Studies Continue to Find Genetic Causes for Autism in the Search for a Cure
Autism is a physical condition linked to abnormal biology and chemistry in the brain. The exact cause of this developmental disorder remains unknown, but is an active area of research, especially in the area of genetics where researchers are honing in on the gene mutations linked to autism which one day could lead to improved diagnosis and potentially a medical cure.
A recent report released by the Centers for Disease Control and Prevention (CDC) suggests that autism and related disorders is more common than previously thought. About one in 88 children in the United States are diagnosed with an autism spectrum disorder (ASD).
Researchers are fairly certain that autism has genetic roots, as the condition appears to run in families. However, genetic causes found so far have been so varied that it is hard to find enough people with a given mutation to establish a clear pattern.
One study conducted by researchers at Boston Children’s Hospital and published in the journal Neuron have found evidence for several inherited mutations using whole-exome sequencing to study large Middle Eastern families with autism. There is some evidence of the same mutations in US families as well, however American families aren’t usually the best for studies as they aer typically much smaller.
Tim Yu MD PhD and colleagues began by looking at three Middle Eastern families that had two or more children with an ASD. In all three families, the parents were first cousins, a common tradition in the Middle East. The researchers used genetic mapping techniques to narrow the search for specific chromosomal locations, and then proceeded to sequence the genes in those areas.
The team found recessive mutations in three genes not previously known to be involved in autism, but rather in severe genetic syndromes:
• Mutations in AMT, a gene classically associated with a severe metabolic syndrome known as nonketotic hyperglycinemia, marked by severe seizures and death during infancy.
• Mutations in PEX7. Typical PEX7 mutations cause rhizomelic chondrodysplasia punctata, a severe syndrome causing metabolic and bone abnormalities, cataracts, severe epilepsy and early death.
• Mutations in SYNE1, a gene associated with brain malformation, severe motor and muscle problems, and possibly bipolar psychiatric disease.
"This is the first time these genes have been associated with autism," says Dr. Maria Chahrour PhD who shares first authorship of the study with Yu. "The AMT and PEX7 mutations weren't picked up by standard tests for metabolic disorders, but when you're able to sequence the entire exome, you can find them."
These findings inspired the team to look further for other genetic syndromes affecting cognition and behavior. They screened 163 additional Middle Eastern families with autism for mutations in 70 known genes associated with the disorder. This approach revealed even more genetic abnormalities, including:
• Two families with recessive mutations in VPS13B (known to cause Cohen syndrome, which includes intellectual disability, obesity, vision and joint problems, and small head size)
• A family with a recessive mutation in POMGNT1 (known to cause muscle-eye-brain disease, marked by brain malformation, intellectual disability, muscle and vision problems)
• A family with an X-linked mutation in MECP2 in two boys (MECP2 mutations are known to cause Rett syndrome in girls, but are typically lethal in boys)
"It's not clear yet how many U.S. families have these recessive mutations," says Yu. "Further studies could begin to estimate what fraction of autism cases might fall under this model."
This study is only one among the many searching for a genetic cause for autism. Genetic scientists from the Children’s Hospital of Philadelphia and the University of Utah, together with Lineagen Inc (a biotech company) have identified 25 additional genetic variants that occur in some people with autism. These are described as “individually rare” but of “high impact” – meaning that each one has a strong effect in increasing a person’s risk for autism.
Scientists from the Seaver Autism Center at Icahn School of Medicine at Mount Sinai suggest that there are potentially hundreds of mutated genes linked to ASDs.
Researchers with the George Washington University have found that another genetic defect associated not only with autism, but also with other conditions of interrupted brain development could be due to a disruption at the cellular level. The team found that one type of cortical neuron, the projection neuron, is not generated in appropriate numbers during development in a mouse model of 22q11 Deletion Syndrome causing autistic symptoms.