Gene Found to Explain Why Boys Have Autism More than Girls
For every female that has classic autism there are four males. When including the entire spectrum of autism disorders, boys are diagnosed 10 times more often than girls. Researchers from George Washington University an found a gene that is affected by the sex hormones, which may help better explain the gender differences in autism rates.
RORA Gene Function Suppressed by Testosterone
Valerie Hu, a professor of biochemistry and molecular biology, and colleagues at GWU’s School of Medicine and Health Sciences studied a gene implicated in autism called retinoic acid-related orphan receptor-alpha, or RORA. Previous research has found that RORA is important for the development of the cerebellum and that the brains of people with autism expressed less of it than normal.
To test how RORA is affected by hormones, Hu bathed human brain cells expressing the gene in either estradiol, a form of estrogen, or dihydrotestosterone (DHT). They found that estradiol enhanced the expression of the gene where DHT suppressed it, protecting females against RORA deficiency.
The team also found that RORA regulates another gene which controls aromatase, an enzyme which converts testosterone to estrogen. If RORA is under-expressed, aromatase cannot function properly, causing testosterone levels to accumulate in dangerous amounts which are thought to contribute to the development of autism.
"For a long time elevated fetal testosterone has been a proposed as risk factor for autism, but the problem is that there has been no molecular explanation," says Hu. "Now we have evidence for a really exacerbating situation. What we have identified is an inhibitory feedback loop. That is what makes this so fascinating."
RORA is not the only gene that is suspected to play a role in the gender differences in autism rates. In September 2010, researchers found that 1% of boys with autism exhibited mutations in the PTCHD1 gene, a gene that is believed to play a vital role in the development of neural pathways involved in the delivery of information to cells during the brain’s development.
Girls with the same genetic mutation did not display ASD. It is suspected that because girls carry a second X-chromosome that may not be affected, it may shield them from the disorder.
Another gene, called CACNA1G, may increase the risk of autism in boys as well. Researchers from UCLA in 2009 found that a common form of this gene occurs more frequently in the DNA of families that have two or more sons affected by autism, but no affected daughters.
PLoS One February 2011
Science Translational Medicine 2010
Molecular Psychiatry, May 19 2009