This is a guest blog post from Autism Speaks Science Board member John Elder Robison, author of Look Me in the Eye: My Life with Asperger’s and Be Different: Adventures of a Free-Range Aspergian.
As head of clinical programs at Autism Speaks, I oversee a number of vital resources for researchers studying the causes and treatment of autism. Today brought the publication of a new and revealing study made possible by Autism Speaks’ Autism Genetic Resource Exchange (AGRE).
Autism researchers have been studying twins for years for insights into the genetic and nongenetic factors that influence the development of autism. One of the most powerful ways to do so is to study twins (both identical and non-identical) where at least one of the pair has autism. This approach allows us to look at how often both twins receive a diagnosis of autism. Study of identical twins, who share 100 percent of their genes, then helps us determine the degree to which autism is inherited, or genetic; and comparison to fraternal twins, who share around 50 percent of their DNA, allows us to understand how environmental influences add to the risk of autism spectrum disorder (ASD).
But until now we’ve had only three, small twin studies, which together looked at just 66 twin pairs–a number too small to produce reliable conclusions. Still, these studies were the best we had, and theysuggested that when one identical twin develops an ASD, the chance of the other twin developing the disorder is as high as 90 percent. These same studies showed little to no overlap among fraternal twins – leading to the conclusion that inherited genes alone produced the risk.
Now comes the game changer. The California Autism Twins Study (CATS) is the largest ever study of twins with ASD, with scientifically reliable information on 192 twin pairs, both identical and fraternal. It was conducted by a group of renowned researchers in collaboration with the AGRE team. AGRE clinical staff collected DNA and helped perform the home-based diagnostic and cognitive testing on many of the participants, using scientifically validated research measures for diagnosing ASD.
So what were its dramatic findings?
It found that when one identical twin develops autism, the chance of the other twin developing the disorder is 70 percent. More surprisingly, it documented a whopping 35 percent overlap among fraternal twins. This is strong evidence that environmental influences are at play. Moreover, the 35 percent “both twins affected” rate is higher than the 3 percent to 14 percent overlap between different age siblings. (i.e. If one child in a family has autism, there is a 3 percent to 14 percent chance that a younger sibling will develop it.) This suggests that there are environmental influences uniquely shared by twins–for instance, in the womb and perhaps during birth.
In other words, we now have strong evidence that, on top of genetic heritability, a shared prenatal environment may have a greater than previously realized role in the development of autism in twins
This has important implications for future research. For instance, is there a particular time period during the pregnancy when a child’s brain development is particularly vulnerable to environmental influences? And what might these influences be? Already we have evidence implicating such factors as advanced parental age, maternal nutrition, maternal infections (especially flu) during pregnancy, and premature and/or underweight birth. Indeed, multiple-birth pregnancies are themselves associated with increased risk of developmental disorders such as cerebral palsy and autism.
Only by further studying these issues can we begin to provide parents and parents-to-be with the reliable guidance they seek and need. Autism Speaks is currently investing in several studies that are exploring how environmental factors increase the risk for ASD. As we go forward in these endeavors, we greatly value your input. So please write and share your comments on our blog and website. For more on the study, read The Womb as Environment.
On July 5th, NBC Nightly News came to Andy Shih, Autism Speaks’ vice president of scientific affairs, for perspective on the game-changing California Autism Twins study. To view the clip please visit here.
More national television media coverage of the ground-breaking results of the California Autism Twin study–research made possible by the Autism Speaks Autism Genetic Resource Exchange (AGRE) and Autism Speaks’ supporters such as you.
This is a guest post by by Mehreen Kouser, a Dennis Weatherstone Fellow, and Ph.D. Candidate working with Dr. Craig Powell at the UT Southwestern.
This year IMFAR hosted a Scientific Panel titled “Shank synaptic genes in autism: Human genetics to mouse models and therapeutics” organized and chaired by Dr. Craig Powell. This panel consisted of four presentations starting with the unequivocal role of Shank3 in autism and ending with potential treatment strategies in genetically mutated mouse models of Shank3.
