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Rare deletions on chromosome 16 tie autism to obesity

by  /  10 February 2010

Big impact: Children carrying the 16p11.2 chromosomal deletion are likely to become overweight as they get older, researchers say.

Individuals who carry a large and rare deletion on chromosome 16 that is associated with autism are likely to have developmental delay, be obese, or both, according to two studies published last week in Nature1,2.

The deletion, known briefly as 16p, covers a 25-gene stretch of chromosomal region 16p11.2. It crops up in roughly 0.6 percent of all cases of autism3. Some studies have found the variant in individuals with other psychiatric conditions, such as schizophrenia and bipolar disorder, and even in healthy controls.

The new reports are the first to search for obesity-related rare variants across the genome and the first to link 16p — or any other rare DNA deletion or duplication — to obesity.

“The contribution of [rare variants] to obesity, just like for autism and schizophrenia, may be much more important than has been anticipated thus far,” says Jacques Beckmann, chair of medical genetics at University of Lausanne in Switzerland, and lead investigator of one of the new studies. “We should not ignore it.”

Beckmann’s study shows that nearly three percent of people who are both obese and have developmental delays carry the 16p deletion. In an independent study published in the same issue, researchers from the University of Cambridge searched for rare variants in 300 people with severe early-onset obesity. They found four carrying deletions in 16p; all have developmental delay and two have autism.

Scientists are stumped by this surprisingly robust genetic connection between obesity and autism.

“We don’t know whether the genes responsible for an autistic phenotype are the same as those responsible for obesity — maybe yes, maybe not,” Beckmann says. “That’s definitely something that we need to investigate in the future.”

Clinicians have long noticed that people with developmental problems tend to be overweight. A report published in the January issue of Obesity found that 23.4 percent of children with autism and 19.3 percent of children with a learning disability are obese, compared with 12.2 percent of healthy children4.

“Psychologists usually say that obese people have learning disabilities because they are in isolation, because they are depressed, or because they are badly seen by others,” says Beckmann’s co-investigator Philippe Froguel, chair of the department of genomics of common diseases at Imperial College in London. “But one of the lessons learned in last few years of genetic studies is that there is clearly an overlap between the development of the brain and obesity.”

One well-known example is Prader-Willi syndrome, characterized by mental retardation, obsessive behaviors and obesity. Prader-Willi is caused by a seven-gene deletion inherited from the father’s copy of chromosome 15; the same region is also missing in some people with autism5.

Since 2007, four large genome-wide association studies of obese people have identified about a dozen common single-letter nucleotide changes (SNPs) that tend to crop up more often in obese individuals than in healthy controls. Many of these SNPs lie near genes that are expressed in the brain or encode proteins important for brain development6.

Going to extremes:

Beckmann and Froguel’s study began with one obese teenage boy at Beckmann’s clinic. A comprehensive genetic screen revealed that the boy carries the 16p deletion.

Collaborating with clinics across Europe, the researchers searched for the deletion in a sample of 3,947 individuals with developmental delay. They found it in 22 individuals, or about 0.6 percent, which agrees with previous reports. Of the 22, 4 have confirmed autism diagnoses.

Looking further into the clinical data, the researchers found that a substantial number of the 16p carriers are overweight, and that their obesity increases with age. All four of the adult carriers are obese, as are 6 of the 15 adolescents; 3 carriers under age 2 are of normal weight. The researchers saw a similar trend in clinical data from six older studies of individuals with 16p deletions.

“In other words, with this deletion, overweight starts early but is not necessarily fully established by adolescence,” Beckmann says. “With time, [they] all become obese.”

The most unexpected result emerged from a search for the deletion in a group of individuals with both developmental delay and obesity. The researchers found that 9 of 312 of these individuals — or 2.9 percent — carry the deletion.

“With these extreme phenotypes, it’s quite easier to establish that something we find in the genome is responsible,” says Froguel. “Then we can search for it in the normal population.” When the researchers finally searched several large cohorts of thousands of obese people, they found the deletion in about 0.4 percent.

Clinical complexity:

Signature Genomic Laboratories, a private clinical genetic testing company in Spokane, Washington, has studied more than 150 individuals carrying the 16p deletion.

At a genetics conference last fall, the company reported that these carriers show a surprising range of clinical diagnoses, and often inherit the deletions from healthy parents.

