Rare bone disorder reveals new insights into autism

By Heather Buschman, Ph.D.
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Children with multiple hereditary exostoses (MHE), an inherited genetic disease, suffer from multiple growths on their bones that cause pain and disfigurement. But beyond the physical symptoms of this condition, some parents have long observed that their children with MHE also experience autism-like social problems.

Buoyed by the support of these parents, researchers at Sanford-Burnham recently used a mouse model of MHE to investigate cognitive function. They found that mice with a genetic defect that models human MHE show symptoms that meet the three defining characteristics of autism: social impairment, language deficits, and repetitive behavior. The study, published online the week of March 12 in the Proceedings of the National Academy of Sciences, also defines the molecular and physiological basis of this behavior, pinpointing the amygdala as the region of the brain causing autistic symptoms.

“There is growing evidence that many autistic people have related genetic defects, or defects that are exacerbated by this one,” said Yu Yamaguchi, M.D., Ph.D., professor in the Sanford Children’s Health Research Center at Sanford-Burnham. Yamaguchi led this study, along with colleagues Fumitoshi Irie, Ph.D. and Hedieh Badie-Mahdavi, Ph.D.

Measuring social behavior in mice

In humans, MHE is caused by a mutation in one of two genes, Ext1 or Ext2. Together, these genes encode an enzyme necessary to produce heparan sulfate—a long sugar chain that helps bone cells grow and proliferate. In this study, Yamaguchi and his team used mice that lack the Ext1 gene in just a certain type of neuron to understand the mechanism of social problems in MHE patients.

Then the researchers examined the mice’s behavior to test for the three defining characteristics of autism: social impairment, language deficits, and repetitive behavior. Using several different techniques, the team found that the mutant mice were less social than normal mice. They also exhibited language deficiencies, as determined using ultrasound vocalization measurements, a well-characterized substitute for mouse language. Lastly, Yamaguchi’s team took at look at repetitive behaviors in these mice. Using a board covered with holes, they observed that normal mice will poke their noses in many holes at random, while the mutant mice poke their noses in the same hole again and again.

These behavioral measurements clearly demonstrate what the parents of children with MHE have always suspected—the disease affects more than just bones. The genetic defect that causes skeletal deformities also causes social and cognitive problems.

Pinpointing the amygdala

With these definitive simulations of autistic behavior in humans in hand, the researchers went on to define the cellular, molecular, and physiological basis for these symptoms. They did this first using a technique called c-Fos immuno-histochemistry, which illuminates the parts of the brain that are activated by certain experiences.

The researchers stimulated either normal or mutant mice with other normal mice for five minutes, then looked at their brains. In normal mice, the c-Fos immuno-histochemical staining lit up in the hippocampus, the amygdala, and other areas of the brain. In the mutant mice, the hippocampus was activated just as it was in normal mice. However, the amygdala of the mutant mice lacking the Ext1 gene wasn’t nearly as activated as it was in normal mice. This finding pinpoints the amygdala as the region of the brain that contributes to autistic symptoms in this model.

“These results are consistent with the amygdala theory of autism,” Yamaguchi said, referring to the idea that since the amygdala is thought to control a person’s social intelligence, autism could be caused by abnormalities in that part of the brain.

What this means for the general autistic population

Obviously, not all autistic children have MHE, nor are all MHE children autistic. But, according to Yamaguchi, there is evidence that some people who are autistic might have similar defects in heparan sulfate. This is the sugar chain that’s defective in MHE, where it causes bone deformities and—as this study now shows—social deficits.

“There are a few studies that compared the genomes of healthy and autistic people and they revealed differences in some heparan sulfate-related genes,” Yamaguchi said.

There are most likely many different genetic abnormalities that can lead to autism in the general population. This study and others now indicate that for some (presumably a small percentage), the condition could be caused by mutations in genes encoding enzymes and proteins involved in making heparan sulfate.

There’s even more evidence that heparan sulfate is an important factor in autism. According to Yamaguchi, several of the genes thought to be most closely associated with autism have an affinity for heparan sulfate. This means that while a slight reduction in heparan sulfate alone might not be the cause of many cases of autism, heparan sulfate defects could increase the severity of the disease when a person also has mutations in other autism candidate genes.

Yamaguchi’s team is now comparing DNA from autistic and non-autistic volunteers to look for mutations in heparan sulfate genes. So far the initial results have been encouraging.

How patient advocates contributed to this research

Yu Yamaguchi, M.D., Ph.D.

Several years ago, Yamaguchi was studying bone development when he received a call from Sarah Ziegler, co-founder of the MHE Research Foundation. Ziegler’s son, Robert, was born with MHE. Over the years, as Robert endured many surgeries to treat this incurable disease, Ziegler dug deep into the scientific literature to educate herself about MHE and bones. Somewhere along the way, she came across Yamaguchi’s work and had a feeling that his research could impact MHE.

One of Ziegler’s greatest frustrations was that most medical specialists only saw one part of Robert’s disease, when she knew from her experience with him and other MHE patients that the disease affects more than just bones. These kids have social problems, too. She called Yamaguchi and told him all this, setting his research on a new course that led to this latest finding.

