Scholarly article on topic 'Autism in 2016: the need for answers'

Autism in 2016: the need for answers Academic research paper on "Clinical medicine"

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Jornal de Pediatria
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{"Autism spectrum disorder" / Neurobiology / Epidemiology / "Environmental factors" / "Air pollutants" / Epigenetics / "Transtorno do espectro autista" / Neurobiologia / Epidemiologia / "Fatores ambientais" / "Poluentes atmosféricos" / Epigenética}

Abstract of research paper on Clinical medicine, author of scientific article — Annio Posar, Paola Visconti

Abstract Objective Autism spectrum disorders are lifelong and often devastating conditions that severely affect social functioning and self-sufficiency. The etiopathogenesis is presumably multifactorial, resulting from a very complex interaction between genetic and environmental factors. The dramatic increase in autism spectrum disorder prevalence observed during the last decades has led to placing more emphasis on the role of environmental factors in the etiopathogenesis. The objective of this narrative biomedical review was to summarize and discuss the results of the most recent and relevant studies about the environmental factors hypothetically involved in autism spectrum disorder etiopathogenesis. Sources A search was performed in PubMed (United States National Library of Medicine) about the environmental factors hypothetically involved in the non-syndromic autism spectrum disorder etiopathogenesis, including: air pollutants, pesticides and other endocrine-disrupting chemicals, electromagnetic pollution, vaccinations, and diet modifications. Summary of the findings While the association between air pollutants, pesticides and other endocrine-disrupting chemicals, and risk for autism spectrum disorder is receiving increasing confirmation, the hypothesis of a real causal relation between them needs further data. The possible pathogenic mechanisms by which environmental factors can lead to autism spectrum disorder in genetically predisposed individuals were summarized, giving particular emphasis to the increasingly important role of epigenetics. Conclusions Future research should investigate whether there is a significant difference in the prevalence of autism spectrum disorder among nations with high and low levels of the various types of pollution. A very important goal of the research concerning the interactions between genetic and environmental factors in autism spectrum disorder etiopathogenesis is the identification of vulnerable populations, also in view of proper prevention.

Academic research paper on topic "Autism in 2016: the need for answers"

+Model

ARTICLE IN PRESS

J Pediatr (Rio J). 2016;xxx(xx):xxx-xxx

REVIEW ARTICLE

Autism in 2016: the need for answers^

Q1 Annio Posara b'*, Paola Viscontia

a IRCCS Institute of Neurological Sciences of Bologna, Child Neurology and Psychiatry Unit, Bologna, Italy b University of Bologna, Department of Biomedical and Neuromotor Sciences, Bologna, Italy

Received 23 August 2016; accepted 13 September 2016

Jornal de

Pediatría %,

10 11 12

20 21 22

KEYWORDS

Autism spectrum

disorder;

Neurobiology;

Epidemiology;

Environmental

factors;

Air pollutants;

Epigenetics

Abstract

Objective: Autism spectrum disorders are lifelong and often devastating conditions that severely affect social functioning and self-sufficiency. The etiopathogenesis is presumably multifactorial, resulting from a very complex interaction between genetic and environmental factors. The dramatic increase in autism spectrum disorder prevalence observed during the last decades has led to placing more emphasis on the role of environmental factors in the etiopathogenesis. The objective of this narrative biomedical review was to summarize and discuss the results of the most recent and relevant studies about the environmental factors hypothetically involved in autism spectrum disorder etiopathogenesis.

Sources: A search was performed in PubMed (United States National Library of Medicine) about the environmental factors hypothetically involved in the non-syndromic autism spectrum disorder etiopathogenesis, including: air pollutants, pesticides and other endocrine-disrupting chemicals, electromagnetic pollution, vaccinations, and diet modifications. Summary of the findings: While the association between air pollutants, pesticides and other endocrine-disrupting chemicals, and risk for autism spectrum disorder is receiving increasing confirmation, the hypothesis of a real causal relation between them needs further data. The possible pathogenic mechanisms by which environmental factors can lead to autism spectrum disorder in genetically predisposed individuals were summarized, giving particular emphasis to the increasingly important role of epigenetics.

Conclusions: Future research should investigate whether there is a significant difference in the prevalence of autism spectrum disorder among nations with high and low levels of the various types of pollution. A very important goal of the research concerning the interactions between genetic and environmental factors in autism spectrum disorder etiopathogenesis is the identification of vulnerable populations, also in view of proper prevention. © 2016 Sociedade Brasileira de Pediatria. Published by Elsevier Editora Ltda. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

* Please cite this article as: Posar A, Visconti P. Autism in 2016: the need for answers. J Pediatr (Rio J). 2016. http://dx.doi.org/10.1016/j.jped.2016.09.002

* Corresponding author.

E-mail: annio.posar@unibo.it (A. Posar).

http://dx.doi.org/10.1016/jjped.2016.09.002

0021-7557/© 2016 Sociedade Brasileira de Pediatria. Published by Elsevier Editora Ltda. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

+Model ARTICLE IN PRESS

2 Posar A, Visconti P

PALAVRAS-CHAVE

Transtorno do espectro autista; Neurobiologia; Epidemiologia; Fatores ambientais; Poluentes atmosféricos; Epigenética

Autismo em 2016: necessidade de respostas Resumo

Objetivo: Os transtornos do espectro autista (TEAs) sao vitalicios e normalmente sao doencas devastadoras que afetam gravemente o funcionamento social e a autossuficiéncia. A etiopato-genia é presumivelmente multifatorial, resultante de uma interacao muito complexa entre fatores genéticos e ambientais. O aumento drástico na prevaléncia de TEAs observado nas últimas décadas levou a maiorenfase no papel dos fatores ambientais na etiopatogenia. O objetivo desta análise da narrativa biomédica foi resumir e discutir os resultados dos estudos mais recentes e relevantes sobre os fatores ambientais hipoteticamente envolvidos na etiopatogenia dos TEAs.

