Middle Eastern families yield intriguing clues to autism

Study implicates several genes involved in helping the brain learn from experience

July 10, 2008

Research involving large Middle Eastern families, sophisticated genetic analysis and groundbreaking neuroscience has implicated a half-dozen new genes in autism. More importantly, it strongly supports the emerging idea that autism stems from disruptions in the brain's ability to form new connections in response to experience - consistent with autism's onset during the first year of life, when many of these connections are normally made.

Interestingly, not all the affected genes were actually deleted, but only prevented from turning on - offering hope that therapies could be developed to reactivate the genes. The study, led by researchers at Children's Hospital Boston and members of the Boston-based Autism Consortium, appears in the July 11 issue of Science.

Autism genes have been difficult to identify because the disorder is complex, with a variety of causes stemming from many possible genes or combinations of genes. In addition, since people with autism tend not to have children, most of the genes identified thus far aren't inherited from a parent, but instead are mutated during embryonic development, making them hard to track through traditional linkage studies in families.

Christopher Walsh, MD, PhD, chief of genetics at Children's Hospital Boston, approached the problem by studying Middle Eastern families. In traditional Arab societies, it is common for cousins to marry, increasing the likelihood that offspring will inherit rare mutations. Middle Eastern families also tend to have many children, making them ideal for mapping genes.

"To map a gene for autism in American families, averaging two to three kids per family, you would need to pool many families," says Walsh, who is also a Howard Hughes Medical Institute investigator at Beth Israel Deaconess Medical Center (BIDMC). "In larger families, one family alone may be enough to definitively localize a gene."

The Homozygosity Mapping Collaborative for Autism (HMCA) recruited 104 families with a high incidence of autism from the Arabic Middle East, Turkey and Pakistan; 88 of these families have cousin marriages. Local clinicians were rigorously trained in administering standardized autism research assessments. Walsh's team later flew to sites in Turkey, Dubai, Kuwait and Saudi Arabia to confirm the diagnoses.

Using a technique called homozygosity mapping Walsh and colleagues compared the DNA of family members with and without autism, searching for recessive mutations--those that cause disease only when a child inherits two copies.

Experience-based learning
A neuron "fires" and an electrical signal (known as an action potential) travels down the neuron's axon and reaches the synapse - the junction with another neuron. Chemical messengers known as neurotransmitters are released into the synapse, causing special channels to open on the receiving neuron. The resulting influx of calcium starts an action potential in the second neuron. A cascade of signals then travel to its nucleus, launching a program involving multiple genes that communicates back to the surface, enabling the neuron to strengthen, weaken, create or destroy synapses or make a different kind of synapse. In this way, a neuron alters its connections with other neurons in response to the input it receives - the cellular basis for learning.

When things go wrong...
Gene mutations can lead to a variety of cellular mishaps that compromise the brain's ability to form relevant connections in response to experience, leading to cognitive and behavioral disorders such as autism. For example, a mutation might cause proteins at the neuron's surface to malfunction, which could prevent an action potential from occurring, or might disrupt part of the biochemical cascade that is triggered by the action potential. Or, a mutation might lead to problems with messenger RNA from the nucleus, hampering the cell's ability to communicate back to the surface and refine its synapses. Other mutations may affect the transcription factors that participate in messenger RNA synthesis. Interestingly, most of the half-dozen genes newly linked to some cases of autism (Science, July 11, 2008) were not deleted. Instead, nearby "on/off" switches (known as enhancers) were missing. This finding offers the hope that the genes might just need to be turned on or off by different means, such as medication, reducing the effects of autism.

...the proper connections aren't made.
Unless the genetic problem is corrected, the neuron will be unable to communicate properly with its neighbors - ultimately impairing brain function. If not properly maintained, necessary synapses can weaken or malfunction, or, conversely, the brain may be unable to prune connections that aren't needed. In some instances, neurons may build the wrong kind of synapses, causing the brain's balance of excitatory versus inhibitory signals to go askew. Autism is now thought to result from an improper excitatory/inhibitory balance.
Images by Graham Paterson, Children's Hospital Boston

"We check each set of chromosomes from beginning to end, looking for one place where the child has two identical pieces of DNA on both chromosomes," Walsh explains. "Eventually we find a spot where all affected children have two identical chunks of DNA, and where unaffected children have something different."