Over the past few years , Shank3 has emerged as the new “it” gene for autism. Current estimates suggest that Shank3 errors account for 0.5-2 % of autism diagnoses making it a major genetic cause of autism. Several recent human studies have implicated mutations/deletions/duplications in the Shank family of proteins, especially Shank3, to be involved in ASD and 22q13 Deletion Syndrome. Shank3 is a scaffolding protein that is involved in synapse architecture. Mutations in Shank3 are known to affect synaptic connections between neurons in similar ways to other autism-relevant genes such as neuroligin and neurexin. Thus understanding the role of Shank3 in autism is critical.
The first presenter at this panel was Dr. Catalina Betancur from INSERM in France. Dr. Betancur was a major player in the discovery of Shank3’s relevance to autism. She carefully detailed all known human mutations, deletions, and duplications published since the first paper on Shank3 mutations in idiopathic autism was published in 2007.She also outlined the case for Shank3 as a major causative gene in the symptoms of the 22q13 chromosomal deletion syndrome known as Phelan-McDermid Syndrome. In addition, Dr. Betancur detailed the work of her laboratory and others implicating Shank2, another member of the Shank gene family, in autism.
Dr. Joseph Buxbaum from Mount Sinai School of Medicine in New York was the next presenter. His laboratory was the first to publish a genetic mouse model of Shank3 successfully Shank3. Their Shank3 mutant mouse closely mimics autism-associated mutations in this area of the Shank3 gene. His work focused on the heterozygous mutation of Shank3 gene as this is the state of autism patients with Shank3 mutations. Characterization of this mouse model, clearly suggests that Shank3 plays an important role in synapse architecture, function, and plasticity. Among the most intriguing findings in his presentation was his ability to reverse the manifestations of Shank3 mutation in brain slices treated with Insulin-like Growth Factor-1 (IGF-1). This gives us the much needed hope that Shank3 mutation models of autism will lead to identification of novel therapeutic targets that can be validated in these models.
Next, Dr. Yong-hui Jiang from Duke University in North Carolina presented his work on a genetic mouse model very similar to that of Dr. Buxbaum’s group, but his focus was the homozygous mutation of Shank3 mutating both copies of the gene. He noted that the Shank3 gene is more complex than originally thought, with potentially having as many as six variants or isoforms. His careful analysis of this mutant model clarified that only a portion of Shank3 isoforms are affected by this genetic strategy. He identified abnormalities in synaptic connection morphology in his model. Moreover, his lab characterized this mouse model extensively on autism related behaviors and found them to be impaired in the social behaviors, repetitive behaviors, communication, motor coordination and learning and memory. These results demonstrate that human diseases can be successfully modeled in mice. The hope is that if we can reverse them in mice, treatments for humans are not far away.
Dr. Joao Peca from Guoping Feng’s lab at MIT in Massachusetts concluded the session by presenting a completely different Shank3 mutation in mice. He began his presentation by telling us about another synaptic gene called SAPAP3 and showing us its involvement in a repetitive grooming behavior in mice. He showed that SAPAP3 knockout mice continuously groom themselves and that this behavior can be reversed by putting this gene back into the striatum of mice later in life. He also showed that Shank3 is a strong binding partner of SAPAP3 and their Shank3 mutant mice have the same increase in repetitive grooming behaviors. Like the other Shank3 mutations, this mutant does not affect all forms of Shank3, but may mimic a different human mutation.
This panel set the stage for future advances in the area of Shank3 and autism. Only 4 years after the initial study implicating Shank3 in autism, we now have at least 3 different animal models and 4 publications on these models. Although, we may face grave challenges in sorting through the heterogeneity of mutations, deletions, and duplications and their different consequences, these presenters clearly demonstrate that this strategy will lead to identification of potential therapeutic targets that can be readily tested in animal models.
A new Autism Speaks-funded study reveals differences in brain connections in areas that redirect attention, mediate social interactions and modulate emotional responses.