Fontes: Foi realizada uma pesquisa na Biblioteca Nacional de Medicina dos Estados Unidos (PubMed) sobre os fatores ambientais hipoteticamente envolvidos na etiopatogenia dos TEAs nao sindromicos, incluindo: poluentes atmosféricos, pesticidas e outros desreguladores endocrinos, poluicao eletromagnética, vacinas e alteracoes na dieta.

Resumo dos achados: Embora a associacao entre poluentes atmosféricos, pesticidas e outros desreguladores endocrinos e o risco de TEA esteja recebendo cada vez mais confirmacoes, a hipótese de uma relacao causal real entre eles ainda precisa de mais dados. Os possíveis mecanismos patogénicos por meio dos quais os fatores ambientais podem causar TEA em individuos geneticamente predispostos foram resumidos, com énfase especial no papel cada vez mais importante da epigenética.

Conclusoes: Futuras pesquisas devem investigar se há uma diferenca significativa na prevalencia de TEA entre nacoes com níveis altos e baixos de vários tipos de poluicao. Um objetivo muito importante da pesquisa a respeito das interacoes entre fatores genéticos e ambientais na etiopatogenia do TEA é a identificacao de populacoes vulneráveis, também em virtude da prevenccao adequada.

© 2016 Sociedade Brasileira de Pediatria. Publicado por Elsevier Editora Ltda. Este e um artigo Open Access sob uma licenca CC BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

62 Introduction

63 Autism spectrum disorders (ASDs) are lifelong and often dev-

64 astating conditions that severely affect social functioning

65 and self-sufficiency, having a very negative impact on the

66 lives of the entire family of the affected individuals. Accord-

67 ing to the criteria of the Diagnostic and Statistical Manual

68 of Mental Disorders, 5th edition (DSM-5), ASDs are defined

69 by persisting deficits in social communication and inter-

70 action, as well as by restricted and repetitive behaviors,

71 interests, and activities.1 ASDs presumably have a multi-

72 factorial etiopathogenesis, resulting from a very complex

73 interaction between genetic and environmental factors.2,3

74 Only in a minority of cases is the presence of a defined

75 medical condition demonstrable.

76 Epidemiological studies during the last decades have

77 shown a dramatic increase in ASD prevalence, which has

78 reached as much as 1-2% of children in recent years.4

79 The epidemiological study by Nevison suggests that this

80 increase is mainly real,5 and therefore only in small

81 part attributable to better knowledge of the problem.

82 This phenomenon needs further investigation and possible

83 explanatory hypotheses in terms of public health. Of course

84 this prevalence increase cannot be explained based only on

85 genetic factors, and the role of possible environmental fac-

86 tors should be carefully considered. First, it is necessary

87 to understand what has changed in the environment and

88 habits during the last few decades. In literature, several

hypotheses have been considered. In this review, the authors 89

carried out a synthesis of the most intriguing hypothe- 90

ses as follows: all the recent (between January 1, 2013 91

and August 20, 2016) and relevant (preferably case-control 92

studies involving humans) literature available on PubMed 93

(United States National Library of Medicine) was selected, 94

using the following keywords: ''autism'', ''air pollutants'', 95

''pollution'', ''pesticides'', ''endocrine-disrupting chemi- 96

cals'', ''environmental factors'', ''electromagneticfields'', 97

''vaccinations'', ''omega-3'', and ''epigenetics''. 98

Air pollutants 99

Over the last years, the etiopathogenic role of the expo- 100

sure to air pollutants, mainly heavy metals and particulate 101

matter (PM) during the pre-, peri-, and postnatal period, has 102

been seriously considered in the literature, although definite 103

conclusions are lacking. Here follows a brief description of 104

some of the most important recent papers in this regard. 105

Becerra et al. investigated the possible effect of 106

traffic-related air pollution exposure during pregnancy on 107

the development of autism. The authors performed a 108

population-based case -control study using data from air 109

monitoring stations and a land-use regression (LUR) model 110

to estimate exposure rates. They identified children born 111

in Los Angeles, California, United States of America (USA), 112

who were diagnosed with a primary autistic disorder (AD) 113

114 according to the Diagnostic and Statistical Manual of Men- Nurses' Health Study II, a prospective longitudinal cohort ™

115 tal Disorders, 4th Edition, Text Revision (DSM-IV-TR)6 at of female nurses recruited in 1989. The authors accounted 177

116 the age of 3-5 years during the period 1998-2009. They for possible biases by adjusting for family-level and cen- 178

117 included 7603 children with AD and 10 controls (by defini- sus tract-level socioeconomic status, maternal age at birth, 179

118 tion without documentation of autism) per case matched and birth year. Perinatal exposures to the highest vs. low- 180

