The importance of research on environmental factors in a highly heritable disorder

1. The importance of research on environmental factors in a highly heritable disorder Mark A. Corrales, MPPUS Environmental Protection Agency Statements are those of the author, and do not necessarily represent views or policies of the EPA Autism One ConferenceMay 22, 2009

2. Parallel Universes? • General public • Parents & affected kids • Interest groups: • environmental health advocacy • industries • Researchers • Policy/ decision-makers: Lawyers, economists, policy and risk analysts, staff scientists, etc.

3. Getting from here to there • If you believe national-level medical and environmental health interventions are needed… • the autism research literature is a key bottleneck.

4. From evidence to action Drug development Behav./Educ. therapy guidelines Regulations to limit risk factors RCTs Experimental toxicology, mechanisms Prospective epidemiology Retrospective epidemiology Ecologic studies More studies Better studies More types of studies (multiple lines of evidence) Public & scientific consensus Anecdotes, case studies Guesses, theories

5. Is there sufficient evidence for action?Published scientific research literature on environmental factors in autism is still quite limited compared to other diseases or health effects.

6. Why so little research on environmental factors in autism? • All autism research limited (until recently) • Behavioral > molecular level(until recently) • Genes > Environment:High heritabilitymisinterpreted as“genes not environment” • Toxicology and epidemiology not sufficiently integrated with autism research so far

7. Why so little research on environmental factors in autism? • All autism research limited (until recently) • Behavioral > molecular level(until recently) • Genes > Environment:High heritabilitymisinterpreted as“genes not environment” • Toxicology and epidemiology not sufficiently integrated with autism research so far

8. Most autism science is extremely recent As of early 2009, • Half of all the autism research literature indexed in PubMed was published in the past eight years: 2000-2008. • One third was published in just the last five years: 2004-2008.

9. Very RecentExplosion of Interest in Autism In 2008: • Newspaper articles: >3,000 headlines/ year • Scientific journal articles:>1,000 / year • New books:>150 / year

10. Journal articles

11. But so far… • Treatment -- New ASD treatments approved: 0 • Primary prevention (at national level) -- Environmental standards, regulations, or other actions taken based on any potential ASD risk factors: 0

12. Autism Literature is Still Relatively Limited • If you believe national-level medical and environmental health interventions are needed… • the autism research literature is a key bottleneck.

13. What has autism literature focused on?

14. Why so little research on environmental factors in autism? • All autism research limited (until recently) • Behavioral > molecular level(until recently) • Genes > Environment:High heritabilitymisinterpreted as“genes not environment” • Toxicology and epidemiology not sufficiently integrated with autism research so far

15. Autism literature has focused on behavioral, not chemical level • The PDD literature focused on the behavioral and cognitive levels three times as much as did biomedical literature as a whole (as % of articles). • 56% vs. only 38% of publications were indexed with chemical substances in PubMed vs. PDD literature, even recently (2000-2006). Corrales, Ringer, & Herbert 2008 IMFAR poster #2999

16. Moldin vs. Lathe biological level

17. Chem level Comorbid disorders at elevated prevalence in PDD according to Rzhetsky (2007)

18. Why so little research on environmental factors in autism? • All autism research limited (until recently) • Behavioral > molecular level(until recently) • Genetic > Environmental studies:High heritabilitymisinterpreted as“genes not environment” • Toxicology and epidemiology not sufficiently integrated with autism research so far

20. Genetic etiology pie

21. A few molecular substances have dominated the literature (1985-2007) • Approx. 2,000 unique substance terms in ASD literature, occurring 12k times in 4,000 PMIDs • Half the substance terms appear only once. • Avg. substance is in just 6 PMIDs • Chemicals in >100 articles each: • MECP2 gene or protein (>400) • Serotonin (251) • Risperidone (112) Corrales, Ringer, & Herbert 2008 IMFAR poster #2999.

22. Medications vs. environmental pollutants • >130 medications in >1200 PMIDs ( ~10% of articles) • 43 environmental pollutant terms (other than Hg) occur 111 times across 84 PMIDs ( <1% of articles) • Only 36/111 refer to a specific chemical rather than class (“teratogens” or “environmental pollutants” seen up to 13 times each) • Lead and cocaine were the most common specific toxins. • No other specific toxin (except Hg) appeared in more than 4 PMIDs. 3 Hg-related terms in 91 PMIDs (<1%) • 41 dietary substances in 189 PMIDs (1-2%)(e.g., gluten, folic acid) (excluding food additives) • A few potential treatments/ causes(valproic acid, oxytocin, melatonin, secretin) found in 26-91 PMIDs each. • ~20 other terms in 2-14 PMIDs each:(food additives, illegal drugs, infectious agents or allergens, nicotine, alcohol, caffeine) Corrales, Ringer, & Herbert 2008 IMFAR poster #2999. Includes publications 1985-2007.

23. Environment-relevant topics understudied in ASD literature, relative to all biomedical literature

24. Haz subst

25. Cell types

26. Understudied in ASD vs. in 10 comorbid disorders(1985-2007) • IL-6: Studied 9x more often in comorbid literature than in ASD • Glucocorticoids: 52x • Anti-Inflammatory Agents: 36x • Adrenal Cortex Hormones: 17x • Thyroxine: 7x • Steroids: 7x • Herpesviridae Infections: 16x Corrales, Ringer, & Herbert 2008 IMFAR poster #2999

27. Why so little research on environmental factors in autism?

28. Highly-cited statements on autism and environment “Among cognitive diseases, ASDs are the most heritable (about 80%), suggesting that they are determined largely by genes and not by the environment.” [emphasis added]Südhof (2008) Nature Heritability is at least 90%.Moldin, Rubenstein, and Hyman (2006) Journal of Neuroscience

29. Genes and Environmentin Autism:From Pie to Venn

30. G AND EDO NOT FORM A PIE CHART If one is trying to show population attributable fractions (PAF), the sum is not 100%. If one is trying to partition variance, GxE should be explained.

