- Email: [email protected]
Bone Morphogenetic Proteins
Bone Morphogenetic P ro t e i n s : Indications and Uses Christopher Bibbo, Brian Rougeux, BSa
DO, DPM
a,b,
*, Jonas Nelson,
MD
b
, David Ehrlich,
MD
c
,
KEYWORDS Bone morphogenetic protein BMP Foot and ankle surgery Bone healing KEY POINTS One specific family of proteins that is generating interest is bone morphogenetic proteins. Coupled with the current increase in patient comorbidities (eg, diabetes), considerable interest remains focused on improving the bone healing process. In an effort to improve on osseous healing success rates, clinical and basic science studies are beginning to focus on elucidating the role of various growth factors on bone healing.
INTRODUCTION
Although bone healing is generally successful, it is anticipated that 5% to 10% of the estimated 5.6 million fractures occurring annually in the United States is delayed or impaired with an unknown cause.1 Fusions may be considered to functionally represent similar considerations for healing; thus, when coupled with fractures, a considerable number of patients have impaired bone healing. Poor bone healing in high-risk patients, whether of a fusion or a fracture, continues to be a challenge for foot and ankle surgeons (Figs. 1–3). There are several risk factors for poor bone healing,2 which should always be in the forefront of planning when the surgical management of an osseous condition is being contemplated (Box 1). In consideration of the overall 5% to 10% rate for delayed bone healing in the pooled general population, the aforementioned factors pose an even worse risk for failed bone healing in foot and ankle surgery. Coupled with the current increase in
Conflicts of Interest: No conflicts of interest exist with any of the authors. a Department of Orthopaedics, Marshfield Clinic, 1000 North Oak Avenue, Marshfield, WI, USA; b Division of Plastic & Reconstructive Surgery, Department of Surgery, Hospital of the University of Pennsylvania, 10 Penn Tower, 3400 Spruce Street, Philadelphia, PA 19104, USA; c Division of Plastic & Reconstructive Surgery, Department of Surgery, Thomas Jefferson University, 840 Walnut Street, Philadelphia, PA 19107, USA * Corresponding author. Department of Orthopaedics, Marshfield Clinic, 1000 North Oak Avenue, Marshfield 54449, WI. E-mail address: [email protected] Clin Podiatr Med Surg 32 (2015) 35–43 http://dx.doi.org/10.1016/j.cpm.2014.09.005 podiatric.theclinics.com 0891-8422/15/$ – see front matter Ó 2015 Elsevier Inc. All rights reserved.
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Fig. 1. Radiographs and computed tomography of an 80-year-old female diabetic with disabling pain and valgus deformity; this patient is at high risk for poor bone healing.
patient comorbidities (eg, diabetes), considerable interest remains focused on improving the bone healing process. In an effort to improve on osseous healing success rates, clinical and basic science studies are beginning to focus on elucidating the role of various growth factors on bone healing. One specific family of proteins that is generating interest is the bone morphogenetic proteins (BMPs). THE BASIC SCIENCE OF BONE MORPHOGENETIC PROTEINS
Bone development and growth is a highly complex process that may can occur via 2 major pathways: intramembranous or endochondrial. During intramembranous development, bone tissue is formed directly in primitive connective tissue (mesenchyme), whereas endochondrial bone tissue replaces a hyaline cartilage template. Current knowledge suggests that the regulation of intrauterine skeletal patterning is controlled
Fig. 2. Collagen sponges soaked in rhBMP-2 and structural bone needed to fill areas of bone voids; BMPs are adjuvants and are not a substitute for structural bone requirements.
Bone Morphogenetic Proteins
Fig. 3. Intraoperative views of fine-wire circular external fixator used to achieve fusion via compression while allowing weight bearing. Any variety of fixation techniques may be used in conjunction with BMP.
