Human nonunion tissues display differential gene expression in comparison to physiological fracture callus

The healing of bone fractures can become aberrant and lead to nonunions which in turn have a negative impact on patient health. Understanding why a bone fails to normally heal will enable us to make a positive impact in a patient's life. While we have a wealth of molecular data on rodent models of fracture repair, it is not the same with humans. As such, there is still a lack of information regarding the molecular differences between normal physiological repair and nonunions. This study was designed to address this gap in our molecular knowledge of the human repair process by comparing differentially expressed genes (DEGs) between physiological fracture callus and two different nonunion types, hypertrophic (HNU) and oligotrophic (ONU). RNA sequencing data revealed over ∼18,000 genes in each sample. Using the physiological callus as the control and the nonunion samples as the experimental groups, bioinformatic analyses identified 67 and 81 statistically significant DEGs for HNU and ONU, respectively. Out of the 67 DEGs for the HNU, 34 and 33 were up and down-regulated, respectively. Similarly, out of the 81 DEGs for the ONU, 48 and 33 were up and down-regulated, respectively. Additionally, we also identified common genes between the two nonunion samples; 8 (10.8 %) upregulated and 12 (22.2 %) downregulated. We further identified many biological processes, with several statistically significant ones. Some of these were related to muscle and were common between the two nonunion samples. This study represents the first comprehensive attempt to understand the global molecular events occurring in human nonunion biology. With further research, we can perhaps decipher new molecular pathways involved in aberrant healing of human bone fractures that can be therapeutically targeted.

Stem Cells and Their Derivatives: An Implication for the Regeneration of Nonunion Fractures

Despite advances in biomedical research, fracture nonunion rates have remained stable throughout the years. Long-bone fractures have a high likelihood of nonunion, but the specific biological pathways involved in this severe consequence are unknown. Fractures often heal in an organized sequence, including the production of a hematoma and an early stage of inflammation, the development of a soft callus and hard callus, and eventually the stage of bone remodeling. Deficient healing can result in a persistent bone defect with instability, discomfort, and loss of function. In the treatment of nonunions, mesenchymal stem cells (MSCs) prove to be a promising and safe alternative to the standard therapeutic strategies. Moreover, novel scaffolds are being created in order to use a synergistic biomimetic technique to rapidly generate bone tissue. MSCs respond to acellular biomimetic matrices by regenerating bone. Extracellular vesicles (EVs) derived from MSCs have recently gained interest in the field of musculoskeletal regeneration. Although many of these techniques and technologies are still in the preclinical stage and have not yet been approved for use in humans, novel approaches to accelerate bone healing via MSCs and/or MSC derivatives have the potential to reduce the physical, economic, and social burdens associated with nonhealing fractures and bone defects. In this review, we focus on providing an up-to-date summary of recent scientific studies dealing with the treatment of nonunion fractures in clinical and preclinical settings employing MSC-based therapeutic techniques.

Comparison of radiographic appearance and bone scintigraphy in fracture nonunions

Many surgeons assess the biological activity of fracture nonunions by the presence or absence of callus on radiographs. However, the assessment of biological activity by radiographic appearance alone is controversial. Bone scintigraphy reflects blood flow and new bone formation; therefore, it is useful in assessing such biological activity in nonunion cases. This retrospective study compared radiographs with Tc-99m bone scintigraphy in 48 patients with uninfected nonunions. Positive uptake was observed in all cases. The uptake patterns were classified into 4 types: type 1, intense, uniform uptake; type 2A, a definite photon-deficient cleft between 2 areas of intense uptake; type 2B, a photon-deficient area other than type 2A; and type 3, an intermediate pattern with uneven, distributed uptake. The percentage of type 1 with intense uptake does not decrease with time, and type 2 does not increase. When uptake was compared in patients with a nonunion and a united fracture, higher uptake in nonunion was seen in 46% and lower uptake was seen in 27%. All cases of hypertrophic and more than half of oligotrophic nonunions were type 1. Type 2 was seen in 17% of oligotrophic, 67% of comminuted, 100% of defect, and 57% of atrophic nonunions. Poor callus visualization may not preclude biological activity. Long duration from injury may not equate to declines in biological activity. Comparing uptake in nonunions and united fractures in the same patient may help to assess biological activity. The photon-deficient area is helpful to assess the necessity of bone graft or other osteogenic supplementation.

