A meta-analysis of the evidence on the impact of prenatal and early infancy exposures to mercury on autism and attention deficit/hyperactivity disorder in the childhood

A Two-Phase Case-Control Study of Autism Risk Among Children Born From the Late 1990s Through the Early 2000s in the United States

Background

This study evaluated the hypothesis that the 1999 recommendation by the American Academy of Pediatrics (AAP) and US Public Health Service (PHS) to reduce exposure to mercury (Hg) from Thimerosal in US vaccines would be associated with a reduction in the long-term risk of being diagnosed with autism.

Material/Methods

A two-phase assessment utilizing a case (n=73) -control (n=11,783) study in the Vaccine Adverse Event Reporting System (VAERS) database (for hypothesis generating) and a more rigorous, independent matched case (n=40) -control (n=40) study (hypothesis testing) was undertaken.

Results

Analysis of the VAERS database using logistic regression revealed that the odds ratio (OR) for being an autism case in the VAERS database significantly decreased with a more recent year of vaccination in comparison to controls (OR=0.65) from 1998 to 2003. Sex-separated analyses revealed similar significant effects for males (OR=0.62) and females (OR=0.71). Analyses of the matched case-control data revealed, using the t-test statistic, that the mean date of birth among cases diagnosed with an autism spectrum disorder (ASD) (2000.5±1.2) was significantly more in the past than in controls (2001.1±1.3). Logistic regression also revealed that the OR for being diagnosed with ASD significantly decreased with a more recent date of birth in comparison to controls (OR=0.67) from 1998–2003.

Conclusions

This study reveals that the risk of autism during from the late1990s to early 2000s in the US significantly decreased with reductions in Hg exposure from Thimerosal-containing childhood vaccines, but future studies should examine this phenomenon in other US populations. Vaccine programs have significantly reduced the morbidity and mortality associated with infectious disease, but Thimerosal should be removed from all vaccines.

Altered urinary porphyrins and mercury exposure as biomarkers for autism severity in Egyptian children with autism spectrum disorder

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder that affects social, communication, and behavioral development. Recent evidence supported but also questioned the hypothetical role of compounds containing mercury (Hg) as contributors to the development of ASD. Specific alterations in the urinary excretion of porphyrin-containing ring catabolites have been associated with exposure to Hg in ASD patients. In the present study, the level of urinary porphyrins, as biomarkers of Hg toxicity in children with ASD, was evaluated, and its correlation with severity of the autistic behavior further explored. A total of 100 children was enrolled in the present study. They were classified into three groups: children with ASD (40), healthy controls (40), and healthy siblings of the ASD children (20). Children with ASD were diagnosed using DSM-IV-TR, ADI-R, and CARS tests. Urinary porphyrins were evaluated within the three groups using high-performance liquid chromatography (HPLC), after plasma evaluation of mercury (Hg) and lead (Pb) in the same groups. Results showed that children with ASD had significantly higher levels of Hg, Pb, and the porphyrins pentacarboxyporphyrin, coproporphyrin, precoproporphyrin, uroporphyrins, and hexacarboxyporphyrin compared to healthy controls and healthy siblings of the ASD children. However, there was no significant statistical difference in the level of heptacarboxyporphyrin among the three groups, while a significant positive correlation between the levels of coproporphyrin and precoproporphyrin and autism severity was observed. Mothers of ASD children showed a higher percentage of dental amalgam restorations compared to the mothers of healthy controls suggesting that high Hg levels in children with ASD may relate to the increased exposure to Hg from maternal dental amalgam during pregnancy and lactation. The results showed that the ASD children in the present study had increased blood Hg and Pb levels compared with healthy control children indicating that disordered porphyrin metabolism might interfere with the pathology associated with the autistic neurologic phenotype. The present study indicates that coproporphyrin and precoproporhyrin may be utilized as possible biomarkers for heavy metal exposure and autism severity in children with ASD.

