Thursday June 10 2010
Only part of the genetic risk was inherited from parents
“Scientists have discovered the first significant link between autism and DNA,” says The Independent. The newspaper suggests that their results “could eventually lead to early diagnostic tests for autism and new forms of treatment”.
The current study looked at more than 5,000 rare genetic differences (called copy number variations, or CNVs) in the DNA of 996 people with autistic spectrum disorders (ASD) and 1,287 people without the condition. It found that people with ASD had a greater number of genes affected by these differences than those without ASD. Some of the CNVs affected genes that were already thought to play a role in ASD, while others affected genes not previously known to play a role.
This important study provides a lot of information about genes that may play a role in the development of ASD. More research will be needed to examine these genes. While this may eventually suggest new targets for drug treatments, their development would still be some way off. The study also showed that different people with ASD had different sets of variations, suggesting that a single, universal genetic test for ASD is unlikely to be developed in the near future.
Where did the story come from?
The study was carried out by a large international consortium of researchers. The main funders were Autism Genome Project Consortium, Autism Speaks (US), the Health Research Board (Ireland), The Medical Research Council (UK), Genome Canada/Ontario Genomics Institute, and the Hilibrand Foundation (US). The study was published in the peer-reviewed journal Nature.
The Independent, Daily Mirror, The Guardian, Daily Mail and The Daily Telegraph have covered this research. The Mirror and Daily Mail suggest in their headlines that a new test for autism could be developed based on these results. Most of the other papers discussed this possibility in their articles. However, not all individuals with ASD carry the same genetic variations, meaning a single diagnostic test based on these results seems unlikely to be developed in the near future. At best, such a test might indicate the chance of developing ASD, but would not be able to predict definitively whether a person would or would not develop ASD. Much more research into how well such a test might perform would be needed before it could become a reality.
The Independent importantly highlights that caution should be applied when interpreting the implications of this study, which the researchers themselves describe as preliminary. They say that “it will take many more years of intensive investigation to understand and treat the genetic alterations that increase an individual's susceptibility to the disorder”.
What kind of research was this?
This was a case-control study looking for genetic differences that could contribute to the development of autistic spectrum disorders. In particular, they were looking at sections of DNA that can exist in different numbers of copies in different individuals. These are called “copy number variations” or CNVs, and they can arise when pieces of DNA are duplicated or deleted. They wanted to determine whether individuals with autistic spectrum disorders have different numbers of copies at rare CNV sites than people without the condition.
This type of study design is commonly used to investigate the potential genetic causes of disease. This type of study has been made easier by advances in genetic technology, which means that researchers can now look for a large number of differences across multiple DNA samples in a relatively short space of time.
What did the research involve?
The researchers analysed DNA that was collected from 996 people with autistic spectrum disorders (ASD cases) and their parents, plus DNA from 1,287 matched individuals without ASD (the control group). To ensure that people’s ethnic backgrounds did not contribute to any genetic differences seen, the researchers only looked at people with European ancestry.
The researchers looked at CNV sites throughout the DNA and compared these between the cases and controls. They then specifically focused on 5,478 rare CNVs, which occurred in less than 1% of the sample population. They also looked for differences between cases and controls in the number of rare CNVs per individual, how long the CNV DNA regions were, or the number of genes affected by CNVs (that is, where the CNV was within or near to the gene).
What were the basic results?
The researchers found no differences between people with autistic spectrum disorders and controls in the number of CNVs per individual. Cases and controls each had 2.4 CNVs on average. There was also no difference between cases and controls in the length of their CNVs.
However, compared with controls, people with autistic spectrum disorders were found to have 19% more genes affected by CNVs (i.e. a CNV near to or within a gene). In some cases, these variations had been inherited from their parents. In other cases, the variations had arisen spontaneously in the affected individual.
Of the rare CNVs, 226 were found to affect a single gene and were found only in people with ASD, not the controls. Several of these CNVs were in genes or areas of the DNA previously thought to be involved in ASD or intellectual development. Some of the CNVs were in areas of the DNA containing genes not previously suspected to be involved in ASD. Other CNVs were in and around genes with roles in cell division, movement and signalling, and some played roles in nerve cells.
How did the researchers interpret the results?
The researchers concluded that their results “provide strong support for the involvement of multiple rare genic CNVs” in ASD. They say that this and subsequent research “may broaden the targets amenable to genetic testing and therapeutic intervention”.
There is much ongoing research into the genetic risk factors for complex conditions such as ASD, which is helping us to understand their role. One school of thought suggests that the genetic basis of these conditions is due to the cumulative influence of numerous common genetic factors contributing to the overall risk of developing the disease. The current study also suggests a role for rare copy number variations.
This particular study advances researchers’ knowledge of genes that may play a role in ASD, and they will use these results to target specific genetic areas for further study. Ideally, these results should now be verified by replicating the study in other groups of cases and controls.
Some news sources have suggested that these findings could lead to a test for ASD. However, not all individuals with ASD carry the same genetic variations, so a single diagnostic test based on these results seems unlikely in the near future. At best, such a test might indicate the chance of developing ASD, but it would not be able to predict definitively whether a person would or would not develop ASD. Much more research would be needed to look at the practical feasibility of a test, and how accurate and predictive such a test might be before it could become a reality.