Research in the past decade has brought us closer to understanding the myriad molecular genetic bases of the autism spectrum disorders. At the annual meeting of the Society for Neuroscience in San Diego in November 2010, leading autism researchers shared their latest findings.
Autism is known to have a very strong genetic basis; for example, its known gene characteristics are shared 70–90 percent of the time between identical twins. Huda Zoghbi, a professor in the department of molecular and human genetics at Baylor College of Medicine, summarized how she and her colleagues have searched for and identified some of the likely hundreds of genetic risk factors.
In 1999, Zoghbi and her colleagues reported that the MECP2 gene is mutated in Rett Syndrome, an autism spectrum disorder that mainly affects girls, who show learning difficulties and stereotyped, repetitive hand movements. The researchers then developed an animal model of the condition, using genetic engineering to introduce an extra copy of the MECP2 gene into mice.
They found that the modified mouse showed progressive neurological signs that faithfully replicate the symptoms of the human condition. This inspired the researchers to investigate the gene in human patients, leading to the discovery that some Rett patients also have duplications of the gene.
In a series of electrophysiological studies, Zoghbi’s team further found that neurons from animals with depleted MECP2 levels have reduced electrical activity due to a smaller number of excitatory synapses, whereas cells from animals carrying an extra copy of the gene have increased electrical activity because of an increase in synapse number.
“Loss or gain of function of the protein have opposite effects on synapse numbers, neuronal activity and molecular signatures,” says Zoghbi, “yet several of the clinical features are the same. This raises the possibility that the symptoms are a manifestation of the inability of neurons to adapt or respond to changing stimuli.”
More recently, Zoghbi's group has used genetic engineering to delete the MECP2 gene from specified types of neurons in the mouse brain to see which groups of cells influence the symptoms.
Animals in which the gene was missing from cells that produce the neurotransmitter dopamine moved in abnormal ways, while those lacking the gene in the hypothalamus behaved aggressively and exhibited abnormal responses to stress. Mice lacking the gene in inhibitory interneurons of the cerebral cortex, however, reproduced nearly all the clinical features of Rett Syndrome. These animals had a 30 percent reduction in levels of the inhibitory neurotransmitter GABA, due to a decrease in the enzymes that synthesize it.
“Deleting MECP2 from various neurons is helping us identify the cells that cause various features of autism, such as social behavior problems and anxiety,” says Zoghbi. “Loss of this protein seems to compromise the function of these neurons, which in turn reveals the resulting clinical features. We are now planning pre-clinical studies of compounds that enhance GABA signaling.”
Finding disruptions on the chromosome
At the meeting, Evan Eichler, a geneticist at the University of Washington, discussed further evidence from humans that some cases of autism are caused by mutations, deletions, or duplications of segments of chromosomes containing multiple genes.
Genomic studies comparing large numbers of autistic patients with controls have identified genetic variations that are common to people diagnosed with autism, but most of these variants contribute only a very small fraction of autism effects. Eichler and his colleagues tried a different approach.
They examined the chromosomes of autistic children, and found that some 5 percent have chromosomal abnormalities that can be seen under the light microscope. By focusing on regions of chromosomes that are predicted to have been unstable during human evolution, they identified 130 genetic ‘hotspots’ and then systematically analyzed them in their data from autistic children. Approximately 1 percent of the cases examined were found to have a deletion of a region chromosome 16, which contains 25 genes.
Eichler’s findings show that the cognitive and neurodevelopmental disabilities that characterize many of the autism spectrum disorders can occur as a result of disruption of many of these hotspots. Those identified already have been implicated in a variety of functions, including development of the brain’s language circuits, neuronal development, signaling, wiring, and synaptic plasticity. The findings support the "two-hit model" of developmental delay.
“The ‘two-hit’, or ‘second site’ model states that individually rare variation is, essentially, a single event that results in a mild phenotype,” a mild version of the disorder, says Eichler. “But the presence of two hits [two such variations] puts an individual into a predisposed state for developing a severe phenotype. The second hit can be spontaneous, but it can also be inherited from the same or the other parent.”
“There’s no question that the presence of these two hits occurs far too often than we would expect by chance,” says Eichler. “It all comes back to this idea that the human genome is loaded with large, impactful mutations, and it is the gestalt of those mutations that, when put together, manifests as the disease.”
“I have a feeling that this will be a recurrent theme for neuropsychiatric and neurobehavioral disease,” he adds, “where multiple severe hits come together to result in a phenotype. It helps to explain aspects of these diseases, which have flummoxed geneticists for a while. One is this idea of heritability—why you sometimes see a phenotype segregated in families and why it sometimes suddenly disappears." Another aspect it might explain is why families with members who have one disorder, like epilepsy, frequently have members that show signs of some other neurological disorder, like schizophrenia”
Eichler believes that next-generation molecular biological techniques, such as exome and transcriptome sequencing, which are cheaper and more efficient than more commonly used methods, will eventually lead to the identification of many more genes involved in autism.
“Next-generation sequencing technologies allow us to examine up to 90 percent of the coding sequences of an individual,” he says. “We think we’ve already identified the point mutations responsible for about 20 percent of autism cases, and maybe up to 45 percent of the genes causing sporadic cases. Another 10-15 percent of cases can be explained by copy number variation. I think we’ll have a good hint at something like 80 percent of the genetic causation within the next few years.”