Methyl mercury exposure and neurodevelopmental outcomes in the Seychelles Child Development Study Main cohort at age 22 and 24years

BACKGROUND

All fish contain methyl mercury (MeHg), a known neurotoxicant at adequate dosage. There is still substantial scientific uncertainty about the consequences, if any, of mothers consuming fish with naturally-acquired levels of MeHg contamination. In 1989-1990, we recruited the Main Cohort of the Seychelles Child Development Study to assess the potential developmental effects of prenatal MeHg exposure. We report here on associations with neurodevelopmental outcomes obtained at 22 and 24years of age.

METHODS

Neurodevelopmental tests at 22years included the Boston Naming Test, Cambridge Neuropsychological Test Automated Battery (CANTAB), and the Profile of Mood States. At 24years, we administered the Stroop Word-Color Test, the Barkley Adult ADHD Rating Scale, the Test of Variables of Attention, and the Finger Tapping test. We also administered a healthy behaviors survey at both ages. Primary analyses examined covariate-adjusted associations in multiple linear regression models with prenatal MeHg exposure. In secondary analyses we also examined associations with recent postnatal MeHg exposure.

RESULTS

We did not observe adverse associations between prenatal MeHg exposure and any of the measured endpoints. Some measures of attention, executive function, and delayed recall showed improved performance with increasing exposure. Secondary analysis did not show consistent patterns of association with postnatal exposure.

CONCLUSIONS

Our cohort has been examined at ten different ages over 24years of follow-up. Findings suggest that prenatal and recent postnatal MeHg exposure from ocean fish consumption is not adversely associated with neurobehavioral development at levels that are about ten times higher than typical U.S. exposures.

Autism genes are selectively targeted by environmental pollutants including pesticides, heavy metals, bisphenol A, phthalates and many others in food, cosmetics or household products

The increasing incidence of autism suggests a major environmental influence. Epidemiology has implicated many candidates and genetics many susceptibility genes. Gene/environment interactions in autism were analysed using 206 autism susceptibility genes (ASG's) from the Autworks database to interrogate ∼1 million chemical/gene interactions in the comparative toxicogenomics database. Any bias towards ASG's was statistically determined for each chemical. Many suspect compounds identified in epidemiology, including tetrachlorodibenzodioxin, pesticides, particulate matter, benzo(a)pyrene, heavy metals, valproate, acetaminophen, SSRI's, cocaine, bisphenol A, phthalates, polyhalogenated biphenyls, flame retardants, diesel constituents, terbutaline and oxytocin, inter alia showed a significant degree of bias towards ASG's, as did relevant endogenous agents (retinoids, sex steroids, thyroxine, melatonin, folate, dopamine, serotonin). Numerous other suspected endocrine disruptors (over 100) selectively targeted ASG's including paraquat, atrazine and other pesticides not yet studied in autism and many compounds used in food, cosmetics or household products, including tretinoin, soy phytoestrogens, aspartame, titanium dioxide and sodium fluoride. Autism polymorphisms influence the sensitivity to some of these chemicals and these same genes play an important role in barrier function and control of respiratory cilia sweeping particulate matter from the airways. Pesticides, heavy metals and pollutants also disrupt barrier and/or ciliary function, which is regulated by sex steroids and by bitter/sweet taste receptors. Further epidemiological studies and neurodevelopmental and behavioural research is warranted to determine the relevance of large number of suspect candidates whose addition to the environment, household, food and cosmetics might be fuelling the autism epidemic in a gene-dependent manner.

