Tibial dyschondroplasia (TD) appears to involve a failure of the growth plate chondrocytes within growing long bones to differentiate fully to the hypertrophic stage, resulting in a mass of prehypertrophic chondrocytes which form the avascular TD lesion. Many biochemical and molecular markers of chondrocyte hypertrophy are absent from the lesion, or show reduced expression, but the cause of the disorder remains to be identified. As differentiation to the hypertrophic state is impaired in TD, we hypothesised that chondrocyte genes that are differentially expressed in the growth plate should show altered expression in TD. Using differential display, four genes, B-cadherin, EF2, HT7 and Ex-FABP were cloned from chondrocytes stimulated to differentiate to the hypertrophic stage in vitro, and their differential expression confirmed in vivo. Using semi-quantitative RT-PCR, the expression patterns of these genes were compared in chondrocytes from normal and TD growth plates. Surprisingly, none of these genes showed the pattern of expression that might be expected in TD lesion chondrocytes, and two of them, B-cadherin and Ex-FABP, were upregulated in the lesion. This indicates that the TD phenotype does not merely reflect the absence of hypertrophic marker genes, but may be influenced by more complex developmental mechanisms/defects than previously thought.

The technique of RNA differential display has been used extensively to clone differentially expressed genes from a wide variety of cells and tissues. Recently, a simplified method of cloning differential display products, separated on agarose gels, was described. Here we report an adaption of this method, using total RNA, to clone differentially expressed genes. The approach is simple and rapid, and requires only small quantities of total RNA. Utilising this approach, we have cloned three differentially regulated genes from chondrocytes stimulated to hypertrophy in vitro, and confirmed their pattern of expression by Northern blotting. These gene fragments were sequenced and found to correspond to known genes, although only one has previously been isolated from chondrocytes.