Overexpression of an osteogenic morphogen in fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva

A 2-year-old boy presented with low-grade fever and multiple progressive painful swellings over upper dorsal trunk and supraclavicular region with progressive stiffening of skin for the last 2 months. Examination revealed dysmorphic face, proximally placed thumb and bilateral hallux valgus. Hence, a diagnosis of Fibrodysplasia Ossificans Progressiva was entertained.

TGF-beta signaling in human skeletal and patterning disorders

Members of the transforming growth factor beta (TGF-beta) family of multifunctional peptides are involved in almost every aspect of development. Model systems, ranging from genetically tractable invertebrates to genetically engineered mice, have been used to determine the mechanisms of TGF-beta signaling in normal development and in pathological situations. Furthermore, mutations in genes for the ligands, receptors, extracellular modulators, and intracellular signaling molecules have been associated with several human disorders. The most common are those associated with the development and maintenance of the skeletal system and axial patterning. This review focuses on the mechanisms of TGF-beta signaling with special emphasis on the molecules involved in human disorders of patterning and skeletal development.

MyoD enhances BMP7-induced osteogenic differentiation of myogenic cell cultures

The muscle-specific, basic helix-loop-helix transcription factor MyoD can induce cells from other mesenchymal lineages to express a skeletal muscle phenotype. Interestingly, MyoD is initially upregulated in myogenic cells incubated with bone morphogenetic proteins (BMPs), a treatment that induces osteogenic differentiation, suggesting that MyoD has a role in BMP-induced osteogenesis of myogenic cells. This possibility is supported by our observations that muscle satellite cells derived from adult MyoD(-/-) mice show severely impaired osteogenic induction by BMP-7 (osteogenic protein 1; OP-1) as indicated by the decreased gene expression of the bone markers alkaline phosphatase, osteocalcin, Runx2/Cbfa1, and Osterix. Ectopic expression of MyoD increased alkaline phosphatase activity and Osterix mRNA expression in response to BMP treatment. Similarly, ectopic expression of MyoD in the pluripotent mesenchymal cell line C3H10T1/2 increased alkaline phosphatase activity induced by BMP-7. Transcription assays showed that transfection with a MyoD-expression vector, but not other myogenic basic helix-loop-helix transcription factors (Myf5, myogenin) increased Runx2/Cbfa1 transactivation of a reporter gene construct containing either six OSE sequences in tandem or a single OSE site. This effect was enhanced by BMP treatment. These studies, therefore, demonstrate that the muscle transcription factor MyoD is required for efficient BMP-induced osteogenesis of myogenic cells and indicate that MyoD might exert its effects through co-operative interactions with Runx2/Cbfa1.

Heterotopic ossification

Heterotopic ossification, the formation of bone in soft tissue, requires inductive signaling pathways, inducible osteoprogenitor cells, and a heterotopic environment conducive to osteogenesis. Little is known about the molecular pathogenesis of this condition. Research into two rare heritable and developmental forms, fibrodysplasia ossificans progressiva and progressive osseous heteroplasia, has provided clinical, pathologic, and genetic insights. In fibrodysplasia ossificans progressiva, overexpression of bone morphogenetic protein 4 and underexpression of multiple antagonists of this protein highlight the potential role of a potent morphogenetic gradient. Research on fibrodysplasia ossificans progressiva also has led to the identification of the genetic cause of progressive osseous heteroplasia: inactivating mutations in the alpha subunit of the gene coding for the stimulatory G protein of adenylyl cyclase. Better understanding of the complex developmental and molecular pathology of these disorders may lead to more effective strategies to prevent and treat other, more common forms of heterotopic ossification.

Retroviral delivery of Noggin inhibits the formation of heterotopic ossification induced by BMP-4, demineralized bone matrix, and trauma in an animal model

BACKGROUND

The heterotopic ossification of muscles, tendons, and ligaments is a common problem faced by orthopaedic surgeons. We investigated the ability of Noggin (a BMP [bone morphogenetic protein] antagonist) to inhibit heterotopic ossification.

METHODS

Part 1: A retroviral vector carrying the gene encoding human Noggin was developed and used to transduce muscle-derived stem cells. Part 2: Cells transduced with BMP-4 were implanted into both hind limbs of mice along with either an equal number, twice the number, or three times the number of Noggin-expressing muscle-derived stem cells (treated limb) or with nontransduced muscle-derived stem cells (control limb). At four weeks, the mice were killed and radiographs were made to look for evidence of heterotopic ossification. Part 3: Eighty milligrams of human demineralized bone matrix was implanted into the hind limbs of SCID (severe combined immunodeficiency strain) mice along with 100,000, 500,000, or 1,000,000 Noggin-expressing muscle-derived stem cells (treated limbs) or nontransduced muscle-derived stem cells (control limbs). At eight weeks, the mice were killed and radiographs were made. Part 4: Immunocompetent mice underwent bilateral Achilles tenotomy along with the implantation of 1,000,000 Noggin-expressing muscle-derived stem cells (treated limbs) or nontransduced muscle-derived stem cells (control limbs). At ten weeks, the mice were killed and radiographs were made.

RESULTS

Part 1: An in vitro BMP inhibition assay demonstrated that Noggin was expressed by muscle-derived stem cells at a level of 280 ng per million cells per twenty-four hours. Part 2: Three varying doses of Noggin-expressing muscle-derived stem cells inhibited the heterotopic ossification elicited by BMP-4-expressing muscle-derived stem cells. Heterotopic ossification was reduced in a dose-dependent manner by 53%, 74%, and 99%, respectively (p < 0.05). Part 3: Each of three varying doses of Noggin-expressing muscle-derived stem cells significantly inhibited the heterotopic ossification elicited by demineralized bone matrix. Heterotopic ossification was reduced by 91%, 99%, and 99%, respectively (p < 0.05). Part 4: All eleven animals that underwent Achilles tenotomy developed heterotopic ossification at the site of the injury in the control limbs. In contrast, the limbs treated with the Noggin-expressing muscle-derived stem cells had a reduction in the formation of heterotopic ossification of 83% and eight of the eleven animals had no radiographic evidence of heterotopic ossification (p < 0.05).

CONCLUSIONS

The delivery of Noggin mediated by muscle-derived stem cells can inhibit heterotopic ossification caused by BMP-4, demineralized bone matrix, and trauma in an animal model.

CLINICAL RELEVANCE

Gene therapy to deliver Noggin may become a powerful method to inhibit heterotopic ossification in targeted areas of the body.

Deletion mutants of BMP folding variants act as BMP antagonists and are efficient inhibitors for heterotopic ossification

Heterotopic ossification is a frequent complication in patients who have suffered head and neck traumas or have undergone total hip replacement. In this report, stable folding variants of the natural occurring osteoinductive BMPs were shown to act as inhibitors for heterotopic ossification. The most effective BMP folding variant construct performed even better than the natural occurring BMP antagonist Noggin because it also inhibited calcium deposition of pre-osteoblastic cells.