Decreased pool of mesenchymal stem cells is associated with altered chemokines serum levels in atrophic nonunion fractures

Nonunion fractures can cause severe dysfunction and are often difficult to treat mainly due to a poor understanding of their physiopathology. Although many aspects of impaired fracture healing have been extensively studied, little is known about the cellular and molecular mechanisms leading to atrophic nonunion. Therefore, the aim of the present study was to assess the pools and biological functions of bone marrow-derived mesenchymal stem cells (hMSCs) and circulating endothelial progenitor cells (EPCs) in atrophic nonunion patients compared to healthy subjects, and the systemic levels of growth factors involved in the recruitment, proliferation and differentiation of these cells. In nonunions, the pool of hMSCs was decreased and their proliferation delayed. However, once committed, hMSCs from nonunions were able to proliferate, differentiate into osteoblastic cells and mineralize in vitro as efficiently as hMSCs from healthy subjects. In parallel, we found altered serum levels of chemokines and growth factors involved in the chemotaxis and proliferation of hMSCs such as leptin, interleukin-6 (IL-6) and its soluble receptor, platelet-derived growth factor-BB (PDGF-BB), stem cell factor (SCF) and insulin-like growth factor-1 (IGF-1). Moreover, we showed that the number of EPCs and their regulating growth factors were not affected in nonunion patients. If nonunion is generally attributed to a vascular defect, our results also support a role for a systemic mesenchymal and osteogenic cell pool defect that might be related to alterations in systemic levels of factors implicated in their chemotaxis and proliferation.

Enhancement of fracture healing with the diamond concept: the role of the biological chamber

Bone regeneration presents a unique challenge to both clinicians and scientists. Recently, a vast amount of knowledge has been attained with regard to the molecular mediators, cell populations and the overall cascade of events participating in the bone repair processes. For the treatment of bone non-unions or bone defects, the 'diamond concept' for biological enhancement supports the implantation of mesenchymal stem cells, a scaffold and a growth factor. Prior to the implantation of any or all of these materials however, the surgeon must develop the ideal biological environment (non-union bed) where molecular and physiological processes will evolve facilitating an early and successful osteogenesis leading to bone continuity and functional restoration of the affected limb. At the end of the surgical procedure the non-union bed should have been transformed to a 'biological chamber' active enough to support efficiently all the necessary physiological processes for a successful outcome. The notion of creating the optimum 'biological chamber' represents the centre of the highest biological activity and in a sense the heart of the diamond concept.

Osteoinduction properties of different growth factors on cells from non-union patients: in vitro study for clinical application

This report compares the effect of rhBMPs and PRG on cells derived from human non-union sites. Treatment of non-union continues to be a challenging task for the trauma surgeon often resulting in unsatisfactory results and long-term morbidity. Over the past two decades, the possibility to use growth factors in bone regeneration has been investigated. In this study we compared the in vitro capability of two recombinant human bone morphogenetic proteins (rhBMP-2 and rhBMP-7) and activated platelet-rich plasma (PRG) to stimulate proliferation and/or differentiation of cells derived from non-union patients. Cells derived from the lesion sites, osteoblasts and mesenchymal stem cells (MSCs) derived from other bone sites of the same patients were used. Treatment with rhBMP-7 or rhBMP-2 showed an improvement in the expression of osteoblastic markers (osteonectin and osteocalcin) in cells derived from human non-union sites. This enhancement was more marked in MSCs, while no significant changes were observed in osteoblast cultures. The PRG treatment produced in all analysed samples a considerable increase in cell proliferation without affecting cell differentiation. On the basis of our results, for an effective biological treatment of non-unions, small amounts of autologous bone marrow (MSCs) are necessary in the lesion site in order to provide both growth factors and a sufficient number of responsive cells. Finally, our results prove that sequential timing administration of PRG and rhBMPs may be used in new therapeutic strategy.