by a plethora of signaling molecules. Within the molecular framework for skeletal development are some well-known transcription factors, including Wnt, hedgehog signaling factors, homeobox (HOX) transcription factors, members of the transforming growth factor beta (TGF-b), and fibroblast growth factors (FGF) families (Box 2). However, the manipulation of bone, vis-a`-vis bone healing, has always been at the forefront of clinical challenges, and the subject of early investigations. Through a series of experiments, Marshall Urist3 (in 1965) postulated the presence of a natural
Box 1 Risk factors for poor bone healing Smoking High-energy injury History of a delayed/nonunion Immune suppression Suboptimal arterial inflow Multiple medical comorbidities Diabetes Multiple surgeries Alcohol abuse Chronic infections Collagen disorders Inherited metabolic bone disorders
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Box 2 Regulatory factors in skeletal patterning during bone development and growth Wnt signaling: limb axis planning and stem cell differentiation (in conjunction with BMPs) HOX transcription factors (homeobox genes): influence limb axis development Sonic Hedgehog protein signaling: helps regulate osteoblast and osteoclast activity TGF-b: stimulates osteoblasts and inhibits osteoclasts FGF: stimulates osteoblasts during intramembraneous and endochondral ossification
biological agent or BMP, which could act to promote bone formation. Gene sequencing to characterize the molecular nature of BMPs did occur until the 1980s.4 Since that time, it has been recognized that the BMPs are a group of highly conserved (stable across species), inducible proteins that belong to the TGF-b supergene family. It is know known that the molecules fitting the physical structure of BMPs also influence a wide range of growth factor functions. BMPs have been previously classified into 4 subgroups based on their molecular structure (groups I–IV).5 From a bench top standpoint, these classification schemes are helpful in describing newly discovered protein structure/function, but they have not yet become common parlance in the clinical setting. Nonetheless, the effects of BMPs are far-reaching and delicately intertwined. Through transgenic and knockout mice models, BMP signaling/pathways have been shown to play critical roles in the development of multiple organ systems: hair cell growth and pigmentation (BMP-2, BMP-4, BMP-7, BMP-11),6 ovarian development (BMP-15)7 and cardiac myocyte differentiation and development (BMP-2, BMP-4).8,9 Several BMPs have a selectively profound role in the development/maintenance of the musculoskeletal system. To date, more than 20 BMPs have been characterized, with each BMP having a highly regulated molecular pathway, with variable levels of effect on the musculoskeletal system.10 It is now recognized that BMP-14 has a strong influence on cartilage growth, whereas other BMPs (eg, BMP-2 and BMP-7) possess more osteoselective properties. Expanding this knowledge further, Cheng and colleagues11 showed that BMPs (except BMP-1 and BMP-3) may also possess selective functional/temporal influences on bone, stimulating osteogenesis in mature osteoblasts, whereas BMP-2, BMP-4, BMP-6, BMP-7, and BMP-9 have greater osteoinductive properties.11 Temporally (in relation to bone healing), BMP-2, BMP-6, and BMP-9 have been found to have their most pronounced influence on pluripotent mesenchymal cells, progenitor cells, and preosteoblastic cells; a phenomenon that has been termed the osteogenic hierarchy of BMPs.11 Bone Morphogenetic Protein Mechanism of Action: Receptor Binding and Downstream Signaling
BMPs regulate bone development by the attraction (chemotaxis) of mesenchymal stem cells, followed by the stimulation of osteoblastic, osteoclastic, and progenitor cell lines. The interaction of several key regulatory proteins that interact with the BMPs is highly regulated. First, BMPs bind to specific cell wall receptors (Box 3). BMPs ultimately stimulate osteoblast differentiation via a signal transduction chain reaction that is initiated by the 3 serine threonine kinase receptors BMP receptor (BMPR)-IA, BMPR-IB and BMPR-II, which heterotetramerize on binding to dimeric BMP; after cell receptor binding, a highly complex intracellular system of signaling (downstream signaling) and counter-regulation occurs (Box 4).10
Bone Morphogenetic Proteins
