Down Syndrome Ups & Downs

Oct 25, 2004 -- Some things are gradually getting better with Down syndrome (once called mongolism). Medical and surgical advances are helping remedy certain features such as heart defects. People with the syndrome now live longer (with an average life expectancy into the 50s). And education and vocational training are improving for people with Down syndrome. But it is still not possible to prevent the mental retardation characteristic of Down syndrome.

The Basic Mystery

The basic mystery as to what causes Down syndrome and what to do about it also remains elusive. While most cases are due to an extra chromosome 21, in some cases there is triplication of only a small part of chromosome 21 This has led to the logical thought that there is a Down syndrome critical region on the long (q) arm of chromosome 21 that is responsible for some, if not all, of the features of Down syndrome. A number of genes in this region may be involved in the production of the Down syndrome. 

Down Syndrome Critical Region

Things are not simple (or well understood). Triplication of a segment of chromosome 21 containing 33 critical-region genes does not result in the craniofacial abnormalities typical of the syndrome, according to mouse research published this past week in  Science. This mouse model would seem to refute the notion that triplication of the 33 critical-region genes is directly responsible for the craniofacial abnormalities of Down syndrome.

Of Mice and Men

However, it is difficult to tell whether a mouse is mentally retarded and alterations in behavior and learned tasks must suffice to flag differences in mental acuity between mutant mice and their normal counterparts. Another challenge is the difference in colinearity of the human and mouse genomes, and the lack of conservation of gene order between mice and men. The more we learn, the more we realize we have much more to learn about Down syndrome.


  1. Olson LE, Richtsmeier JT, Leszl J, and Reeves RH.  A Chromosome 21 Critical Region Does Not Cause Specific Down Syndrome Phenotypes. Science 22 October 2004: 687-690.
  2. Nelson DL and Gibbs RA. The Critical Region in Trisomy 21. Science 22 October 2004: 619-621.

Barbara K. Hecht, Ph.D.
Frederick Hecht, M.D.
Medical Editors,

Related Links

  • Down Syndrome 
  • Down Syndrome Was Not Discovered By Dr. Down
  • Genetic Disease

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