Accepted Manuscript Title: Prenatal factors associated with Autism Spectrum Disorder (ASD) Author: A. Ornoy L. Weinstein-Fudim Z. Ergaz PII: DOI: Reference:
S0890-6238(15)00075-1 http://dx.doi.org/doi:10.1016/j.reprotox.2015.05.007 RTX 7129
To appear in:
Reproductive Toxicology
Received date: Revised date: Accepted date:
3-3-2015 14-5-2015 14-5-2015
Please cite this article as: Ornoy A, Weinstein-Fudim L, Ergaz Z, Prenatal factors associated with Autism Spectrum Disorder (ASD), Reproductive Toxicology (2015), http://dx.doi.org/10.1016/j.reprotox.2015.05.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Prenatal factors associated with Autism Spectrum
2
Disorder (ASD)
ip t
3 Ornoy A¹, Weinstein– Fudim L¹, Ergaz Z².
5
¹Laboratory of Teratology, Department of Medical Neurobiology, Hebrew University Hadassah
6
Medical School ²Department of Neonatology, Hadassah-Hebrew University Medical Center,
7
Jerusalem, Israel
8
Correspondence: Ornoy A [email protected]
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Page 1 of 55
Headings:
2
Abstract
3
1. Introduction
4
1a. Definition of ASD
5
1b Epidemiology of ASD
6
1c: Etiology of ASD
7
1d: Pathogenesis of ASD
8
2. Maternal diseases and ASD in the offspring
9
2a. ASD in offspring of diabetic mothers 3. The infectious and inflammatory origin of ASD
11
3a. infection and inflammation
12
3b. Population studies
13
3c1. Case control studies that found associations between maternal infection and ASD
14
3c2. Studies that did not find increased ASD rate
15
3d. Congenital Rubella
16
3e. CMV infection
17
3f. Influenza
18
3g. Toxoplasma
19
24 25
4c. Animal Models of inflammation4d. Conclusion
20 21 22 23
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3h. Parvovirus
3i. Tic borne infections
3j. Other viral infections in pregnancy and ASD
4. Inflammation
4a. Studies evaluating maternal immune system 4b. In vitro studies of inflammation
2
Page 2 of 55
5. Exposure to drugs and chemicals during pregnancy and ASD in the offspring
2
5a. Exposure to Selective serotonin reuptake inhibitors (SSRIs)
3
5b. Exposure to Valporic acid (VPA):
4
5c. Exposure to thalidomide
5
5d. Exposure to cocaine
6
5e. Exposure to ethanol
7
5f. Exposure to misoprostol and Möbius sequence
8
5g. Exposure to cigarette smoking
9
5h. Exposure to air pollution
10
5i. Exposure to heavy metals
11
5j. Exposure to pesticides and insecticides
12
5k. Vitamin D deficiency
13
5l. Folic acid deficiency
14
6. Conclusions
15
7. References
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Page 3 of 55
Abstract
2
Autism spectrum disorder (ASD) affecting about 1% of all children is associated, in
3
addition to complex genetic factors, with a variety of prenatal, perinatal and postnatal
4
etiologies. We discuss the known associated prenatal factors affecting the fetus
5
throughout pregnancy; whenever relevant, also summarize some animal data. Among
6
the maternal diseases in pregnancy associated with ASD are pregestational and/or
7
gestational diabetes mellitus (PGDM, GDM), maternal infections (i.e. rubella,
8
cytomegalovirus (CMV)), prolonged fever and maternal inflammation, which cause
9
changes in a variety of inflammatory cytokines. Among the drugs are valproic acid,
10
thalidomide, and possibly misoprostol and serotonin reuptake inhibitors (SSRIs).
11
Associations were described with ethanol, and possibly cocaine heavy metals heavy
12
smoking, and Folic acid deficiency. Heavy exposure to pesticides and air pollution
13
during pregnancy was recently associated with ASD. We need more epidemiologic
14
data to establish many of these associations; if proven, they might be promising
15
avenues for prevention.
16
Key words: ASD, prenatal factors, inflammation, infections, drugs, chemicals
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Page 4 of 55
1
1. Introduction
3
1a. Definition of ASD:
4
ASD is defined by the Diagnostic Statistical Manual of Mental Disorders 5 (DSM 5)
5
which is a book published by the American Psychiatric Association and contains the
6
clinical definition of all mental and neurobehavioral disorders, as a neurobehavioral
7
disorder manifested by persistent deficits in social and communication interaction,
8
deficits in developing, understanding and maintaining relationships, as well as
9
abnormal and fixed interests and repetitive behavior [1] . Symptoms must be present
an
us
cr
ip t
2
at early childhood and interfere with daily function.
