d for motifs, domains, and biochemical characteristics that may suggest function. The most challenging problem will then be determining the functions of these genes and the other 'known' genes. While it is tempting to focus on genes whose protein characteristics suggest a hypothesis for relevance to some aspect of DS, the more than 100 genes distributed throughout the chromosome that have no functional association are too large a dataset to ignore. For these and other genes on 21q, detailed expression analysis may be informative. Demonstration that a gene shows increased expression in the trisomic state by northern blot or RT-PCR analysis, followed by RNA tissue in situ hybridization to define specific cell types, brain regions and developmental stages of expression, may help in selecting genes of greater or lesser interest.
The most direct assessment of function will require mutation or overexpression of individual genes or sets of genes. For these experiments, the 'complete' protein databases for S. cerevisiae, C. elegans and Drosophila will provide homologous genes that can be analyzed in more tractable systems. The increasing complexity of the zebrafish EST database will add another model organism system of increasing utility. Issues remain with all model organisms, however, of verifying correct gene structures, identifying orthologous genes versus merely homologous genes, and interpreting mutation and knockout data in one system versus overexpression in another. The ultimate model organism, of course, will remain the mouse. Multiple genes can be 'added' to the Ts65Dn using transgenics carrying bacterial chromosomes (BACs), to look for enhanced DS-relevant phenotypes. The human sequence will be useful here in ensuring that clones are extensive enough to contain appropriate regulatory regions. Single-gene knockouts can also be 'subtracted' from the Ts65Dn mouse model, to search for amelioration of phenotype. With good biological intuition and luck, it may not be necessary to understand all of the genes within chromosome 21 before promising candidates are identified and the design of potential therapeutics can begin.
This work was supported by the Boettcher Foundation and Grant No. HD17449 from the National Institutes of Health.