Every year the Interagency Autism Coordinating Committee (IACC) updates its Strategic Plan for Autism Spectrum Disorder Research, identifying progress and new opportunities across the range of autism spectrum disorder (ASD) research. Each year this task gets more difficult. In 2012, the speed of progress was so rapid that each draft of the Plan was out of date by the time the IACC reviewed it. The sheer volume of research was overwhelming. According to PubMed, there were over 1,000 ASD papers related to genetics or brain imaging since January 2011 – more than three times the number of papers from the same interval a decade ago.
What have we learned from this recent wave of research? A great deal. Although clinicians usually identify 18 to 24 months as the earliest time to make a diagnosis of ASD, electroencephalography (EEG) and brain imaging studies of children at risk show reproducible differences in brain activity before 12 months.1,2 Early detection means the possibility of early intervention. One of Time magazine’s top 10 medical breakthroughs of this year was a randomized controlled trial (RCT) of intensive behavioral treatment in 18 - 30 month old toddlers with ASD, which showed striking results.3,4 Relative to a comparison group of children with ASD who were receiving standard treatment, children receiving an early, intensive treatment showed twice the level of improvement, with some losing their diagnosis altogether. In addition to the profound behavioral gains, the early intervention children also showed EEG responses that resembled the EEG patterns seen in typically-developing children.
We still do not know enough about what causes ASD. The high concordance in identical twins and the association with several genetic disorders, like Fragile X and Rett syndrome, have encouraged a search for genomic causes. With the power of modern technology, we are getting an early picture of the genomic architecture of ASD. The emerging picture is more complex than most scientists would have predicted even a few years ago: over 100 genes have now been implicated.5,6,7 Genomic variations are present in at least 25 percent of children,8,9 but none of these variants accounts for more than 1 or 2 percent of cases and some appear to be “private” – occurring in only a single child or single family. Not only do many genes contribute to this one disorder, but each of these genes appears to be a risk factor for many neurodevelopmental disorders, including schizophrenia, attention deficit hyperactivity disorder (ADHD), and epilepsy.10
One of the most surprising findings from ASD genomics has been the high rate of spontaneous, or de novo,variation. Sequencing studies have found a great many base strands of DNA that are duplicated or deleted, so called copy number variations (CNVs), in addition to spontaneous single base or short strands of variants.11,12,13 These changes are not found in parental DNA, hence the idea that these mutations are not being inherited, but instead are arising spontaneously prior to or just after conception. Some of these changes may accumulate in the father’s sperm, which replicate throughout life and may acquire mutations with increasing parental age. Dads over 40 have more of these mutations and they are at higher risk for having a child with ASD, relative to fathers under age 30.14
What about environmental causes? Several environmental factors, such as exposure to air pollution,15agricultural pesticides,16 and antidepressants,17 have been reported to increase ASD risk, mostly based on exposure during pregnancy. While most scientists assume that environmental factors interact with genetic susceptibility, we still know far too little about either genetic or environmental factors to make this link. Reports of specific genetic mutations that cause alterations in the synthesis of carnitine, a chemical involved in fat metabolism, suggest that dietary treatments may be helpful in treating some forms of ASD, reminding us that genetic diseases often have environmental corrections.18
The increasing prevalence of ASD adds urgency to finding the causes. The most recent report from the Centers for Disease Control and Prevention, based on the 2008 survey of children born in 2000, describes 1 in 88 children affected, a 78 percent increase since 2002.19 The drivers for this increasing prevalence are still unclear. A recent study from England reported a prevalence of roughly 1 percent in adults, many of them not previously diagnosed.20 These results suggest the possibility that there may not be much of an increase in the population affected by ASD, just much better ascertainment. Indeed, the highest prevalence figures come from population studies with the most careful ascertainment. A population-based study in South Korea found a rate of 1 in 38, with two-thirds not previously detected.21 While the rate in the population here in the U.S. is lower than this, the 1 in 88 figure in 2008 may still fall short of the mark. In the absence of a more complete explanation, as I have said in an earlier blog, the safest assumption is “more affected not just more detected” – a disorder that affects 1 in 88 children needs urgent attention.
If there is a true increase in children affected, what environmental factor or factors are driving this increase? The absence of the kind of clusters we see with infectious diseases or environmental toxicants makes the search for environmental contributors to ASD risk especially difficult. But we need to search carefully, following the clues that emerge. Most experts believe that prenatal exposure is the critical period of risk. And the growing evidence for brain changes in the first year, well before behavioral changes, would be consistent with an early period of risk.