The relationship between mercury and autism: A comprehensive review and discussion

The brain pathology in autism spectrum disorders (ASD) indicates marked and ongoing inflammatory reactivity with concomitant neuronal damage. These findings are suggestive of neuronal insult as a result of external factors, rather than some type of developmental mishap. Various xenobiotics have been suggested as possible causes of this pathology. In a recent review, the top ten environmental compounds suspected of causing autism and learning disabilities were listed and they included: lead, methyl-mercury, polychorinated biphenyls, organophosphate pesticides, organochlorine pesticides, endocrine disruptors, automotive exhaust, polycyclic aromatic hydrocarbons, polybrominated diphenyl ethers, and perfluorinated compounds. This current review, however, will focus specifically on mercury exposure and ASD by conducting a comprehensive literature search of original studies in humans that examine the potential relationship between mercury and ASD, categorizing, summarizing, and discussing the published research that addresses this topic. This review found 91 studies that examine the potential relationship between mercury and ASD from 1999 to February 2016. Of these studies, the vast majority (74%) suggest that mercury is a risk factor for ASD, revealing both direct and indirect effects. The preponderance of the evidence indicates that mercury exposure is causal and/or contributory in ASD.

Environmental pollutants and child health-A review of recent concerns

In recent years, many new studies have evaluated associations between environmental pollutants and child health. This review aims to provide a broad summary of this literature, comparing the state of epidemiological evidence for the effects of a wide range of environmental contaminants (air pollutants, heavy metals, organochlorine compounds, perfluoroalkyl substances, polybrominated diphenyl ethers, pesticides, phthalates and bisphenol A) on child health outcomes. The review addresses effects on foetal growth and prematurity, neurodevelopment, respiratory and immune health, and childhood growth and obesity. Findings of recent prospective studies and meta-analyses have corroborated previous good evidence, often at lower exposure levels, for effects on foetal growth of air pollution and polychlorinated biphenyls (PCBs), for neurotoxic effects of lead, methylmercury, PCBs and organophosphate pesticides, and for respiratory health effects of air pollution. Moderate evidence has emerged for a potential role of environmental pollutants in attention deficit hyperactivity disorder and autism (lead, PCBs, air pollution), respiratory and immune health (dichlorodiphenyldichloroethylene - DDE - and PCBs), and obesity (DDE). In addition, there is now moderate evidence that certain chemicals of relatively recent concern may be associated with adverse child health outcomes, specifically perfluorooctanoate and foetal growth, and polybrominated diphenyl ethers and neurodevelopment. For other chemicals of recent concern, such as phthalates and bisphenol A, the literature is characterised by large inconsistencies preventing strong conclusions. In conclusion, since most of the recent literature evaluates common exposures in the general population, and not particularly high exposure situations, this accumulating body of evidence suggests that the unborn and young child require more protection than is currently provided. Large, coordinated research efforts are needed to improve understanding of long-term effects of complex chemical mixtures.

Thimerosal-Preserved Hepatitis B Vaccine and Hyperkinetic Syndrome of Childhood

(1) BACKGROUND: Hyperkinetic syndrome of childhood (HKSoC) is an International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9) category in which the majority of the children are also diagnosed under the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, Text Revision (DSM-IV-TR), where the umbrella term is "Attention-Deficit and Disruptive Behavior Disorders". The diagnostic criteria for HKSoC are developmentally inappropriate inattention, hyperactivity, and impulsivity. Some studies have implicated mercury (Hg) exposure as a risk factor. (2) METHODS: This hypothesis testing study; using the Vaccine Safety Datalink; assessed the toxicological effects of bolus exposure to organic-Hg from Thimerosal-containing vaccines (TCVs) by examining the relationship between Thimerosal-preserved hepatitis B vaccines (TM-HepB) given at varying levels and at specific intervals in the first six months after birth and the risk of a child being diagnosed with HKSoC. (3) RESULTS: Children diagnosed with HKSoC were significantly more likely to be exposed to increased organic-Hg from TM-HepB doses given within the first month (odds ratio = 1.45; 95% confidence interval (CI) = 1.30-1.62); within the first two months (odds ratio = 1.43; 95% CI = 1.28-1.59); and within the first six months (odds ratio = 4.51; 95% CI = 3.04-6.71) than controls. (4) CONCLUSION: The results indicate that increasing organic-Hg exposure from TCVs heightens the risk of a HKSoC diagnosis.

DNA methylation: a mechanism linking environmental chemical exposures to risk of autism spectrum disorders?