Box 3 BMP-2 receptors Serine/threonine kinase receptors: bind BMP ligand Form heterotetrameric-activated complexes Type IA BMP receptor (BMPR-IA) BMPR-IB Type IA activin receptor (ActR-IA) BMPR-II ActR-II ActR-IIB
Throughout the signaling process, a highly constrained regulatory and counterregulatory array of proteins exist,10 resulting in a finely tuned balance of systems that so far has not been fully elucidated. The heterotetramerization of the BMPR ultimately results in the activation of intrinsic receptor serine/threonine kinases, allowing phosphorylation of transcription-regulating Smad proteins.10 The net result of Smad complexes translocating into the nucleus is an interaction with sequence-specific transcription factors that regulate gene expression, including Runx2, Osterix (Osx), and Dlx5 regulating gene activation or repression.12 The p38 mitogen-activated protein (MAP) kinase pathway has also been shown to influence BMP-induced gene expression.12,13 The BMP signaling cascade is tightly regulated at every step by soluble inhibitory proteins such as Noggin and other cysteine knot–containing proteins that prevent the binding of BMP proteins to specific receptors.12,14 In addition, several cryptic (poorly characterized) counter-regulatory proteins exist that block the action of the Smads; Smurfs proteins (Hect type E3 ubiquitin ligases) also interact to degrade Smad proteins,15–17 BMPRs and the transcription factor Runx2.10,18 In general, Smurf proteins are thus considered inhibitory to bone formation (Box 5).19 It has recently been shown that the inflammatory cytokine tumor necrosis factor alpha is also capable of downregulating BMPR-IA and BMPR-II transcripts.20 CLINICAL INDICATIONS AND USES OF BONE MORPHOGENETIC PROTEINS
The BMPs are an important subclass of growth factors within the TGF-b superfamily. BMPs that are commercially available include BMP-2 and BMP-7, both having positive influences on bone healing. At present, the most facile and readily available BMP for surgical use is recombinant BMP-2 (rhBMP-2). Although there are only a few studies in the foot and ankle literature that attest to the efficacy of rhBMP-2 to
Box 4 BMPR substrates and downstream processing Phosphorylated Smads1,5,8 binds to intracellular Smad4 Bound together, P-Smads1,5,8/Smad4 travel to cell nucleus P-Smads1,5,8/Smad4 complex binds to transcription factors; for example, Runx2 in osteoblasts, activating target gene transcription
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Box 5 Antagonists to BMP/Smad -
Noggin: with cystine knot–containing elements, binds BMP-2, BMP-4, and BMP-7 and blocks BMP signaling
-
Smad6: binds type I BMPR, preventing Smads1,5,8 activation (phosphorylation)
-
Tob: an antiproliferative protein, Tob binds Smad1 and Smad5, inhibiting BMP signaling
-
Smurf1 (Smad ubiquitin regulatory factor 1): E3 ubiquitin ligase, binds/degrades Smad1 and Smad5
assist with bone healing, the data are compelling. Several key points must be remembered when considering BMP in clinical practice (see Figs. 1–3):
BMPs are indicated only in patients at high risk for poor bone healing BMPs are an adjuvant modality: growth factors do not substitute for bone loss All aspects of a failed prior bony surgical procedure must be evaluated BMPs do not correct for poor planning/technical execution of a surgery BMP use is often off-label and may be costly, which is a health care consideration
CLINICAL STUDIES AND OUTCOMES
The benefit of BMPs in improving bone healing have been well shown experimentally in animal fracture models.21,22 The use of BMPs in at-risk patients with complex fractures has also been observed clinically to be of benefit in fracture healing (Bibbo C, unpublished data, 2005). However, the greatest utility for BMPs in foot and ankle surgery may be as an adjuvant in arthrodesis procedures. To date, only 2 studies have specifically examined the use of rhBMP-2 in foot and ankle surgery. After first reporting positive results in a pilot study,23 Bibbo and colleagues24 published a retrospective analysis of the adjuvant use of rhBMP-2 in 69 high-risk patients who underwent 112 fusions. Using computed tomography fusion criteria, an overall 96% union rate was observed at a mean time of 11 weeks. All fusion sites were pooled for average time to union but the difference between foot and ankle fusion sites was not statistically significant. In addition, the investigators found that, when indicated, bone graft (including autograft and allograft) did not statistically lessen the fusion rate. Thus, when bone is indicated for structural alignment and construct integrity, it may be safely used in conjunction with rhBMP-2 without an undue negative impact on bone healing rates. COMPLICATIONS AND CONCERNS