Decreased osteogenesis, increased cell senescence and elevated Dickkopf-1 secretion in human fracture non union stromal cells

The delicately orchestrated process of bone fracture healing is not always successful and long term non union of fractured bone occurs in 5-20% of all cases. Atrophic fracture non unions have been described as the most difficult to treat and this is thought to arise through a cellular and local failure of osteogenesis. However, little is known about the presence and osteogenic proficiency of cells in the local area of non union tissue. We have examined the growth and differentiation potential of cells isolated from human non union tissues compared with normal human bone marrow mesenchymal stromal cells (BMSC). We report the isolation and culture expansion of a population of non union stromal cells (NUSC) which have a CD profile similar to that of BMSC, i.e. CD34-ve, CD45-ve and CD105+ve. The NUSC demonstrated multipotentiality and differentiated to some extent along chondrogenic, adipogenic and osteogenic lineages. However, and importantly, the NUSC showed significantly reduced osteogenic differentiation and mineralization in vitro compared to BMSC. We also found increased levels of cell senescence in NUSC compared to BMSC based on culture growth kinetics and cell positivity for senescence associated beta galactosidase (SA-beta-Gal) activity. The reduced capacity of NUSC to form osteoblasts was associated with significantly elevated secretion of Dickkopf-1 (Dkk-1) which is an important inhibitor of Wnt signalling during osteogenesis, compared to BMSC. Conversely, treating BMSC with levels of rhDkk-1 that were equivalent to those levels secreted by NUSC inhibited the capacity of BMSC to undergo osteogenesis. Treating BMSC with NUSC conditioned medium also inhibited the capacity of the BMSC to undergo osteogenic differentiation when compared to their treatment with BMSC conditioned medium. Our results suggest that the development of fracture non union is linked with a localised reduced capacity of cells to undergo osteogenesis, which in turn is associated with increased cell senescence and Dkk-1 secretion.

Are patients with a nonunion after a femoral neck fracture more osteoporotic than others? BMD measurement before the choice of treatment?A pilot study of hip BMD and biochemical bone markers in patients with femoral neck fractures

The incidence of nonunion is high after a femoral neck fracture. If reliable predictors were available, fractures that heal well could be subjected to internal fixation while fractures that do not could undergo an arthroplasty. In this pilot study of 28 patients, those with a low hip BMD on admission had a higher risk of developing non-union.

Cell viability, osteoblast differentiation, and gene expression are altered in human osteoblasts from hypertrophic fracture non-unions

Recent studies have provided evidence that the number and proliferation capacity of bone marrow-derived mesenchymal stem cells, as well as the number of osteoprogenitor cells are reduced in patients with fracture non-unions. For fracture non-unions that do not heal after appropriate surgical intervention, the question arises as to what extent systemic cellular dysfunctions should be considered as being pathogenetic factors. For this purpose, we have examined the hypothesis that the cell function of osteoblasts isolated from patients with fracture non-unions may differ from those of normal control individuals in an identical and controlled in vitro situation. We analyzed the osteoblast cell viability, formation of alkaline phosphatase-positive (CFU-ALP) and mineralization-positive (CFU-M) colony forming units, as well as global differences of gene expression in osteoblasts from patients with fracture non-unions and from control individuals. We found that cell viability and CFU-M-formation were significantly reduced in non-union osteoblasts. This was accompanied by significant differences in osteoblast gene expression as revealed by Affymetrix-microarray analysis and RT-PCR. We identified a set of significantly down-regulated factors in non-union osteoblasts that are involved in regulation of osteoblast proliferation and differentiation processes (canonical Wnt-, IGF-, TGF-beta-, and FGF-signaling pathways). The results of the present study strongly support the hypothesis that cell viability, differentiation, and gene expression of osteoblasts may be altered in patients who develop recurrent and recalcitrant fracture non-unions. Proteins involved in Wnt-, IGF, TGF-beta-, and FGF-signaling pathways may be of particular interest and may unveil new potential therapies.

The anabolic and catabolic responses in bone repair

The literature on fracture repair has been reviewed. The traditional concepts of delayed and nonunion have been examined in terms of the phased and balanced anabolic and catabolic responses in bone repair. The role of medical manipulation of these inter-related responses in the fracture healing have been considered.

