The Down syndrome reference article from the English Wikipedia on 24-Jul-2004
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Down syndrome

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Down syndrome (also called Down's syndrome) encompasses a number of genetic disorders, of which trisomy 21 (a nondisjunction) is the most representative, causing highly variable degrees of learning difficulties and physical disabilities. This genetic disorder was named after John Langdon Haydon Down, the British doctor who described it.

Table of contents
1 Overview
2 Medical research
3 Down syndrome's sociology
4 Down syndrome in fiction
5 External links

Overview

Incidence of Down syndrome is estimated at 1 per 660 births and is the most common chromosomal abnormality. It is a commonly known observation that maternal age influences the risk of conceiving a baby with the syndrome. At age 20-24, it is only 1/1490, while at age 40 it is 1/106 and at age 49 it is 1/11. (Source: Hook EB. Rates of chromosomal abnormalities at different maternal ages. Obstet Gynecol 1981;58:282.)

Down syndrome is named after John Langdon Haydon Down, who first described the condition. It was originally called mongolism, after a perceived resemblance observed by Down between the faces of some of his patients with Down syndrome and the mongoloid race. This usage, like other medical terms used at the time, has become a term of abuse, and is now generally viewed as both offensive and medically meaningless.

Trisomy 21 is the existence of a third copy of the chromosome 21 in cells throughout the body of an affected person. The condition puts children with Down syndrome at an immediate disadvantage compared with children who do not have DS. The IQ of a child with Down syndrome is rarely measured above 60. The brain of children with Down syndrome is usually small and underweight. The cerebellum and brain stem are unusually small. So is the superior temporal gyrus. Educational progress may also be damaged by illness and disabilities such as recurring infectious diseases, heart problems, eyesight, and hearing problems.

Early educational intervention, screening for common problems such as thyroid functioning, medical treatment where indicated, a conducive family environment, vocational training, etc. can produce excellent progress in the overall development of children with Down syndrome. On the one hand, Down syndrome shows that we cannot jump over genetic limitations; on the other, it shows that education can produce excellent progress whatever the starting point. The commitment of parents, teachers and therapists to individual children has produced previously unexpected positive results.

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 representative (95% of cases). 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 learning disability may range from mild to severe.

Trisomy 21 results in over-expression of genes located on chromosome 21. One of these is the superoxide dismutase gene. Some (but not all) studies have shown that the activity of the superoxide 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 one of 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. Whilst 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, none appear to lead to any proven lasting benefits, and they all remain highly controversial.

Down syndrome's sociology

A child with Down syndromeEnlarge

A child with Down syndrome

Advocates for people with Down syndrome stress that they have the same human rights, emotions, dignity and value as any other human being. The abuse and forcible institutionalisation of people with Down syndrome was closely linked to early twentieth-century racial and eugenic theory, culminating in the murder of many people with Down syndrome and other disabilities by the Nazi government in Germany in the period late 1930s-1945, and the creation of compulsory sterilization programs around the world which targeted the mentally disabled.

The housing of people with Down syndrome in institutions and their exclusion from society is currently regarded by most democratic governments and social care agencies as highly damaging to the social status and human rights of people with Down syndrome. As social models of disability and the philosophies of social role valorisation and inclusion have gradually taken hold, people with Down syndrome are increasingly achieving their potential for personal and social development. Many children in the UK are now educated in mainstream schools, learn to read and write, and are likely to live productive and valued lives as part of their families and communities.

Individuals with Down syndrome share many of the characteristics of their parents, with average life expectancy now approaching 70 in most developed societies thanks mainly to improved diet, housing, health and social care. Many children and adults with Down's syndrome enjoy an excellent quality of life, and the extra chromosome may confer some health benefits, for example, reduced incidence of certain cancers caused by double immunity: lung cancer for example is virtually unknown in people with Down syndrome.

Down syndrome in fiction

External links