11
ASD is 4-5 times more prevalent in males than in females; It is now one of the most
12
common childhood morbidities presenting in various degrees of severity. Mental
13
impairment is more common in the children with severe presentations of symptoms.
14
The etiology is diverse, largely unknown, and seems to be the result of genetic and
15
environmental interaction [2, 3].
16
Environmental exposures are increasingly being recognized as potential risk factors
18 19 20 21
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M
10
for ASD, and the possibility that the prenatal environment affects fetal programming is a promising direction for research. Prenatal environment includes maternal use of medication, maternal infection and inflammations, and exposure to various substances such as alcohol and heavy smoking during pregnancy. 1b Epidemiology of ASD
22
The reported prevalence of autism has increased dramatically over time, from 4 to 5
23
cases per 10,000 in 1966 to approximately 100/10,000 (1%) today [2-4] . The increase
24
is thought to result from a mixture of factors that include increased public awareness
5
Page 5 of 55
and changed diagnostic standards. It is still unclear what proportion of the increase
2
can be explained by these factors and what proportion is due to a true increase in risk.
3
In the US, for example, the autism and developmental disabilities monitoring network
4
(ADDM network) found in children aged 8 years in the 2008 monitoring at 14 sites in
5
the US an increase from 67/10,000 (0.67%)) in 2000 to 114/10,000 (1.14%) in 2008
6
[5]. Moreover, the same network found in the 2010 monitoring of 8 years old children
7
at 11 sites in the US a further increase of ASD prevalence to 147 per 10,000 children,
8
raising the rate to 1.47% of children. They also found different prevalence of ASD
9
among the different participating sites, ranging from 57 to 219 per ten thousand
10
children! [6]. Significant ethnic differences were also found, with the highest rate
11
among non-Hispanic white children and the lowest among Hispanic children. The
12
reason for this wide geographic and ethnic variation in the rate of ASD is not clear,
13
but could be explained by ascertainment variability due to different diagnostic
14
practices, socioeconomic disparities and differences in the access to medical services.
15
Another study, looking at the prevalence of ASD in metropolitan New Jersey in 2006
16
in comparison to 2002, found an increase from 106/10,000 in 2002 to 174/10,000 in
18 19 20 21
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2006, with boys outnumbering girls by nearly 5:1 [7]. In Israel, judging from the number of children who received childhood disability benefits by the Israeli Insurance Institute because of ASD, the cumulative incidence at 8 years of age at 2011 has increased 10 folds from 1991 and reached 0.49% with a ratio of males/females of about 5 [8]. In a different study on children that receive
22
medical services from Maccabi health services the rate in 2010 was found to be
23
0.48% [9]; of 423,524 children aged 1-12 years, 2,034 children were diagnosed as
24
having ASD, with a ratio of males to females of 4.9.
6
Page 6 of 55
The more recent global prevalence of autism was estimated to be 0.62% [5, 10]. In
2
spite of a wide variation in prevalence between the studies, the authors conclude that
3
there is no evidence of a significant impact of ethnic or socioeconomic factors on the
4
rate of ASD.
5
The increasing rate of ASD is mostly attributed to the changes in the clinical
6
definition in both the Diagnostic Statistical Manual of Mental Disorders (DSM) and
7
International Classification of Diseases (ICD) which is the internationally used
8
classification of all diseases, congenital malformations and syndromes. The DSM 4
9
definition of ASD was more general, including additional clinical symptoms
10
compared to earlier versions of the DSM. Moreover, the increase in awareness,
11
increase in available services for children with ASD and the decrease in the age of
12
diagnosis are very important contributors to the elevated prevalence of ASD [5]. It
13
was also shown that many of the children and adults who were previously diagnosed
14
as having mental impairment have severe presentations of ASD according to the
15
diagnostic criteria of DSM 4 [10, 11].The wide variation in the prevalence of ASD in
16
the different US States that have similar populations [5, 6] is an additional proof that
18 19 20 21
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the increased prevalence of ASD mainly stems from better ascertainment following the clinical definitions of ASD [5]. Hence, it is expected that the changes in the definition of ASD symptoms, as recently defined in the DSM 5, will have some influence on the prevalence of ASD, although exact figures demonstrating these changes are still lacking [12, 13]. Indeed, from an analysis of 14 studies where ASD
22
was diagnosed on the basis of DSM 4 criteria, applying the DSM 5 criteria
23
significantly reduced the rate of ASD, especially of the formerly group of Pervasive
24
Developmental Disorder not Otherwise Specified (PDD NOS) [14].