Global gene profiling reveals a downregulation of BMP gene expression in experimental atrophic nonunions compared to standard healing fractures

Nonunion is a challenging problem that may occur following certain bone fractures. However, there has been little investigation of the molecular basis of nonunions. Bone morphogenetic proteins (BMPs) play a significant role in osteogenesis. However, little is known about the expression patterns of BMPs in abnormal bone healing that results in nonunion formation. These facts prompted us to investigate and compare the gene expression patterns of BMPs and their antagonists in standard healing fractures and nonunions using rat experimental models. Standard closed healing fractures and experimental atrophic nonunions produced by periosteal cauterization at the fracture site were created in rat femurs. At postfracture days 3, 7, 10, 14, 21, and 28, total RNA was extracted from the callus of standard healing fracture and fibrous tissue of nonunion (n=4 per each time point and each group). Gene expression of BMPs, BMP antagonists, and other regulatory molecules were studied by methods including Genechip microarray and real-time quantitative RT-PCR. Gene expression of BMP-2, 3, 3B, 4, 6, 7, GDF-5, 7, and BMP antagonists noggin, drm, screlostin, and BAMBI were significantly lower in nonunions compared to standard healing fractures at several time points. Downregulation in expression of osteogenic BMPs may account for the nonunions of fracture. The balance between BMPs and their endogenous antagonists is critical for optimal fracture healing.

Cyclooxygenase-2 inhibitors in human skeletal fracture healing

This article identifies the underlying molecular events responsible for fracture nonunions in a subset of fracture patients. Expression profiling of fracture callus tissue from both uneventful fracture repair and nonunion outcomes showed a decrease of COX-2 expression and an inability to mount an immune response in nonunion fractures. Validation in vitro with Saos-2 osteoprogenitor cell lines showed a decrease in osteogenesis potential after the cells were treated with celecoxib, a COX-2 specific inhibitor and anti-inflammatory agent. This article recapitulates that an initial immune response is crucial to fracture healing and suggests limited usage of COX-2 inhibitors in patients with healing fractures.

Growth factor expression during the development of atrophic non-union

The aim of the study was to determine, for the first time, the distribution of expression of several important growth factors during the development of atrophic non-unions using an animal model. The sites of expression of TGFbeta, PDGF, FGFb, and BMP 2/4 were determined at the osteotomy sites of both normally healing bones and within atrophic non-unions at 1 and 8 weeks after operation using immunolocalization techniques. At 1 week after operation, the osteotomy gaps of the control group contained fracture haematoma and surrounding granulation tissue, whereas the osteotomy gaps of the non-union group contained only haematoma. The tissues of both the non-union and control groups demonstrated the same presence and distribution of growth factors. By 8 weeks after the operation, the control group osteotomy gaps were filled with bone within which the active osteoblasts stained positively for each of the growth factors. At 8 weeks, the osteotomy gaps of the non-union group contained only fibrous tissue, which failed to stain positively for any of the factors. These findings suggest that the development of atrophic non-union is not directly due to a lack of these four growth factors.

Pulsed electromagnetic fields increase growth factor release by nonunion cells

The mechanisms involved in pulsed electromagnetic field stimulation of nonunions are not known. Animal and cell culture models suggest endochondral ossification is stimulated by increasing cartilage mass and production of transforming growth factor-beta 1. For the current study, the effect of pulsed electromagnetic field stimulation on cells from human hypertrophic (n = 3) and atrophic (n = 4) nonunion tissues was examined. Cultures were placed between Helmholtz coils, and an electromagnetic field (4.5-ms bursts of 20 pulses repeating at 15 Hz) was applied to 1/2 of them 8 hours per day for 1, 2, or 4 days. There was a time-dependent increase in transforming growth factor-beta 1 in the conditioned media of treated hypertrophic nonunion cells by Day 2 and of atrophic nonunion cells by Day 4. There was no effect on cell number, [3H]-thymidine incorporation, alkaline phosphatase activity, collagen synthesis, or prostaglandin E2 and osteocalcin production. This indicates that human nonunion cells respond to pulsed electromagnetic fields in culture and that transforming growth factor-beta 1 production is an early event. The delayed response of hypertrophic and atrophic nonunion cells (> 24 hours) suggests that a cascade of regulatory events is stimulated, culminating in growth factor synthesis and release.

Expression of the gene encoding the matrix gla protein by mature osteoblasts in human fracture non-unions

BACKGROUND

Osteoblast phenotypic abnormality, namely the expression of collagen type III, has been shown previously in fracture non-union woven bone.