In the aforementioned sentinel article,24 an overall 8.7% (6 of 69 patients) complication rate occurred a high-risk patient population. Five of 112 joint fusion sites (in only 3 patients) went on to develop a nonunion, which equaled an overall 4% nonunion rate (4% nonunion rate per fusion site, as well as a 4% nonunion rate per patient). Anatomic fusion sites that went on to nonunion included the transverse tarsal joint complex (talonavicular joint [n 5 1], calcaneocuboid joint [n 5 1], and the subtalar joint [n 5 2]). The transverse tarsal joint nonunions were painless; these were not revised and successfully managed with an in-shoe orthotic. The subtalar joint was partial nonunion and mildly painful, and to date has not needed revision. Wound healing complications occurred in 2 insulin-dependant diabetic patients (2 of 69 patients 5 3%), not proved causal by the use of the BMP. Deep infection
Bone Morphogenetic Proteins
occurred in 1 patient (1 of 69 patients 5 1.5%). Wound and infectious complications were successfully managed with local wound care, negative-pressure dressings, and antibiotics. There were no other adverse events recorded that were related to the use of rhBMP-2. Two articles have recently called into question an association of BMP use and the development of cancer in patients after spine fusion.25,26 Although a complicated topic that potentially has great merit, from an epidemiologic and a vetted scientific standpoint, these studies were poorly constructed and do not prove a causal relationship. In addition, other subsequent published articles have refuted the association of cancer development with BMP use.27–29 An initial database review of the lead author’s foot and ankle patients who were treated with adjuvant rhBMP-2 does not show any correlation with BMP use and the development of cancer (Bibbo C, unpublished data, 2006). The salient outcome points from these studies are (see Figs. 1–3):
BMPs are adjuvants for bone healing, not a substitute for structural bone needs BMPs are useful in many fusion procedures and techniques BMPs are indicated only when risk factors exist for a nonunion BMPs are currently thought to be safe for use in foot and ankle surgery
SUMMARY
The BMPs are a group of growth factors that have varied roles in the development and maintenance of many organ systems. Several of the BMPs have osteogenic potential, and exert their effects via complex and highly regulated pathways. At present, only rhBMP-2 and rhBMP-7 are available for clinical use, but only rhBMP-2 is readily available, and from a practical standpoint is considered the only commercially available BMP. Only a few studies exist on BMP use in foot and ankle surgery, but these have shown promising results with low complication rates. BMP is an adjuvant to bone healing, and does not substitute for structural bone needs. In addition, rhBMP-2 outside spinal fusions is considered to be US Food and Drug Administration off-label, and should be used only in patients who are at high risk for bone healing problems. REFERENCES
1. Einhorn TA. Enhancement of fracture-healing. J Bone Joint Surg Am 1995;77: 940–56. 2. Bibbo C, Anderson RB, Davis WH. Complications of midfoot and hindfoot arthrodesis. Clin Orthop Relat Res 2001;391:45–58. 3. Urist MR. Bone: formation by autoinduction. Science 1965;150:893–9. 4. Wozney JM, Rosen V, Celeste AJ, et al. Novel regulators of bone formation: molecular clones and activities. Science 1988;242:1528–34. 5. Marsell R, Einhorn TA. The role of endogenous bone morphogenetic proteins in normal skeletal repair. Injury 2009;40(Suppl 3):S4–7. 6. Sharov AA, Sharova TY, Mardaryev AN, et al. Bone morphogenetic protein signaling regulates the size of hair follicles and modulates the expression of cell cycle-associated genes. Proc Natl Acad Sci U S A 2006;103:18166–71. 7. Yoshino O, McMahon HE, Sharma S, et al. A unique preovulatory expression pattern plays a key role in the physiological functions of BMP-15 in the mouse. Proc Natl Acad Sci U S A 2006;103:10678–83. 8. Taha MF, Valojerdi MR, Mowla SJ. Effect of bone morphogenetic protein-4 (BMP-4) on cardiomyocyte differentiation from mouse embryonic stem cell. Int J Cardiol 2007;120:92–101.
