Autoimmune Thyroid Disease

An Unfortunate and Lengthy Adventure in Misdiagnosis

What are the odds of new mutations causing autism?

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This is to follow on from previous posts I’ve made about the Autism Genome Project and the two genetic theories of autism.

The “new mutations” genetic theory of autism is one pioneered by scientists such as Professors Michael Wigler and Jonathan Sebat at the Cold Spring Harbor Laboratory on Long Island, New York. This short piece from the New Scientist explains their theory:

Duplications or deletions of portions of the genome may cause many – if not most – cases of autism.

Such errors can alter the number of copies of particular genes in the regions affected. These copy-number variations are 10 times as common in autistic children as in other children.

A team led by Jonathan Sebat of Cold Spring Harbor Laboratory on Long Island, New York, examined 118 families that have one autistic child and 99 families with children who are not autistic. Ten per cent of the autistic children, but only 1 per cent of the other children, had copy-number variants in their genomes that didn’t appear in their parents (Science, DOI: 10.1126/science.1138659).

The copy-number variants tend to affect different genes in each autistic child. This suggests that autism is not caused by a single genetic defect. Clues to autism revealed in copied genes

I’ve spent most of my time recently talking about single nucleotide polymorphisms – where one letter of a gene is substituted for another letter. Here, the scientists studied deletions and duplications of genes. Deletions and duplications and variants of genes are normal, very common, and widely found in the genetic code of all human beings. What the scientists have found here is that new deletions and duplications are present in a small but significant minority of autistic children – ten percent of autistic children to be exact, whereas they are normally present in about one percent of births. This is not at all surprising, considering the correlation of low-functioning DNA methylation genes with autism.

What we have to be cautious about is concluding causation from this – deletions/duplications rarely cause any observable changes in people, and they can occur anywhere in our genome – which consists of tens of thousands of different genes. For deletions/duplications to actually have some effect on the likelihood of being born with autism, they need to occur in genes that affect personality or the brain and nervous system.

What does the rest of the scientific community think?

Much of the autism research community believes there may be roughly six major genes involved in autism, and maybe 30 others that may confer some risk. A combination of mutations in any of these genes could contribute to the likelihood of being born with autism. Largest Ever Autism Study Identifies Two Genetic Culprits

Even if we included all of the personality and nervous system genes, that’s still a pretty narrow selection of genes that would have to be deleted or duplicated in order to produce changes in the child’s personality, let alone changes that cause autism.

One of the regular genes identified in autism by the Autism Genome Project is CNTNAP2. CNTNAP2 encodes neurexins – synaptic cell surface proteins – which are involved in glutamate functioning in the nervous system and brain. According to Entrez Gene, “This gene encompasses almost 1.5% of chromosome 7 and is one of the largest genes in the human genome.” That’s a lot of room for variation isn’t it?


Written by alienrobotgirl

7 February, 2008 at 9:08 pm

Posted in Autism Genetics

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  1. […] 2008 Following on from the two genetic theories of autism I’ve discussed before, and the questioning of the odds involved in new mutations causing autism, here’s a little more news that slipped under the radar recently. There is a new wrinkle to […]

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