AIMS

To investigate osteoblasts from fracture non-unions for evidence of gene expression of non-collagenous bone matrix proteins that have been implicated in mineralisation, namely matrix gla protein (MGP), osteonectin, osteopontin, and osteocalcin. MGP is a consistent component of bone matrix, but there are no reports of osteoblasts in the skeleton expressing the gene for MGP, and the site of synthesis of skeletal MGP (perhaps the liver) has yet to be determined.

METHODS

Biopsies from normally healing human fractures and non-unions were examined by means of in situ hybridisation, using 35S labelled probes and autoradiography to disclose levels of gene expression.

RESULTS

In normally healing fractures, mature osteoblasts on woven bone were negative for MGP mRNA, but positive for osteonectin, osteopontin, and osteocalcin mRNA molecules. In non-unions, osteoblasts displayed a novel phenotype: they were positive for MGP mRNA, in addition to osteonectin, osteopontin, and osteocalcin mRNA molecules.

CONCLUSIONS

Mature osteoblasts in slowly healing fractures have an unusual phenotype: they express the gene encoding MGP, which indicates that control of osteoblast gene expression in non-unions is likely to be abnormal. This might be of importance in the pathogenesis of non-uniting human fractures, and is of current interest given the emerging status of MGP as an inhibitor of mineralisation.

Mature osteoblasts in human non-union fractures express collagen type III

AIMS

High levels of collagen type III are biochemically detectable in biopsies of non-uniting fractures, and in the serum of patients suffering from this condition. The aim of this study was to determine whether the expression of collagen type III was limited to fibrous tissue in non-unions, or whether some was present in bone.

METHODS

Biopsies from normally healing human fractures and non-unions were examined using in situ hybridisation and immunohistochemistry.

RESULTS

The mesenchymal cell population, which includes fibroblast and osteoblast precursors, expressed mRNA for collagen type III. However, mature osteoblasts on the surface of woven bone varied profoundly between normally healing fractures (in which they were negative or occasionally weakly positive) and non-unions (in which they were strongly positive). Areas of woven bone that had osteoblasts positive for collagen type III mRNA also immunostained positively for the protein.

CONCLUSIONS

This study shows that non-union fracture callus osteoblasts on the surfaces of woven bone exhibit an unusual phenotype: they express collagen type III, a molecule characteristic of an earlier stage of osteoblast differentiation, which is not expressed by osteoblasts on woven bone surfaces of bone that develops normally. This finding may be useful in developing an early clinical test for impending non-union.

Microstructural features of non-union of human humeral shaft fracture

Microstructures of non-unions of human humeral shaft fractures were investigated by using scanning electron microscopy, transmission electron microscopy, and X-ray microdiffraction. The non-union has a trabeculae structural framework similar to woven bone. Among the trabeculae are cavities that are subdivided into small chambers by thin plates of collagen fibrils. Some chambers are filled with variously shaped mineralized particles several micrometers in size. The collagen fibrils in both the trabeculae and the thin plates were only slightly mineralized by hydroxyapatite. Vesicles loaded with noncrystalline calcium phosphate (NCP) were observed in most mineralized particles, and brushite crystals with special morphology were seen to be embedded in some particles in irregular shapes. X-ray microdiffraction results indicated that the mineral phases in the non-unions were mainly NCP in addition to small amounts of hydroxyapatite and brushite. NCP deposition and insufficient mineralization of the collagen fibrils may be two important microstructural features of the non-unions of human humeral shaft fractures different from normally repaired bone callus.

[Several indicators of oxidative processes in fractures of the long bones]

The authors have observed 55 patients with fresh fractures, with disturbances in consolidation (delayed consolidation, non-union of fractures, false joints) and with defects of diaphyses of the long bones. In the fracture area the dynamics of oxidation-reduction potential (ORP) was studied and volarographic investigations (determination of the volumetric speed of the local blood flow by hydrogen, a test with oxygen load) were carried out. Certain correlation between the quantity and the dynamics of the values under study and the state of the consolidation process was established. Coincidence of the dynamics of ORP and the local blood flow, presence of the quantity of volumetric speed of the local blood flow of 25-30 ml/min per 100 g of tissue with rather high ORP and a positive test under oxygen load in the polarographic study were prognostically favourable. The applied methods allow to control the course of the process of consolidation of the fragments and to a certain extent prognosticate the results of treatment of the patients with fractures of the long bones.

