Clinical Studies Of Osteogenic Cells

Dental &Craniofacial Research

Investigators

Linked publications, trials & patents

Abstract

The Skeletal Clinical Studies program focuses on the role of skeletal stem cells and osteogenic cells in the establishment of various forms of skeletal dysplasias, in particular, in fibrous dysplasia of bone (FD) and the McCune-Albright Syndrome (MAS). MAS is defined clinically by the triad of cafe-au-lait skin pigmentation, polyostotic FD, hyperfunctioning endocrinopathies, and varying degrees of renal phosphate wasting (phosphaturia). The disease is caused by post-zygotic activating mutations of the GNAS gene product, Gs alpha, at R201. These mutations lead to overproduction of cAMP, and cause the focal formation of abnormal bone and fibrotic marrow. Due to the scientific interest in determining the effect of these mutations on bone formation and bone resorption, and the impact of abnormal endocrine and renal function on both normal and fibrous dysplastic bone, a series of clinical protocols for the study and treatment of FD are ongoing (98-D-0145, 98-D-0146, 01-D-0197). These studies, several of which are highlighted below, have provided insight not only into the pathophysiology of the disease, but have also led to recommendations for evidence-based changes in clinical management of this complex group of patients. The dental and craniofacial abnormalities were extensively examined in a large series of with FD/MAS patients. Eighty-four percent had FD in the maxilla and/or mandible. By image analysis, four types of radiographic changes were observed in lesions in the jawbones: ground glass (granular/condensed trabeculae), radiolucent (lytic), mixed radiolucent/radio-opaque (mixed density) or radio-opaque (sclerotic). Masking or displacement of the maxillary sinus and mandibular canal were prevalent in FD sites. Current studies are aimed at determining if these varying appearances reflect different stages of lesion progression. FD in its polyostotic form can cause severe deformity and crippling. Patients underwent intensive skeletal screening to further characterize the progression of the bone disease. Previously, both FD in the spine, and scoliosis in patients with polyostotic FD, were thought to be uncommon entities. Scoliosis, if left untreated, contributes to significant deformity and morbidity, and is not often recognized due to the magnitude of skeletal problems exhibited by the patients. By examining the bone scans of our large cohort, we found that 63% of the patients had FD lesions in the spine, and that 40% had scoliosis. Since there was a strong correlation between a spinal lesion and scoliosis, it was recommended that patients with polyostotic FD be screened for spinal lesions at least once with a bone scan, and if a spine lesion is found, they should be followed clinically for scoliosis over their lifetimes. It was also noted that although fractures occur frequently in FD/MAS, fracture incidence and the effect of coexisting metabolic abnormalities (endocrinopathy and/or renal phosphate wasting) on fractures had been ill defined. The medical records and the endocrine and phosphorus metabolism of the patients were examined. From this study, it was found that the occurrence of fractures in FD/MAS peaks between 6-10 years of age, and declines thereafter. Fractures occur earlier and more frequently in the presence of renal phosphate wasting. These data have implications for long-term prognosis, clinical management, and interpretation of therapeutic interventions in patients with this disease. Considering the impact that renal phosphate wasting has on the severity of the skeletal disease, it has been of interest to determine factors that regulate phosphate metabolism. Fibroblast Growth Factor-23 (FGF-23) has been proposed as a circulating factor (?phosphotonin?) causing renal phosphate wasting not only in autosomal dominant hypophosphatemic rickets (as a result of inadequate degradation), but also in tumor-induced osteomalacia (as a result of excess synthesis by tumor cells). Because FD bone is osteomalacic, and renal phosphate wasting occurs in ~50% of patients FD/MAS, production of FGF-23 in FD tissue and cells was investigated, and serum levels of FGF-23 were measured in FD/MAS patients with or without renal phosphate wasting. It was found that FGF-23 is produced by normal bone forming cells, by bone marrow osteoprogenitor cells, and by FD osteoprogenitor cells in vivo and in vitro. In situ hybridization analysis of FGF-23 mRNA expression identified ?fibrous? cells, osteogenic cells and cells associated with microvascular walls as specific cellular sources of FGF-23. Serum levels of FGF-23 were found to be increased in FD/MAS patients compared to normal age-matched controls, and significantly higher in FD/MAS patients with renal phosphate wasting compared to those without, and correlated with the % of the skeleton that was affected. Production of FGF-23 by FD tissue may play an important role in the renal phosphate wasting syndrome associated with FD/MAS. However, its levels in serum are confounded by the fact that high serum calcium, PTH, and other putative phosphotonins, have similar effects on serum and urine phosphorus. To circumvent these factors, serum levels of FGF-23 were studied in patients with chronic hypoparathyroidism and hyperphosphatemia. These patients had significantly higher levels of FGF-23 compared to normal controls and patients with hyperparathyroidism, suggesting that there is a feedback system in which serum FGF-23 responds to serum phosphorus and regulates it. However, in the setting of chronic hypoparathyroidism, the degree of elevation of FGF-23 is insufficient to normalize serum phosphorus. Future studies are aimed at further clarifying the mechanisms by which FGF-23 and other putative phosphotonins regulate phosphate metabolism and hard tissue calcification. Based on the fact that skeletal stem cells have the remarkable ability to regenerate bone and support bone marrow formation after extensive ex vivo expansion, it has been a long term goal of the group to use them clinically in patients requiring extension bone reconstruction. Furthermore, it is envisioned that skeletal stem cells may be engineered to either produce a product that is deficient in certain patients, or that genetic mutations could be either silenced or corrected. Towards these goals, the group has developed a number of in vivo transplantation protocols that are undergoing testing in animal model systems. However, based on the fact that the transplant construct contains hydroxyapatite/tricalcium phosphate (HA/TCP), it is difficult to determine the amount of new bone formation due to the opacity of the carrier. For this reason, studies were conducted o determine whether qCT (quantitative computerized tomography) can be used to estimate the extent of new bone formation in HA/TCP based transplants. Bone-forming transplants were generated by attaching cultured human bone marrow stromal cells (BMSCs) to aliquots of HA/TCP particles and placed in subcutaneous pockets in immunocompromised mice. After 8 weeks, the transplants were individually imaged in a GE CTI; each scan included a phantom. An overall BMD of each transplant was obtained using QCT Pro software. H&E-stained sections of the same transplants were then examined histologically and the extent of bone within each transplant was scored on a semi-quantitative, exponential scale ranging from 0 to 4 by 3 blinded observers. A total of 120 transplants were evaluated. An average BMD ? 600 mg/cc was noted in transplants with appreciable bone formation (bone score ? 3). It was reported that use of qCT offers the first practical approach for the non-invasive determination of new bone formation in mineralizing BMSC/HA-TCP transplants.