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The genetics of autism

Autism and autism spectrum disorders (ASDs) actually represent a rather large continuum of conditions that range from very severe neurodevelopmental delay and abnormalities to the relatively mild. In severe cases, the child is nonverbal and displays a fairly well-characterized set of behaviors, including repetitive behaviors such as “stimming” (for example, hand flapping, making sounds, head rolling, and body rocking.), restricted behavior and focus, ritualistic behavior, and compulsive behaviors. In more mild cases, less severe compulsion, restriction of behavior and focus, and ritualistic behaviors do not necessarily preclude functioning independently in society, but such children and adults may have significant difficulties with social interactions and communication. Because ASDs represent a wide spectrum of neurodevelopmental disorders whose symptoms typically first manifest themselves to parents between the ages of two and three, the idea that vaccines cause autism and ASDs has been startlingly difficult to dislodge and has fueled an anti-vaccine movement, both here in the U.S. and in other developed nations, particularly the U.K. and Australia. This movement has been stubbornly resistant to multiple scientific studies that have failed to find any link between vaccines in autism or the other favorite bogeyman of the anti-vaccine movement, the mercury-containing thimerosal preservative that used to be in many childhood vaccines in the U.S. until the end of 2001. Add to that the rising apparent prevalence of ASDs, and, confusing correlation with causation, the anti-vaccine movement concludes that vaccines must be the reason for the “autism epidemic.”

In reality, autism and ASDs appear to be increasing in prevalence due to diagnostic substition, better screening, and the broadening of the diagnostic criteria that occurred in 1994. Autism prevalence does not appear to be rising, at least not dramatically, at all, as the prevalence of ASDs, when assessed carefully, appears to be similar in adults as it is in children. If the true prevalence rate of autism and ASDs has increased, it has not increased by very much. In reality autism appears to have a major and probably predominant genetic component, and several scientific studies over the last few years have linked autism with various genetic abnormalities. Not surprisingly, given the varied presentation and severity of ASDs, these studies have not managed to identify single genes that produce autism or ASDs with a high degree of penetrance (probability of causing the phenotype if the gene is present). Indeed, one can argue that the state of current evidence is that ASDs are due to multiple genes, perhaps dozens or hundreds. Again, this is not surprising given the heterogeneity of ASD severity, presentation, and symptoms.

One of the more surprising studies supporting a genetic basis for autism appeared to much fanfare in Nature last week. The study by Pinto et al, looks at the functional impact of global rare copy number variation in autism spectrum disorders. Its results are rather surprising in that the large team of investigators (studies of this type take a lot of people to carry out) found that it may be relatively uncommon copy number variations in various genes that lead to the phenotype of autism or ASDs.
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Posted in: Neuroscience/Mental Health, Science and Medicine, Vaccines

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