INTRODUCTION

Signal transduction through receptor and ligand binding depends on the proper folding of all partners, especially when it involves the formation of a heterotetramer. In the case, the receptor binding of the ligand can be uncoupled from signal transduction, and folding variants of a ligand can be developed into antagonists of the natural bioactivity of the ligand. Here we present a deletion mutant of a bone morphogenetic protein (BMP) folding variant capable of inhibiting the bone-inducing action of natural occurring BMPs.

MATERIALS AND METHODS

Deletion mutants and site-directed mutants of BMP folding variants were generated and tested for their ability to reduce alkaline phosphatase activity and mineralization in a pre-osteoblastic cell line. In vivo activity of the optimized folding variant was determined in a heterotopic ossification model in rodents and in two Xenopus laevis model systems. Biosensor interaction analysis was used to determine the affinity of the optimized BMP folding variant to the extracellular domain of BMP receptors.

RESULTS

In vitro and in vivo tests in rodents revealed that the structural elements of the wrist epitope combined with finger 2 and a positive charge proximal to the tip of this finger are sufficient to induce osteoinhibition with deletion mutants and folding variants of mature BMP-4. The inhibitor designed to suppress heterotopic ossification showed BMP antagonist activity in embryos and animal caps of X. laevis. Binding studies of the inhibitor to ectodomains of type I and type II BMP receptors revealed a concentration-dependent binding, especially to the high-affinity BMP receptor.

CONCLUSIONS

Deletion mutants of BMP folding variants are a new form of BMP antagonists and act through competition with osteoinductive BMP for BMP receptor binding. The excellent in vivo performance of the optimized folding variant is because of its ability to block signaling of endogenous BMPs deposited in the extracellular matrix even more effectively than the natural occurring BMP antagonist Noggin.

Ankylosis of the jaw in a patient with fibrodysplasia ossificans progressiva

A case of fibrodysplasia ossificans progressiva (FOP) is presented. This uncommon connective tissue disease tends to produce progressing ectopic osteogenesis. Because there are no reported curative procedures for TMJ ankylosis occurring in this condition, a palliative surgical approach is described. Etiology, diagnosis, and prognosis of the disease is reviewed. Recent research in BMP cytokine-induced bone repair may allow new approaches to treating this debilitating disease in the future.

In vivo somatic cell gene transfer of an engineered Noggin mutein prevents BMP4-induced heterotopic ossification

BACKGROUND

The formation of the skeleton requires inductive signals that are balanced with their antagonists in a highly regulated negative feedback system. Inappropriate or excessive expression of BMPs (bone morphogenetic proteins) or their antagonists results in genetic disorders affecting the skeleton, such as fibrodysplasia ossificans progressiva. BMP signaling mediated through binding to its receptors is a critical step in the induction of abnormal ossification. Therefore, we hypothesized that engineering more effective inhibitors of this BMP-signaling process may lead to the development of therapies for such conditions.

METHODS

BMP4-induced heterotopic ossification was used as a model for testing the ability of the BMP antagonist Noggin to block de novo bone formation, either by local or systemic delivery. Since Noggin naturally acts locally, a Noggin mutein, hNOGDeltaB2, was engineered and was shown to circulate systemically, and its ability to block heterotopic ossification was tested in a mouse model with use of adenovirus-mediated somatic cell gene transfer.

RESULTS

A mouse model of BMP4-induced heterotopic ossification was developed. Local delivery of wild-type NOG inhibited heterotopic ossification, but systemic administration was ineffective. In contrast, systemic delivery of the adenovirus encoding hNOGDeltaB2 resulted in systemic levels that persisted for more than two weeks and were sufficient to block BMP4-induced heterotopic ossification.

CONCLUSIONS

BMP4-induced heterotopic ossification can be prevented in vivo either by local delivery of wild-type Noggin or after somatic cell gene transfer of a Noggin mutein, hNOGDeltaB2. Furthermore, the data in the present study provide proof of concept that a naturally occurring factor can be engineered for systemic delivery toward a desirable pharmacological outcome.

CLINICAL RELEVANCE

Blocking bone formation is clinically relevant to disorders of heterotopic ossification in humans, such as fibrodysplasia ossificans progressiva. Furthermore, development of BMP antagonists as therapeutic agents may provide modalities for the treatment of other pathologic conditions that arise from aberrant expression of BMPs and/or from a lack of their antagonists.

Structural basis of BMP signaling inhibition by Noggin, a novel twelve-membered cystine knot protein

BACKGROUND

The activity of bone morphogenetic proteins (BMPs) is regulated extracellularly by several families of secreted, negatively-acting factors. These BMP antagonists participate in the control of a diverse range of embryonic processes, such as establishment of the dorsal-ventral axis, neural induction, and formation of joints in the developing skeletal system. The ongoing process of neurogenesis in the adult brain also requires inhibition of BMP ligand activity. To date, the three-dimensional structures of these antagonists as well as the nature of their interaction with ligand have remained unknown. Toward that end, we have determined the crystal structure of the antagonist Noggin bound to BMP-7.

METHODS

The complex of the two homodimeric proteins was preformed, isolated by size exclusion chromatography, and crystallized at neutral pH. To probe the molecular interface of the complex and to quantitate the activity of a human mutant form, variant Noggin proteins were produced and their binding affinities were measured in vitro. The correlation between binding affinity and biological activity was examined with Noggin-soaked beads implanted in the developing chick limb bud.

RESULTS AND CONCLUSIONS

The structure of the complex reveals that Noggin inhibits BMP signaling by blocking the binding sites of both types of receptors (Type I and Type II), mimicking their modes of binding. The affinity of Noggin variants for BMP-7 correlated well with the inhibition of BMP-induced chondrogenesis in the chick limb bud, confirming that Noggin acts by sequestering the ligand in an inactive state. Interestingly, the scaffold of Noggin was found to contain a cystine knot topology and protein fold similar to that of BMPs, indicating that ligand and antagonist may have evolved from a common ancestral gene.

Stromal cells of fibrodysplasia ossificans progressiva lesions express smooth muscle lineage markers and the osteogenic transcription factor Runx2/Cbfa-1: clues to a vascular origin of heterotopic ossification?

Fibrodysplasia ossificans progressiva (FOP) is a rare heritable genetic disorder, which is characterized pathologically by sporadic episodes of explosive growth of mesenchymal cells in skeletal muscle followed by cellular differentiation to heterotopic bone through an endochondral process. This study examined the histological origin and differentiation state of stromal cells in early FOP lesions and investigated the association between the phenotype of these FOP cells and bone formation. Interestingly, FOP lesional stromal cells were found to display characteristics of the smooth muscle (SM) cell lineage and are therefore potentially of vascular origin. These cells co-express multiple SM lineage markers along with multiple proteins associated with bone formation including the obligate osteogenic transcription factor Runx2/Cbfa-1. It is hypothesized that the stromal cells of early FOP lesions may be locally recruited vascular cells or cells of the bone marrow stroma and that these cells maintain the potential (given the correct environmental stimuli) to differentiate along an endochondral ossification pathway.

