NEL-2 GENE IN BONE FORMATION & CRANIAL SUTURE CLOSURES

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The applicant proposes to investigate the function of NEL-2, a novel gene isolated in craniosynostosis (CS), in bone formation and premature suture closure. CS, which affects roughly one of every 3,000 infants, is the premature obliteration of cranial sutures in association with cranial dysmorphism. We have isolated and identified cDNA whose expression is upregulated in the premature suture fusion sites of patients with a prevalent type of CS, non-familial unilateral coronal synostosis (UCS). The nucleotide sequence of the full length cDNA of this gene has approximately 61 percent homology with the chicken nel gene, and thus has been named this cDNA human NEL-2. Both chicken nel and human NEL-2 consist of 6 EGF-like repeats. We demonstrated that the human NEL-2 messages are primarily localized in the mesenchymal cells and osteoblasts at the osteogenic front, along the parasutural bone margins and within the condensing mesenchymal cells of the newly formed bone. Human NEL-2 is specifically expressed in fetal brain but not in fetal lung, kidney or liver. We have also shown that rat NEL-2 is expressed in rat calvarial osteoprogenitor cells and is largely absent in rat tibia and fibroblast cell culture. Mammalian expression vectors expressing rat NEL-2 were constructed and transfected into MC3T3 cell line. An approximate two and a half-fold increase in mineralization was seen as compared to the control. In addition, overexpression of rat NEL-2 induced BMP-2 gene expression. Our data suggest that the NEL-2 gene is preferentially expressed in cranial intramembranous bone and neural tissue, and it may be associated with membranous bone formation. Therefore, we hypothesize that the overexpression of the NEL-2 gene product induces the overproduction of cranial membranous bone. In addition premature cranial suture closure, as seen in craniosynostosis (CS) and non-familial unilateral coronal synostosis (UCS), may be due to the overproduction of cranial membranous bone and the overexpression of NEL-2. To test this hypothesis, we propose the following specific aims: 1) Determining expression of NEL-2 in osteoblasts of non-familial UCS patients and at different stages of bone formation in vitro; 2) Determining the effect of NEL-2 on bone formation in vitro; 3) Determining the effect of NEL-2 on bone formation in human calvaria osteoblastic cells from CS patients; 4) Characterizing the regulation of NEL-2 expression; and 5) investigating the overexpression effect of NEL-2 on the rat cranial suture closure.