This disorder is a dramatic example of a language development disorder, but millions of children have much more mild language disorders. Many of these kids outgrow their language problems but not all. A group of researchers working in the UK and the US asked whether some of these relatively mild language disorders are somehow connected to the FOXP2 gene, which so far has only been found to be involved in dramatic speech disorders. Sure enough, they found that variants in a gene regulated by FOXP2 are strongly correlated with mild language disorders.
It is easy to see how FOXP2 might be involved in different types of language disorders. Mutations in the FOXP2 gene itself cause major problems, but you don't need mutations in FOXP2 itself to have issues. The FOXP2 gene produces a transcription factor, a protein that regulates other genes. The FOXP2 transcription factor presumably regulates genes involved in language development. If you mutate FOXP2, then you are likely to have regulatory problems with all of the many genes FOXP2 controls, so you're likely to get a severe language disorder. But what if FOXP2 itself is good, while one of the target genes has a mutation? In that case, most of your FOXP2-regulated language development genes do their job just fine, but one of them doesn't, possibly leading to a mild language disorder.
In other words, break FOXP2 and you have a major failure; break one of the downstream genes, and you only have a minor failure.
The research group set out to look for those downstream genes by using a technique called chromatin immunoprecipitation (also known as ChIP). This technique lets you identify which genes are directly bound (and therefore regulated) by the FOXP2 transcription factor. The researchers found that FOXP2 binds a gene called CNTNAP2. CNTNAP2 is known to be linked to some speech disorders in autism, so the researchers considered it a good candidate for being involved in language disorders in non-autistic children.
The next step was to find genetic variants in this gene that correlte with language disorders. To do this, the researchers were able to use a large group of study subjects who had already been tested for language disorders and had contributed DNA. Sure enough, the research group found nine genetic variants (or SNPs - changes in a single DNA base) that were significantly correlated with a language disorder.
Interestingly, all nine of these SNPs were found in an intron, a portion of the gene that does not encode protein, which suggests that the SNPs have an impact on how the gene is regulated or processed into a protein.
So what does this research mean? It's unlikely that this information is going to lead to a gene therapy or drug treatment any time soon. But the authors argue that it is important to break down the genetic components of language disorders so that we can better understand what the specific problems are. For example, children with autism frequently have language disorders, but there are many children with language disorders who are not autistic. Is the genetic source of the autistic and non-autistic language disorders the same? This study suggests that the answer is yes, at least in some cases: no autistic children were included in this study, but the results zeroed in on a gene, CNTNAP2, which has also been implicated in language disorders in autistic children. And thus this research supports the idea that autism is a disorder with multiple genetic components - it's not caused by mutations in a single gene. To understand what causes autism, we need to identify those components and understand how they work together.
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