119 by sex, year of birth, and minimum gestational age. The est quintile of diesel particulate, lead, manganese, nickel, 181

120 authors performed conditional logistic regression, adjusting and cadmium - as well as an overall measure of metals - 182

121 for: maternal age, birthplace, race/ethnicity, and educa- were significantly associated with ASD. For most pollutants, 183

122 tion; birth type (single, multiple), parity; insurance type (a the association was stronger for males than for females. The 184

123 socioeconomic status proxy); and gestational age at birth. authors concluded that perinatal exposure to air pollutants 185

124 They calculated a 12-15% relative increase in odds of autism may increase ASD risk, suggesting further studies for the pos- 186

125 per interquartile range (IQR) increase for ozone and PM2.5 sible sex-specific biological pathways associating perinatal 187

126 (PM with an aerodynamic diameter less than 2.5 ^m) when exposure to air pollutants with ASD.11 188

127 mutually adjusting for both pollutants. Moreover, they cal- Von Ehrenstein et al. evaluated the risks for autism 189

128 culated a 3-9% relative increase in the odds of AD per IQR related to the exposure to monitored ambient air pollut- 190

129 increase for LUR-derived estimates of exposure to nitric ants during pregnancy in Los Angeles County, California. 191

130 oxide and nitrogen dioxide. LUR-derived associations were Among the cohort of children born between 1995 and 2006, 192

131 most robust for children born to mothers who had less than the authors considered 148 722 individuals whose mothers 193

132 high school education. The authors concluded by suggesting were living in a 5-km buffer radius around air pollution 194

133 the presence of associations between prenatal exposure to monitoring stations during pregnancy. The authors included 195

134 mostly traffic-related air pollution and autism.7 768 children diagnosed with AD according to DSM-IV-TR 196

135 Volk et al. examined the possible association between criteria6 between 1998 and 2009. The risks for autism were 197

136 air pollution and autism. They performed a population- heightened per interquartile range increase in average con- 198

137 based case-control study in California (USA) including 279 centrations of several pollutants during pregnancy, including 199

138 preschool children with autism (according to both the Autism 1,3-butadiene, meta/para-xylene, other aromatic solvents, 200

139 Diagnostic Observation Schedule [ADOS] and the Autism lead, perchloroethylene, and formaldehyde, adjusting for 201

140 Diagnostic Interview-Revised [ADI-R]) and 245 normal con- maternal age, race/ethnicity, nativity, education, insurance 202

141 trols, frequency matched by sex, age, and broad geographic type, parity, child sex, and birth year. The authors con- 203

142 area. Odds ratios for autism were adjusted for children's cluded that autism risks in children may increase following 204

143 sex and ethnicity, parents' education level, maternal age, in utero exposure to traffic- and industry-related ambient 205

144 and prenatal smoking. Children with autism, during gesta- air toxics.12 206

145 tion and the first year of life, were more likely to live at Talbott et al. studied the possible association between 207

146 residences with the highest quartile of exposure to traffic- prenatal and early childhood exposure to PM2.5 and risk for 208

147 related air pollution compared with controls. In the same ASD. ASD diagnosis was made if a child scored >15 on the 209

148 periods, the exposure to nitrogen dioxide, PM2.5, and PM10 Social Communication Questionnaire (SCQ) and had writ- 210

149 (PM with an aerodynamic diameter less than 10 ^m) were ten documentation, including the ADOS or other diagnostic 211

150 also associated with autism. The authors concluded that tools results, of an ASD diagnosis from a child psychologist 212

151 during pregnancy and the first year of life, the exposure or psychiatrist. The authors performed a population-based 213

152 to traffic-related air pollution, nitrogen dioxide, PM2.5, and case-control study, considering 217 ASD children, born in 214

153 PM10 is associated with autism.8 Southwestern Pennsylvania (USA) between 2005 and 2009, 215

154 In Taiwan, Jung et al. studied the possible association in comparison with 226 controls without ASD, frequency 216

155 between children's long-term postnatal exposure to air pol- matched for age, sex, and race. Odds ratios (adjusted for 217

156 lution and newly diagnosed ASD. From 2000 through 2010, maternal age, education, race, and smoking) were high, but 218

157 they performed a prospective population-based cohort study not significant, for specific prenatal and postnatal intervals 219

158 considering 49 073 individuals aged less than 3 years. Within (pre-pregnancy, pregnancy, and year one), while postna- 220

159 this cohort, 342 children developed ASD according to the tal year two was significant. The authors also evaluated 221

160 criteria of the International Classification of Diseases, Ninth the effect of cumulative pregnancy periods; starting three 222

161 Revision, Clinical Modification (ICD-9-CM).9 Hazard ratios months before pregnancy through pregnancy, the adjusted 223

162 were adjusted for age, anxiety, gender, intellectual disabil- odds ratios were significant for pre-pregnancy through year 224

163 ity, preterm birth, and socioeconomic status. The risk of two. The authors concluded that both prenatal and post- 225

164 new diagnoses of ASD augmented according to increasing natal PM2.5 exposures are associated with increased ASD 226

165 levels of ozone, carbon monoxide, nitrogen dioxide, and sul- risk, suggesting that future studies should consider multi- 227

166 fur dioxide. The results of the study suggest that children's ple pollutant models as well as the elucidation of the PM2.5 228

167 exposure to these four pollutants in the preceding 1 -4 years involvement in the etiopathogenesis of ASD.13 229