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9. Wang YX, Qian LX, Liu D, et al. Bone morphogenetic protein-2 acts upstream of myocyte-specific enhancer factor 2a to control embryonic cardiac contractility. Cardiovasc Res 2007;74:290–303. 10. Chen D, Zhao M, Mundy GR. Bone morphogenetic proteins. Growth Factors 2004;22:233–41. 11. Cheng H, Jiang W, Phillips FM, et al. Osteogenic activity of the fourteen types of human bone morphogenetic proteins (BMPs). J Bone Joint Surg Am 2003;85A: 1544–52. 12. Ryoo HM, Lee MH, Kim YJ. Critical molecular switches involved in BMP-2induced osteogenic differentiation of mesenchymal cells. Gene 2006;366:51–7. 13. Nohe A, Keating E, Knaus P, et al. Signal transduction of bone morphogenetic protein receptors. Cell Signal 2004;16:291–9. 14. Groppe J, Greenwald J, Wiater E, et al. Structural basis of BMP signaling inhibition by Noggin, a novel twelve-membered cysteine knot protein. J Bone Joint Surg Am 2003;85A(Suppl 3):52–8. 15. Zhao M, Qiao M, Harris SE, et al. Smurf1 inhibits osteoblast differentiation and bone formation in vitro and in vivo. J Biol Chem 2004;279:12854–9. 16. Zhao M, Qiao M, Oyajobi BO, et al. E3 ubiquitin ligase Smurf1 mediates corebinding factor alpha1/Runx2 degradation and plays a specific role in osteoblast differentiation. J Biol Chem 2003;278:27939–44. 17. Zhu H, Kavsak P, Abdollah S, et al. A SMAD ubiquitin ligase targets the BMP pathway and affects embryonic pattern formation. Nature 1999;400:687–93. 18. Shen R, Chen M, Wang YJ, et al. Smad6 interacts with Runx2 and mediates Smad ubiquitin regulatory factor 1-induced Runx2 degradation. J Biol Chem 2006;281: 3569–76. 19. Kaneki H, Guo R, Chen D, et al. Tumor necrosis factor promotes Runx2 degradation through up-regulation of Smurf1 and Smurf2 in osteoblasts. J Biol Chem 2006;281:4326–33. 20. Singhatanadgit W, Salih V, Olsen I. Bone morphogenetic protein receptors and bone morphogenetic protein signaling are controlled by tumor necrosis factoralpha in human bone cells. Int J Biochem Cell Biol 2006;38:1794–807. 21. Bouxsein ML, Turek TJ, Blake CA, et al. Recombinant human bone morphogenetic protein-2 accelerates healing in a rabbit ulnar osteotomy model. J Bone Joint Surg Am 2001;83A:1219–30. 22. Gerhart TN, Kirker-Head CA, Kriz MJ, et al. Healing segmental femoral defects in sheep using recombinant human bone morphogenetic protein. Clin Orthop Relat Res 1993;293:317–26. 23. Bibbo C, Haskell MD. Recombinant bone morphogenetic protein-2 (rhBMP-2) in high-risk foot & ankle surgery: techniques & preliminary results of a prospective, intention to treat study. Tech Foot Ankle Surg 2007;6:71–9. 24. Bibbo C, Patel DV, Haskell MD. Recombinant bone morphogenetic protein-2 (rhBMP-2) in high-risk ankle and hindfoot fusions. Foot Ankle Int 2009;30:597–603. 25. Mesfin A, Buchowski JM, Zebala LP, et al. High-dose rhBMP-2 for adults: major and minor complications: a study of 502 spine cases. J Bone Joint Surg Am 2013;95:1546–53. 26. Carragee EJ, Chu G, Rohatgi R, et al. Cancer risk after use of recombinant bone morphogenetic protein-2 for spinal arthrodesis. J Bone Joint Surg Am 2013;95: 1537–45. 27. Cooper GS, Kou TD. Risk of cancer after lumbar fusion surgery with recombinant human bone morphogenic protein-2 (rh-BMP-2). Spine (Phila Pa 1976) 2013;38: 1862–8.
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28. Veeravagu A, Cole T, Jiang B, et al. The use of bone morphogenetic protein in thoracolumbar spine procedures: analysis of the Marketscan Longitudinal Database. Spine J 2014. http://dx.doi.org/10.1016/j.spinee.2014.05.010. pii:S1529– 9430(14)00472-0. 29. Lad SP, Bagley JH, Karikari IO, et al. Cancer after spinal fusion: the role of bone morphogenetic protein. Neurosurgery 2013;73:440–9.
43
DO, DPM
a,b,
*, Jonas Nelson,
MD
b
, David Ehrlich,
MD
c
,
KEYWORDS Bone morphogenetic protein BMP Foot and ankle surgery Bone healing KEY POINTS One specific family of proteins that is generating interest is bone morphogenetic proteins. Coupled with the current increase in patient comorbidities (eg, diabetes), considerable interest remains focused on improving the bone healing process. In an effort to improve on osseous healing success rates, clinical and basic science studies are beginning to focus on elucidating the role of various growth factors on bone healing.