Science is one step closer to understanding how the brains of individuals with autism process information. Over the past decade it became clear that neural communication is disrupted in autism, although the details, and the causes, remained elusive. Two hypotheses emerged to explain the information processing differences found in individuals with autism spectrum disorder (ASD). First, scientists identified over-activity at the level of the synapse—the point where two neurons connect. Second, imaging studies showed that the balance of local versus long-range connections was skewed in autism in favor of strong local connections but weak long-range connections between distant brain regions. Complex behavior, such as language and social interaction, depend upon such long range connections. Although the findings supporting the two hypotheses were likely related, the mechanism relating over-activity at the synapse and biased local versus long-range connectivity was unclear.
Now, taking advantage of recent technical developments that facilitate the measurement of small structures within the brain, Autism Speaks’-funded researchers at Boston University have bridged these two hypotheses. Basilis Zikopoulos, Ph.D., and Helen Barbas, Ph.D., reconstructed the intricate fabric of neural “wires”—called axons—that connect prefrontal cortex with neighboring and distant brain regions. The prefrontal cortex is a collection of brain areas that are involved in higher cognitive functions such as attention, planning, social interactions, and abstract thinking. Using postmortem brain tissue generously donated by families of individuals with and without autism, the team measured features of the different axons traveling beneath the cortical surface.
Talking to itself
In comparison to the control samples, autism brain tissue had fewer large axons connecting regions of the prefrontal cortex to the other areas of cortex. Added to this connection imbalance is a thinner coat of axon insulation, called myelin. Myelin is a fatty substance that insulates axons to ensure faithful delivery of neural signals over long distances. Prefrontal axons from autism brain tissue traveling to distant brain areas have less myelin. Zikopoulos and Barbas also found more thin axons that connect neighboring regions in the prefrontal cortex in the autism brain tissue versus the control samples. This biased pattern of connectivity creates a situation in which prefrontal cortex is not efficiently exchanging information with other brain areas, but rather is operating in a solo fashion.
Pursuing the reason for the abundance of thin axons, the researchers first checked to see whether this was due to more neurons in the cortex just above. Detailed examination of neuron density showed no more neurons, just more axons. This finding led the team to speculate that atypical axon branching had occurred in the autism samples. By painstakingly tracing individual axon branches through a volume of tissue, Zikopoulos and Barbas confirmed that the abundance of thin axons emerged from overzealous branching. The thin axons found in autism samples formed tree-like braches that connected with many more cells in neighboring areas than is typical. This pattern of branching furthered the bias for local versus long-range communication.
Differences in axons and their insulation may offer some links between the two hypotheses that aim to explain disrupted communication in autism. Fewer large axons travel to distant brain regions. Thin axons form too many branches that make extra connections locally. These factors can handily explain the local over-connectivity and long-range under-connectivity observation made through imaging studies. The fact that small axons seem to branch more frequently would also lead to an over-abundance of synaptic connections in a neighboring region. This result can help to explain the synaptic connection hypothesis. Taken together, the results lead us to a deeper understanding of how information may be processed quite differently in the brains of individuals with autism.
Anatomy offers a closer look
Imaging studies aiming to understand brain connectivity use techniques such as magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI). These techniques focus on the large swaths of axons, often called “white matter” because of myelin’s fatty white hue. While many of these studies have suggested an imbalance favoring local connectivity in the brains of individuals with autism and served as a stimulus for this study, the level of detail that comes from quantitative anatomy is much richer. “The study of brain tissue at high resolution is needed to pinpoint defects in the brain’s communication system, which is prerequisite to developing rational therapies. Studying pathways using DTI is comparable to looking down from a plane at 35,000 feet – you can see some of the major highways below but not the exits, and certainly not the driveway to your house to see if the car is there,” states Dr. Barbas.
Martha Herbert, M.D., Ph.D., of Massachusetts General Hospital and LADDERS, an imaging expert who was not involved with this study, was enthusiastic about the new results. “Zikopoulos and Barbas have given us a paper about the cellular level of white matter changes in autism that has needed to be written for quite some time. Their analysis is meticulous and comprehensive, and goes far in providing microscopic details to answer provocative questions raised by the larger scale observations in so many brain scan studies in autism. Their findings suggest that the insult occurs later in brain development, after the neurons migrate to where they will live in the brain. The time when these neurons hook up with each other through white matter is crucial and during this period environment and immune factors as well as genes may play a role in the observed changes. These specific details about white matter in autism matter greatly for how we understand autism and how we go about providing the help people with autism need.”
