Discovery of Sclerostin
G enomics technologies and DNA sequencing have had a transformative impact on biological research, giving us unprecedented access to the genomes of numerous organisms and ushering in such new fields as systems biology (1) and, more recently, synthetic biology. (2,3) In the 1990s, the advent of highthroughput sequencing spurred application of this powerful new technology to the challenging endeavor of trying to understand the genetic basis of various inherited human diseases. One such disease was sclerosteosis, a very rare, recessively inherited highbone-mass disorder that was thought to be caused primarily by excessive osteoblast-mediated bone formation rather than by defective osteoclast-mediated bone resorption. (4,5) Within the realm of inherited high-bone-mass disorders, (6) it was the hope for a discovery in the area of osteoblast biology that made sclerosteosis particularly interesting.
Indeed, the identification of new bone-building pathways that might yield novel anabolic agents had been a long-standing goal in bone biology, with hopes for therapeutic application in fracture healing, orthopedic procedures, and low-bone-mass conditions such as osteoporosis. Against this backdrop came the exciting news, independently from Mary Brunkow's group at Celltech R&D, Inc., (7,8) and from Wim Van Hul's group at the University of Antwerp, (9) that sclerosteosis was caused by mutations in a single gene (SOST) encoding a novel secreted protein. Because these were inactivating mutations, it was clear that this protein, aptly given the name sclerostin, functioned either directly or indirectly as an inhibitor of bone formation. During this same general time period, large-scale cDNA/ expressed sequence tag (EST) (10)(11)(12) and genomic DNA sequencing efforts were being made in academia and industry to rapidly identify new human genes. A novel secreted protein of unknown function, which turned out to be sclerostin, was discovered by computational mining of large DNA sequence databases using either homology-based programs (eg, BLAST) (13,14) or a special CxGxC-class cystine-knot search pattern (C Paszty, unpublished) (15) designed to identify new families of cystine-knot proteins. (16,17) Thus sclerostin emerged during an exciting era of gene discovery that was fueled, in part, by the development of important genomics technologies and the advent of high-throughput DNA sequencing.
Similar to sclerostin inactivation in humans, mice with a targeted deletion of the sclerostin gene (SOST knockout mice) were found to have high bone mass, demonstrating evolutionary conservation of sclerostin's function as a negative regulator of bone formation. (18) Analysis of bones from these mice revealed that bone formation was markedly increased on each of the key skeletal surfaces where new bone is normally formed (surface of trabecular bone and internal and external surfaces of cortical bone). Consistent with the increases in bone formation and bone mass, robust increases in bone strength also were found in these animals. These data from SOST knockout mice further highlighted sclerostin's critical role as an inhibitor of bone formation throughout the skeleton and its potential as a pharmacologic target for increasing bone mass and bone strength.
Molecular Interactions and Signaling Pathways
Efforts to understand the genetic control of bone mass were, of course, not restricted to studying sclerosteosis. Van Buchem disease, another very rare, recessively inherited high-bone-mass disorder, was determined to be caused by a 52-kb deletion 35 kb downstream of SOST, (19,20) resulting in absence of postnatal sclerostin expression. (21,22) For osteoporosis pseudoglioma syndrome (OPPG), a recessively inherited low-bone-mass disease, loss-of-function mutations were found in a Wnt signaling coreceptor, the low-density lipoprotein receptor-PERSPECTIVE J JBMR