The serum protein alpha 2-Heremans-Schmid glycoprotein/fetuin-A is a systemically acting inhibitor of ectopic calcification

Ectopic calcification is a frequent complication of many degenerative diseases. Here we identify the serum protein alpha2-Heremans-Schmid glycoprotein (Ahsg, also known as fetuin-A) as an important inhibitor of ectopic calcification acting on the systemic level. Ahsg-deficient mice are phenotypically normal, but develop severe calcification of various organs on a mineral and vitamin D-rich diet and on a normal diet when the deficiency is combined with a DBA/2 genetic background. This phenotype is not associated with apparent changes in calcium and phosphate homeostasis, but with a decreased inhibitory activity of the Ahsg-deficient extracellular fluid on mineral formation. The same underlying principle may contribute to many calcifying disorders including calciphylaxis, a syndrome of severe systemic calcification in patients with chronic renal failure. Taken together, our data demonstrate a critical role of Ahsg as an inhibitor of unwanted mineralization and provide a novel therapeutic concept to prevent ectopic calcification accompanying various diseases.

Anesthetic management in fibrodysplasia ossificans progressiva (FOP): a case report

Fibrodysplasia ossificans progressiva (FOP) is a rare congenital disorder of connective tissue, which has significant implications on anesthetic management of affected patients, e.g., application of difficult airway algorithm, avoiding any trauma during venous cannulation. FOP has received very limited reviews in the published anesthesia literature. We describe perioperative care of a 21-year-old female with established diagnosis of FOP presenting for hysteroscopy followed by dilatation and curettage. Multiorgan abnormalities of FOP are reviewed and available anesthetic options are analyzed and compared.

Bone morphogenetic proteins, their antagonists, and the skeleton

Skeletal homeostasis is determined by systemic hormones and local factors. Bone morphogenetic proteins (BMP) are unique because they induce the differentiation of mesenchymal cells toward cells of the osteoblastic lineage and also enhance the differentiated function of the osteoblast. However, the activity of BMPs needs to be tempered by intracellular and extracellular antagonists. BMPs bind to specific receptors and signal by phosphorylating the cytoplasmic proteins mothers against decapentaplegic (Smad) 1 and 5, which form heterodimers with Smad 4, and after nuclear translocation regulate transcription. BMP antagonists can be categorized as pseudoreceptors that compete with signaling receptors, inhibitory Smads that block signaling, intracellular binding proteins that bind Smad 1 and 5, and factors that induce ubiquitination and proteolysis of signaling Smads. In addition, a large number of extracellular proteins that bind BMPs and prevent their binding to signaling receptors have emerged. They are the components of the Spemann organizer, noggin, chordin, and follistatin, members of the Dan/Cerberus family, and twisted gastrulation. The antagonists tend to be specific for BMPs and are regulated by BMPs, indicating the existence and need of local feedback mechanisms to temper BMP cellular activities.

Paresis of a bone morphogenetic protein-antagonist response in a genetic disorder of heterotopic skeletogenesis

BACKGROUND

Fibrodysplasia ossificans progressiva is a rare genetic disorder characterized by congenital malformations of the great toes and by progressive heterotopic bone formation. Bone morphogenetic protein-4 (BMP-4) messenger ribonucleic acid (mRNA) and protein are uniquely overexpressed in lymphocytes and lesional cells from patients who have fibrodysplasia ossificans progressiva. However, the BMP-4 gene is not mutated in fibrodysplasia ossificans progressiva. The activities of BMPs are specified in part by the formation of morphogen gradients that are further regulated by an array of secreted antagonists. Recent studies have indicated that BMP-4 upregulates the expression of the BMP antagonists noggin, gremlin, and follistatin, thereby establishing an autoregulatory feedback loop. Therefore, a defect in the feedback pathway between BMP-4 and one or more of its extracellular antagonists could contribute to the elevated BMP-4 activity characteristic of fibrodysplasia ossificans progressiva.

METHODS

Basal and BMP-4-induced expression of noggin, gremlin, follistatin, and chordin mRNA were investigated in control and fibrodysplasia ossificans progressiva lymphoblastoid cell lines with use of reverse transcriptase-polymerase chain reaction and Northern analysis.

RESULTS

In the absence of exogenous BMP-4 stimulation (basal state), steady-state levels of all of the BMP antagonists that were investigated were similar in fibrodysplasia ossificans progressiva and control cell lines. Upon stimulation with recombinant human BMP-4, control lymphoblastoid cell lines exhibited a marked increase in expression of noggin and gremlin mRNA. Fibrodysplasia ossificans progressiva cells, however, showed a dramatically attenuated response to BMP-4 stimulation compared with that of controls.

CONCLUSIONS

These data indicate a paresis of a BMP-antagonist response, suggesting the loss of a negative feedback mechanism by which cells normally regulate the magnitude and boundaries of ambient morphogenetic signals. This paresis may account in part for the increased BMP-4 activity in fibrodysplasia ossificans progressiva.

Bone morphogenetic protein-4 regulation in fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva is a rare genetic disorder in which connective tissues are replaced with heterotopic bone through an endochondral process. Bone morphogenetic protein-4 messenger ribonucleic acid and protein levels are elevated in the cells of patients with fibrodysplasia ossificans progressiva, but the molecular mechanism of this steady-state elevation is unknown. Nuclear run-on assays and messenger ribonucleic acid stability assays were done to examine the molecular mechanisms of increased bone morphogenetic protein-4 messenger ribonucleic acid. The bone morphogenetic protein-4 transcription rate in patient cells was found to be enhanced fivefold to sevenfold over normal control cells, suggesting that elevated steady-state levels of this transcript were attributable at least in part to an enhancement in transcription initiation. The stability of bone morphogenetic protein-4 messenger ribonucleic acid was found to be similar for patient and control cells and to have an extremely brief half-life, with bone morphogenetic protein-4 messenger ribonucleic acid almost completely decayed (75%) by 40 minutes. This unusually brief half-life suggests that a high fidelity control over temporal expression of the bone morphogenetic protein 4-message can be maintained. The data document that enhanced transcription rather than increased messenger ribonucleic acid stability is responsible for the elevation in steady-state levels of bone morphogenetic protein-4 messenger ribonucleic acid, and suggest that an inappropriate enhancement of the rate of bone morphogenetic protein-4 transcription plays a critical role in the molecular pathophysiology of fibrodysplasia ossificans progressiva.