Just over 6 percent of the 88 families showed rare, inherited deletions within DNA regions linked to autism. These affected DNA regions varied among families, further indication of autism's large variety of genetic causes. In all, the technique identified five chromosome deletions affecting at least six identifiable genes (C3orf58, NHE9, PCDH10, contactin-3 [CNTN3], RNF8, and genes encoding a cluster of cellular sodium channels).

One of the genes, NHE9, was also found to be mutated in European and American children with autism (particularly those with both autism and seizures).

Experience-dependent learning: A common thread
The genes discovered are diverse in function, but all seem to be part of a fundamental molecular network that orchestrates the refinement and maturation of brain connections, or synapses, in response to input from the outside world. It is the refinement of these synaptic connections that is the basis of learning and memory, suggesting that autism at its heart may represent molecular defects of learning.

"This network can be disrupted in a myriad of ways, and may be one mechanism that people with a variety of autism-linked mutations share," says Michael Greenberg, PhD, a coauthor on the paper and director of the Neurobiology Program at Children's Hospital Boston.

Normally, as a neuron (brain cell) receives an incoming message at the synapse, a network of reactions is sparked that extends all the way to its nucleus. Greenberg and his colleagues had long been mapping this network, and had previously found that it activates at least 300 genes. These genes then communicate back to the neuron's surface, telling the cell to make a new synapse, strengthen the synapse that's already there, eliminate a synapse, or make a different kind of synapse. This give-and-take system is how the brain builds its circuitry; neuroscientists call it "experience-dependent learning."

Working independently of Walsh, Greenberg and his colleagues had already identified three of the same genes found in the Middle Eastern patients (c3orf58, NHE9, and PCDH10) while looking for genes that turn on or off in neurons as part of this network - either in response to synaptic activity or through so-called transcription factors that are activated by synaptic activity.

The work bolsters a growing body of evidence that autism may represent a disruption of the brain's ability to modify its synaptic connections in response to experience.

"Taken together, our findings suggest that experience-dependent learning could be relevant to autism, and that autism might result from the deregulation of any one of a number of genes that are part of the same signaling pathway," Greenberg says.

Can normal function be revived?
Interestingly, only one chromosome deletion found in the Middle Eastern families actually removed a gene - in most cases, what was lost was a region adjacent to the gene that contains its "on/off" switches. This has important implications for therapy, because it suggests that autism mutations don't always remove a gene altogether, but only inhibit its activity in certain contexts, says Eric Morrow, MD, PhD, of Massachusetts General Hospital, who is co-first author of the paper with Seung-Yun Yoo, PhD. "This means that we would not need to replace the gene, if we could only figure out how to reactivate it, perhaps with medications," says Morrow, who also holds appointments at BIDMC and Children's.

The findings also support the use of behavioral therapies in autism, which expose children to a rich environment and highly repetitive activities that may help turn on the genes and strengthen synaptic connections, Morrow adds.

"This publication a big event in the world of autism research," says Clarence Schutt, PhD, Scientific Advisor to the Nancy Lurie Marks Family Foundation, which funded work by both the Walsh and Greenberg labs. "To have discovered a connection between autism and activity-related gene expression at the synapse will put this field at the center of neuroscience."

The study was supported by the National Institutes of Health, the National Institutes of Mental Health, Cure Autism Now, the F.M. Kirby Foundation, the Simons Foundation, the Harvard Kuwait Project, the Developmental Disabilities Research Center of Children's Hospital Boston, the Clinical Investigator Training Program of Harvard and MIT in collaboration with Pfizer Inc. and Merck & Co., the Charles H. Hood Foundation and the Howard Hughes Medical Institute. The research is particularly indebted to Nancy Lurie Marks, whose NLM Family Foundation fostered the interdisciplinary collaboration at Children's that made the discoveries possible.

Contact:
Keri Stedman
617-919-3110
keri.stedman@childrens.harvard.edu

Children's Hospital Boston is home to the world's largest research enterprise based at a pediatric medical center, where its discoveries have benefited both children and adults since 1869. More than 500 scientists, including eight members of the National Academy of Sciences, 11 members of the Institute of Medicine and 12 members of the Howard Hughes Medical Institute comprise Children's research community. Founded as a 20-bed hospital for children, Children's Hospital Boston today is a 397-bed comprehensive center for pediatric and adolescent health care grounded in the values of excellence in patient care and sensitivity to the complex needs and diversity of children and families. Children's also is the primary pediatric teaching affiliate of Harvard Medical School. For more information about the hospital and its research visit: www.childrenshospital.org/newsroom.

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