Modeling the development of axon differences
The new axon data may also offer suggestions of what occurred to lead to this state of altered connectivity. Combining the axon data with what is known about the development of the cerebral cortex, the researchers devised a model that links differences in axons below frontal areas to a cascade of events in development that affects the growth of axons and their insulation. The hypotheses proposed are testable, and point to specific pathways that could to be targeted for the development of therapeutic interventions in autism.
One pathway that Zikopoulos and Barbas analyzed in detail was GAP-43, one of a myriad of signals that are exquisitely timed to encourage the proper growth and integration of developing neurons. GAP-43 stimulates axon branching. GAP-43 also interacts with other signals to prevent premature myelination, and is turned down after myelination begins. In the anterior cingulate cortex, a region of the prefrontal cortex that was the most profoundly affected in the brains studied, the neurons are sensitive to GAP-43 signals for an especially long duration in development. This is an important point because environmental agents such as estrogen-like compounds can increase the GAP-43 signal altering the timing of different aspects of neural development.
One take-home message from this study is how important it is to have diverse lines of inquiry investigating autism. The axon results were consistent across individuals studied. For example, the same changes were found regardless of whether or not the individual experienced seizures. Zikopoulos and Barbas also note that many of the genes that have been associated with autism exert effects on neural connectivity. Dr. Barbas says, “Genome-wide association studies are valuable in pointing out potential vulnerability in affected individuals. Our study suggests further that environmental factors (in utero, after birth, or both) can lead to abnormal connectivity because external factors up-regulate signals such as GAP-43 that regulate axon growth. We need studies to take into account all of these aspects of autism, not just one.”
This work was published in the Journal of Neuroscience and featured on the cover of the November 3 issue.
This research was made possible through funding from Autism Speaks and brain tissue supplied by Autism Speaks’ Autism Tissue Program. These groundbreaking results came from the brain tissue of ten generous individuals who chose to make a lasting gift to science. Importantly, only a small portion of each donated brain was needed for this study, making other sections of this incredibly valuable research resource available to others to uncover new insights into autism spectrum disorders. Please consider making a lasting gift to science by participating in the ATP.
Autism Speaks has partnered with SAGE Labs, a division of Sigma Life Science, to create and validate the first line of rat models of autism. Previous rodent models of ASD, which are important for understanding the biological basis of autism and drug discovery, have used mice. Such models allow scientists to examine the downstream effects of genetic mutations on brain development and behavior. For studies of behavior, learning and cognition, the rat is the animal of choice due to its complex behavioral repertoire. With these new genetically modified rats, the richness of previous behavioral and physiological research can be leveraged and applied directly to autism. These new rat models will now be validated in the laboratories of several scientists who will be studying brain development and behavior of the animals.
Edward Weinstein, Ph.D., the Director of SAGE labs, answers some questions about the new model animals and how they will benefit autism research.
1) Generally speaking, why do we need a rat model of autism when mouse models exist?
Dr. Weinstein: Researchers have been able to create genetically modified mice for the past 25 years, so a large number of mouse models have been created, addressing all sorts of human diseases and disorders. But mice are not always the best model system for every condition. For example, researchers who study ocular disease greatly prefer the rabbit. And the rat is an excellent model for toxicology, cardiovascular, and cognitive research. In the past, researchers would create the best genetically modified mouse model possible to answer their scientific questions. Now they can create the most relevant genetically modified model in whatever species best models the condition they are studying.
2) Why can’t the same technology that one uses to make a genetically modified mouse work with rats?
Dr. Weinstein: Genetically modified mice, also sometimes called knockout mice, are created by making changes to the DNA in a mouse embryonic stem cell. Researchers tried unsuccessfully for decades to develop rat models using the same technology. SAGE labs has developed a new technology, called zinc finger nuclease technology, which allows us to create knockout rats without the use of embryonic stem cells.
3) Can you describe how the Sigma technology works in a manner accessible to our community?