INTRODUCTION
Although bone healing is generally successful, it is anticipated that 5% to 10% of the estimated 5.6 million fractures occurring annually in the United States is delayed or impaired with an unknown cause.1 Fusions may be considered to functionally represent similar considerations for healing; thus, when coupled with fractures, a considerable number of patients have impaired bone healing. Poor bone healing in high-risk patients, whether of a fusion or a fracture, continues to be a challenge for foot and ankle surgeons (Figs. 1–3). There are several risk factors for poor bone healing,2 which should always be in the forefront of planning when the surgical management of an osseous condition is being contemplated (Box 1). In consideration of the overall 5% to 10% rate for delayed bone healing in the pooled general population, the aforementioned factors pose an even worse risk for failed bone healing in foot and ankle surgery. Coupled with the current increase in
Conflicts of Interest: No conflicts of interest exist with any of the authors. a Department of Orthopaedics, Marshfield Clinic, 1000 North Oak Avenue, Marshfield, WI, USA; b Division of Plastic & Reconstructive Surgery, Department of Surgery, Hospital of the University of Pennsylvania, 10 Penn Tower, 3400 Spruce Street, Philadelphia, PA 19104, USA; c Division of Plastic & Reconstructive Surgery, Department of Surgery, Thomas Jefferson University, 840 Walnut Street, Philadelphia, PA 19107, USA * Corresponding author. Department of Orthopaedics, Marshfield Clinic, 1000 North Oak Avenue, Marshfield 54449, WI. E-mail address: [email protected] Clin Podiatr Med Surg 32 (2015) 35–43 http://dx.doi.org/10.1016/j.cpm.2014.09.005 podiatric.theclinics.com 0891-8422/15/$ – see front matter Ó 2015 Elsevier Inc. All rights reserved.
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Bibbo et al
Fig. 1. Radiographs and computed tomography of an 80-year-old female diabetic with disabling pain and valgus deformity; this patient is at high risk for poor bone healing.
patient comorbidities (eg, diabetes), considerable interest remains focused on improving the bone healing process. In an effort to improve on osseous healing success rates, clinical and basic science studies are beginning to focus on elucidating the role of various growth factors on bone healing. One specific family of proteins that is generating interest is the bone morphogenetic proteins (BMPs). THE BASIC SCIENCE OF BONE MORPHOGENETIC PROTEINS
Bone development and growth is a highly complex process that may can occur via 2 major pathways: intramembranous or endochondrial. During intramembranous development, bone tissue is formed directly in primitive connective tissue (mesenchyme), whereas endochondrial bone tissue replaces a hyaline cartilage template. Current knowledge suggests that the regulation of intrauterine skeletal patterning is controlled
Fig. 2. Collagen sponges soaked in rhBMP-2 and structural bone needed to fill areas of bone voids; BMPs are adjuvants and are not a substitute for structural bone requirements.
Bone Morphogenetic Proteins
Fig. 3. Intraoperative views of fine-wire circular external fixator used to achieve fusion via compression while allowing weight bearing. Any variety of fixation techniques may be used in conjunction with BMP.
by a plethora of signaling molecules. Within the molecular framework for skeletal development are some well-known transcription factors, including Wnt, hedgehog signaling factors, homeobox (HOX) transcription factors, members of the transforming growth factor beta (TGF-b), and fibroblast growth factors (FGF) families (Box 2). However, the manipulation of bone, vis-a`-vis bone healing, has always been at the forefront of clinical challenges, and the subject of early investigations. Through a series of experiments, Marshall Urist3 (in 1965) postulated the presence of a natural
Box 1 Risk factors for poor bone healing Smoking High-energy injury History of a delayed/nonunion Immune suppression Suboptimal arterial inflow Multiple medical comorbidities Diabetes Multiple surgeries Alcohol abuse Chronic infections Collagen disorders Inherited metabolic bone disorders
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Box 2 Regulatory factors in skeletal patterning during bone development and growth Wnt signaling: limb axis planning and stem cell differentiation (in conjunction with BMPs) HOX transcription factors (homeobox genes): influence limb axis development Sonic Hedgehog protein signaling: helps regulate osteoblast and osteoclast activity TGF-b: stimulates osteoblasts and inhibits osteoclasts FGF: stimulates osteoblasts during intramembraneous and endochondral ossification
