All ETDs from UAB

Advisory Committee Chair

Amjad Javed

Advisory Committee Members

Mohammad Hassan

Chung H Kau

Somsak Sittitavornwong

Document Type

Thesis

Date of Award

2017

Degree Name by School

Master of Science (MS) School of Dentistry

Abstract

The Runx2 transcription factor is essential for the differentiation of mesenchymal stem cells into functional chondrocytes and osteoblasts. In humans, the haploinsufficiency of the Runx2 gene is associated with cleidocranial dysplasia (CCD). The CCD is characterized by short stature, open sutures and fontanels in the skull, hypoplastic clavicles and supernumerary teeth. Skeletal elements disrupted in CCD patients are developed by intramembranous and/or endochondral ossification. However, the cell type specific roles of the Runx2 gene in the development of CCD-associated skeletal defects remain unknown. The main goal of this research was to define the specific role of the Runx2 gene in chondrocytes, immature osteoblasts and mature osteoblasts, and determine how each cell type contributes to the development of the skeletal phenotypes associated with CCD. The Runx2 gene was deleted in chondrocytes, immature osteoblasts and mature osteoblasts, using Cre-recombinase driven by collagen type II (Col2a), collagen type I (Col1a) and osteocalcin (OC) promoters respectively. Here, we report the cell-specific contribution of Runx2 deficiency and haploinsufficiency to the CCD phenotype. Runx2 deficiency in chondrocytes leads to dwarfism, a loss of endochondral ossification and perinatal lethality. Impairment in skull development was also observed and included open anterior and posterior fontanels and sutures. At birth, a Runx2 gene dose dependent decrease in mineralization of all endochondral-derived bone was noted. Due to the perinatal lethality of the Runx2 homozygous mice, skeletal phenotypes in postnatal life could not be studied. Importantly, the craniofacial defect due to Runx2 haploinsufficiency in chondrocytes was recovered postnatally. Fontanels and sutures were closed in Runx2 heterozygous mice by 1-month of age. Surprisingly, clavicles were formed in Runx2 heterozygous and homozygous mice, but they were misshapen and hypoplastic. Therefore, contrary to the popular notion that clavicles are primarily formed through endochondral ossification, we show that clavicle development is mainly regulated by Runx2 activity in osteoblasts and not chondrocytes. Mice with Runx2 deficiency in immature osteoblasts exhibited normal endochondral ossification at birth. However, skull bones formed via intramembranous ossification were impaired, resulting in reduced mineralization of the frontal, parietal and occipital bones. Poor skull development was coupled with open sutures and fontanels in Runx2 homozygous mice. To understand if Runx2 deficiency in immature osteoblasts is associated with a delayed or failed closure of fontanels and sutures, we evaluated littermates until adulthood. To our surprise, impaired skull development due to Runx2 deficiency in immature osteoblasts persisted throughout postnatal life. By 1-month, fontanels and sutures were nearly closed in wildtype and heterozygous littermates. In sharp contrast, anterior and posterior fontanels along with the sagittal and lamboid sutures remained open in homozygous mice, even at the 3-months of age. These results confirm that Runx2 deficiency in immature osteoblast causes a failure of suture and fontanel closure. Interestingly, wormian bones with abnormal mineralization, orientation and positioning were present in place of the interparietal and occipital bones along the posterior fontanel. Although not evident by µCT, a mild defect in the closure of the sagittal suture was noted in heterozygous mice. Histological analysis revealed that only a thin layer of fibrous tissue covered the open fontanel and suture in homozygous mice. Thus, the presence of Runx2 in immature osteoblasts is crucial for the closure of sutures and fontanels. Clavicles were extremely thin and fragile in both Runx2 heterozygous and homozygous mice at birth. Three-month old homozygous mice also exhibited a range of hypoplasticity with runted and fractured clavicles. Runx2 deficiency or haploinsufficiency in mature osteoblasts did not cause any impairment in embryonic cranial development or any failure in the closure of sutures or fontanels. However, loss of Runx2 in mature osteoblasts affected skull growth in post-natal life. Skull bones in both male and female homozygous mice presented a moth eaten like bone phenotype. A progressive osteolysis of the skull bones occurred throughout postnatal life. Clavicles were developed in heterozygous and homozygous mice; however, severe osteoporosis and loss of bone mass resulted in frequent fracturing of the clavicles in the adult homozygous mice. Histological analysis confirmed the loss of skull bone and the apoptosis of osteocytes. Thus, life long activity of Runx2 in mature osteoblasts and osteocytes is necessary for the maintenance of adult bone mass and to prevent skull and clavicular osteoporosis. In conclusion the CCD phenotypes of open fontanels, open sutures and hypoplastic clavicles are caused by the cell specific function and dosages of the Runx2 gene.

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