Muscle-based gene therapy and tissue engineering to improve bone healing

Failed fracture healing is a significant problem in orthopaedics, often seen in patients with scaphoid fractures, high-energy injuries, and osteoporosis. Current treatments often result in poor outcomes and donor site morbidity. Gene therapy has been the focus of much recent research to improve bone healing. In the current review, the authors specifically evaluate the use of muscle-derived cells as a gene delivery vehicle and inducible osteoprogenitor cell that can enhance bone regeneration. Muscle-derived cells have been used to deliver bone morphogenetic protein-2 and produce ectopic bone. These cells express osteocalcin and have been found within newly generated bone in locations normally occupied by osteoblasts and osteocytes. Finally, it is shown that muscle-derived cells coupled with ex vivo gene therapy can heal critical-sized calvarial defects.

Fibrodysplasia ossificans progressiva and corneal keloid

PURPOSE

To report on the formation of a corneal keloid in a patient with fibrodysplasia ossificans progressiva after excision of a pterygium-like lesion.

METHODS

Clinical and pathophysiological observations and hypothesis concerning pathophysiological mechanisms.

RESULTS

An 11-year-old boy with fibrodysplasia ossificans progressiva presented with progressive development of a growing white scar in the central part of the left cornea after excision of a pterygium-like lesion.

DISCUSSION

To the best of our knowledge, this is the first case report of a corneal keloid formation in a patient with fibrodysplasia ossificans progressiva. Pathophysiological mechanisms are considered. Therapeutic options are discussed.

BMP4-expressing muscle-derived stem cells differentiate into osteogenic lineage and improve bone healing in immunocompetent mice

Recent advances in molecular biology have led the way for novel approaches to improve bone healing. The ideal growth factor, vector, and delivery systems for producing bone in an immune competent animal model, however, have yet to be identified. Using a retrovirus encoding BMP4 and recently isolated muscle-derived stem cells (MDSCs), we demonstrated the following: MDSCs undergo osteogenic differentiation in response to BMP4 in a dose-dependent manner; retrovirus encoding BMP4 can efficiently transduce MDSCs, both enhancing osteogenic differentiation and inhibiting myogenic differentiation; transduced MDSCs can produce high levels of functional BMP4 as they differentiate toward an osteogenic lineage; allogeneic transduced MDSCs can induce robust de novo bone formation in immunocompetent mice despite the presence of an immune reaction, demonstrating the ability of this retroviral-BMP4-muscle construct to provide sufficient stimuli for osteoinduction in vivo; MDSCs appear to deliver BMP4, respond to the human BMP4 in an autocrine manner, and actively participate in bone formation, thus serving both osteoinductive and osteoproductive roles; and the BMP4-expressing MDSCs can induce bone formation and improve bone healing in a critical-sized skull defect in immunocompetent mice. Therefore, we believe that technology based on the MDSCs and vector system has great potential for promoting bone healing in a variety of musculoskeletal conditions.

Muscle development genes: their relevance in neuromuscular disorders

Myogenesis is a complex cascade of events that involves the specification and differentiation of muscle precursor cells or myoblasts, their fusion to form primary and secondary myotubes and subsequent maturation into muscle fibres. In addition, the development of axial muscle requires the migration of muscle precursor cells. These events are under strict genetic control. The contribution of individual genes to this process has been highlighted both by the phenotype of mice with targeted inactivation of individual myogenic regulatory factors and by rare human disorders in which the involvement of these genes has been demonstrated. The inactivation of known myogenic regulatory genes is associated with abnormal regulation of skeletal muscle differentiation and has an effect on regeneration but does not cause progressive muscle weakness or wasting. This review summarises recent developments in this field and will be of particular relevance to those interested in neuromuscular disorders. We also examine the possibility that some rare human conditions associated with abnormal muscle formation may be due to genetic defects in one of the myogenic regulatory genes.

Regulatory mechanisms of osteoblast and osteoclast differentiation

Bone is continuously destroyed and reformed to maintain constant bone volume and calcium homeostasis in vertebrates throughout their lives. Osteoblasts and osteoclasts are specialized cells responsible for bone formation and resorption, respectively. Recent developments in bone cell biology have greatly changed our conceptions of the regulatory mechanisms of the differentiation of osteoblasts and osteoclasts. Bone morphogenetic proteins (BMPs) play critical roles in osteoblast differentiation. The discovery of Smad-mediated signals revealed the precise functions of BMPs in osteoblast differentiation. Transcription factors, Runx2 and Osterix, are found to be essential molecules for inducing osteoblast differentiation, as indicated by the fact that both Runx2-null mice and Osterix-null mice have neither bone tissue nor osteoblasts. Smad transcriptional factors are shown to interact with other transcription regulators, including Runx2. Also, the recent discovery of receptor activator of NF-kappaB ligand (RANKL)-RANK interaction confirms the well-known hypothesis that osteoblasts play an essential role in osteoclast differentiation. Osteoblasts express RANKL as a membrane-associated factor. Osteoclast precursors that express RANK, a receptor for RANKL, recognize RANKL through the cell-cell interaction and differentiate into osteoclasts. Recent studies have shown that lipopolysaccharide and inflammatory cytokines such as tumor necrosis factor receptor-alpha and interleukin I directly regulate osteoclast differentiation and function through a mechanism independent of the RANKL-RANK interaction. Transforming growth factor-beta super family members and interferon-gamma are also shown to be important regulators in osteoclastogenesis. These findings have opened new areas for exploring the molecular mechanisms of osteoblast and osteoclast differentiation.

Bone grafting alternatives in spinal surgery

BACKGROUND CONTEXT

Bone grafting is used to augment bone healing and provide stability after spinal surgery. Autologous bone graft is limited in quantity and unfortunately associated with increased surgical time and donor-site morbidity. Alternatives to bone grafting in spinal surgery include the use of allografts, osteoinductive growth factors such as bone morphogenetic proteins and various synthetic osteoconductive carriers.

PURPOSE

Recent research has provided insight into methods that may modulate the bone healing process at the cellular level in addition to reversing the effects of symptomatic disc degeneration, which is a potentially disabling condition, managed frequently with various fusion procedures. With many adjuncts and alternatives available for use in spinal surgery, a concise review of the current bone grafting alternatives in spinal surgery is necessary.

STUDY DESIGN/SETTING

A systematic review of the contemporary English literature on bone grafting in spinal surgery, including abstract information presented at national meetings.

METHODS

Bone grafting alternatives were reviewed as to their efficacy in extending or replacing autologous bone graft sources in spinal applications.

RESULTS

Alternatives to autologous bone graft include allograft bone, demineralized bone matrix, recombinant growth factors and synthetic implants. Each of these alternatives could possibly be combined with autologous bone marrow or various growth factors. Although none of the presently available substitutes provides all three of the fundamental properties of autograft bone (osteogenicity, osteoconductivity and osteoinductivity), there are a number of situations in which they have proven clinically useful.

