Analysis reveals overlap between genetic and epigenetic risk maps of autism
Scientists at the University of Massachusetts Medical School are the first to map epigenetic changes in neurons from the brains of individuals with autism, providing empirical evidence that epigenetic alterations-changes in gene expression caused by mechanisms other than changes in the underlying DNA sequence-may play an important role in the disease.
Analysis of these variations revealed hundreds of genetic sites that overlap with many of the genetic regions known to confer risk for Autism Spectrum Disorders. The study was published in Online First by the Archives of General Psychiatry.
Autism spectrum disorders are a group of complex biological illnesses with a variety of origins. People with a disorder on the autism spectrum often struggle with social interactions and communication. Many suffer from delayed language skills, as well as restricted interests and repetitive behavior. It's estimated that only 10 percent of cases are a result of genetic mutations. The cause of the remaining 90 percent of cases is unknown.
"We know that autism is a biological disorder," said Schahram Akbarian, MD, PhD, director of the Irving S. and Betty Brudnick Neuropsychiatric Research Institute and professor of psychiatry at the University of Massachusetts Medical School. "But very little is known about the genetic and molecular underpinnings associated with the disorder. It's been hypothesized that an epigenetic model of autism could potentially explain why genetic screening strategies for the disorder have been so difficult and frustrating. Our study is the first clear evidence gained exclusively from nerve cells pointing to a link between epigenetic changes and known genetic risk sites for autism."
In order to see if epigenetic changes were occurring in individuals with autism, Akbarian and colleagues developed a novel method for extracting chromatin - the packaging material that compresses DNA into a smaller volume so it can fit inside a cell's nucleus - from the nuclei of postmortem nerve cells. Using tissue samples obtained through the Autism Tissue Program from 16 individuals diagnosed with an autism spectrum disorder, Akbarian and colleagues used deep sequencing technology to compare these tissue samples with 16 control samples for changes in histone methylation, a small protein that attaches to DNA and controls gene expression and activity.
After analyzing the sequenced DNA data, Zhiping Weng, PhD, director of the Program in Bioinformatics and Integrative Biology and professor of biochemistry & molecular pharmacology at UMass Medical School, found hundreds of sites along the genome affected by an alteration in histone methylation in the brain tissue from the autistic individuals. However, less than 10 percent of the affected genes they observed were the result of a mutation to the DNA sequence.
"Neurons from subjects with autism show changes in chromatin structures at hundreds of loci genome-wide, revealing considerable overlap between genetic and epigenetic risk maps of developmental brain disorders," said Akbarian.
"Our understanding of psychiatric disorders, such as autism, is burdened by the fact that we often can't see the structural changes that lead to disease," said Akbarian. "It's only by studying these diseases on the molecular level that scientists can begin to get a handle on how they work and understand how to treat them."
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