168 may increase the risk of developing ASD. No association has Again in Southwestern Pennsylvania, Talbott et al. per- 230

169 been found between PM10 and the risk of new diagnoses of formed a population-based case-control study to estimate 231

170 ASD.10 the possible association between exposure to 30 environ- 232

171 In the USA, Roberts et al. studied the possible associa- mental neurotoxicants and ASD.13 The authors studied 217 233

172 tion between perinatal exposure to air pollutants and ASD. children with ASD born between 2005 and 2009 in compar- 234

173 They estimated associations between levels of hazardous ison with 224 controls, frequency matched by birth year, 235

174 air pollutants at the birth time/place and ASD in the chil- sex, and race, and 5007 controls from a random sam- 236

175 dren (325 cases vs. 22101 controls) of participants in the ple of birth certificates using birth residence. The authors 237

4 Posar A, Visconti P

238 calculated odds ratios, adjusted for mother's age, educa- diagnosed according to DSM-IV-TR,6 born from 1994 to 2000 300

239 tion, race, smoking status, child's year of birth, and sex. (645 in North Carolina and 334 in California), compared with 301

240 They found that living during pregnancy in areas with higher 14666 randomly sampled controls born in the same counties 302

241 styrene and chromium levels was associated with increased and years (12434 in North Carolina and 2232 in California, 303

242 ASD risk, while borderline effects were found for polycyclic respectively). The authors calculated odds ratios of autism 304

243 aromatic hydrocarbons (PAHs) and methylene chloride. How- for a 10 ^g/m3 increase in PM10 concentration within periods 305

244 ever, based on these findings, it is unclear whether the of three months from the preconceptional period through 306

245 mentioned chemicals represent risk factors in themselves the child's first birthday. Odds ratios were adjusted for 307

246 or reflect the effect of a pollutant mixture.14 year, state, mother's education and age, race/ethnicity, 308

247 Considering that in the USA children with ASD appear to and neighborhood-level urbanization and median household 309

248 live in spatial clusters and that the reason for this clus- income. In addition, a nonparametric term for week of birth 310

249 tering is hard to determine due to boundless variations in was included to account for seasonal trends. The authors 311

250 healthcare access and diagnostic practices, Schelly et al. found that PM«, exposure during the third-trimester, but not 312

251 explored ASD diffusion in Costa Ric^ a smaU setting where in earlier pregnancy, was associated with a higher risk of 313

252 no variations in healthcare access or diagnostic practices autism. The authors pointed out that their data did not allow 314

253 are present. In addition, in Costa Rica the potential for them to know the PM composition (PM10 arises from traf- 315

254 exposure to mercury from the source hypothetically impli- fic, wood smoke, and power plants), but further research in 316

255 cated in ASD (see coal-fired power plants) is absent, and this field is also important in order to facilitate prevention 317

256 areas with high levels of air pollution are spatially concen- efforts against the different sources from which the PM can 318

257 trated. The study included 118 children with ASD, diagnosed arise.18 319

258 according to ADOS, assessed in the period of 2010-2013. In the USA, Raz et al. examined the possible asso- 320

259 The authors identified spatial clusters suggesting a mecha- ciation between mother's PM exposure and odds of ASD 321

260 nism that does not depend on factors such as information in her child, performing a nested case -control study of 322

261 about ASD, healthcare access, diagnostic practices, or envi- the Nurses' Health Study II participants.11 They included 323

262 ronmental toxicants. The results that emerged from the 245 children with a diagnosis of ASD, confirmed by ADI-R 324

263 study do not support the most likely environmental cluster- and Social Responsiveness Scale (SRS), and 1522 randomly 325

264 ing cause, which is air pollution.15 selected children without ASD. During pregnancy the expo- 326

265 Dickerson et al. performed an ecologic study in five sure to PM2.5 was associated with higher odds of ASD, with 327

266 sites located in the USA: in Arizona, Maryland, New Jer- an adjusted (for child sex, year of birth, month of birth, 328

267 sey, South Carolina, and Utah, respectively. They examined maternal age at birth, paternal age at birth, and census 329

268 the association during the 1990s between prevalence of ASD income) odds ratio for ASD of 1.57 per IQR increase in 330

269 (diagnosed according to DSM-IV-TR criteria6), at the level of pm2.5, considering only the women who had maintained the 331

270 census tract, and nearness of tract centroids to the clos- same address before and after pregnancy (160 cases and 332

271 est industrial facilities emitting arsenic, lead, or mercury. 986 controls). The association between PM2.5 exposure and 333

272 They analyzed 2489 census tracts with 4486 ASD cases, ASD was stronger in the third trimester than in the first 334

273 adjusting for demographic and socio-economic area-based two trimesters of pregnancy when mutually adjusted. Little 335

274 characteristics. The authors found that the prevalence of association was found between PM10-2.5 (PM with an aerody- 336

275 ASD was increased in census tracts located in the closest namic diameter between 10 ^m and 2.5 ^m) and ASD. The 337

276 10th percentile compared to those located in the furthest authors concluded that during pregnancy, especially in the 338

277 50th percentile. The authors concluded that these find- third trimester, a higher maternal exposure to PM2.5 is asso- 339

278 ings suggest an association between residential proximity ciated with increased odds of a child with ASD.19 340