CONCLUSIONS

Alternatives to autogenous bone grafting find their greatest appeal when autograft bone is limited in supply or when acceptable rates of fusion may be achieved with these substitutes (or extenders) despite the absence of one or more of the properties of autologous bone graft. In these clinical situations, the morbidity of autograft harvest is reasonably avoided. Future research may discover that combinations of materials may cumulatively result in the expression of osteogenesis, osteoinductivity and osteoconductivity found in autogenous sources.

[Neurogenic ectopic ossification: a physiatry update]

The first aim of this paper is to present a critical review of the existing literature on neurogenic ectopic ossification. The second aim is to introduce the two papers published in this issue of the journal. In addition, etiopathogenic aspects are described as well as some insights in classical therapies and new therapeutic options which could prove to be useful in the near future.

How does recombinant human bone morphogenetic protein-4 enhance posterior spinal fusion?

STUDY DESIGN

A rabbit posterolateral intertransverse process fusion model was used to evaluate the effect that different doses of recombinant human bone morphogenetic protein-4 delivered in a porous hydroxyapatite-tricalcium phosphate ceramic had on osteogenesis and spinal fusion.

OBJECTIVE

To study the biologic effect and threshold dose of recombinant human bone morphogenetic protein-4 in enhancing spinal fusion.

SUMMARY OF BACKGROUND DATA

Biologic manipulation for spinal fusion is an area undergoing active research. The enhancing effects of recombinant human bone morphogenetic proteins 2 and 7 on spinal fusion have been proved, and clinical trials of their application are in progress. Recombinant human bone morphogenetic protein-4 is another osteoinductive protein that has the ability to induce heterotopic bone formation, and its potential for enhancing spinal fusion has not yet been studied.

METHODS

For this study, 24 adult New Zealand white rabbits underwent single-level unilateral posterior intertransverse process spinal fusion at L5-L6. The animals were divided into four groups using different graft materials: allograft as well as hydroxyapatite-tricalcium phosphate augmented with 0, 1.25, and 5 microgram of recombinant human bone morphogenetic protein-4, respectively. The local changes were evaluated by sequential radiograph, manual palpation, histomorphology, and microradiography.

RESULTS

At week 7, ossification in the intertransverse process area ceased in groups without recombinant human bone morphogenetic protein-4, whereas active multicentric endochondral bone formation was demonstrated in groups with this growth factor. The success rate of contiguous bony bridging was found to correlate positively with the dose of recombinant human bone morphogenetic protein-4.

CONCLUSIONS

Recombinant human bone morphogenetic protein-4 effectively enhances new bone formation and accelerates fusion in the rabbit posterolateral posterior spinal fusion model. The effective dose of recombinant human bone morphogenetic protein-4 is 10 times lower than the reported dosage of recombinant human bone morphogenetic proteins 2 and 7.

Age-related bone loss: old bone, new facts

The human skeleton serves several functions for the body: support, locomotion, protection of vital organs, and housing of bone marrow. Bone remodeling is the result of the interactions of multiple elements, including osteoblasts, osteoclasts, hormones, growth factors, and cytokines, the end result being the maintenance of the bone architecture and to maintain systemic calcium homeostasis. In early life, a careful balance exists between bone formation by osteoblasts and bone resorption by osteoclasts. With aging, the process of coupled bone formation is affected by the reduction of osteoblast differentiation, activity, and life span which is further potentiated in the perimenopausal years with hormone deprivation and increased osteoclast activity. Age-related bone loss is thus not only a consequence of hormone deprivation, but also the result of changes in bone formation and cell-cell interactions with a unique pathophysiology. In this review, we describe the cellular and metabolic changes associated with aging bone and present recent evidence regarding cell differentiation within the bone marrow. We also consider the mechanism of programmed cell death, apoptosis, as being an important determinant of aging in bone as well as describe possible future interventions to prolong the life span of osteoblasts.

Current status of human chromosome 14

Over the past three decades, extensive genetic, physical, transcript, and sequence maps have assisted in the mapping of over 30 genetic diseases and in the identification of over 550 genes on human chromosome 14. Additional genetic disorders were assigned to chromosome 14 by studying either constitutional or acquired chromosome aberrations of affected subjects. Studies of benign and malignant tumours by karyotype analyses and by allelotyping with a panel of polymorphic genetic markers have further suggested the presence of several tumour suppressor loci on chromosome 14. The search for disease genes on human chromosome 14 has also been achieved by exploiting the human-mouse comparative maps. Research on uniparental disomy and on the search for imprinted genes has supported evidence of epigenetic inheritance as a result of imprinting on human chromosome 14. This review focuses on the current developments on human chromosome 14 with respect to genetic maps, physical maps, transcript maps, sequence maps, genes, diseases, mouse-human comparative maps, and imprinting.

The skeletal muscle satellite cell: stem cell or son of stem cell?

The concept of the adult tissue stem cell is fundamental to models of persistent renewal in functionally post-mitotic tissues. Although relatively ignored by stem cell biology, skeletal muscle is a prime example of an adult tissue that can generate terminally differentiated cells uniquely specialized to carry out tissue-specific functions. This capacity is attributed to satellite cells, a population of undifferentiated, quiescent precursors that become activated to divide and differentiate in response to the demands of growth or damage. The aim of this review is to discuss the role of the satellite cell as an adult tissue-specific stem cell. We examine evidence for the presence of behaviourally and phenotypically distinct subpopulations of precursor within the satellite cell pool. Further, we speculate on the possible identity, origins and relevance of multipotent muscle stem cells, a population with both myogenic and hematopoietic potentials that has been isolated from whole muscle. Taken together, current evidence suggests the possibility that the regenerative compartment of adult skeletal muscle may conform to an archetypal stem cell-based hierarchy, maintained within a stem cell niche. It therefore remains to be seen whether all satellite cells are skeletal muscle-specific stem cells, or whether some or all are the progeny of an as yet unidentified muscle stem cell.

Born again bone: tissue engineering for bone repair

Destruction of bone tissue due to disease and inefficient bone healing after traumatic injury may be addressed by tissue engineering techniques. Growth factor, cytokine protein, and gene therapies will be developed, which, in conjunction with suitable carriers, will regenerate missing bone or help in cases of defective healing.