“The phenotypes are all so complex, it makes it very difficult to figure out what’s going on,” says Trilochan Sahoo, a laboratory director at the company. Sahoo is analyzing the laboratory’s records to see how many individuals with 16p deletions are also obese.

It may be premature to assume that obesity and developmental delay are mechanistically linked, rather than just co-morbid conditions, Sahoo says. For instance, an obsession with food of a particular type could be causing a child with autism to overeat.

“I am a little apprehensive that they have oversold the idea that there is a strong causal relationship between this deletion and obesity,” Sahoo says. “But with these publications and more awareness, I’m sure that it will be looked at more closely.”

Clinical geneticist Marwan Shinawi is also studying individuals with 16p deletions. His team has also shown that 16p variants are associated with not just autism, but mental retardation, speech delays and face and body malformations.

“It’s a fascinating story,” notes Shinawi, assistant professor of pediatrics at Washington University in St. Louis. “We started really with just autism and keep expanding the spectrum of clinical manifestations of this condition.”

Of the 17 cases of 16p deletion Shinawi’s group has seen so far, 5 are obese, all of them adolescents or adults. In response to the new reports, his team plans to alert the families of the younger children that they’re at risk of obesity.

“It’s now going to be part of the genetic counseling with these families,” Shinawi says. “We can tell them to watch their diet, to start intervention early on, maybe to avoid obese consequences in the future.”

References:


  • Anonymous

    I would like my Autistic Son to lose weight. Do you have a solution?

    • Steve

      Less food, more moving

  • Anonymous

    My 6 year old son was diagnosed with 16p 11.2-12.1, interstitial deletion. The only reason he was diagnosed was because all he did was eat, eat, eat and had many other symptoms that his doctor thought he might have prader willi disease. He had a chromosome micro array done and they found the deletion. He has lost alot of weight now but we credit that to him taking concerta everyday, he also has ADHD, hypotonia, speech and language delay, and asthma.

  • Anonymous

    My son has 16 11.2 micro deletion, he is 14 months old.. he is the complete oposite on the eatting he doesnt want to eat and also has alot of texture issues with food. He has hypotonia and sever delay in growth and stuck around 6 months on skills. Not sure about adhd or autism yet but he does have a heart defect that is being watched closely by doctors which think the deletion was the cause of it at birth. Seems there isnt a whole lot about this deletion, just trying to get in contact with other parents that have some experience with this deletion so I can help my son. Thank you

  • Anonymous

    For families with a 16p11.2 deletion or duplication, I would like to refer you to Simons VIP Connect (http://www.simonsvipconnect.org/).

    This is an online community for individuals with copy number variation (CNV) deletions or duplications and their families. The goal of the community is to allow families to contact each other, provide support and learn more about individuals with CNVs. The community is initially reaching out to individuals with a 16p11.2 deletion or duplication. Registering allows you to connect with other individuals with similar CNVs and to learn about research opportunities.
    ———–
    John E. Spiro, PhD
    Senior Associate Director for Research
    Simons Foundation Autism Research Initiative (SFARI)
    https://sfari.org/

  • Nick Buckley

    For those parents of an autistic child who has diet or obesity issues, I would like to point out that there have been several studies showing a correlation between autism, gluten or gliadin sensitivity (problems with eating wheat, barley, or rye), and casein sensitivity (problems with digesting the casein in milk products). On the other hand, there has been no conclusive evidence showing that diet restrictions are beneficial for children with autism. It may be a good idea to ask your doctor if your child may have one of these issues, or see how they respond to a gluten- or casein-free diet. I sincerely hope that this helps in some way!

  • Alicia Kelley

    Our son, who is now 21, finally got genetic testing and has the deletion. We have had a diagnosis around 18 years for Autism Spectrum/Aspergers, but never got him tested as no one ever suggested it and just provided med checks, no real answers. When he was little, we were on government insurance, and couldn’t get much back then, did’nt have the internet/access/good healthcare we have now. Our son Liam lost nearly 30 lbs, had more energy,and felt better when he did Atkins with my husband. This is in response to previous post from Nick Buckley, mentioning the gluten/casein sensitivity. Unfortunately, because of Liam’s insatiable appetite, we have to lock our fridge and pantry, and keeping him “honest” on a strict diet can be hard. His appetite dialed back a lot on the diet, but again, it was hard to maintain. He has gained it all back, and considering a low glycemic diet that might be more “livable” for him.

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