Medical Research

Of the inborn disorders that affect intellectual capacity, Down syndrome is the most prevalent and best studied. Down syndrome is a term used to encompass a number of genetic disorders of which trisomy 21 is the most frequent (95% of cases). Discovered by the Parisian physician Jerome Lejeune in 1959, Trisomy 21 is the existence of the third copy of the chromosome 21 in cells throughout the body of the affected person. Other Down syndrome disorders are based on the duplication of the same subset of genes (e.g., various translocations of chromosome 21). Depending on the actual etiology, the degree of impairment may range from mild to severe. In rare cases trisomy 21 is present in some cell lines but not all, due to an anomalous early cell division in the zygote. There is evidence that this variant, called mosaic Down syndrome, may produce less developmental delay, on average, than full trisomy 21.

Trisomy 21 results in over-expression of genes located on chromosome 21. One of these is the super oxide dismutase gene. Some (but not all) studies have shown that the activity of the super oxide dismutase enzyme (SOD) is elevated in Down syndrome. SOD converts oxygen radicals to hydrogen peroxide and water. Oxygen radicals produced in cells can be damaging to cellular structures, hence the important role of SOD. However, the hypothesis says that once SOD activity increases disproportionately to enzymes responsible for removal of hydrogen peroxide (e.g., glutathione peroxidase), the cells will suffer from a peroxide damage. Some scientists believe that the treatment of Down syndrome neurons with free radical scavengers can substantially prevent neuronal degeneration. Oxidative damage to neurons results in rapid brain aging similar to that of Alzheimer's disease.

Another chromosome 21 gene that might predispose Down syndrome individuals to develop Alzheimer's pathology is the gene that encodes the precursor of the amyloid protein. Neurofibrillary tangles and amyloid plaques are commonly found in both Down syndrome and Alzheimer's individuals. Layer II of the entorhinal cortex and the subiculum, both critical for memory consolidation, are among the first affected by the damage. A gradual decrease in the number of nerve cells throughout the cortex follows. A few years ago, Johns Hopkins scientists created a genetically engineered mouse called Ts65Dn (segmental trisomy 16 mouse) as an excellent model for studying the Down syndrome. Ts65Dn mouse has genes on chromosomes 16 that are very similar to the human chromosome 21 genes. With this animal model, the exact causes of Down syndrome neurological symptoms may soon be elucidated. Naturally, Ts65Dn research is also likely to highly benefit Alzheimer's research.

While there are a number of commercially promoted dietary supplements on the market, especially in the USA, mainly involving various combinations of vitamins and minerals, none of these have been medically approved for use in the UK for the mass treatment of people with Down syndrome and none appear to lead to any proven lasting benefits. All remain highly controversial.

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