Muscle satellite cells are multipotential stem cells that exhibit myogenic, osteogenic, and adipogenic differentiation

Muscle satellite cells are believed to represent a committed stem cell population that is responsible for the postnatal growth and regeneration of skeletal muscle. However, the observation that cultured myoblasts differentiate into osteocytes or adipocytes following treatment with bone morphogenetic proteins (BMPs) or adipogenic inducers, respectively, suggests some degree of plasticity within the mesenchymal lineage. To further investigate this phenomenon, we explore the osteogenic and adipogenic potential of satellite cells isolated from adult mice. Our experiments clearly demonstrate that satellite cell-derived primary myoblasts, expressing myogenic markers such as MyoD, Myf5, Pax7 and desmin, differentiated only into osteocytes or adipocytes following treatment with BMPs or adipogenic inducers, respectively However, satellite cells on isolated muscle fibers cultured in Matrigel readily differentiated into myocytes as well as osteogenic and adipogenic lineages, whereas primary myoblasts did not. Satellite cell-derived primary myoblasts isolated from mice lacking the myogenic transcription factor MyoD (MyoD-/-) differentiate into myocytes poorly in vivo and in vitro (Megeney et al., Genes Dev. 1996; Sabourin et. al, J. Cell Biol., 1999). Therefore, we tested whether MyoD-/- primary myoblasts display increased plasticity relative to wild type cells. Unexpectedly, the osteogenic or adipogenic differentiation potential of MyoD-/- primary myoblasts did not increase compared to wild-type cells. Taken together, these results strongly suggest that muscle satellite cells possess multipotential mesenchymal stem cell activity and are capable of forming osteocytes and adipocytes as well as myocytes.

Identification of three novel mutations of the noggin gene in patients with fibrodysplasia ossificans progressiva

We report noggin mutations in three Spanish families with fibrodysplasia ossificans progressiva (FOP). The three propositi have typical FOP findings; in the first and third families the parents are unaffected, while in the second family the father is partially affected. DNA of the three propositi and their parents was screened by sequencing for mutations in the noggin gene (NOG). Sequencing indicated a G to C mutation at nucleotide 274 of the NOG gene in the first propositus, encoding for the G92R substitution at the peptide level; this first mutation is de novo, the corresponding change not being observed in parents. In the second propositus, a G to T mutation at nucleotide 271 encodes for the G91C substitution, transmitted in the corresponding family by the partially affected father. In the third propositus, sequencing indicated a G to A mutation at nucleotide 275, encoding for the G92E substitution; this third mutation is de novo. All three mutations, as well as the Delta42 deletion already reported, resulted in the alteration of the portion of the NOG gene at positions 265-282, encoding for the potential N-myristoylation site at residues 89-GGGGGA-94.

Mast cell involvement in fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (FOP) is a catastrophic genetic disorder of progressive heterotopic ossification associated with dysregulated production of bone morphogenetic protein 4 (BMP4), a potent osteogenic morphogen. Postnatal heterotopic ossification in FOP is often heralded by hectic episodes of severe post-traumatic connective tissue swelling and intramuscular edema, followed by an intense and highly angiogenic fibroproliferative mass. The abrupt appearance, intense size, and rapid intrafascial spread of the edematous preosseous fibroproliferative lesions implicate a dysregulated wound response mechanism and suggest that cells and mediators involved in inflammation and tissue repair may be conscripted in the growth and progression of FOP lesions. The central and coordinate role of inflammatory mast cells and their mediators in tissue edema, wound repair, fibrogenesis, angiogenesis, and tumor invasion prompted us to investigate the potential involvement of mast cells in the pathology of FOP lesions. We show that inflammatory mast cells are present at every stage of the development of FOP lesions and are most pronounced at the highly vascular fibroproliferative stage. Mast cell density at the periphery of FOP lesional tissue is 40- to 150-fold greater than in normal control skeletal muscle or in uninvolved skeletal muscle from FOP patients and 10- to 40-fold greater than in any other inflammatory myopathy examined. These findings document mobilization and activation of inflammatory mast cells in the pathology of FOP lesions and provide a novel and previously unrecognized target for pharmacologic intervention in this extremely disabling disease.

Familial calcium crystal diseases: what have we learned?

The spectrum of heterotopic calcification or ossification is expanding because of the reports of several kindreds with calcium pyrophosphate deposition disease, apatite deposition disease, and others with less common syndromes associated with extracellular matrix calcification, such as fibrodysplasia ossificans progressiva and related syndromes. Genomic DNA studies in both humans and mice provide a shortcut to understanding the genetic basis of promotion and prevention of ECM calcification. Mutation in the COL2A1 gene has been identified in one family with spondyloepiphyseal dysplasia and calcium pyrophosphate and apatite crystalline deposits. In another kindred with precocious osteoarthritis without spondyloepiphyseal dysplasia, the phenotype was linked to markers of chromosome 8. In four other kindreds, the phenotypes were linked to an area of chromosome 5p. Two genes located in this region, which are expressed in articular cartilage, are being investigated as possible calcium pyrophosphate deposition disease genes. The results of linkage studies in three kindreds with articular/periarticular ADD with the COL2A1 gene were noninformative. Two different mouse mutations, the ank/ank and the ttw/ttw mice, are associated with intra-articular and ligament apatite deposits caused by a decrease in extracellular pyrophosphate concentrations, mimicking human arthritis caused by apatite deposition disease. Mutations in the matrix GLA protein, both in mice and in humans, are also associated with vascular and articular calcification. These mouse mutations provide cutting-edge information in the investigation of the mechanisms of apatite deposition in humans.

Bone regeneration: new findings and potential clinical applications

Bone is a biologically privileged tissue in that it has the capacity to undergo regeneration as part of a repair process. Fracture healing is the most common and recognizable form of bone regeneration, but several other examples of bone regeneration have been observed in humans, suggesting that the ability to regulate bone regeneration as a therapeutic tool should be possible. Historically, efforts at limb lengthening have led to procedures for regenerating bone, such as the method of Ilizarov. This procedure, known as distraction osteogenesis, has applications in a variety of skeletal conditions, including the restoration of large skeletal defects, the transport of bone in cases of severe trauma with bone loss, and the correction of skeletal deformities. Fibrodysplasia ossificans progressiva is an example of how an abnormal metabolic condition can be viewed as evidence for the capacity of humans to regenerate large amounts of bone if the cellular and molecular signaling events are altered. Elucidation of the cellular and molecular basis for bone regeneration in humans - particularly the role of the human genome in relation to the expression of various growth factors and cytokines, such as the bone morphogenetic proteins - offers great potential for the treatment of orthopaedic conditions. Development of specific bone morphogenetic proteins as therapeutic substances to induce bone regeneration in patients is well under way. As methods for enhancing fracture healing, distraction osteogenesis, and other procedures are refined, the development of protein- and gene-based therapies for regulating bone formation should lead to a new era of orthopaedic practice.

Expression of IL-5 alters bone metabolism and induces ossification of the spleen in transgenic mice

We have developed a transgenic mouse line, NJ.1638, which expresses high levels of IL-5 from T cells, with profound hematological consequences. Eosinophils comprise more than 60% of circulating white blood cells in these animals, with the total peripheral white blood cell counts increasing more than 40-fold relative to wild-type littermates. This extraordinary proliferative capacity is sustained by expanded sites of extramedullary hematopoiesis and is accompanied by multifocal, ectopic bone formation in the spleen. Histology of the splenic nodules revealed the presence of osteoid matrices and osteocytes trapped within mineralized trabecular plates. In addition, polarized light microscopy of calcified tissue sections revealed both woven bone and areas of organized lamellar bone. Morphometric assessments demonstrated that both the growth and mineralization of splenic bone occurred at rates nearly an order of magnitude higher than in skeletal bone. Skeletal bone metabolic parameters were also perturbed. We also observed heterotopic ossification of the spleen and perturbation of skeletal bone homeostasis following adoptive engraftment of transgenic marrow to wild-type recipients. These data suggest that IL-5 overexpression mediates bone formation through the mobilization of marrow-derived osteogenic progenitors and/or the inhibition of recruited osteoclasts.

