Bone morphogenetic protein
Bone morphogenetic proteins (BMPs) are a group of growth factors also known as cytokines and as metabologens. Originally discovered by their ability to induce the formation of bone and cartilage, BMPs are now considered to constitute a group of pivotal morphogenetic signals, orchestrating tissue architecture throughout the body. The important functioning of BMP signals in physiology is emphasized by the multitude of roles for dysregulated BMP signalling in pathological processes. Cancerous disease often involves misregulation of the BMP signalling system. Absence of BMP signalling is, for instance, an important factor in the progression of colon cancer, and conversely, overactivation of BMP signalling following reflux-induced esophagitis provokes Barrett's esophagus and is thus instrumental in the development of adenocarcinoma in the proximal portion of the gastrointestinal tract.
Recombinant human BMPs are used in orthopedic applications such as spinal fusions, nonunions and oral surgery. rhBMP-2 and rhBMP-7 are Food and Drug Administration (FDA)-approved for some uses. rhBMP-2 is more effective at growing bone than any other BMPs and because of its efficacy it is widely used off-label.
Originally, seven such proteins were discovered. Of these, six (BMP2 through BMP7) belong to the Transforming growth factor beta superfamily of proteins. BMP1 is a metalloprotease. Since then, thirteen more BMPs have been discovered, bringing the total to twenty.
|BMP||Known functions||Gene Locus|
|BMP1||*BMP1 does not belong to the TGF-β family of proteins. It is a metalloprotease that acts on procollagen I, II, and III. It is involved in cartilage development.||Chromosome: 8; Location: 8p21|
|BMP2||Acts as a disulfide-linked homodimer and induces bone and cartilage formation. It is a candidate as a retinoid mediator. Plays a key role in osteoblast differentiation.||Chromosome: 20; Location: 20p12|
|BMP3||Induces bone formation.||Chromosome: 14; Location: 14p22|
|BMP4||Regulates the formation of teeth, limbs and bone from mesoderm. It also plays a role in fracture repair, epidermis formation, dorsal-ventral axis formation, and ovarian follical development.||Chromosome: 14; Location: 14q22-q23|
|BMP5||Performs functions in cartilage development.||Chromosome: 6; Location: 6p12.1|
|BMP6||Plays a role in joint integrity in adults. Controls iron homeostasis via regulation of hepcidin.||Chromosome: 6; Location: 6p12.1|
|BMP7||Plays a key role in osteoblast differentiation. It also induces the production of SMAD1. Also key in renal development and repair.||Chromosome: 20; Location: 20q13|
|BMP8a||Involved in bone and cartilage development.||Chromosome: 1; Location: 1p35–p32|
|BMP8b||Expressed in the hippocampus.||Chromosome: 1; Location: 1p35–p32|
|BMP10||May play a role in the trabeculation of the embryonic heart.||Chromosome: 2; Location: 2p14|
|BMP15||May play a role in oocyte and follicular development.||Chromosome: X; Location: Xp11.2|
For a detailed history of the discovery and isolation of bone morphogenetic proteins read "Bone Morphogenetic Proteins: an Unconventional Approach to Isolation of First Mammalian Morphogens" in Cytokine & Growth Factor Reviews, or "Bone morphogenetic proteins in tissue engineering: the road from laboratory to the clinic" in Journal of Tissue Engineering and Regenerative Medicine.
From the time of Hippocrates it has been known that bone has considerable potential for regeneration and repair. Nicholas Senn, a surgeon at Rush Medical College in Chicago, described the utility of antiseptic decalcified bone implants in the treatment of osteomyelitis and certain bone deformities. Pierre Lacroix proposed that there might be a hypothetical substance, osteogenin, that might initiate bone growth.
The biological basis of bone morphogenesis was shown by Marshall R. Urist. Urist made the key discovery that demineralized, lyophilized segments of bone induced new bone formation when implanted in muscle pouches in rabbits. This discovery was published in 1965 by Urist in Science. Urist proposed the name "Bone Morphogenetic Protein" in the scientific literature in the Journal of Dental Research in 1971.
Bone induction is a sequential multistep cascade. The key steps in this cascade are chemotaxis, mitosis, and differentiation. Early studies by Hari Reddi unraveled the sequence of events involved in bone matrix-induced bone morphogenesis. On the basis of the above work, it seemed likely that morphogens were present in the bone matrix. Using a battery of bioassays for bone formation, a systematic study was undertaken to isolate and purify putative bone morphogenetic proteins.
A major stumbling block to purification was the insolubility of demineralized bone matrix. To overcome this hurdle, Hari Reddi and Kuber Sampath used dissociative extractants, such as 4M guanidine HCL, 8M urea, or 1% SDS. The soluble extract alone or the insoluble residues alone were incapable of new bone induction. This work suggested that the optimal osteogenic activity requires a synergy between soluble extract and the insoluble collagenous substratum. It not only represented a significant advance toward the final purification of bone morphogenetic proteins by the Reddi laboratory, but ultimately also enabled the cloning of BMPs by John Wozney and colleagues at Genetics Institute.
BMPs interact with specific receptors on the cell surface, referred to as bone morphogenetic protein receptors (BMPRs).