While there are still more questions than answers, ASD has emerged as one of the hottest areas for biomedical research. The debate over ASD is frequently contentious, with some seeing it as an illness, some as an injury, and some as an identity. Many scientists now see ASD as a source of insight, teaching investigators from many disciplines, and studying a wide variety of disorders, new lessons about genetics, brain development, and behavior. Whatever your perspective on this complex issue, this is a period of unprecedented scientific progress, with many people from diverse fields now joining the ASD research community. This growth in our community, and the surge of investments made in ASD research over the past decade, are paying off by deepening our understanding of this complex disorder and laying the groundwork for future advances that will ultimately improve the lives of people with ASD and their families.
1Bosl W, Tierney A, Tager-Flusberg H, et al. EEG complexity as a biomarker for autism spectrum disorder risk.BMC Med. 2011 Feb 22;9:18. PMID: 21342500
2Tierney AL, Gabard-Durnam L, Vogel-Farley V, et al. Developmental trajectories of resting EEG power: an endophenotype of autism spectrum disorder. PLoS One. 2012;7(6):e39127. PMID: 22745707
3Park, Alice (2012 Dec 4) Top 10 Medical Breakthroughs: Hope for Reversing Autism. Time magazine. Available at: http://healthland.time.com/2012/12/04/top-10-health-lists/slide/hope-for-reversing-autism.
4Dawson G, Jones EJ, Merkle K, et al. Early behavioral intervention is associated with normalized brain activity in young children with autism. J Am Acad Child Adolesc Psychiatry. 2012 Nov;51(11):1150-9. PMID: 23101741
5Simons Foundation. SFARI Gene Website. Accessed on December 14, 2012.
6Betancur C. Etiological heterogeneity in autism spectrum disorders: more than 100 genetic and genomic disorders and still counting. Brain Res. 2011 Mar 22;1380:42-77. PMID: 21129364
7Geschwind DH. Genetics of autism spectrum disorders. Trends Cogn Sci. 2011 Sep;15(9):409-16. PMID: 21855394
8Sanders SJ, Murtha MT, Gupta AR, et al. De novo mutations revealed by whole-exome sequencing are strongly associated with autism. Nature. 2012 Apr 4;485(7397):237-41. PMID: 22495306
9Devlin B, Scherer SW. Genetic architecture in autism spectrum disorder. Curr Opin Genet Dev. 2012 Jun;22(3):229-37. PMID: 22463983
10Malhotra D, Sebat J. CNVs: harbingers of a rare variant revolution in psychiatric genetics. Cell. 2012 Mar 16;148(6):1223-41. PMID: 22424231
11Coe BP, Girirajan S, Eichler EE. The genetic variability and commonality of neurodevelopmental disease. Am J Med Genet C Semin Med Genet. 2012 May 15;160C(2):118-29. PMID: 22499536
12Neale BM, Kou Y, Liu L, et al. Patterns and rates of exonic de novo mutations in autism spectrum disorders.Nature. 2012 Apr 4;485(7397):242-5. PMID: 22495311
13O'Roak BJ, Vives L, Girirajan S, et al. Sporadic autism exomes reveal a highly interconnected protein network of de novo mutations. Nature. 2012 Apr 4;485(7397):246-50. PMID: 22495309
14Kong A, Frigge ML, Masson G, et al. Rate of de novo mutations and the importance of father’s age to disease risk. Nature. 2012 Aug 23;488(7412):471-5. PMID: 22914163
15Volk H, Lurmann F, Penfold B, et al. Traffic related air pollution, particulate matter, and autism. Arch Gen Psychiatry. 2012. Epub ahead of print.
16Shelton JF, Hertz-Picciotto I, Pessah IN. Tipping the balance of autism risk: potential mechanisms linking pesticides and autism. Environ Health Perspect. 2012 Jul;120(7):944-51. PMID: 22534084
17Croen LA, Grether JK, Yoshida CK, et al. Antidepressant use during pregnancy and childhood autism spectrum disorders. Arch Gen Psychiatry. 2011 Nov;68(11):1104-12. PMID: 21727247
18Celestino-Soper PB, Violante S, Crawford EL, et al. A common X-linked inborn error of carnitine biosynthesis may be a risk factor for nondysmorpohic autism. Proc Natl Acad Sci USA. 2012 May 22;109(21):7974-81. PMID: 22566635
19Centers for Disease Control and Prevention (CDC); Autism and Developmental Disabilities Monitoring Network - Surveillance Year 2008 Principal Investigators. Prevalence of autism spectrum disorders - Autism and Developmental Disabilities Monitoring Network, 14 Sites, United States, 2008. MMWR Surveill Summ. 2012 Mar 30; 61(3):1-19 PMID: 22456193
20Brugha TS, McManus S, Bankart J, et al. Epidemiology of autism spectrum disorders in adults in the community in England. Arch Gen Psychiatry. 2011 May;68(5):459-65. PMID: 21536975
21Kim YS, Leventhal BL, Koh YJ, et al. Prevalence of autism spectrum disorders in a total population sample.Am J Psychiatry. 2011 Sep;168(9):904-12. PMID: 21558103
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