Thoraco-pulmonary disproportion, scoliosis, kyphosis, frozen spine, dystonia and other muscular disorders, including those of the universal contractile chambers

A nervous system with muscle to do its bidding elevates the animal above the plant world. Much of what happens in the body, good or bad, involves muscular action. A muscle may underact or quit. It may also overact, even to the point of rupturing. Since muscles come in pairs, a muscular action is a double effect requiring coordination between the mated muscles. Cooperation between paired muscles is sometimes lacking and both muscles contract or relax simultaneously so the desired effect does not happen, or destructive conflict develops which leads to various secondary disorders and chain reactions. Muscles react favorably/unfavorably to whatever stimuli they get.

Bone formation and inflammation in cardiac valves

BACKGROUND

For nearly a century, the mechanical failure of calcified heart valves was attributed to a passive degenerative process. Recently, several case reports described bone formation in surgically excised heart valves and suggested an unexpected process of tissue repair.

METHODS AND RESULTS

We studied the prevalence and pathology of heterotopic ossification in 347 surgically excised heart valves (256 aortic, 91 mitral) in 324 consecutive patients (182 men, 142 women; mean age 68 years) who underwent cardiac valve replacement surgery between 1994 and 1998. The valves were examined microscopically to determine the prevalence and features of bone formation and remodeling. Two hundred eighty-eight valves (83%) had dystrophic calcification. Mature lamellar bone with hematopoietic elements and active bone remodeling were present in 36 valves (13%) with dystrophic calcification. Endochondral bone formation, similar to that seen in normal fracture repair, was identified in 4 valves. Microfractures were present in 92% of all valves with ossification. Neoangiogenesis was found in all valves with ossification. Bone morphogenetic proteins 2 and 4 (BMP 2/4), potent osteogenic morphogens, were expressed by myofibroblasts and preosteoblasts in areas adjacent to B- and T-lymphocyte infiltration in valves where ossification was identified. Mast cells were present in calcified and ossified valves and were especially prominent in atheromatous regions.

CONCLUSIONS

Heterotopic ossification consisting of mature lamellar bone formation and active bone remodeling is a relatively common and unexpected finding in end-stage valvular heart disease and may be associated with repair of pathological microfractures in calcified cardiac valves.

Morphogenesis of the turtle shell: the development of a novel structure in tetrapod evolution

The turtle shell is an evolutionary novelty that is synapomorphic for chelonians. The carapace is initiated by the entrapment of the ribs by the carapacial ridge (CR), a lateral bulge of the dorsal ectoderm and dermal mesoderm. The mechanisms by which the CR is initiated, the ribs entrapped and the dorsal dermis ossified, remains unknown. Similarly, the formation of the plastron remains unexplained. Here, we present a series of anatomical investigations into plastron and carapace formation in the red-eared slider, Trachemys scripta, and the snapping turtle, Chelydra serpentina. We document the entrapment of the ribs by the CR and the formation of the plastron and carapacial bones by intramembranous ossification. We note the formation of the ossification centers around each rib, which suggest that the rib is organizing dermal ossification by secreting paracrine factors. The nuchal ossification center is complex and appears to involve multiple bone-forming regions. Individual ossification centers at the periphery of the carapace form the peripheral and pygial bones. The intramembranous ossification of the plastron proceeds from nine distinct ossification centers, and there appear to be interactions between the spicules of apposing centers as they draw near each other.

Osteoprogenitor cells within skeletal muscle

The formation of ectopic bone within skeletal muscle is a widely observed phenomenon. However, the source of the osteoprogenitor cells responsible for ectopic bone formation remains unknown. This study was designed to test for osteogenic differentiation among cells isolated from skeletal muscle tissue. Different subpopulations of cells derived from an adult mouse skeletal muscle were tested for induction of alkaline phosphatase activity after exposure to bone morphogenetic protein-2 in vitro. A responsive subpopulation was identified, transduced with a retrovirus encoding for beta-galactosidase (Rv-lacZ) and an adenoviral construct encoding for one bone morphogenetic protein-2, and injected into the hindlimb of immune compromised (severe combined immunodeficient, or SCID) mice. The injected cells appeared to actively participate in the ectopic bone formation. The existence of lacZ-positive muscle-derived cells colocalized with osteocalcin-producing cells within lacunae of newly formed bone matrix suggests osteoblast and osteocyte differentiation. Although a specific cell was not isolated, these data support the contentions that osteoprogenitor cells reside within skeletal muscle and that muscle may represent a source other than bone marrow for the harvest of these cells.

Progressive osseous heteroplasia

Progressive osseous heteroplasia (POH) is a recently described genetic disorder of mesenchymal differentiation characterized by dermal ossification during infancy and progressive heterotopic ossification of cutaneous, subcutaneous, and deep connective tissues during childhood. The disorder can be distinguished from fibrodysplasia ossificans progressiva (FOP) by the presence of cutaneous ossification, the absence of congenital malformations of the skeleton, the absence of inflammatory tumorlike swellings, the asymmetric mosaic distribution of lesions, the absence of predictable regional patterns of heterotopic ossification, and the predominance of intramembranous rather than endochondral ossification. POH can be distinguished from Albright hereditary osteodystrophy (AHO) by the progression of heterotopic ossification from skin and subcutaneous tissue into skeletal muscle, the presence of normal endocrine function, and the absence of a distinctive habitus associated with AHO. Although the genetic basis of POH is unknown, inactivating mutations of the GNAS1 gene are associated with AHO. The report in this issue of the JBMR of 2 patients with combined features of POH and AHO--one with classic AHO, severe POH-like features, and reduced levels of Gsalpha protein and one with mild AHO, severe POH-like features, reduced levels of Gsalpha protein, and a mutation in GNAS1--suggests that classic POH also could be caused by GNAS1 mutations. This possibility is further supported by the identification of a patient with atypical but severe platelike osteoma cutis (POC) and a mutation in GNAS1, indicating that inactivating mutations in GNAS1 may lead to severe progressive heterotopic ossification of skeletal muscle and deep connective tissue independently of AHO characteristics. These observations suggest that POH may lie at one end of a clinical spectrum of ossification disorders mediated by abnormalities in GNAS1 expression and impaired activation of adenylyl cyclase. Analysis of patients with classic POH (with no AHO features) is necessary to determine whether the molecular basis of POH is caused by inactivating mutations in the GNAS1 gene.