31. G AND EFORM A VENN DIAGRAM G & E G E

32. G AND Enot G vs. E G&E G E • “What % is genetic?” • Variance studies misinterpreted as ruling out E, or as if heritability were PAF • Sum of PAFs is not 100% • A case is not G or E • G and E are needed to explain the risk

33. Tuberculosis as “a genetic disease”? • TB was thought/ known to be “a genetic disease,” because it was seen to run in families. • Later, we realized some people are genetically more susceptible to the environmental cause (infectious agent). • TB in the population is caused by the combination of G and E (“GxE”). • Many examples of GxE have been discovered (mostly in cancers & cardiovascular disease, but also PKU, depression, ADHD, etc.)

34. How can environment be important if heritability is so high? • G x E (GE interaction) Most heritability studies assume no GxE. • epiG E (epigenetic change) • G E (genetic damage) • G E (GE correlation)

35. I. Gene-environment interaction (GxE) • TB example • Most heritability estimates ignore GxEShared • Heritability is overestimated where genes interact with shared environment. • GxE inflates heritability estimates when environmental risk factors are shared by dizygotic twins. Looks like the impact of their genes alone, but is actually impact of GxE. • Shared environment would include: • Prenatal risk factors • Widespread risk factors

36. G x E: One genotype is more sensitive to environment Haynes 2003 EHP

37. epiGE II. Epigenetic modifications (de novo or inherited) • Epigenetic tags add to apparent heritability. • Epigenetic tags may be set by prenatal or other environment. • May be de novo or possibly inherited • Epigenetic tags may be amenable to change via environmental interventions (e.g. diet). (e.g., see Corrales 2009 Autism File magazine)

38. GE III. Genome damaged by environment • Environmental factors can cause DNA damage (point mutations or structural variations such as copy number variants, CNVs, due to duplications or deletions) • This initially appears to be a de novo genetic cause of autism. • If inherited by subsequent generations it appears to be an inherited genetic cause of autism. • The root cause actually would be environmental and potentially preventable, not genetic, for such cases. • Unclear if mutation or CNV rates have risen. • Signatures of mechanism may be informative (type of SNP, or NHEJ vs. NAHR, FoSTeS, etc.). ROS causes DSBs which precede NHEJ and possibly NAHR. (Wenli Gu et al., 2008 PathoGenetics)

39. GE IV. G-E correlation:Genes can affect exposure to environmental factors • Genes that affect non-shared environmental exposures increase apparent heritability, but prevention through environmental interventions may be possible in such cases. • Non-shared exposures driven by genes are those where one DZ twin is more exposed than other, due to their genotype. • G-> Pica (tendency to put soil, etc. in mouth, causing exposure to lead, etc.) • G-> Dietary preferences (nutritional deficiency?) • G-> Preference for social interaction, eye contact, etc.

40. Historical trends in exposure or environmental interventions are hidden in heritability estimates if GxE exists • Irony: If GxE is important, growing environmental exposures will increase disease prevalence, but make heritability estimates higher over time. Apparent impact of environment is reported as smaller and smaller, even though in reality it is growing. • Twin studies only examine differences attributable to environmental conditions that differ between twins, but some environmental exposures are ubiquitous. If some interventions have not been tried in the studied twins, the potential impacts of those may not be seen.

41. Recap: How can environment be important if heritability is so high? • G x E (interaction) • epiG E (epigenetic changes) • G E (genetic damage) • G E (correlation)

42. Why so little research on environmental factors in autism? • All autism research limited (until recently) • Behavioral > molecular level(until recently) • Genes > Environment:High heritabilitymisinterpreted as“genes not environment” • Toxicology and epidemiology not sufficiently integrated with autism research so far

43. Toxicology literature and autism literature have almost no overlap Published 1985-2007, in PubMed: • Toxicology literature: 1.5 million articles • Autism literature: 10 thousand • Overlap: Just 690 articles… • Mostly on drug toxicity, not about environmental chemicals • Perhaps 2-4% of ASD literature is on chemicals as risk factors? See also: Herbert, Ringer, Corrales IMFAR 2008 poster #2979

45. Integrating autism with toxicology or epidemiology literature ASD - Symptom - Risk Factor One can seek potential risk factors using this logic:

46. ASD - Comorbidity - Risk Factor is oftencomorbid with... has asrisk factor... In this type of analysis, one identifies known risk factors for diseases that are often comorbid with or autism. The reasoning here is that prenatal environmental tobacco smoke, alcohol, lead, or PCBs might be risk factors for autism because they have been associated with ADHD.

47. ASD studies rarely investigate environmental risk factors MOST STUDIES VERY FEW VERY FEW

48. Much literature linking environmental toxins to autism-related features, just not to autism per se Toxins

49. ASD – Gene – Toxin AUTISM Gene Expression Toxins

50. Search for Chemicals that Interact with Autism Gene Candidates • Objective, comprehensive search for risk factors? • 142 genes (autism candidates) selected • Toxicogenomics database queried for reports of chemicals “affecting” any of these genes(e.g., gene expression changes in animal studies) • Approx. 500 chemicals were identified, for 122 genes: • xenobiotics (pollutants, pesticides, illegal drugs, etc.) • medications • nutrients • endogenous substances Corrales, IMFAR 2009 poster