Autism
TEDS - Main autism behaviour types are not genetically linked
By Medical Research Council (MRC), UK
Sep 6, 2005, 01:38

Scientists at the MRC Social, Genetic and Developmental Psychiatry Centre at the Institute of Psychiatry, King's College London, have discovered that two sets of behaviours that co-occur in autism spectrum conditions appear to be caused by different sets of genes.

The report by Dr Angelica Ronald in collaboration with Professor Robert Plomin and Dr Francesca Happ� - published in Developmental Science - could help advance future diagnosis, treatment and understanding of autism spectrum conditions.

In an autism diagnosis, two types of behaviours must be displayed; those that reflect social impairment such as a difficulty in making friends and non-social obsessive and repetitive behaviours such as sticking to rigid routines. These two types of symptoms can both have massive impact on children's development. The researchers were motivated to investigate these two components separately because they represent two very different types of behaviours and it is not clear why they co-occur in autism spectrum conditions.

The study collected data from the UK-based Twins Early Development Study (TEDS) through parent and teacher reports on 3000 pairs of seven-year-old twins. Participants completed a questionnaire designed to assess social and non-social behaviours that are characteristic of autism spectrum conditions but also seen in the general population. The questions assessed the extent to which the twins displayed a range of behaviours, such as how considerate of other people's feelings they are or whether they are fussy and over-particular.

The researchers found that identical twins (where each twin shares the same set of genes) tended to show similar levels of social impairments to each other: i.e. both twins would show either many or few social impairments. In contrast, fraternal twins (where only a proportion of their genes are shared) often had very different levels of social impairments to each other. The same pattern of results was found in the twins for non-social behaviours. This demonstrated that both social and non-social behaviours are highly heritable, that is, a large proportion of the variation of these types of behaviours in the general population is due to genetic influences.

The researchers then posed themselves a new question: whether social and non-social behaviours are influenced by the same set of genes. If the same genes operate on both, one would expect high correlations between social impairments in one twin and non-social behaviours in the other twin in identical twin pairs. Lower correlations would be expected in fraternal twins because they do not share all their genes.

The researchers did not find evidence to suggest that the same genes are involved. They found that social and non-social impairments did not correlate very highly and in many cases, for example, if one identical twin showed social impairments, their co-twin did not show any non-social impairments. The results of this study indicated that most of the genes influencing social impairments are different to those that influence non-social behaviours.

Dr Angelica Ronald said: "This study suggests for the first time that social and non-social behaviours, which are both shown in autism spectrum conditions, are caused by mainly different sets of genes. It suggests that 'genes for autism' is a misnomer: there are several genetically distinct components involved. This finding has important implications for DNA and brain studies: it may be better to study the social and non-social components separately rather than requiring that a child has both components, which is what traditional diagnosis requires."

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