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Abstract

Down syndrome (DS) or trisomy 21 is a complex congenital disorder affecting 1:700 live births, and a leading genetic cause of mental retardation. The identification of genes and molecular mechanisms responsible for the DS phenotypes has therefore been a high priority for genome research. Using Ts65Dn, a well-established mouse model of DS, we carried out gene expression profiling by cDNA arrays and quantitative Real Time PCR (qPCR). We focused on the chromosome 21 genes orthologs (mmu21) because they are primary contributors of trisomy. We analyzed RNAs from several tissues of Ts65Dn and controls, either as pools or as individual samples. With pools, we observed a trend of 1.5 fold overexpression for the majority of trisomic genes with however exceptions to this rule for a few genes (Kahlem, Sultan et al., 2004). For many genes, it is unlikely that such a modest change in gene expression levels will have drastic effects on the fitness of the organism. To select genes, where slight changes in gene activity could have a more dramatic phenotypic effect, we focused on the normal variation of gene expression levels. Genes whose level of gene expression is critical are more likely to be tightly controlled than those for which slight variation of expression will not have a detrimental effect. Our working hypothesis is that genes relevant for DS reach an expression threshold in trisomic individuals that is not or rarely encountered in controls. Hence, we determined inter-individual expression differences for 50 chr21 gene orthologs in the cortex, midbrain and cerebellum of individual Ts65Dn mice and controls by qPCR (TaqMan). Our study enabled the identification of a short list of potential key contributor genes of the trisomy phenotypes in the brain. Among those, we found App, the amyloid precursor protein (sultan et al., submitted). Although the systematic analysis of mmu21 gene expression profiles contributes to an understanding of how cells and organisms respond to structural gene dosage imbalance, it does not give direct information on gene function. Systematic functional genomic approaches are being carried out in parallel in the laboratory. As part of this, we attempted to contribute a functional analysis study in Caenorhabditis elegans for nine selected genes. We monitored tissue specific expression GFP fusions under the control of the respective endogenous promoters. The selected genes were knocked-down by means of RNA interference (RNAi). The use of C. elegans provides an excellent experimental model for the initial haracterization of gene function and may become an important tool in assessing the contribution of genes in complex phenotypes such as DS.