TARGETED INHIBITION OF THE P21WAF1/CIP1 GENE EXPRESSION IN CHONDROCYTES CARRYING THE ACHONDROPLASIA AND THANATOPHORIC DYSPALSIA MUTATIONS: DEVELOPING A STRATEGY TO PREVENT GROWTH RETARDATION IN BONE DEVELOPMENT IN FGFR3-RELATED SKELETAL DISORDERS

Fibroblast growth factors (FGF) regulate cell proliferation, differentiation and development in many different tissues. The signalling to the cell nucleus by FGF is mediated by specific transmembrane receptors (FGFR). Mutations in the FGFR3 have been detected in patients affected by achondroplasia (ACH) or thanatophoric dysplasia (TD). Experimental evidences indicate that the molecular mechanisms for the defect in bone elongation in ACH and TD are due to the following: 1) The mutations in the FGFR3 make it constitutively activated: it means that it keeps signalling to the nucleus without the need of its own ligand (FGF). This can be figured out as an engine always switched on even in the absence of the starting key. 2) As a consequence of the constitutive FGFR3 activation, some genes are switched on in the nuclei of chondrocytes. One of them is the p21 gene that is a universal negative regulator of the cell cycle; indeed, it causes growth arrest of chondrocytes and the subsequent inhibition of bone elongation. The work hypothesis is based on the following observations: a) the carboxyl-terminus of Stat5 protein is the one responsible for gene activation. Naturally occurring variants that are missing such a carboxyl-terminus, recognise the same target sequences but inhibit gene activation: these truncated forms are dominant negative variants. b) p21 gene is a natural target for Stat5. The rational of the project is to introduce the DN Stat5 variants in ACH or TD chondrocytes in order to inhibit p21 gene expression, thus to allow, through proliferative chondrocytes, a normal bone elongation.