biological agent or BMP, which could act to promote bone formation. Gene sequencing to characterize the molecular nature of BMPs did occur until the 1980s.4 Since that time, it has been recognized that the BMPs are a group of highly conserved (stable across species), inducible proteins that belong to the TGF-b supergene family. It is know known that the molecules fitting the physical structure of BMPs also influence a wide range of growth factor functions. BMPs have been previously classified into 4 subgroups based on their molecular structure (groups I–IV).5 From a bench top standpoint, these classification schemes are helpful in describing newly discovered protein structure/function, but they have not yet become common parlance in the clinical setting. Nonetheless, the effects of BMPs are far-reaching and delicately intertwined. Through transgenic and knockout mice models, BMP signaling/pathways have been shown to play critical roles in the development of multiple organ systems: hair cell growth and pigmentation (BMP-2, BMP-4, BMP-7, BMP-11),6 ovarian development (BMP-15)7 and cardiac myocyte differentiation and development (BMP-2, BMP-4).8,9 Several BMPs have a selectively profound role in the development/maintenance of the musculoskeletal system. To date, more than 20 BMPs have been characterized, with each BMP having a highly regulated molecular pathway, with variable levels of effect on the musculoskeletal system.10 It is now recognized that BMP-14 has a strong influence on cartilage growth, whereas other BMPs (eg, BMP-2 and BMP-7) possess more osteoselective properties. Expanding this knowledge further, Cheng and colleagues11 showed that BMPs (except BMP-1 and BMP-3) may also possess selective functional/temporal influences on bone, stimulating osteogenesis in mature osteoblasts, whereas BMP-2, BMP-4, BMP-6, BMP-7, and BMP-9 have greater osteoinductive properties.11 Temporally (in relation to bone healing), BMP-2, BMP-6, and BMP-9 have been found to have their most pronounced influence on pluripotent mesenchymal cells, progenitor cells, and preosteoblastic cells; a phenomenon that has been termed the osteogenic hierarchy of BMPs.11 Bone Morphogenetic Protein Mechanism of Action: Receptor Binding and Downstream Signaling
BMPs regulate bone development by the attraction (chemotaxis) of mesenchymal stem cells, followed by the stimulation of osteoblastic, osteoclastic, and progenitor cell lines. The interaction of several key regulatory proteins that interact with the BMPs is highly regulated. First, BMPs bind to specific cell wall receptors (Box 3). BMPs ultimately stimulate osteoblast differentiation via a signal transduction chain reaction that is initiated by the 3 serine threonine kinase receptors BMP receptor (BMPR)-IA, BMPR-IB and BMPR-II, which heterotetramerize on binding to dimeric BMP; after cell receptor binding, a highly complex intracellular system of signaling (downstream signaling) and counter-regulation occurs (Box 4).10
Bone Morphogenetic Proteins
Box 3 BMP-2 receptors Serine/threonine kinase receptors: bind BMP ligand Form heterotetrameric-activated complexes Type IA BMP receptor (BMPR-IA) BMPR-IB Type IA activin receptor (ActR-IA) BMPR-II ActR-II ActR-IIB
Throughout the signaling process, a highly constrained regulatory and counterregulatory array of proteins exist,10 resulting in a finely tuned balance of systems that so far has not been fully elucidated. The heterotetramerization of the BMPR ultimately results in the activation of intrinsic receptor serine/threonine kinases, allowing phosphorylation of transcription-regulating Smad proteins.10 The net result of Smad complexes translocating into the nucleus is an interaction with sequence-specific transcription factors that regulate gene expression, including Runx2, Osterix (Osx), and Dlx5 regulating gene activation or repression.12 The p38 mitogen-activated protein (MAP) kinase pathway has also been shown to influence BMP-induced gene expression.12,13 The BMP signaling cascade is tightly regulated at every step by soluble inhibitory proteins such as Noggin and other cysteine knot–containing proteins that prevent the binding of BMP proteins to specific receptors.12,14 In addition, several cryptic (poorly characterized) counter-regulatory proteins exist that block the action of the Smads; Smurfs proteins (Hect type E3 ubiquitin ligases) also interact to degrade Smad proteins,15–17 BMPRs and the transcription factor Runx2.10,18 In general, Smurf proteins are thus considered inhibitory to bone formation (Box 5).19 It has recently been shown that the inflammatory cytokine tumor necrosis factor alpha is also capable of downregulating BMPR-IA and BMPR-II transcripts.20 CLINICAL INDICATIONS AND USES OF BONE MORPHOGENETIC PROTEINS
The BMPs are an important subclass of growth factors within the TGF-b superfamily. BMPs that are commercially available include BMP-2 and BMP-7, both having positive influences on bone healing. At present, the most facile and readily available BMP for surgical use is recombinant BMP-2 (rhBMP-2). Although there are only a few studies in the foot and ankle literature that attest to the efficacy of rhBMP-2 to
Box 4 BMPR substrates and downstream processing Phosphorylated Smads1,5,8 binds to intracellular Smad4 Bound together, P-Smads1,5,8/Smad4 travel to cell nucleus P-Smads1,5,8/Smad4 complex binds to transcription factors; for example, Runx2 in osteoblasts, activating target gene transcription
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Box 5 Antagonists to BMP/Smad -
Noggin: with cystine knot–containing elements, binds BMP-2, BMP-4, and BMP-7 and blocks BMP signaling
-
