Cutting edge

June 11, 1999

The study of chromosomal abnormalities is broadening our understanding of the human brain and child development.

One of the challenges confronting those with an interest in child development and education is the role of genes. While it is easy to exaggerate the state of knowledge about how genes influence our abilities or behaviour, it is likely that major advances will be made in the next few years.

Our research aims to identify genes that could contribute to the development of cognitive skills. Our approach is unconventional. It is based on the fact that there are naturally occurring chromosomal abnormalities among humans that can be associated with the loss of many genes. Some of these genes may be needed for brain development or function, and the individuals in whom they are lost will suffer loss or impairment of cognition.

If our senses have sent us misleading information about the world around us because of a genetic disorder, this could cause a variety of adjustment problems, learning difficulties or behavioural quirks. The way that others respond to them could lead to secondary consequences such as poor self-esteem.

Neuropsychologists have tried to understand the organisation of the adult brain by studying individuals whose brains are damaged in adulthood. Their research has largely been confined to a study of the developed rather than the developing brain.

The trouble with applying this technique to children, whose brains retain plasticity of function, is that an acquired lesion may have variable and unpredictable consequences, and localised events such as strokes are very rare. Our approach is to locate genes that are responsible for the development of cognitive skills, the building blocks of our cognition.

The human genome comprises perhaps 100,000 genes, and these are organised into 23 pairs of chromosomes in each of our brain cells. To work normally, these genes must be expressed in the correct dosage, which often means both of a complementary pair (one from each parent) should be functioning normally. Many genes are important to the development of our central nervous system. If part or one of the chromosomes carrying such genes is missing, the dosage may be inadequate and the brain cannot develop.

We can identify the consequences of maldevelopment by setting cognitive tasks, and by brain imaging of individuals who are known to have lost specific chromosome regions. The aim is to find a consistent cognitive deficit. Modern molecular genetic techniques allow us to define the region of the chromosome that is missing. Information from the Human Genome Project allows us to identify which genes have been deleted. If we know one or more of those genes is normally expressed in the brain at some stage of development, then the gene is a candidate.

The conditions that allow us to apply this technique are rare. To date, we have focused on the X chromosome, which is normally present in two complete copies in females. This chromosome may be wholly or partially lost, with minor consequences on cognition and behaviour. Some of our findings suggest that there may be different X-linked genes that influence specific cognitive skills such as the ability to recognise and copy patterns, the recall of visuospatial information, the ability to recognise social cues and memory for verbally encoded information.

Locating the genes is still some way off, but we are optimistic we will be able to do so in the next few years.

David Skuse is professor at the Institute of Child Health, University College London.

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