Research shows autism spectrum disorders are largely genetic

Studies have found that some families carry very high genetic risks for ASD due to inherited mutations

Screening for rare mutations in families has become a standard of care in many countries and is increasingly being conducted in Ireland. Photograph: Getty Images

Screening for rare mutations in families has become a standard of care in many countries and is increasingly being conducted in Ireland. Photograph: Getty Images

Tue, Apr 1, 2014, 01:00

Until recently, autism spectrum disorders (ASD) were viewed as a relatively rare condition. Increased awareness and recognition has led to increased diagnostic rates and now the prevalence of ASD is thought to be relatively common, affecting about 1/150 people in the population.

Genetic risk factors are well recognised as playing a predominant role in ASD. Early evidence for this was identified in twin studies in the 1970s.

These studies compare the rate of a condition such as ASD in identical twins (who share 100 per cent of their DNA) with non-identical twins (who typically share 50 per cent of their DNA).

A large number of such studies have consistently found that if one of a pair of identical twins is autistic, the chance that the other one will be too is 60-90 per cent, while the concordance rate in non-identical twins is less than 30 per cent.

Family studies have shown that the risk of having ASD is 10-20 per cent if you have an affected first-degree relative.

Modern genetic tools, especially DNA sequencing technologies, now provide the means to investigate the role of genetic factors in a large number of individuals at a much higher resolution than previously.

The costs of these studies have fallen considerably and this has led to several large-scale international studies of autism genetics that have included Irish families.

A major finding to emerge is that many cases of autism are caused by rare genetic mutations, which can be recognised currently in 15-20 per cent of cases and this is likely to increase as more studies are completed.


Inherited mutations
Some of the time, these mutations are inherited from the parents, but in many cases they are associated with new mutations – ones that arose de novo (afresh) in the generation of sperm or egg cells.

These de novo mutations, which are not carried by the parents, can give rise to sporadic cases of autism, with a genetic cause but no family history. It is now possible to screen for many of these mutations and to advise people about recurrence risks.

This is hugely important to couples with a child with ASD who frequently overestimate their risk of having another child with ASD.

Often couples choose not to have more children but if a child has been affected by a de novo mutation, the risk of another child being affected is no higher than the general population.

It is also clear that some families carry very high genetic risks for ASD, due to inherited mutations.

Screening for rare mutations in families with autism has become a standard of care in many countries and is increasingly being conducted in Ireland.

This has led to the identification of a growing number of specific genetic conditions, which can sometimes cause ASD.

Such mutations do not always specifically lead to autism, however. They can also occur in patients with other symptoms, such as epilepsy, schizophrenia or other neurological or psychiatric conditions, and sometimes occur in a very small number of typically developing people.

Autism is thus one of a range of possible clinical manifestations of mutations that alter brain development or function.

Researchers at Trinity College Dublin, NUI Galway and Autism Speaks are currently holding a consultation regarding the establishment of an autism registry and biobank (iarb.ie). A clinical registry will help establish the scale of autism in Ireland and the needs of the community while a biobank of biological materials, such as blood and saliva, would provide researchers with data to investigate the genetic factors that may contribute to autism.

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