Dr. Weinstein: The technology is actually fairly straightforward. We create an enzyme called a zinc finger nuclease (ZFN), which has been especially constructed to target a specific gene. We can then inject that ZFN into a one-cell rat embryo where it will bind the targeted gene and cause a break in the DNA. This effectively “knocks out”, or inactivates, the gene. The one-cell embryo is then transferred into a surrogate rat and 21 days later you have a genetically modified rat that is born. This genetically modified rat will be exactly like every other rat, with the exception of the inactivation of that one specific gene that was targeted. Part of the beauty of this technology is that it not only works for creation of genetically modified rats, but also mice and rabbits. It should work in virtually any animal model system.
4) What do you see are the next steps in using genetically modified rats for autism research?
Dr. Weinstein: We have worked to create a suite of genetically modified rats that we hope will be useful for autism research. The real challenging work, however, comes in the validation of the models. Testing these rats to see if there really are behavioral differences in them will be key to determining their usefulness.
In addition, autism research is still at the stage where new genetic links are continuously being uncovered. We hope to continue to serve the research community by expanding our platform of autism models as rapidly as possible.
5) Genetics is only one component of the autism puzzle. Research has identified environmental agents that either alone or in combination with a genetic vulnerability lead to the altered neurodevelopment we see in autism. Can environmental and gene-environment interaction questions be posed using these new rats?
Dr. Weinstein: We get kind of hung-up when we say “genetic model.” We absolutely can do these experiments. Using the genetically modified rats, we can carefully time exposures both in utero and during early post-natal life. Gene-environment interactions are extremely important to understand and we hope that researchers will seek answers to those questions with these new autism model rats.
6) Why did you partner with Autism Speaks for your foray into rat models of genes important in autism?
Dr. Weinstein: We have a lot of expertise in the use of Zinc Finger Nucleases and the creation of genetically modified animals. However, we don’t have the in-depth expertise or background in autism that is necessary for determining which are the best models to create. Working with Autism Speaks has been critical from a scientific aspect. Also, members of Autism Speaks are so well connected to the research community, you know that when you get scientific advice from Autism Speaks, you are getting advice from an entire field of researchers.
We look forward to sharing the results of the behavioral and physiological evaluations of these new rats as scientists begin working with them.
Guest blog by Dr. John Vincent, who is a Scientist and Head of the Molecular Neuropsychiatry and Development Laboratory of the Psychiatric Neurogenetics Section in the Neuroscience Research Department and an Associate Scientist of The Centre for Applied Genomics at The Hospital for Sick Children, Toronto.
The group here in the Molecular Neuropsychiatry & Development Lab at CAMH, along with our collaborative partners at Sick Kids and elsewhere report the identification of PTCHD1 as a gene for autism spectrum disorder, as well as intellectual disability, on the X-chromosome. The finding stems from attempts to find differences in specific strands of DNA, called copy number variants (CNVs) in the DNA of autistic individuals that might be linked to the condition. If the human genome can be thought of as a book containing the DNA code written out in words and sentences, we each of us have our own unique book, with many words spelled differently from each other, but mostly without changing the overall meaning of the words and sentences. Traditional genetic studies would try to identify spelling mistakes that compromise the meaning of a sentence, and thus lead to an incorrect message, i.e. resulting in a clinical condition. In our current study we have been looking instead for whole sentences, paragraphs or pages that are either deleted or duplicated (i.e. CNVs), thus altering the meaning of the message, and leading in this case to autism.
We were particularly interested, in our study, to look at CNVs on one of the 2 sex chromosomes, the X, as it might explain some of the bias towards boys having autism over girls. In our initial screen of over 400 autism patients, we identified a large deletion disrupting the PTCHD1 gene. In an analysis of CNVs in genomes from over 1000 more autism individuals, deletions just next to this gene were the most significant finding. These deletions are likely to disrupt DNA sequences that may regulate how the PTCHD1 gene is expressed. In addition, we identified many single letter changes in the PTCHD1 gene that may affect the “meaning”. These changes were not found in the DNA of many control individuals.
