Diagnosis is all in the chromosomes;Briefing;Research focus

29th October 1999 at 01:00
Finn Egil Tonnessen on how genetic research has unlocked the secrets of dyslexia

IT IS now almost a century since researchers into dyslexia first highlighted the tendency for reading and writing problems to run in families and its higher prevalence among boys. But a raft of subsequent statistical studies on the alleged heritability of dyslexia produced inconclusive findings.

Some researchers argued that dyslexics' reading and writing problems were inherited. Others maintained that home and school factors were the main causes of the problem.

These competing claims are, in part, linked to different ways of defining and diagnosing dyslexia, as well as to alternative methods for studying the phenomenon. Some scholars conducted studies of multi-generational families, while others primarily studied identical and non-identical twins. The sizes of these researchers' samples varied greatly.

During the past decade, however, researchers have been more concerned with finding one or more "dyslexia genes" that can be localised on certain chromosomes. Until now, chromosome 6 has been the candidate most studies have pointed to. Chromosomes 1 and 15 have also been mentioned.

Over the past three years, my colleagues and I at the Centre for Reading Research in Stavanger, Norway, together with Professor Herbert Lubs from the Department of Genetics at the University of Miami, have studied the largest family with reading problems ever known.

The reading and writing abilities of about 80 family members across four generations have been scrutinised in detail, using blood samples and the latest genetic testing technology.

Our findings, reported in the current issue of the British Journal of Medical Genetics, have shown, for the first time, that chromosome 2 can be involved in the inheritability of dyslexia.

We have discovered that a gene on chromosome 2 affects the development and functioning of some parts of the brain responsible for language skills.

Among dyslexics, a fault on this gene leads to difficulties in processing written language. For example, the dyslexic typically reads "god" for "dog".

Although our research does not indicate that chromosomes one, six and 15 are particularly important, we cannot exclude them from the discussion. We also recognise that dyslexia might be connected in some way to several other genes on other chromosomes.

Reading is a complex, multi-layered activity involving many skills. The genetics of dyslexia are therefore also likely to be complex and multi faceted.

Our findings, while original and exciting, need to be supplemented with more genetic research in order to obtain a better understanding of how genes affect the development of reading ability. We hope we will then be in a better position to make earlier and more precise diagnoses of dyslexia, which in turn will allow timely interventions to prevent the onset of the condition and to treat it when it does occur. finn-egil.tonnessen@slf.his.no

Special Needs magazine, page 4

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