Sclerostin

Structure

The sclerostin protein, with a length of 213 residues, has a dssp secondary structure that is 28% beta sheet (6 strands; 32 residues).

Function

Sclerostin, the product of the SOST gene, located on chromosome 17q12–q21 in humans, was originally believed to be a non-classical bone morphogenetic protein (BMP) antagonist. More recently sclerostin has been identified as binding to LRP5/6 receptors and inhibiting the Wnt signaling pathway. The inhibition of the Wnt pathway leads to decreased bone formation. Although the underlying mechanisms are unclear, it is believed that the antagonism of BMP-induced bone formation by sclerostin is mediated by Wnt signaling, but not BMP signaling pathways. Sclerostin is expressed in osteocytes and some chondrocytes and it inhibits bone formation by osteoblasts.

Sclerostin production by osteocytes is inhibited by parathyroid hormone, mechanical loading and cytokines including prostaglandin E2, oncostatin M, cardiotrophin-1 and leukemia inhibitory factor. Sclerostin production is increased by calcitonin. Thus, osteoblast activity is self regulated by a negative feedback system.

Clinical significance

Mutations in the gene that encodes the sclerostin protein are associated with disorders associated with high bone mass, sclerosteosis and van Buchem disease. Sclerosteosis is an autosomal recessive disorder characterized by bone overgrowth. It was first described in 1958 but given the current name in 1967. Excessive bone formation is most prominent in the skull, mandible and tubular bones. It can cause facial distortion and syndactyly. Increased intracranial pressure can cause sudden death in patients. It is a rare disorder that is most prominent in the Afrikaner population in South Africa (40 patients), but there have also been cases of American and Brazilian families.

van Buchem disease is also an autosomal recessive skeletal disease characterized by bone overgrowth. It was first described in 1955 as "hyperostosis corticalis generalisata familiaris", but was given the current name in 1968. Excessive bone formation is most prominent in the skull, mandible, clavicle, ribs and diaphyses of long bones and bone formation occurs throughout life. It is a very rare condition with about 30 known cases in 2002. In 1967 van Buchem characterized the disease in 15 patients of Dutch origin. Patients with sclerosteosis are distinguished from those with van Buchem disease because they are often taller and have hand malformations.

An antibody for sclerostin is being developed because of the protein’s specificity to bone. Its use has increased bone growth in preclinical trials in osteoporotic rats and monkeys. In a Phase I study, a single dose of anti-sclerostin antibody from Amgen (Romosozumab) increased bone density in the hip and spine in healthy men and postmenopausal women and the drug was well tolerated. In a Phase II trial, one year of the antibody treatment in osteoporotic women increased bone density more than bisphosphonate and teriparatide treatment; it had mild injection side effects. A Phase II trial of a monoclonal human antibody to sclerostin from Eli Lilly had positive effects on post-menopausal women. Monthly treatments of the antibody for one year increased the bone mineral density of the spine and hip by 18 percent and 6 percent, respectively, compared to the placebo group. In a Phase III trial, one year of Romosozumab treatment in post-menopausal women reduced the risk of vertebral fractures compared to the placebo group. It also increased the bone mineral density in the lumbar spine (13.3% versus 0.0%), femoral neck (5.2% versus -0.7%) and total hip (6.8% versus 0.0%) compared to the placebo group. Adverse events were balanced between the groups.

The Amgen drug is expected to be on the market in 2017 and is predicted to be the gold standard in osteoporosis treatment by 2021. In addition, OsteoGeneX is developing small molecule inhibitors of sclerostin.