Smad6: binds type I BMPR, preventing Smads1,5,8 activation (phosphorylation)
-
Tob: an antiproliferative protein, Tob binds Smad1 and Smad5, inhibiting BMP signaling
-
Smurf1 (Smad ubiquitin regulatory factor 1): E3 ubiquitin ligase, binds/degrades Smad1 and Smad5
assist with bone healing, the data are compelling. Several key points must be remembered when considering BMP in clinical practice (see Figs. 1–3):
BMPs are indicated only in patients at high risk for poor bone healing BMPs are an adjuvant modality: growth factors do not substitute for bone loss All aspects of a failed prior bony surgical procedure must be evaluated BMPs do not correct for poor planning/technical execution of a surgery BMP use is often off-label and may be costly, which is a health care consideration
CLINICAL STUDIES AND OUTCOMES
The benefit of BMPs in improving bone healing have been well shown experimentally in animal fracture models.21,22 The use of BMPs in at-risk patients with complex fractures has also been observed clinically to be of benefit in fracture healing (Bibbo C, unpublished data, 2005). However, the greatest utility for BMPs in foot and ankle surgery may be as an adjuvant in arthrodesis procedures. To date, only 2 studies have specifically examined the use of rhBMP-2 in foot and ankle surgery. After first reporting positive results in a pilot study,23 Bibbo and colleagues24 published a retrospective analysis of the adjuvant use of rhBMP-2 in 69 high-risk patients who underwent 112 fusions. Using computed tomography fusion criteria, an overall 96% union rate was observed at a mean time of 11 weeks. All fusion sites were pooled for average time to union but the difference between foot and ankle fusion sites was not statistically significant. In addition, the investigators found that, when indicated, bone graft (including autograft and allograft) did not statistically lessen the fusion rate. Thus, when bone is indicated for structural alignment and construct integrity, it may be safely used in conjunction with rhBMP-2 without an undue negative impact on bone healing rates. COMPLICATIONS AND CONCERNS
In the aforementioned sentinel article,24 an overall 8.7% (6 of 69 patients) complication rate occurred a high-risk patient population. Five of 112 joint fusion sites (in only 3 patients) went on to develop a nonunion, which equaled an overall 4% nonunion rate (4% nonunion rate per fusion site, as well as a 4% nonunion rate per patient). Anatomic fusion sites that went on to nonunion included the transverse tarsal joint complex (talonavicular joint [n 5 1], calcaneocuboid joint [n 5 1], and the subtalar joint [n 5 2]). The transverse tarsal joint nonunions were painless; these were not revised and successfully managed with an in-shoe orthotic. The subtalar joint was partial nonunion and mildly painful, and to date has not needed revision. Wound healing complications occurred in 2 insulin-dependant diabetic patients (2 of 69 patients 5 3%), not proved causal by the use of the BMP. Deep infection
Bone Morphogenetic Proteins
occurred in 1 patient (1 of 69 patients 5 1.5%). Wound and infectious complications were successfully managed with local wound care, negative-pressure dressings, and antibiotics. There were no other adverse events recorded that were related to the use of rhBMP-2. Two articles have recently called into question an association of BMP use and the development of cancer in patients after spine fusion.25,26 Although a complicated topic that potentially has great merit, from an epidemiologic and a vetted scientific standpoint, these studies were poorly constructed and do not prove a causal relationship. In addition, other subsequent published articles have refuted the association of cancer development with BMP use.27–29 An initial database review of the lead author’s foot and ankle patients who were treated with adjuvant rhBMP-2 does not show any correlation with BMP use and the development of cancer (Bibbo C, unpublished data, 2006). The salient outcome points from these studies are (see Figs. 1–3):
BMPs are adjuvants for bone healing, not a substitute for structural bone needs BMPs are useful in many fusion procedures and techniques BMPs are indicated only when risk factors exist for a nonunion BMPs are currently thought to be safe for use in foot and ankle surgery
SUMMARY
The BMPs are a group of growth factors that have varied roles in the development and maintenance of many organ systems. Several of the BMPs have osteogenic potential, and exert their effects via complex and highly regulated pathways. At present, only rhBMP-2 and rhBMP-7 are available for clinical use, but only rhBMP-2 is readily available, and from a practical standpoint is considered the only commercially available BMP. Only a few studies exist on BMP use in foot and ankle surgery, but these have shown promising results with low complication rates. BMP is an adjuvant to bone healing, and does not substitute for structural bone needs. In addition, rhBMP-2 outside spinal fusions is considered to be US Food and Drug Administration off-label, and should be used only in patients who are at high risk for bone healing problems. REFERENCES
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