279 to industrial facilities that emit air pollutants and increased Guxens et al. performed a study on four Euro- 341

280 ASD prevalence.16 pean population-based birth/child cohorts. They assessed 342

281 Dickerson et al., again considering the same sample whether prenatal exposure to nitrogen oxides and PM, esti- 343

282 of 4486 children with ASD living in 2489 census tracts, mated between 2008 and 2011, was associated with autistic 344

283 using multi-level negative binomial models, studied the traits. The authors included 8079 children aged 4-10 years in 345

284 possible association between lead, mercury, and arsenic the study. Autistic traits were assessed using respectively: 346

285 air concentrations and ASD prevalence. When adjusting the ASD module of the Autism-Tics, Attention Deficit and 347

286 for demographic and socio-economic factors, tracts with Hyperactivity Disorders, and Other Comorbidities (A-TAC) 348

287 lead concentrations in the highest quartile had significantly inventory in the Swedish cohort; the Pervasive Developmen- 349

288 increased ASD prevalence compared to tracts with concen- tal Problems subscale of the Child Behavior Checklist for 350

289 trations of lead in the lowest quartile. Furthermore, tracts Toddlers (CBCL1 ^-5) in the Dutch and in the Italian cohort; 351

290 with mercury concentrations over the 75th percentile and an adapted 18-item version of the SRS in the Dutch cohort; 352

291 arsenic concentrations over the 75th percentile had a sig- and the Childhood Autism Spectrum Test (CAST) in the Span- 353

292 nificantly increased ASD prevalence compared to tracts with ish cohorts. Prenatal exposure to nitrogen oxides and PM 354

293 concentrations of arsenic, lead, and mercury under the 75th was not associated with autistic traits. The authors hypothe- 355

294 percentile. The authors concluded by suggesting a possible sized that prenatal air pollution exposure could be related to 356

295 association between air lead concentrations and ASD preva- ASD but not to broad autistic traits. They suggested further 357

296 lence, and sustaining that multiple metal exposure may research, including: European studies on children diagnosed 358

297 produce synergistic effects on the development of ASD.17 with ASD; comparing manifestation and detection of ASD 359

298 In the US^ Kalkbrenner et al. examined the exp°sure between the USA and Europe; and exploring the mechanisms 360

299 to PM10 at the birth address in 979 children with ASD, underlying the association between air pollutants and ASD in 361

362 order to better understand the different results compared matched by sex, age at interview, and region of birth. The 421

363 to the previous studies.20 authors used Bayesian logistic models to evaluate the associ- 422

ation between imidacloprid and ASD as well as to correct for 423

potential differential exposure misclassification because of 424

364 Pesticides and other endocrine-disrupting recall in a case-control study of ASD. At interview, control 425

365 chemicals cases were slightly younger than ASD children (mean: 3 years 426

7 months and 3 years 10 months, respectively). Data were 427

366 Endocrine disrupting chemicals (EDCs) are generally man- adjusted for: child's gender, birth regional center, and age; 428

367 made substances that can interfere with the endocrine mother's education, race/ethnicity, and parity; and owner- 429

368 system, and are present in a large number of home and ship of pets during the prenatal period. The odds of prenatal 430

369 industrial products. The possible association between EDCs, exposure to imidacloprid among cases with ASD were only 431

370 in particular pesticides, and autism has been discussed at slightly higher than among controls. A susceptibility win- 432

371 length in recent years, so far without reaching definite dow analysis showed higher odds ratios for exposures during 433

372 conclusions. gestation than for exposures during early life, and while 434

373 In the context of the population-based case-control considering only consistent users of imidacloprid, the odds 435

374 study called Childhood Autism Risks from Genetics and Envi- ratio raised to 2.0. The authors concluded that the associa- 436

375 ronment (CHARGE), Shelton et al. explored, in California, tion between exposure to imidacloprid and ASD could result 437

376 the possible association between proximity of residence to from exposure misclassification alone, due to recall bias. 438

377 agricultural pesticides during gestation and ASD or devel- However, they suggested further investigation about this 439

378 opmental delay (DD). For 970 participants, aged 2-5 years, association and emphasized the need for validation studies 440

379 commercial pesticide application data collected using the concerning prenatal imidacloprid exposure in patients with 441

380 California Pesticide Use Report (from 1997 to 2008) were ASD.22 442

381 connected with the addresses during gestation. The pounds Unfortunately, both these studies21,22 were based on 443

382 of active ingredient applied regarding organophophates, data collected retrospectively, without biological samples 444

383 organochlorines, pyrethroids, and carbamates (four pesti- acquired during pregnancy. In this regard, the work of 445

384 cide families selected by the authors) were aggregated Braun et al. represents a step forward. In this study on 446

385 within buffer distances of 1.25-km, 1.5-km, and 1.75-km the gestational exposure to EDCs and autistic behaviors, the 447

386 from the residence. The authors used multinomial logistic authors considered, besides six organochlorine pesticides, 448

387 regression to calculate the exposure odds ratio compar- eight phthalate metabolites (phthalates are mainly utilized 449

388 ing cases affected by ASD (486, diagnosed according to as plasticizers), bisphenol A (used to produce some plas- 450

389 both ADI-R and ADOS), or by DD (168, assessed by the tics and epoxy resins), 25 polychlorinated biphenyls (PCBs) 451

