses on the roles of these three VKDPs, osteocalcin, matrix Gla protein and Ucma/GRP, in skeletal development and disease but will also summarize the roles the other skeletal VKDPs (periostin, protein S and GAS6) in skeletal biology.

DEPTOR in Skeletal Development and Diseases

Discovered in 2009, the DEP-domain containing mTOR-interacting protein, DEPTOR, is a known regulator of the mechanistic target of rapamycin (mTOR), an evolutionarily conserved kinase that regulates diverse cellular processes in response to environmental stimuli. DEPTOR was originally identified as a negative regulator of mTOR complexes 1 (mTORC1) and 2 (mTORC2). However, recent discoveries have started to unravel the roles of DEPTOR in mTOR-independent responses. In the past few years, mTOR emerged as an important regulator of skeletal development, growth, and homeostasis; the dysregulation of its activity contributes to the development of several skeletal diseases, both chronic and genetic. Even more recently, several groups have reported on the relevance of DEPTOR in skeletal biology through its action on mTOR-dependent and mTOR-independent pathways. In this review, we summarize the current understanding of DEPTOR in skeletal development and disease.

Specificity for the Epiphyseal Round Cell Layer is Significantly Associated With Height GWAS​

Human height is a model polygenic trait with thousands of height-related SNPs identified in GWAS to date. An important determinant of height is the proliferation and hypertrophy of growth plate chondrocytes during childhood long bone elongation. Connecting the expression of specific genes that affect skeletal biology to associated variants in GWAS remains a difficult challenge. To connect the genetics of height and growth plate gene expression, we studied the relationship between gene expression in the murine growth plate and common-variant associations from GWAS of height. To obtain gene expression data from the growth plate, we dissected three layers of murine tibial growth plates, extracted RNA from each layer, and measured expression using the Affymetrix GeneChip 430 3.0. For each gene, we derived a specificity score for each growth plate layer, and SNP-level p-values from a published GWAS of height (N~700000) were combined into gene-level p-values using MAGMA. We then used MAGMA to test for association between specificity of expression for each growth plate layer and the GWAS gene level p-values for height. We found that specificity for the round cell layer is significantly associated with height GWAS p-values (p = 8.5x10-9). This association remains when we condition on each of the other cell layers and on membership in a set of genes from OMIM that cause skeletal growth disorders (3.3x10-8 < p < 4.1x10-6). We replicated this result in a RNA-seq dataset of maturing chondrocytes sampled at three time points during development in vitro (days 3, 5, and 10): we found that z-scores for expression in the earliest two days of development are significantly associated with gene-level p-values from height GWAS (pDay3 = 1.2x10-21 and pDay5 = 2.0x10-20) and that this association remains after conditioning on the other timepoints and on the OMIM gene set (3.1x10-20 < pDay3 < 8.3x10-5; 3.7x10-19 < pDay5 < 0.002). We then performed pathway analysis of genes that are both highly specific to the round layer and highly significant in GWAS using Enrichr. Together, our results suggest that genes expressed in early chondrocyte development (the round cell layer) are particularly relevant to the contribution of growth plate-expressed genes to height. This conclusion both sheds light on the regulation of human skeletal growth and also helps prioritize relevant genes implicated from the height GWAS in skeletal biology.