Down's syndrome: trisomy 21

Down's syndrome is the most common single cause of learning disability in children of school age. Children with the syndrome typically have a round, flat face, and eyelids that appear to slant upwards. In addition to some learning disability, they also have an increased risk of infection (particularly respiratory and ear infections), and heart defects occur in about one-quarter of those with the syndrome.

The syndrome is named after John Langdon Down, a nineteenth century doctor who first described the condition in 1866. In 1959, the French physician Lejeune used chromosome-staining techniques to show that Down's syndrome is caused by an extra chromosome 21. Having one extra chromosome is known as trisomy,hence Down's syndrome is also known as trisomy 21.The extra chromosome usually comes from the egg cell due to non-disjunction of chromosome 21. About 70% of the non-disjunctions occur during meiosis I, when homologous chromosomes fail to separate; 30% occur during meiosis II, when sister chromatids fail to separate. Whether it occurs during meiosis I or meiosis II, non-disjunction leads to trisomy. In a few cases, the extra chromosome comes the father.

In about 3% of cases, Down's syndrome results from translocationof an extra chromosome 21. A region of the chromosome breaks off and rejoins with either the end of the other chromosome 21 or with another non-homologous chromosome (commonly chromosome 15). In these cases, a person may have the normal number of chromosomes, but one of the chromosomes will be abnormally long.

Genetic screening: amniocentesis and chorionic villus sampling

Because of the high risk of Down's syndrome among the babies of older mothers, in the UK mothers over the age of 35 years are usually offered free genetic screening by the National Health Service. Genetic screeningrefers to procedures used to examine an individual for the presence of a genetic disease or disorder. The most widely available genetic screening procedure for Down's syndrome is amniocentesis.

Amniocentesisis usually carried out at 15-16 weeks of pregnancy. It involves passing a very fine needle into the uterus, observed with an ultrasound image, and withdrawing a sample of amniotic fluid containing fetal cells. The karyotype of the fetal cells is then analysed to test for Down's syndrome. The fetal cells can also be cultured in a suitable medium in a laboratory so that further tests, such as DNA analysis, can be carried out.

Amniocentesis is performed under local anaesthetic and most women do not find it too uncomfortable. However, there is a 0.5-1 per cent risk of spontaneous miscarriage after the procedure. Therefore, amniocentesis is usually recommended only for those at high risk of carrying a Down's baby-In the 1970s, chorionic villus sampling (CVS)was developed in China. In CVS, a sample of cells is taken from the chorionic villus (small finger-like processes which grow from the embryo into the mothers uterus). The sample is obtained either by inserting a needle through the abdomen, or inserting a catheter. The fetal cells in the sample can then be analysed in the same way as for amniocentesis.

CVS can be carried out between week 8 and week 12 of pregnancy. If the test shows the fetus has Down's syndrome, a decision about abortion can be made earlier than with amniocentesis. Early abortions are usually less difficult, both physically and mentally, than later abortions. However, a higher risk of miscarriage is associated with CVS than with amniocentesis.

Until recently, a mother's age was the only factor available to assess the risk for Down's syndrome. Now biochemical markers are being discovered for the condition. For example, women with a high risk of Down's syndrome pregnancies tend to have about twice as much chorionic gonadotrophin (a sex hormone produced in placenta cells) in their blood serum as women with normal pregnancies. Tests for these biochemical markers cannot show the presence of a Down's baby, but they can be used in conjunction with the mother's age to predict the probable risk of having a baby with Down's syndrome. If the risk is high, the mother can then decide whether to have an amniocentesis or CVS.

Genetic counselling

Genetic screening should be followed by genetic counselling,the giving of advice and information about the risks of a genetic disease and its outcome. Counselling is a very challenging task. Counsellors must have a good understanding of medical genetics and need to be well trained in sympathetic counselling techniques. They must give information which helps clients come to their own decision rather than imposing their own views on the clients. Clients should be made aware that the features of Down's syndrome vary widely. The condition often results in individuals with severe mental disability who require a great deal of support, but many people with Down's syndrome lead independent, long, and fulfilling lives, and they are often very loving individuals. It should not be assumed that mothers carrying a fetus with Down's syndrome would automatically opt for termination of pregnancy.

■ Glossary of essential terms for you to know

■ Your Essential Assignments

I. Quick check

1. What is non-disjunction?

2. Compare amniocentesis and chorionic villus sampling with respect to:

a) When they can be carried out.

b) The risk of inducing a miscarriage.

3. Name one biochemical marker which can help genetic counselor assess the risk of Down’s syndrome for a client.

II. Find the missing words:

III. Use a monolingual English dictionary and give the definitions of the following words:

frequency; range; common; disorder; sympathetic; support.

IV. Suggest Russian equivalents for the following word combinations:

V. Fill in the gaps with the words and expressions from the text:

1. Advances in DNA technology have brought …in preventive medicine.

2. We can now … a large range of inherited diseases before birth.

3. In addition to some learning disability they also have ….

4. The syndrome is named after John Langdon Down, …who first described the condition in 1866.

5. Down’s syndrome is caused by ….

6. Because of the high risk of Down’s syndrome among the babies of older mothers, in the UK mothers over the age of 35 years are usually
offered ….

7. Genetic screening should be followed by genetic counseling? The giving of advice and information about ….

8. Counselling is ….

9. They must give information which helps clients … rather than ….

10. It should not be assumed that mothers carrying a fetus with Down’s syndrome would ….

VI. Find English equivalents for the following word combinations:

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