390 Mullen Scales of Early Learning and the Vineland Adap- (for example, once widely utilized in electrical equipment 452

391 tive Behavioral Scale), with normal controls (316) frequency as dielectric and coolant fluids), eight brominated flame 453

392 matched to the ASD cases by sex, age, and the regional retardants (widely used for example in electronic products), 454

393 catchment area. Data were adjusted for education of the and four perfluoroalkyl substances (utilized in plastic lab- 455

394 father, home ownership, birthplace of the mother, child oratory materials and pipes). In the context of the study 456

395 race/ethnicity, maternal prenatal vitamin intake, and year called Health Outcomes and Measures of the Environment 457

396 of birth. During pregnancy, about one-third of the moth- (HOME), the authors measured the blood or urine con- 458

397 ers lived within 1.5 km of an application of agricultural centrations of the mentioned substances in 175 pregnant 459

398 pesticides. Living near organophosphates at some time dur- women living in the Cincinnati metropolitan area (Ohio, 460

399 ing pregnancy was associated with 60% increased ASD risk, USA), enrolled in the period of 2003-2006 (prospective 461

400 higher for third-trimester exposure to organophosphates birth cohort study). When the children were aged 4 and 462

401 overall, and for second-trimester applications of chlorpyri- 5 years, mothers were administered the SRS. Data were 463

402 fos (an organophosphate explored independently). Children adjusted for: maternal age at delivery, race, marital status, 464

403 of mothers living near applications of pyrethroids during the education, parity, insurance status, employment, house- 465

404 preconception period or pregnancy at third trimester were hold income, prenatal vitamin use, mother's depressive 466

405 at higher risk of both ASD and DD. DD risk was higher in symptoms and IQ, caregiving environment, and gesta- 467

406 those living near applications of carbamates, with no specific tional exposure to tobacco smoke. The authors explored 468

407 vulnerable periods. The authors concluded that their study confounder-adjusted associations between the mentioned 469

408 supports the association between pesticide exposure during EDCs and autistic behaviors assessed by SRS utilizing a two- 470

409 pregnancy, particularly organophosphates, and neurodevel- stage hierarchical regression model to account for repeated 471

410 opmental disorders, as well as providing novel findings about measures and confounding by correlated EDCs. Most of the 472

411 the associations of pyrethroids and carbamates with ASD and EDCs were associated with unimportant differences in SRS 473

412 DD.21 scores. Each two-standard deviations increase in polybromi- 474

413 In California, Keil et al. examined the possible associa- nated diphenyl ether-28 (PBDE-28) (a brominated flame 475

414 tion between mother-reported use of imidacloprid, which retardant) or trans-nonachlor (an organochlorine pesticide) 476

415 is a common household pesticide utilized for the treat- serum concentration was associated with more autistic 477

416 ment of fleas and ticks for pets, and ASD. Participants were behaviors. Conversely, the authors found fewer autistic 478

417 enrolled as part of the aforementioned CHARGE study.21 The behaviors in the children of women with detectable vs. 479

418 analytic dataset included complete information, collected non-detectable PCB-178 (a polychlorinated biphenyl), p- 480

419 before September 2011, for 407 children with ASD (assessed hexachlorocyclohexane (an organochlorine pesticide), or 481

420 using ADI-R and ADOS) and 262 normal controls, frequency PBDE-85 (a brominated flame retardant) concentrations. 482

6 Posar A, Visconti P

483 Increasing perfluorooctanoate (PFOA) (a perfluoroalkyl sub- a correlation between job-related electromagnetic expo- 541

484 stance) concentrations were also associated with fewer sure of the parents, in particular the father, and ASD in 542

485 autistic behaviors. These latter associations with fewer their children.30 However, the study shows some limita- 543

486 autistic behaviors may suggest a possible protective action tions in terms of methodology, including the fact that the 544

487 of the involved substances. The authors concluded that some authors used data from a single center (Ciudad Real) and 545

488 EDCs were associated with greater or fewer childhood autis- employed a tool (Modified Checklist for Autism in Toddlers: 546

489 tic behaviors, but the modest size of the sample did not M-CHAT) not entirely reliable for the diagnosis of ASD. A 547

490 allow them to reach definite conclusions.23 systematic epidemiological study involving multiple cen- 548

491 The intriguing hypothesis that EDCs may modify the ters located in different geographical areas with diverse 549

492 axes of endogenous hormones, interfering with steroid- electromagnetic exposure levels is needed to evaluate the 550

493 dependent neurodevelopment and increasing the ASD hypothesis of an association between the electromagnetic 551

494 risk,23,24 still requires experimental confirmation. pollution extent and autism prevalence.26 But unfortunately, 552

until now these kinds of studies on this topic have been 553

495 Other environmental factors lacking. 554

The possible role of childhood vaccinations in the ASD 555

496 Gao et al. investigated the association between prenatal etiopathogenesis has been much discussed over the years 556

497 environmental risk factors and autism in Tianjin (China). and the debate is stiW ongoing. In their evidence-based 5«

498 They performed a case-control study including 193 chil- meta-analysis, Taylor et al. considered five cohort and five 558

499 dren with autism and 733 typically developed controls case-control studies on this topic, involving 1 256407 and 559

500 matched by age and sex from 2007 to 2012 by using a ques- 9920 ^Mren respectively. The cohort data did not show 560

