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Down Syndrome Abstract
of the Month: May 2000

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The DNA sequence of human chromosome 21

M. Hattori, A. Fujiyama, T. D. Taylor, et al.
Nature, May 18, 2000, (405)311-319

RIKEN, Genomic Sciences Center, Sagamihara, Japan; Institut fur Molekulare Biotechnologie, Genomanalyse, Germany (and 11 other collaborating institutions)

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Chromosome 21 is the smallest human autosome. An extra copy of chromosome 21 causes Down syndrome, the most frequent genetic cause of significant mental retardation, which affects up to 1 in 700 live births. Several anonymous loci for monogenic disorders and predispositions for common complex disorders have also been mapped to this chromosome, and loss of heterozygosity has been observed in regions associated with solid tumours. Here we report the sequence and gene catalogue of the long arm of chromosome 21. We have sequenced 33,546,361 base pairs (bp) of DNA with very high accuracy, the largest contig being 25,491,867 bp. Only three small clone gaps and seven sequencing gaps remain, comprising about 100 kilobases. Thus, we achieved 99.7% coverage of 21q. We also sequenced 281,116 bp from the short arm. The structural features identified include duplications that are probably involved in chromosomal abnormalities and repeat structures in the telomeric and pericentromeric regions. Analysis of the chromosome revealed 127 known genes, 98 predicted genes and 59 pseudogenes.

My comments:

If you missed the news reports, the researchers stated they were surprised that there were only 225 genes present on the entire 21st chromosome. Previous estimates had been as high as 500 genes. The next smallest chromosome, the 22nd one, was mapped completely late last year and was found to have 545 genes. These are the only two human chromosomes to be completely mapped as of May 2000.

This is a major step forward in what may be the eventual "gene therapy" of Down syndrome. Researchers now have a "phone book" for the 21st chromosome: they know where the genes live, but not their occupation.

From this point, research will take a two-pronged approach to the research. One approach will be to determine what each gene does and whether the proteins produced have any contribution to the features of Down syndrome, including associated diseases; this may lead the way for learning how to counteract the effects of the triplicated genes in DS, gene by gene. The second approach will be to determine if the genes can be turned down, or completely off. This can be done without knowing what the genes do.

Either approach may led to new treatments of DS in the near future. How far-reaching these treatments may be are merely speculation at this point.


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