There is now compelling evidence that gene by environment interactions are important in the etiology of autism spectrum disorders (ASDs). However, the mechanisms by which environmental factors interact with genetic susceptibilities to confer individual risk for ASD remain a significant knowledge gap in the field. The epigenome, and in particular DNA methylation, is a critical gene expression regulatory mechanism in normal and pathogenic brain development. DNA methylation can be influenced by environmental factors such as diet, hormones, stress, drugs, or exposure to environmental chemicals, suggesting that environmental factors may contribute to adverse neurodevelopmental outcomes of relevance to ASD via effects on DNA methylation in the developing brain. In this review, we describe epidemiological and experimental evidence implicating altered DNA methylation as a potential mechanism by which environmental chemicals confer risk for ASD, using polychlorinated biphenyls (PCBs), lead, and bisphenol A (BPA) as examples. Understanding how environmental chemical exposures influence DNA methylation and how these epigenetic changes modulate the risk and/or severity of ASD will not only provide mechanistic insight regarding gene-environment interactions of relevance to ASD but may also suggest potential intervention strategies for these and potentially other neurodevelopmental disorders.

Tooth matrix analysis for biomonitoring of organic chemical exposure: Current status, challenges, and opportunities

Epidemiological evidence supports associations between prenatal exposure to environmental organic chemicals and childhood health impairments. Unlike the common choice of biological matrices such as urine and blood that can be limited by short half-lives for some chemicals, teeth provide a stable repository for chemicals with half-life in the order of decades. Given the potential of the tooth bio-matrix to study long-term exposures to environmental organic chemicals in human biomonitoring programs, it is important to be aware of possible pitfalls and potential opportunities to improve on the current analytical method for tooth organics analysis. We critically review previous results of studies of this topic. The major drawbacks and challenges in currently practiced concepts and analytical methods in utilizing tooth bio-matrix are (i) no consideration of external (from outer surface) or internal contamination (from micro-odontoblast processes), (ii) the misleading assumption that whole ground teeth represent prenatal exposures (latest formed dentine is lipid rich and therefore would absorb and accumulate more organic chemicals), (iii) reverse causality in exposure assessment due to whole ground teeth, and (iv) teeth are a precious bio-matrix and grinding them raises ethical concerns about appropriate use of a very limited resource in exposure biology and epidemiology studies. These can be overcome by addressing the important limitations and possible improvements with the analytical approach associated at each of the following steps: (i) tooth sample preparation to retain exposure timing, (ii) organics extraction and pre-concentration to detect ultra-trace levels of analytes, (iii) chromatography separation, (iv) mass spectrometric detection to detect multi-class organics simultaneously, and (v) method validation, especially to exclude chance findings. To highlight the proposed improvements we present findings from a pilot study that utilizes tooth matrix biomarkers to obtain trimester-specific exposure information for a range of organic chemicals.

Prenatal and childhood polybrominated diphenyl ether (PBDE) exposure and attention and executive function at 9-12 years of age

OBJECTIVE

California children's exposures to polybrominated diphenyl ether flame retardants (PBDEs) are among the highest measured worldwide. We previously reported associations for prenatal and childhood PBDE exposures with decrements in attention, processing speed, fine motor coordination, and cognition in children at ages 5 and 7 years. Here, we investigate associations of PBDEs with attention and executive function at ages 9 to 12 years in the expanded CHAMACOS cohort.

METHODS

We measured PBDEs in prenatal and child age 9 year serum samples for families enrolled in the study since pregnancy ("CHAM1", N=321). In a subsequent cohort for which families were enrolled at child age 9 ("CHAM2", N=301), we measured PBDEs in maternal and child samples collected at child age 9, and used predictive modeling to estimate prenatal exposure levels. We examined associations of measured and estimated PBDE concentrations on children's attention and executive functioning at ages 9, 10½, and 12 years.