501 tionnaire. The case group consisted of children diagnosed any relations between vaccination and autism or ASD, nor 561

502 by a pediatrician according to the criteria of DSM-IV- between multiple vaccines (MMR), thimerosal, or mercury 562

503 TR6 and those who scored >30 on the Childhood Autism and autism or ASD. Similarly, the case-control data did not 563

504 Rating Scale (CARS). Statistical analysis was performed show an increased autism or ASD risk following exposure to 564

505 using Quick Unbiased Efficient Statistical Tree (QUEST) MMR, mercury, or thimerosal when grouped by condition or 565

506 and logistic regression analysis in order to calculate the by type of exposure. The authors concluded suggestingthat 566

507 odds ratio of each risk factor, adjusted for socioeconomic vaccinations as well as the vaccine components (thimerosal 567

508 factors. By combining a classification tree and logistic or mercury) or MMR are not associated with an increased risk 568

509 regression analysis, the authors found that mother's depres- for autism or ASD.31 It is however to be noted ^^ according 569

510 sion during pregnancy and neonatal complications (anoxia, to the observation of Turville and toUten the meta-analysis 570

511 jaundice, and aspiration pneumonia) were associated with of Taylor et al. confirmed previous studies showing that 571

512 increased risk for child autism, while mother's air condi- ASD incidence is similar in groups of children who have 572

513 tioner use and father's freshwater fish diet before pregnancy been differently ^drate^ but it did not compare the 573

514 were associated with reduced risk for child autism. The ASD incidence in vaccinated and unvaccinated children.32 574

515 authors mentioned that air conditioner use might decrease Therefore, there is still some uncertainty concerning this 575

516 the concentration of air pollutants, resulting from lower matter and according to Sealey et al^ this topk should be 576

517 air PM filters. Another hypothesis is that a better con- studied further and the scientific community should still be 577

518 trolled indoor environment could reduce dampness and vigilant concerning the possible association between vac- 578

519 conditions that favor microbe growth. The fish diet of cines and ASD.33 However, at present foregoing vaccinations 579

520 fathers might help to reduce or prevent paternal obe- appears to be an unjustified and dangerous choice for public 580

521 sity; alternatively, fish oil might improve the quality of health. 581

522 sperm.25 Van Elst et al. developed an enticing theory according to 582

523 In the present authors' opinion, the possible involve- which there may be a link between polyunsaturated fatty 583

524 ment of electromagnetic pollution in ASD etiopathogenesis acids (PUFA) and ASD. The authors have pointed out that 584

525 is one of the most intriguing hypotheses, but at the same over the last decades the increase in ASD prevalence seems 585

526 time one of the least studied.26 The great increase in to have been concomitant with modifications in dietary 586

527 electromagnetic pollution, related to the huge deployment fatty acid composition, namely the substitution of choles- 587

528 of wireless technologies, seems to overlap chronologically teral with omega-6 in many foodstuffs, which has caused 588

529 with the increase in autism prevalence detected over the a remarkable increase in the omega-6/omega-3 ratio. The 589

530 last decades. A number of studies in the literature have authors hypothesized that in particular during the life's ear- 590

531 suggested possible biological and health effects, including liest stages an omega-3 deficit may lead to alterations of: 591

532 carcinogenicity, attributable to electromagnetic exposure, neurogenesis, synaptogenesis, myelination; neurotransmit- 592

533 probably at least in part mediated by damages to the ter synthesis and turnover; brain connectivity; expression 593

534 DNA.27-29 In particular with regard to ASD, the hypothe- of peroxisome proliferator-activated receptors; responses 594

535 sized pathogenic mechanisms of electromagnetic pollution of inflammation; and cognitive functioning and behavior. 595

536 include: damages to the DNA, oxidative stress, intracellu- All of the foregoing factors appear to be related to ASD.34 596

537 lar calcium increase, dysfunction of the immune system, Unfortunately, to date randomized clinical trials as a whole 597

538 and disruption of the blood-brain barrier.28 29 The obser- have shown no significant effects of supplementation with 598

539 vational case-control study performed by Pino-Lopez and omega-3 in children affected by ASD even though one can- 599

540 Romero-Ayuso in Spain, involving 70 cases with ASD and 136 not exclude the existence of a subgroup of patients with ASD 600 controls (aged 16-36 months), suggested the presence of who may respond to this treatment.35 601

Figure 1 Etiopathogenesis of non-syndromic autism spectrum disorder. Note: It summarizes various possible interactions among genetic and environmental factors involved in the etiopathogenesis of non-syndromic autism spectrum disorder. Each arrow represents a facilitating effect.