Signal transduction through BMPRs results in mobilization of members of the SMAD family of proteins. The signaling pathways involving BMPs, BMPRs and Smads are important in the development of the heart, central nervous system, and cartilage, as well as post-natal bone development.
They have an important role during embryonic development on the embryonic patterning and early skeletal formation. As such, disruption of BMP signaling can affect the body plan of the developing embryo. For example, BMP4 and its inhibitors noggin and chordin help regulate polarity of the embryo (i.e. back to front patterning). Specifically BMP-4 and its inhibitors play a major role in neurulation and the development of the neural plate. BMP-4 signals ectoderm cells to develop into skin cells, but the secretion of inhibitors by the underlying mesoderm blocks the action of BMP-4 to allow the ectoderm to continue on its normal course of neural cell development.
Mutations in BMPs and their inhibitors (such as sclerostin) are associated with a number of human disorders which affect the skeleton.
Several BMPs are also named 'cartilage-derived morphogenetic proteins' (CDMPs), while others are refer to as 'growth differentiation factors' (GDFs).
Clinical uses 
BMPs for clinical use are produced using recombinant DNA technology (recombinant human BMPs; rhBMPs). rhBMP-2 and rhBMP-7 have been shown in clinical studies to be beneficial in the treatment of a variety of bone-related conditions including spinal fusions and nonunions. In 2001, the Food and Drug Administration (FDA) approved rhBMP-7 (aka OP-1; Stryker Biotech) for a humanitarian device exemption as an alternative to autograft in long bone nonunions. In 2004, the humanitarian device exemption was extended as an alternative to autograft for posterolateral fusion. In 2002, rhBMP-2 (Infuse; Medtronic) was approved for anterior lumbar interbody fusions (ALIFs) with a lumbar fusion device. In 2008 it was approved to repair posterolateral lumbar pseudarthrosis, open tibia shaft fractures with intramedullary nail fixation. In these products, BMPs are delivered to the site of the fracture by being incorporated into a bone implant, and released gradually to allow bone formation, as the growth stimulation by BMPs must be localized and sustained for some weeks. The BMPs are eluted through a purified collagen matrix which is implanted in the site of the fracture. rhBMP-2 helps grow bone better than any other rhBMP so it is much more widely used clinically. There is "little debate or controversy" about the effectiveness of rhBMP-2 to grow bone to achieve spinal fusions, and Medtronic generates $700 million in annual sales from their product.
While rhBMPs are approved for specific applications (spinal lumbar fusions with an anterior approach and tibia nonunions), up to 85% of all BMP usage is off-label. rhBMP-2 is used extensively in other lumbar spinal fusion techniques (e.g., using a posterior approach, anterior or posterior cervical fusions). At between $6000 and $10,000 for a typical treatment, BMPs can be costly compared with other techniques such as bone grafting. However, this cost is often far less than the costs required with orthopaedic revision in multiple surgeries.
While there is little debate that rhBMPs are successful clinically, there is controversy about their use. It is common for orthopedic surgeons to be paid for their contribution to the development of a new product, but some of the surgeons responsible for the original Medtronic-supported studies on the efficacy of rhBMP-2 have been accused of bias and conflict of interest. For example, one surgeon, a lead author on four of these research papers, did not disclose any financial ties while with the company on three of the papers; he was paid over $4 million by Medtronic. In another study, the lead author did not disclose any financial ties to Medtronic; he was paid at least $11 million by the company. In a series of 12 publications, the median financial ties of the authors to Medtronic were $12–16 million. In those studies that had more than 20 and 100 patients, one or more authors had financial ties of $1 million and $10 million, respectively. Early clinical trials using rhBMP-2 underreported adverse events associated with treatment. In the 13 original industry-sponsored publications related to safety, there were zero adverse events in 780 patients. It has since been revealed that potential complications can arise from the use including implant displacement, subsidence, infection, urogenital events, and retrograde ejaculation.
rhBMPs are also used in oral surgeries. BMP-7 has also recently found use in the treatment of chronic kidney disease (CKD). BMP-7 has been shown in murine animal models to reverse the loss of glomeruli due to sclerosis. Curis has been in the forefront of developing BMP-7 for this use. In 2002, Curis licensed BMP-7 to Ortho Biotech Products, a subsidiary of Johnson & Johnson.
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|Wikimedia Commons has media related to: Bone morphogenetic proteins|
- BMP: The What and the Who
- BMPedia - the Bone Morphogenetic Protein Wiki
- Bone Morphogenetic Proteins at the US National Library of Medicine Medical Subject Headings (MeSH)
- Chen, Di, Zhao, Ming, and Mundy, Gregory R. (2004). "Bone Morphogenetic Proteins". Growth Factors 22 (4): 233–241. doi:10.1080/08977190412331279890. PMID 15621726.
- Cheng H, Jiang W, Phillips F, Haydon R, Peng Y, Zhou L, Luu H, An N, Breyer B, Vanichakarn P, Szatkowski J, Park J, He T (2003). "Osteogenic activity of the fourteen types of human bone morphogenetic proteins (BMPs)". J Bone Joint Surg Am 85–A (8): 1544–52. PMID 12925636. link