The PTCHD1 gene makes a protein with as yet unknown function, however it shows similarities to several known proteins that function as cell-surface receptors for an inter-cellular signaling pathway known as Hedgehog, crucial in determining how brain cells develop and mature. The preliminary data we present in the paper shows that PTCHD1 appears to have similar properties to the two Hedgehog receptors already known, leading us to speculate on a role for Hedgehog-related processes in autism.
These findings give us another important gene that we can screen for in at risk children, and will allow earlier therapeutic interventions, thus increasing the likelihood of success.
For more information, please see the press release.
The past several years have seen significant advances in our understanding of the genetics of autism spectrum disorders (ASD). As result, many researchers and companies have begun exploring the possibilities of translating these discoveries into clinical applications, especially those that could help with diagnosis. By detecting and understanding genetic risk factors for autism, it is possible to provide earlier diagnosis and intervention, which typically leads to better outcomes. In addition, it can help identify subgroups within our community, that because of shared risk factors, may benefit from more targeted or specific treatment. Finally, some genetic etiologies are associated with specific medical conditions, such as seizures, cancer, and GI problems. Thus, information about specific genetic etiologies can have significant clinical implications. Using this kind of approach, it may also be possible to identify individuals who are predisposed to particular environmental risk factors and help improve our understanding of the role of gene-environment interactions in autism.
Most importantly, Autism Speaks is interested in encouraging the development of real world solutions that can help families make more informed healthcare decisions and improve their overall quality of life. While it has long been known that autism involves genetic factors and genetic testing is already being conducted for some genetic etiologies, such as Fragile X syndrome, recent studies have identified many other autism risk genes that are not routinely screened for during a diagnostic exam. Including more comprehensive genetic screening could be of real benefit to persons with ASD. However, we recognize that the science is still evolving, and there are many uncertainties and important issues such as ethics and risk communication that must be carefully weighed and considered to maximize potential benefits to individuals as well as families in our community.
In order to better understand the state of the science and opportunities and barriers in translating autism genetics discoveries into diagnostic tests, Autism Speaks and several partners, including the National Institutes of Health (NIH), Canadian Institutes for Health Research (CIHR), and the UK’s Medical Research Council (MRC), organized an educational symposium, “Genetic Risk Factors for Autism Spectrum Disorders: Translating Genetic Discoveries into Diagnostics,” in Toronto, Canada, on September 1-2, 2010. By bringing together a multidisciplinary group of about 80 international experts and stakeholders, including government officials, funding agencies, scientists, clinicians, ethicists, lawyers, biotech companies, parents and self-advocates, the symposium aimed to develop some consensus around two key questions: (1) Is the science ready for translation of genetic findings to clinical diagnostics? And if so, (2) how can the professional and stakeholder communities work best together to ensure that genetic diagnostic testing is a benefit to persons with ASD?
With an audience as diverse as the one at this symposium, a variety of opinions was expected. Some researchers thought this discussion was a bit premature because the science is still evolving and the interpretation of microarray-based clinical genetics findings can be oftentimes be complicated. Others, however, advocated for a cautious but firm push to help families by establishing an authoritative reference database and clarifying regulatory guidelines. Developing guidelines for the use of interpretation of genetic testing is especially important since several companies are already providing genetics testing services for the medical community. The FDA acknowledged the new terrain, but clearly preferred greater oversight to help protect consumers. Medical geneticists and genetic counselors pushed back that more regulations may not be necessary because of the professionals’ well-established track record in helping individuals and families interpret and understand complicated genetic findings. Bioethicists and communications experts left the attendees more appreciative of important non-technical issues that should be a part of any deliberation on the risks and benefits of genetic diagnostics for autism. Parents and self-advocates, while optimistic about the promise of the technology, also highlighted the need to address the implications of this technology on important community issues about identity and choice [Read John Elder Robison’s blog on this topic in Psychology Today].
After a packed and intense one and a half days, including breakout sessions where many vigorous and fascinating discussions ranged from community impact to next generation of diagnostic devices, the participants achieved consensus on several important issues, the details of which will be published via a peer reviewed journal and other dissemination channels in the near future. Suffice to say, while there was a general appreciation for how much work we still need to do, there was also enthusiasm for an opportunity to work together to help individuals and families benefit from the translation of latest genetics discoveries.