Nuclear magnetic resonance studies of experimentally induced delayed fracture union

Delayed fracture union was produced in the rat tibia with parenteral administration of prednisolone. Phosphorus nuclear magnetic resonance spectroscopy was used to monitor the local pH changes that occurred at the fracture site throughout repair. During the fracture healing in control animals, the pH of the fracture site changed from 7.2 on Day 2 to 7.5 on Day 20 following fracture, i.e., alkaline with respect to the pH of normal extracellular fluid. Deposition of radioopaque callus occurred mainly during the alkaline phase. In the steroid-treated rats the rate and extent of pH change were significantly less and the fracture site became alkaline five to ten days later than in the control animals. Because radiopaque callus deposition still occurred predominantly in the alkaline phase, union was also delayed by about the same interval of time. The rat prednisolone model differs significantly from the human condition of delayed union. Clinical investigations are indicated to ascertain whether abnormalities of fracture site pH occur in delayed union in humans.

Vitamin D metabolites in patients with established non-union of fracture

The serum concentrations of vitamin D metabolites and biochemical markers of bone metabolism were measured in 15 patients with established non-union of fracture and in 15 age- and sex-matched controls. No significant differences between the two groups were observed in the concentrations of 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3; or in the biochemical indices of skeletal metabolism. These results suggest that patients with established non-union have normal bone turnover with no evidence of disturbed production or utilization of 24,25-dihydroxyvitamin D3 or other major vitamin D metabolites. Disturbances in vitamin D metabolism are unlikely therefore to make a major contribution to the maintenance of non-union of fracture.

Collagen types present at human fracture sites--a preliminary report

Collagen has been examined from normally and abnormally healing fractures. A higher proportion of type III collagen than was expected was found in abnormally healing fractures. Very little is known about collagen synthesis in healing fractures and how it is altered in abnormal healing states such as delayed and non-union.

Delayed fracture healing following jejunoileal bypass surgery for obesity

A 38-year-old woman, who 5 years earlier had undergone a jejunoileal bypass for gross obesity, fractured the distal forearm by a minor trauma. Circulating 25-hydroxycholecalciferol was undetectable without vitamin D3 supplement but increased to the lower normal range on a daily dose of 1200 units of vitamin D3. Serum 1,25-dihydroxycholecalciferol was, however, in the upper normal range, both without and with vitamin D supplement. After intestinal reanastomosis the fracture healed and the biochemical changes normalized. Malabsorption due to reduced amount of functioning intestine may cause severe metabolic bone disease, which may not always be reverted by a high-calcium diet and vitamin D supplementation.

Electrical stimulation of experimental nonunions

Hypertrophic and oligotrophic nonunions were prepared by resection of a portion of the proximal ulna in dogs. In the hypertrophic nonunions, 20 muamps of direct current for eight weeks produced an increase in bone formation compared to the opposite control limb by radiography, photometry, point counting of new bone, and growth rate by sequential fluorochrome labeling and the dynamic uptake of 99mTc-labeled methylene disphosphonate. Oligotrophic nonunions were treated by plating and aspiration grafting in addition to direct-current stimulation. Ony the point counting of new bone showed a significant increase in bone formation with stimulation. Sequential fluorochrome labeling demonstrated that the new bone was laid down on existing bone and not primarily adjacent to the cathode within the fibrous nonunion. This finding supports the cell-mediated rather than physicochemical effect of electrostimulation.

Role of bone scintigraphy in the evaluation and treatment of nonunited fractures: concise communication

Seventy-seven patients with nonunited fractures underwent percutaneous low-grade, direct-current stimulation; the response was correlated with the scintigraphic findings obtained before the treatment. Two distinct patterns of osseous activity were noted: those with intense activity at the fracture site (Group 1) and those with a line of decreased activity surrounded by increased uptake on both sides (Group 2). The scintigrams that did not fit into either of the two patterns were considered as indeterminate (Group 3). Whereas 95% of the patients in Group 1 showed an excellent response to electric stimulation, none of the patients in Group 2 had evidence of healing. The response rate in the third group was 50%. On the basis of these preliminary data, bone scintigraphy is recommended as an important initial examination for the proper selection of patients for percutaneous electric stimulation.