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Monday, September 27, 2010

X-linked autism risk

Study after study has shown a genetic component for autism spectrum disorder (ASD). So far, the data hasn’t explained why boys suffer from the condition more frequently than girls. However, that may be changing. Doctors and researchers from the Centre for Addiction and Mental Health and from the Hospital for Sick Children, both in Toronto, have found a gene on the X chromosome that is associated with some types of ASD.

As I’m sure you know, girls have two X chromosomes, whereas boys have only one. This means that if a girl and a boy each inherit a mutation on one X chromosome, the girl may be able to compensate with the normal version of the gene on her other X. The boy, on the other hand, will have no choice but to express the mutant phenotype. Such traits are called ‘sex-linked’ or ‘X-linked’, and occur most often in males. Females only express sex-linked traits if they inherit the same mutation from both parents (and thus have it on both of their X chromosomes).

By comparing the gene sequences of 2000 ASD patients to that of thousands of controls, the scientists discovered that about 1% of boys with ASD carried a mutation on the PTCHDI gene located on the X chromosome. Sisters carrying the same mutation on only one of their X chromosomes were not affected. Obviously, this one mutation cannot be responsible for the overall difference in numbers between ASD-affected boys and girls. However, it may be only the first of many X-linked genes involved in ASD.

On the face of it, the fact that ASD is looking more and more like a series of genetic disorders can be discouraging. After all, there’s little that can be done to prevent a baby from inheriting these mutations. However, there is a silver lining. Because very young babies can be tested for genetic mutations, once researchers know what sequences to screen for, therapeutic treatments can begin immediately. There is evidence that early intervention is extremely beneficial in helping kids with ASD.

Image above: Chromosomes from a human female. Note the two identical X chromosomes. In males, there would be one large and one small chromosome in that position.