7
Page 7 of 55
In spite of the fact that most of the increase in the prevalence of ASD can be
2
explained by better ascertainment, studies on the incidence (new cases per year) of
3
ASD also showed, in spite of only few changes in the available services, a steady
4
increase in the last years, implying that some of the heightened prevalence may result
5
from a true increase in ASD rate. For example, in the UK in 1988-92 the incidence
6
was 0.40/10,000 person years and it raised to 2.98/10,000 person years in 2000-2001
7
[15]. The authors attributed this increase in incidence to better ascertainment but
8
could not exclude a real increased incidence. An increased incidence was also
9
reported in Israel where diagnostic and treatment methods are well developed for the
10
last 10-15 years [9]. If this is correct, it may largely result from prenatal
11
environmental causes because genetic, ethnic, socioeconomic and geographic factors
12
did not change significantly during that time.
13
1c: Etiology of ASD
14
The main cause of ASD is genetic. Twin studies have shown a high concordance
15
among homozygous twins which is much lower in discordant twins [3]. However, in
16
search for specific genes responsible for ASD it became obvious that there are
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numerous candidate genes on many chromosomes, and only rare cases of ASD can be related to specific genes. Thus, ASD seems to result from the interaction of genetic factors and the prenatal and postnatal environment [3]. The prenatal causes of ASD can be divided into environmental chemicals (i.e. drugs such as valproic acid, thalidomide, misoprostol; alcohol, cocaine and toxic metals
22
taken by the mother during pregnancy), exposure to particulate matter (PM) air
23
pollution of up to 2.5 micron in diameter (PM2.5), maternal infections during
24
pregnancy (i.e. rubella, CMV), maternal and fetal inflammation [16] and maternal
25
diseases (i.e. diabetes mellitus), including allergic diseases such as asthma [17].
8
Page 8 of 55
Advanced maternal and paternal age was also found to increase the risk of ASD [18,
2
19]. An association of ASD with prematurity was also reported by several
3
investigators [20]. In addition, maternal depression and/or emotional strain also seem
4
to play a role. Many studies pointed to different additional possibilities, but some of
5
these possibilities are rare and difficult to prove, and some have no biological
6
plausibility [3, 19, 21-24]
7
1d: Pathogenesis of ASD
8
Numerous mechanisms for ASD have been offered, based on human studies as well as
9
experimental animal models. In addition to complex genetic susceptibility, epigenetic
10
changes have also been proposed [2]. In the recent years it is proposed that ASD is a
11
result from general immune and metabolic disturbances that affect the brain. One of
12
the mechanisms is immune dysregulation that includes abnormal levels of cytokines
13
and growth factors, fetal and maternal antibodies to brain tissue [16, 17], microglial
14
activation, abnormal numbers of CD4 and CC8 cells and a variety of antibodies [21].
15
Additional proposed mechanisms are increased oxidative stress, with or without
16
mitochondrial dysfunction. Frye et al found [24] that at least one third of children
18 19 20
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with ASD meet partial or full criteria for mitochondrial dysfunction as a sign of a systemic metabolic problem in ASD. These children also have a variety of markers for increased oxidative stress, and many also have specific mitochondrial DNA abnormalities.
21
Other mechanisms, more related to the brain, include abnormal white matter
22
connectivity and altered synapses, as demonstrated by different imaging techniques
23
[25]. For example, by using Diffusion Tensor Imaging- an acceptable method for the
24
demonstration of white matter structure, in 22 young children with ASD, Billeci et al
25
[25] found increased fiber length in some areas of the corpus callosum, increased 9
Page 9 of 55
fractional anisotropy of the fibers and a variety of connectivity modifications. Some
2
investigators found abnormalities in brain serotonin, zinc deficiency or alterations in
3
the gut microbiome – brain –interactions, which might lead to anxiety, reduced
4
sociability and social cognition deficits as demonstrated in mice [21, 23, 26].
5
The purpose of the present review is to summarize the data associating antenatal
6
exposure to different injurious agents in the etiology of ASD. In addition, some
7
animal studies will also be presented as they often support the findings in humans
8
(e.g. prenatal exposure to valproic acid in rats and mice, and exposure
9
of zebrafish embryo/larva). It should be emphasized that the possibility to reproduce
10
in pregnant animals the data observed in human strengthens the epidemiological and
11
pathogenetic data. However, the lack of capability to reproduce the human data in
12
animals does not necessarily decrease the relevance of the human studies.
