A study of twins aims to clarify the roles genes, environment play in autism

On a chilly and sunless school holiday the Dixon twins were entertaining themselves like many fourth graders might: watching cartoons on television, playing with various toys, chasing each other around the house, and no doubt wearing on the nerves of their mother, Kristen Dixon.

Viewed from across this living room in the East Oakland Hills, or as a whizzing blur moving throughout the house, the boys looked as similar as twins could be. Both wore jeans and plaid button-down shirts, both had closely cropped hair and stood at about the same height. But after some time, when the boys slowed down and took up individual activities, they gradually began revealing their differences.

Christopher busied himself by drawing and talking to his mother. Meanwhile, as Gerald Jr. sat on the couch watching SpongeBob Squarepants, he was talking as well, but to no one in particular. Upon closer listening he wasn’t speaking a recognizable language, either.

Kristen and Gerald Dixon noticed differences in their fraternal twin sons when the boys were just over a year old. Doctors diagnosed Gerald Jr. with autism when he was two years old, the earliest age that autism experts say an accurate diagnosis can be made. Many parents find out later than this, around the age of four, or even after their children begin school. The Dixons had an advantage that helped them diagnose Gerald early: a perfect comparison they saw every day.

“Having the twins side by side, you know you have a comparison all the time,” said Kristen. “They’re in the same household, they’re in the same environment, they’re getting the exact same things so if one starts falling behind you wonder, ‘Well what’s going on?’”

One of her first clues came when Gerald Jr. was a toddler. “I noticed that he had begun talking and then he just stopped talking,” said Kristen. “I said to Gerald Sr. one day, ‘Gerald hasn’t been talking, have you noticed that? Christopher is kind of blabbering on and on and getting more words but Gerald isn’t really saying anything anymore, like nothing.”

The term autism refers to Autistic Disorder, or Classic Autism, which is one of the Autism Spectrum Disorders (ASDs). These also includes Asberger Syndrome and Pervasive Development Disorder—Not Otherwise Specified (PDD-NOS). Researchers have made great gains in understanding this disorders in the past several years, but much uncertainty still persists. Scientists and medical professionals have struggled to understand the exact causes of autism for many years. They once thought that genes were the primary cause, but they recently found that genetics only account for 37 percent of cases, while the environment accounts for 55 percent of them.

Last summer researchers in California revealed the results of the largest ever study of autism in twins, the California Autism Twins Study (CATS). Twins are the perfect subjects to use in scientific studies of disorders relating to genetics and the environment. Identical twins, with 100 percent of the same genes, are used to test genetic-based hypotheses, while fraternal twins, with roughly 50 percent of shared genes and usually the same environment, both inside the womb and out, are ideal subjects for testing environmental theories. Christopher and Gerald Dixon Jr. were one of 1,100 twin pairs who participated in the six year long CATS study.

Stanford University’s Dr. Joachim Hallmayer, the principal researcher of the CATS study, said that while searching for answers regarding the role that genetics and the environment play in autism was important, he is also interested in digging deeper. “We hope to discover not just whether genetic or environmental factors cause autism, but what role genes play in the specific characteristics of autism.”

Now that the CATS study is over, Hallmayer hopes to spend more time researching the emerging field of epigenetics and its role in autism. Epigenetics is sort of a combination of genes and the environment—researchers think that epigenetics cause differences in the expression and suppression of genes based on chemical reactions inside the body. Hallmayer describes epigenetics as a light switch that can turn a gene on or off. In other words, someone like Christopher Dixon could actually have the gene that causes autism in his body but it has been suppressed through an epigenetic process. While the CATS study has helped researchers and people in the medical community understand autism Disorder more than they previously did, Hallmayer hopes to pursue the study of epigenetics and autism further.

Despite its length and tedious paperwork involved in being clinical participants, the Dixons were happy to participate in the CATS study. “We just hope it helps other families get the help they need for their children,” said Kristen Dixon.