The Science Bit: Part 5 – Understanding Down’s Syndrome

26 10 2010

Down’s Syndrome is a chromosomal disorder that affects approximately 1 in 700 live births. Normally, people have 23 pairs of chromosomes in almost every cell in their body, but in Down’s Syndrome, something goes wrong and the child is born with an extra copy of chromosome 21. In this month’s Science Bit (a bit later in the month than usual, sorry!), I explore the causes of Down’s Syndrome and new research that could provide an answer to this unsolved problem.

Chromosomes are long lengths of DNA that contain hundreds or even thousands of genes, which in turn provide the genetic code for making the proteins that carry out thousands of different functions in our bodies. Normally, we have 23 pairs of chromosomes in almost every cell – 22 pairs of “autosomes”, plus the sex chromosomes; XX if you’re female, and XY if you’re male.

The sex cells, eggs (or oocytes) and sperm (short for spermatozoa), normally have half the number of chromosomes of other cell types. Through a special kind of cell division called meiosis, each egg and sperm only contains one of each chromosome pair so that when one of each of these sex cells combine, the resulting zygote (fertilised egg) has the correct number of chromosomes, neatly arranged into their pairs again. This single fertilised egg cell then undergoes another type of cell division called mitosis in which the chromosomes and cell contents are duplicated, the cell grows and divides over and over again, and packages 23 pairs of chromosomes – identical to the first cell – into every new cell produced.

As miraculous as the cellular mechanisms that give rise to new life are, the process is not flawless, and there are a number of genetic conditions that can arise from faulty chromosomes. One of the best known chromosomal disorders is Down’s Syndrome, in which each cell has an extra copy of chromosome 21. People affected by Down’s Syndrome have the characteristic Down’s features including short stature, a long tongue and sloping eyes; they are also more prone to suffering from respiratory disorders, heart defects, learning difficulties and a much reduced life expectancy.

It has been known for some time that babies born to mothers in their 30s and 40s have an increased risk of Down’s Syndrome compared to younger mothers, and we also know that having the extra chromosome, called “trisomy”, is because the chromosomes don’t split properly during cell division from the fertilised egg cell. So far however, the reason why this “non-disjunction” of chromosomes occurs in older mothers has remained unclear.

New research from the University of Newcastle however, recently published in Current Biology (boo, paywall alert!), may have the answer. Working with mouse eggs, researchers from the University’s Institute for Ageing and Health have identified a group of proteins called “cohesins”, which seem to be important in holding chromosomes together during cell division. They have discovered that, in mice at least, a female’s level of cohesin declines with age, which in turn prevents chromosomes from moving normally such that they become “trapped”. Low levels of cohesin may be responsible for a number of chromosome disorders including not only Down’s Syndrome, but others such as Klinefelter’s Syndrome and Turner Syndrome (intersex conditions both associated with non-disjunction of the sex chromosomes).

It’s far too early to suggest that these findings may lead to a “cure” for chromosomal disorders like Down’s Syndrome, or that they will help to prevent the condition from occurring in the first place, but knowledge of the mechanisms by which cohesin decreases with age could lead to the discovery of ways to prevent this loss. Consequently, this may one day provide reassurance for an ageing population and a generation of families who now tend to have children much later in life.

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