The role of SOX9 in chondrocyte proliferation and hypertrophic differentiation in growth plate development.

Sex reversal Y-related high-mobility group box protein (SOX9) is a transcription factor required for proper development of mesenchymal precursors into chondroblasts [90]. Knockout models indicate that SOX9 may prevent inappropriate apoptosis [145, 146]. Several studies have shown that SOX9 acts as a critical regulator of proliferation in chondroblasts [85, 121, 147]. In humans, loss of SOX9 function causes campomelic dysplasia, a disease that results in dwarfism and lack of sexual maturity as well as premature death in some cases [148]. In proliferative chondroblasts, SOX9 promotes COL2 expression [145]. COL2 is the most abundant

protein in growing chondrocytes and is an important component of the ECM. SOX9 also appears to regulate effectors of the IHH and PTHrP pathways, which are responsible for proper chondrocyte development [145]. Other genes regulated by SOX9 alter ECM formation and potentially chondrocyte differentiation. These genes include: aggrecan, COL11, and Cdrap [149-151]. BMPs and noggin, a BMP antagonist, are known to differentially regulate SOX9 expression [152]. In addition, SOX9 regulates chondrocyte differentiation by controlling expression of SOX5 and SOX6 [145]. PRKG2 has been implicated as one kinase that regulates transition to the hypertrophic state by preventing SOX9 nuclear translocation [85]. In rats missing the cyclic guanine monophosphate (cGMP) binding domain of PRKG2, the systematic order of chondrocyte development was lost, presumably due to the inability of SOX9 to function properly [85]. SOX9 is a good example of a protein that needs to be expressed in the right place at the right time. Various mutations in SOX9 also demonstrate the context specific function of genes in biology and the other possible complications that can occur in combination with skeletal disease. Forms of skeletal disease

A. Overview

Skeletal growth and stability is vital to muscle function, protection of internal organs, maintenance of calcium[J14] stores, haematopoiesis in bone marrow, and

self-confidence. Fortunately, advances in modern medicine are beginning to elucidate the molecular pathogenesis of osteogenic diseases. Modern medicine allows parents to supplement their children with growth hormone to offset short stature. Seminal research in bone development was focused largely on nutritional

aspects of bone disease. Rickets, vitamin D deficiency, was the first well- documented bone disease for which a treatment was developed by Steenbock (1924). Advances in the 1950’s-1970’s by Carlsson and DeLuca introduced the importance of Vitamin D in bone growth and stability [153, 154]. These discoveries drastically reduced the number of nutritional bone growth deficiencies

worldwide[J15]. Current research is connecting genetic predispositions to nutritional

bone diseases studied in the past. Advances in transgenics and molecular biology techniques have allowed researchers to better understand early bone development. Exploration of developmental factors in bone growth is now the norm. Future

research will likely combine both disciplines to understand how genetic and environmental interactions effect bone growth[jk16].

B. Genetics of skeletal disease

Genetic causes of bone disease have been difficult to deduce, because of complex inheritance or challenging phenotypic measurements. However, transgenic mouse lines have been developed to investigate the deletion of single genes. There is a vast array of skeletal diseases which suggest that substantial dissection of osteogenic developmental pathways will be required to find treatments to alleviate complications associated with skeletal diseases. The most common form of dwarfism, achondroplasia, is estimated to affect between 1 in 20,000 and 1 in 40,000 children born. Unfortunately, few types of dwarfism have been characterized at the genetic level. Complications associated with dwarfism cause life-long medical problems with varying levels of severity. Complications range from hearing loss

[155] and obesity [156] to neurological disorders and shortened life expectancy [157].

Bone diseases come in many different forms. The most common form of skeletal disease is osteoporosis. Osteoporosis is the loss of mineralization within the bone that leads to weakened bones that are more susceptible to breaks or fractures. It is caused by the inability of bone to replace mineral in the bone during the remodeling process. Allelic variation in the LRP5 gene has been associated with high and low bone mass [158, 159]. Insufficient calcium is most often the cause for this disease. A similar disease, osteomalacia, occurs if there is insufficient calcium and phosphate for mineralization of newly formed bone in the osteoid. The symptomology of rickets is actually very similar. However, rickets tends to occur in immature bone whereas osteomalacia can occur at any time during life. Both disorders are caused by vitamin D deficiency, often as a result of insufficient exposure to sunlight; however, occasionally kidney and liver disorders can cause these diseases. Mutations within the DMP1 gene have been discovered that greatly alter an individuals susceptibility to rickets and osteomalacia [160]. Paget’s disease[J17] is a bone remodeling disorder in which too much bone mineral turnover

occurs, because osteoclasts become overactive. Complications can include increased bone fracture and increased rate of osteosarcoma. Osteoarthritis afflicts millions of people world wide, causing decreased mobility and quality of life. A number of genetic risk factors have been identified, including mutations in COL2 [161]. Osteosarcoma, bone cancer, is perhaps one of the most difficult types of skeletal disease and cancers to treat. There are many causes of osteosarcoma,

both genetic and environmental, making it a particularly difficult disease to treat and one worthy of considerable research.