RESULTS

Geometric means for prenatal and childhood ΣPBDE levels (sum of PBDE-47, -99, -100, -153) for the expanded CHAMACOS cohort were 26.3 and 63.2 ng/g lipid, respectively, and did not differ significantly between CHAM1 and CHAM2 families. We found consistent associations of prenatal exposure to PBDEs with poorer attention and executive function, measured with parent report and direct neuropsychological testing of the child. For example, using GEE models of repeated outcome measures at ages 9 and 12, a 10-fold increase in prenatal ΣPBDE was associated with poorer response consistency on the Conners' Continuous Performance Test II (β=2.9; 95% CI: 0.9, 4.8) and poorer working memory on the Behavioral Rating Inventory of Executive Function (β=2.5; 95% CI: 0.5, 4.4). Child age 9 ΣPBDE levels were associated with poorer parent-reported attention and executive function for girls but not boys.

CONCLUSIONS

Our results suggest that the prefrontal cortex may be a potential target for PBDE exposure and add to a growing literature showing that these ubiquitous toxicants may adversely affect neurodevelopment.

Early exposure to the combined measles-mumps-rubella vaccine and thimerosal-containing vaccines and risk of autism spectrum disorder

OBJECTIVE

This case-control study investigated the relationship between the risk of Autism Spectrum Disorder (ASD) onset, and early exposure to the combined Measles-Mumps-Rubella (MMR) vaccine and thimerosal consumption measured from vaccinations in the highly genetically homogenous Japanese population.

METHODS

Vaccination histories at 1, 3, 6, 12, 18, 24, and 36 months from birth were investigated in ASD cases (189 samples), and controls (224 samples) matching age and sex in each case. Crude odds ratios (ORs) and 95% confidence intervals (95% CIs) were calculated to determine relationship between MMR vaccination and ASD. The differences in mean values of the thimerosal dosage between cases and controls were analyzed using an unpaired t-test. MMR vaccination and thimerosal dosage were also investigated using a conditional multiple-regression model.

RESULTS

There were no significant differences in MMR vaccination and thimerosal dosage between cases and controls at any age. Furthermore, the ORs (95% CIs) of MMR vaccination and thimerosal dosage associated with ASD in the conditional multiple regression model were, respectively, 0.875 (0.345-2.222) and 1.205 (0.862-1.683) at age 18 months, 0.724 (0.421-1.243) and 1.343 (0.997-1.808) at 24 months, and 1.040 (0.648-1.668) and 0.844 (0.632-1.128) at 36 months. Thus, there were no significant differences.

CONCLUSIONS

No convincing evidence was found in this study that MMR vaccination and increasing thimerosal dose were associated with an increased risk of ASD onset.

A dose-response relationship between organic mercury exposure from thimerosal-containing vaccines and neurodevelopmental disorders

A hypothesis testing case-control study evaluated concerns about the toxic effects of organic-mercury (Hg) exposure from thimerosal-containing (49.55% Hg by weight) vaccines on the risk of neurodevelopmental disorders (NDs). Automated medical records were examined to identify cases and controls enrolled from their date-of-birth (1991–2000) in the Vaccine Safety Datalink (VSD) project. ND cases were diagnosed with pervasive developmental disorder (PDD), specific developmental delay, tic disorder or hyperkinetic syndrome of childhood. In addition, putative non-thimerosal-related outcomes of febrile seizure, failure to thrive and cerebral degenerations were examined. The cumulative total dose of Hg exposure from thimerosal-containing hepatitis B vaccine (T-HBV) administered within the first six months of life was calculated. On a per microgram of organic-Hg basis, PDD (odds ratio (OR) = 1.054), specific developmental delay (OR = 1.035), tic disorder (OR = 1.034) and hyperkinetic syndrome of childhood (OR = 1.05) cases were significantly more likely than controls to receive increased organic-Hg exposure. By contrast, none of the non-thimerosal related outcomes were significantly more likely than the controls to have received increased organic-Hg exposure. Routine childhood vaccination may be an important public health tool to reduce infectious disease-associated morbidity/mortality, but the present study significantly associates organic-Hg exposure from T-HBV with an increased risk of an ND diagnosis.