13
2. Maternal diseases and ASD in the offspring:
14
2a. ASD in offspring of diabetic mothers
15
Diabetes Mellitus, during pregnancy has been associated with an increased rate of
16
ASD in the offspring. Pregestational diabetes Mellitus (PGDM) and gestational
18 19 20 21
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diabetes (GDM) are associated with a large number of pregnancy complications. PGDM may increase the rate of congenital anomalies in the offspring, affect fetal well-being and growth. GDM, which usually develops in the second half of pregnancy, is mainly associated with disturbed fetal growth and increased rate of a variety of pregnancy complications. Both PGDM and GDM are associated with slight
22
disturbances in postnatal growth and development, also affecting fine and gross motor
23
development and increasing the rate of learning difficulties and of Attention Deficit
24
Hyperactivity Disorder (ADHD), a common comorbid neurobehavioral problem in
25
ASD [27]. 10
Page 10 of 55
The negative effects of maternal diabetes on the brain may result from intrauterine
2
increased fetal oxidative stress, epigenetic changes in the expression of several genes
3
and other unknown causes. It was shown repeatedly that good control of diabetes in
4
pregnancy may reduce these complications but apparently will not completely prevent
5
them.
6
It is not surprising that in the search of antenatal factors which might be associated
7
with ASD, maternal diabetes, because of its rising incidence, was an obvious
8
candidate. Lyall et al [28] assessed the possible association of maternal diabetes and
9
ASD in 793 children with ASD from a cohort of 66,445 pregnancies. The highest
10
association was found with maternal GDM, as the odds ratio (OR) of ASD among
11
children born to mothers with GD was 1.76. This is in slight contrast to other studies
12
that found that the risk for ASD is higher among offspring of mothers with PGDM
13
compared to mothers with GDM. Gardener et al [19] assessed the possible association
14
of a variety of antenatal maternal factors with ASD in the offspring. The highest
15
association was with advanced parental age but maternal diabetes was among the
16
other leading factors associated with ASD, with an OR of 2.07 (95% confidence
18 19 20 21
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interval (CI) 1.24-3.47). In recent years more studies have been published showing a possible association with ASD [29]. However there are also studies that could not demonstrate such associations. For example, Hultman et al [30] in their case control study on 408 Swedish children with ASD compared to 2,040 matched controls, found an association of ASD with being born small for gestational age (SGA), low Apgar
22
score or congenital anomalies, but not with maternal diabetes. Guinchat et al [31] in
23
their review of 85 studies studying the possible association between prenatal, perinatal
24
and neonatal factors and ASD did not find a strong association of maternal diabetes
25
and ASD. In contrast to this review, Xu and associates [32] culled from 167
11
Page 11 of 55
publications twelve; three cohort and nine case control studies. For the cohort studies,
2
the pooled risk of maternal diabetes resulted in a pooled OD of 1.48 (1.25-1.75,
3
95%CI p
S0890-6238(15)00075-1 http://dx.doi.org/doi:10.1016/j.reprotox.2015.05.007 RTX 7129
To appear in:
Reproductive Toxicology
Received date: Revised date: Accepted date:
3-3-2015 14-5-2015 14-5-2015
Please cite this article as: Ornoy A, Weinstein-Fudim L, Ergaz Z, Prenatal factors associated with Autism Spectrum Disorder (ASD), Reproductive Toxicology (2015), http://dx.doi.org/10.1016/j.reprotox.2015.05.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Prenatal factors associated with Autism Spectrum
2
Disorder (ASD)