610 611 612

620 621 622

Discussion

Today it is widely accepted that both genetic and environmental factors are implicated in the etiopathogenesis of the so-called non-syndromic ASD, which is not secondary to a well-known genetic condition (such as tuberous sclerosis complex or fragile X syndrome). For a long time, the weight of the genetic component was considered clearly prevalent, but in recent years, given the dramatic increase in the prevalence of ASD, the role of environmental factors has gained relatively greater importance. In addition to these obvious epidemiological considerations, as suggested by Wong et al., the hypothesis that the exposure to environmental factors may considerably contribute to the ASD development is supported by the presence of different subsets of candidate genes in each subject with ASD and by the large phenotypical variability of these individuals.36 However, there is still much to discover about the complex interaction between these causal factors. The relatively new concept of epigenetics can be very helpful in this regard, at least in a subpopulation of cases with ASD. Epigenetics represents a fundamental gene regulation mechanism that is based on chemical modifications of DNA and histone proteins, without changing the DNA sequence. It has been proposed that environmental factors, such as heavy metals and EDCs, can modify epigenetic status and gene expression, causing ASD.37,38

However, the etiopathogenic role of environmental factors has still to be considered carefully due to the lack of conclusive data. While the association between, for example, air pollutants and the risk for ASD is receiving increasing confirmation, the hypothesis of a real causal relation between them needs further data, as well as the fact that possible pathogenic mechanisms of the air pollutants involved in ASD remain hypothetical. In humans, air pollutant exposure has been shown to favor oxidative

stress and inflammation,16 which may contribute to ASD pathogenesis.39 Furthermore, lead, mercury, and arsenic are known neurotoxicants that can cross the blood-brain barrier and impair neurodevelopment.16 Epigenetic mechanisms may be hypothesized based on the results of the experimental study on animals carried out by Hill et al.37 Also to be taken into consideration is the possibility that mixtures of air pollutants, and not single pollutants, are related to ASD, due to synergistic effects12,17; of course, this would make the interpretation of data emerging from studies about pollutants and ASD much more difficult. According to Guxens et al., the father's air pollution exposure during the preconception period also may have a role in ASD etiopathogenesis,20 but studies about this topic are scanty. Unfortunately, the current knowledge in this field is still at the level of assumptions or only a little further developed.

On the other hand, the importance of the genetic component in the etiopathogenesis of ASD should not be forgotten, demonstrated by an impressive amount of data in the literature over the years, and also confirmed today by a simple observation: while the prevalence of the disorder has been increasing over the last decades, the large disproportion between males and females in favor of the former has been confirmed.4 If the etiopathogenesis of ASD was related only to environmental factors, with which both males and females are inevitably in contact, there should be no reason for this persisting difference in prevalence between the sexes.

Fig. 1 represents an attempt to summarize various possible interactions among genetic and environmental factors involved in the etiopathogenesis of non-syndromic ASD. The schema shows that in genetically predisposed individuals, environmental factors may lead to autism directly or through epigenetic mechanisms, alterations of the immune function of both the mother during pregnancy (maternal immune

660 661 662

+Model ARTICLE IN PRESS

8 Posar A, Visconti P

671 activation) and the affected individual, modifications of

672 the microbiome (represented by the totality of symbiotic

673 micro-organisms harbored by the human body) that in turn

674 reciprocally interacts with the immune function, oxidative

675 stress, and mitochondrial dysfunction.37"45 This schema does

676 not claim to be exhaustive and certainly will need updates.

677 However, despite the lack of conclusive data, the pos-

678 sible impact of environmental pollutants on public health

679 should be kept in mind and therefore the precautionary

680 principle, according to Suades-González et al., should be

681 applied in order to protect children from possible pathogenic

682 factors.46 First of all, there are obvious ethical and moral

683 reasons to prompt a precautionary approach to the pollut-

684 ants in the event that this can reduce the occurrence of

685 new cases of ASD. However, even apart from these essential

686 considerations, there are also reasons related to the possi-

687 bility of obtaining, with adequate prevention, a savings of

688 human and financial resources due to the high costs to soci-

689 ety caused by the management of individuals with ASD over a

690 lifetime.

691 Conclusions

692 Despite the great efforts performed during the last decades

693 in medical research, involving considerable human and

694 financial resources, today many aspects of ASD etiopatho-

695 genesis are still unknown, while the prevalence of this

696 heterogeneous condition has increased greatly without satis-

697 factory explanations. This led the authors to develop several

698 hypotheses, which are often divergent and still need confir-

699 mation. Thanks to the huge amount of data that has emerged

700 from the recent research on autism, much more informa-

701 tion is now known about the brain functioning of these

702 subjects, but also about the typically-developed individ-

703 uals. However, unfortunately, even today there is no specific

704 treatment that can cure. In 2016, it is still observed that

705 this disabling disorder is more and more frequent, with-

706 out a known etiopathogenesis nor decisive treatments. This

707 situation has indirectly favored, in the families involved,

708 the use of treatments from complementary and alternative

709 medicine,47 a phenomenon that is not always without risks

710 for the patients.

711 Future research should investigate whether there is a sig-

712 nificant difference in the prevalence of ASD among nations

713 with high and low levels of the various types of pollution.

714 So far, the vast majority of studies that have looked for a

715 correlation between pollutants and autism have been con-

716 ducted in the USA. This may be a limiting factor from an

717 epidemiological point of view, especially because most of

718 the few studies conducted elsewhere have not confirmed

719 the results obtained in the USA. It is unclear whether this

720 is due to methodological problems or to real differences

721 between the geographical areas considered. According to

722 Shelton et al.,21 a very important goal of the research con-

723 cerning the interactions between genetics and environment

724 is the identification of vulnerable populations, also in view of

725 proper prevention. Finally, another aspect that the authors

726 consider very important for research in this field is the uni-

727 formity of diagnostic criteria and assessment tools for ASD,

728 in order to make the results of the studies performed across

729 the world comparable.

Conflicts of interest 30

The authors declare no conflicts of interest. 731

Acknowledgments 32

The authors would like to thank Cecilia Baroncini for English 733

revision. 734

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