Role of B lymphocytes in new bone formation

Although there may be a close relationship between B lymphocytes and osteoclasts, or bone resorbing cells, little is known about the role of B lymphocytes in bone formation. We compared in vivo new bone induction in mice homozygous for the B-cell deficient (microMT) gene knockout, which lack functional B lymphocytes, with bone induction in control wild-type (C57BL/6) mice. Our comparison used two models of new bone induction in vivo: endochondral osteoinduction by subcutaneous implantation of recombinant human bone morphogenetic protein (rhBMP-2) and osteogenic regeneration after tibial bone marrow ablation. The expression of bone-specific proteins (bone sialoprotein, osteopontin, and osteocalcin) and inflammatory/immunomodulatory cytokines (interleukin-1alpha and -1beta, interleukin-6, and tumor necrosis factor-alpha) was assessed by Northern blot analysis or reverse transcription-polymerase chain reaction, respectively. Ossicles induced by rhBMP-2 were larger in volume and mass in microMT knockout mice, but relative volumes of the newly induced bone, cartilage, and bone marrow were similar in the two groups. Six days after tibial bone marrow ablation, microMT knockout mice resorbed the initial blood clot faster and formed more trabecular bone, paralleled by greater levels of bone sialoprotein mRNA than in the wild-type mice. microMT knockout and wild-type mice also differed in the expression pattern of inflammatory/immunomodulatory cytokines during the development of the newly induced bone, suggesting that a genetic lack of B lymphocytes may create a change in the immunological milieu at the site of new bone induction, which stimulates the initial accumulation and proliferation of mesenchymal progenitor.

Linkage exclusion and mutational analysis of the noggin gene in patients with fibrodysplasia ossificans progressiva (FOP)

Fibrodysplasia ossificans progressiva (FOP) is an extremely rare and disabling genetic disorder characterized by congenital malformation of the great toes and by progressive heterotopic endochondral ossification in predictable anatomical patterns. Although elevated levels of bone morphogenetic protein 4 (BMP4) occur in lymphoblastoid cells and in lesional cells of patients with FOP, mutations have not been identified in the BMP4 gene, suggesting that the mutation in FOP may reside in a BMP4-interacting factor or in another component of the BMP4 pathway. A powerful antagonist of BMP4 is the secreted polypeptide noggin. A recent case report described a heterozygous 42-bp deletion in the protein-coding region of the noggin gene in a patient with FOP. In order to determine if noggin mutations are a widespread finding in FOP, we examined 31 families with 1 or more FOP patients. Linkage analysis with an array of highly polymorphic microsatellite markers closely linked to the noggin gene was performed in four classically-affected multigenerational FOP families and excluded linkage of the noggin locus to FOP (the multipoint lod score was -2 or less throughout the entire range of markers). We sequenced the noggin gene in affected members of all four families, as well as in 18 patients with sporadic FOP, and failed to detect any mutations. Single-strand conformation polymorphism (SSCP) analysis of 4 of these patients plus an additional 9 patients also failed to reveal any mutations. Among the samples analyzed by SSCP and DNA sequencing was an independently obtained DNA sample from the identical FOP patient previously described with the 42-bp noggin deletion; no mutation was detected. Examination of the DNA sequences of 20 cloned noggin PCR products, undertaken to evaluate the possibility of a somatic mutation in the noggin gene which could be carried by a small subset of white blood cells, also failed to detect the presence of the reported 42-bp deletion. We conclude that mutations in the coding region of noggin are not associated with FOP.

Osteogenic protein-1 differentially regulates the mRNA expression of bone morphogenetic proteins and their receptors in primary cultures of osteoblasts

The mRNA expression patterns of several bone morphogenetic proteins (BMPs) and their receptors (BMPRs) in long-term primary cultures of fetal rat calvaria (FRC) cells were examined by Northern analysis. Their temporal orders of expression were correlated with those of several biochemical markers characteristic of osteoblastic cell differentiation. Distinct temporal patterns of expression of BMPs and BMPRs during osteoblastic cell differentiation were observed. BMP-2 and BMP-7 mRNA levels did not change significantly. BMP-4 mRNA expression increased and reached a peak prior to matrix formation. BMP-5 mRNA expression increased during the mineralization phase and BMP-6 mRNA expression increased throughout all phases of cell differentiation. Effects of BMP-7 (Osteogenic Protein-1; OP-1) on the expression patterns of several other members of the BMP family and the receptors were also studied. OP-1 downregulated the BMP-4, -5, and -6 mRNA levels by a maximal of 2-fold, 1.5-fold, and 6-fold, respectively. OP-1 did not change significantly the OP-1 and BMP-2 mRNA expression. Of the three type I BMPR examined, OP-1 upregulated ActR-I and BMPR-IA mRNA expression slightly but with statistical significance. OP-1 downregulated BMPR-IB mRNA expression slightly. OP-1 upregulated BMPR-II mRNA expression by a maximum of 2-fold. Our findings demonstrate that OP-1 differentially regulates the mRNA expression of several related members of the BMP family and their receptors in osteoblasts. The observations suggest that OP-1 action on osteoblastic cells involves a complex regulation of gene expression of related members of the BMP family and their receptors in a cell differentiation stage dependent manner.

Practical illustrations in tissue engineering: surgical considerations relevant to the implantation of osteoinductive devices

This paper provides practical illustrations in the use of osteoinductive devices (biomaterial carriers coupled with osteoinductive morphogens) for bone tissue engineering. We discuss the considerations relative to the implantation of these devices that may induce tissues that are located outside the boundaries of the osteoinductive device as well as outside boundaries of the normal skeletal envelope. Five reports of osteoinductive devices generating such tissues are reviewed. Histologic and radiographic data from a sixth example are presented and compared with histologic and radiographic findings typical of two varieties of myositis ossificans. A theory is advanced that osteoinductive implants may induce ectopic tissues that resemble fibro-osseous pathologies. Finally characteristics of tissue-engineered bone graft substitutes that may contribute to development of these pathologies and device characteristics that may obviate these ectopic tissues are considered.

Principles of tissue engineering applied to programmable osteogenesis

This article presents a strategy for design, engineering, and fabrication of a bioresorbable, manufactured bone graft substitute (BGS) device. The approach is based on established precepts of osteogenesis, molecular biology of hyaluronic acid and osteoinductive proteins, and theoretical preformance criteria for such a device collated from the literature of 1991 to 1996. Application of this design and engineering strategy results in a composite device consisting of a D,D-L,L-polylactic acid macrostructure optimized to the architecture of cancellous bone, a microstructure composed of a filamentous velour of hyaluronan and a recombinant human bone morphogenetic protein 2 (rhBMP-2). The performance of this construct was tested in vivo in the dog, intertransverse process, spinal fusion model and in a critical sized defect of the rabbit radius. Data from these studies are used to illustrate principle points of the design and engineering concept.