“I can’t emphasize enough how much it helped that the parents of kids with MHE got involved and supported this research,” Yamaguchi said. “As parents, they noticed their kids had social problems that gave them challenges at school. School officials and other people didn’t take these observations seriously—they usually just waved off the problems, assuming that the kids’ bone deformities just make them shy. This latest research doesn’t solve any bone issues for MHE patients, but it does help support what the parents always knew—these children need special care.”

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This research was funded by the Eunice Kennedy Shriver National Institute of Child Health & Human Development at the National Institutes of Health, the Sanford Children’s Health Research Center, the Mizutani Foundation, the MHE Research Foundation, and families of MHE patients.

Original paper:
Irie F, Badie-Mahdavi H, & Yamaguchi Y (2012). Autism-like socio-communicative deficits and stereotypies in mice lacking heparan sulfate. Proceedings of the National Academy of Sciences of the United States of America PMID: 22411800

ResearchBlogging.org

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About Author

Heather Buschman, Ph.D.

Heather was a Sanford-Burnham Communications staff member.

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8 Comments

  1. Deena Larsen on

    Thank you for this important research. It is wonderful to think that research into a rare disease will be able to help millions.

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  4. Fascinating stuff, although obviously we need to be super cautious about anthropomorphizing mouse behaviour. As for the implications for autism in general, if true, would this not predict bone deficits in at least a subset of the autism population?

  5. I hope that more research can be done

    on the linkage of mhe and diabetes type 1

    Because i am thin with mhe and suspect something
    between heparan sulphate and insulin secretion.

  6. Judith Taylor on

    My 45yr old son has MHE and 15 yrs ago he had 75 percent of his scapula and deltoid muscle removed due to osteochrondoscarcima.. His neck bones have now fused and he has scoliosis with advanced arthritis . He lives in pain and has lost two inches in his height.. I have just read about the discovery of social problems.. I too had many problems similar to what I read but it was always thought it was my marriage break up.. What I now am reading I am so sad for my son as he obviously got a double lot of problems.. Five months ago he became a dad of a beautiful little boy.. We pray he does not suffer as my son has. Is there a test that can be performed in which this problem could be picked up ? My sons first evidence was at four years when one arm changed shape.

  7. Interesting article… I am 26 years old and have MHE…. along with the chronic pain and seemingly fatigue associated with the disease, I have often been termed “anti-social” by some of my peers, and certainly exhibit a lack of social out-goingness… I spend alot of time alone and write music and such…. but never really thought anything of it. I tend to dislike social situations and get aggrevated easily in them…. and so this study is interesting and makes sense.

    I also suffer from really bad IBS and digesstion issues. I hope research probes in the this conneciton as well…… I just have this constant feeling of “un-well” on a daily basis…. fatigue, brain fog, digestive pain…. just feels like my chemistry is off is what I tell my GI doc…. I wonder if HS-deficiency also plays a role in that as well.

    My G-PA died of esophogeal / stomach cancer at 59 and had bad MHE… his brother had his MHE metastize on his rib and died from that in his late 50s.

    An interesting facet of the disease, as it is genetic, is that I refuse to have children. The pain and short statutre suck… haha…. it feels like my energy is constantly being sapped and as if my knees are collapsed or crushed by gravity. Its like I feel like I should be taller and stronger, but instead have this dull, chronic sense of feeling compressed…. haha, its’ weird… But i wonder if the “nesting” inability plays into the strong desire to not have children of my own…. which sucks…. because the rest of my genetics are pretty good!

    It’s just nice to see the research and not feel so alone in dealing w/ this on a daily basis. Some days are good, some are bad. It helps connecting w/ others dealing w/ it. Thanks for the article!

  8. Thank you for researching this topic. Your work has helped me connect the dots between some aspects of my extended family’s medical history.

    I’m a 25-year-old woman with MHE. This disease runs in my family, and as a result, all of us are well acquainted with having surgeries and taking care of each other. Most of us have been treated by the same excellent orthopedic surgeon, Dr. George H. Thompson.

    We’re fortunate in that chronic pain, fatigue, and short stature haven’t been issues for us. What has been an issue, however, is unusual behavior that is consistent with autism. Initially, these differences were ascribed to introversion or giftedness. But those labels couldn’t explain other problems, such as hand-flapping and other repetitive movements, sensory hypersensitivity, moderate face blindness, and overall difficulties in social situations.

    After a fair amount of research, I realized that I met many of the criteria for Asperger syndrome. Since self-diagnosis is generally inadvisable, I went to a psychiatrist, who confirmed the diagnosis in early 2013. (Note: The new DSM-V classification has since merged Asperger syndrome into the broader category of Autism Spectrum Disorder.)

    As far as I know, I’m the only member of my family to have received an official diagnosis. However, my sibling (who also has MHE) independently came to a self-diagnosis of autism as well. Knowing the root cause of our issues has helped us understand ourselves better and deal with our problems accordingly.

    On the whole, research into this area is quite relevant to my family, me, and many others. I’m looking forward to seeing what further developments arise from your work. I wish you all the best.

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