Mike Stephens of Amberley asks :-

What causes autism?

Marion Maw, a biochemist at the University of Otago, responded.

Autism is a common neurological disorder, which is thought to arise from abnormal neurodevelopment of the brain. A diagnosis of autism is made when a child meets specific behavioural criteria. The three main areas of dysfunction are social interaction, communication and repetitive behaviours. Patients that fit some but not all of these criteria may be diagnosed with related conditions such as Asperger syndrome or autism spectrum disorder.

The causes of autism are only poorly understood. Because the diagnosis is based solely on behavioural criteria, this term may encompass a number of biologically distinct disorders. Family studies have established that genetic rather than environmental factors are largely responsible for the condition.

Autism spectrum disorders have an incidence of 0.2 per cent in the general population. The incidence is considerably higher in close relatives of an affected individual: 60-90% in an identical twin and 2-3% in a sibling. These figures suggest that autism usually occurs when an individual inherits an unfortunate combination of genetic risk factors, each of which may only moderately increase the risk of developing autism.

Numerous molecular genetic studies have been undertaken with the aim of identifying these risk factors. These studies have identified many chromosomal regions/genes that are worthy of further investigation, but the precise causes of autism remain largely unknown. The few genes that have been identified to date have roles in diverse biological processes including neurodevelopment and neurotransmission.

There have also been many studies examining a role for environmental factors in some cases of autism. A link with viral infections such as rubella, and exposure to certain chemicals has been established. A proposed link with measles-mump-rubella vaccine led to parental concern but is not supported by epidemiological studies.

An interesting clinical feature is that autism is more common in male than in female individuals, with a sex ratio of about 4 to 1. This observation suggests that aspects of the pathology are either sex-linked or sex-dependent. Sex-linked disorders involve a damaged gene located on the X-chromosome (eg red-green color blindness) whereas sex-dependent disorders reflect biological differences between males and females (eg breast cancer).

The markedly higher incidence of autism in male individuals remains to be explained.