ip t
3 Ornoy A¹, Weinstein– Fudim L¹, Ergaz Z².
5
¹Laboratory of Teratology, Department of Medical Neurobiology, Hebrew University Hadassah
6
Medical School ²Department of Neonatology, Hadassah-Hebrew University Medical Center,
7
Jerusalem, Israel
8
Correspondence: Ornoy A [email protected]
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1
Page 1 of 55
Headings:
2
Abstract
3
1. Introduction
4
1a. Definition of ASD
5
1b Epidemiology of ASD
6
1c: Etiology of ASD
7
1d: Pathogenesis of ASD
8
2. Maternal diseases and ASD in the offspring
9
2a. ASD in offspring of diabetic mothers 3. The infectious and inflammatory origin of ASD
11
3a. infection and inflammation
12
3b. Population studies
13
3c1. Case control studies that found associations between maternal infection and ASD
14
3c2. Studies that did not find increased ASD rate
15
3d. Congenital Rubella
16
3e. CMV infection
17
3f. Influenza
18
3g. Toxoplasma
19
24 25
4c. Animal Models of inflammation4d. Conclusion
20 21 22 23
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Ac ce p
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3h. Parvovirus
3i. Tic borne infections
3j. Other viral infections in pregnancy and ASD
4. Inflammation
4a. Studies evaluating maternal immune system 4b. In vitro studies of inflammation
2
Page 2 of 55
5. Exposure to drugs and chemicals during pregnancy and ASD in the offspring
2
5a. Exposure to Selective serotonin reuptake inhibitors (SSRIs)
3
5b. Exposure to Valporic acid (VPA):
4
5c. Exposure to thalidomide
5
5d. Exposure to cocaine
6
5e. Exposure to ethanol
7
5f. Exposure to misoprostol and Möbius sequence
8
5g. Exposure to cigarette smoking
9
5h. Exposure to air pollution
10
5i. Exposure to heavy metals
11
5j. Exposure to pesticides and insecticides
12
5k. Vitamin D deficiency
13
5l. Folic acid deficiency
14
6. Conclusions
15
7. References
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3
Page 3 of 55
Abstract
2
Autism spectrum disorder (ASD) affecting about 1% of all children is associated, in
3
addition to complex genetic factors, with a variety of prenatal, perinatal and postnatal
4
etiologies. We discuss the known associated prenatal factors affecting the fetus
5
throughout pregnancy; whenever relevant, also summarize some animal data. Among
6
the maternal diseases in pregnancy associated with ASD are pregestational and/or
7
gestational diabetes mellitus (PGDM, GDM), maternal infections (i.e. rubella,
8
cytomegalovirus (CMV)), prolonged fever and maternal inflammation, which cause
9
changes in a variety of inflammatory cytokines. Among the drugs are valproic acid,
10
thalidomide, and possibly misoprostol and serotonin reuptake inhibitors (SSRIs).
11
Associations were described with ethanol, and possibly cocaine heavy metals heavy
12
smoking, and Folic acid deficiency. Heavy exposure to pesticides and air pollution
13
during pregnancy was recently associated with ASD. We need more epidemiologic
14
data to establish many of these associations; if proven, they might be promising
15
avenues for prevention.
16
Key words: ASD, prenatal factors, inflammation, infections, drugs, chemicals
cr
us
an
M
d
te
Ac ce p
17
ip t
1
4
Page 4 of 55
1
1. Introduction
3
1a. Definition of ASD:
4
ASD is defined by the Diagnostic Statistical Manual of Mental Disorders 5 (DSM 5)
5
which is a book published by the American Psychiatric Association and contains the
6
clinical definition of all mental and neurobehavioral disorders, as a neurobehavioral
7
disorder manifested by persistent deficits in social and communication interaction,
8
deficits in developing, understanding and maintaining relationships, as well as
9
abnormal and fixed interests and repetitive behavior [1] . Symptoms must be present
an
us
cr
ip t
2
at early childhood and interfere with daily function.
11
ASD is 4-5 times more prevalent in males than in females; It is now one of the most
12
common childhood morbidities presenting in various degrees of severity. Mental
13
impairment is more common in the children with severe presentations of symptoms.
14
The etiology is diverse, largely unknown, and seems to be the result of genetic and
15
environmental interaction [2, 3].
16
Environmental exposures are increasingly being recognized as potential risk factors
18 19 20 21
d
te
Ac ce p
17
M
10
for ASD, and the possibility that the prenatal environment affects fetal programming is a promising direction for research. Prenatal environment includes maternal use of medication, maternal infection and inflammations, and exposure to various substances such as alcohol and heavy smoking during pregnancy. 1b Epidemiology of ASD
22
The reported prevalence of autism has increased dramatically over time, from 4 to 5
23
cases per 10,000 in 1966 to approximately 100/10,000 (1%) today [2-4] . The increase
24
is thought to result from a mixture of factors that include increased public awareness
5
Page 5 of 55
and changed diagnostic standards. It is still unclear what proportion of the increase
2
can be explained by these factors and what proportion is due to a true increase in risk.
3
In the US, for example, the autism and developmental disabilities monitoring network
4
(ADDM network) found in children aged 8 years in the 2008 monitoring at 14 sites in
5
the US an increase from 67/10,000 (0.67%)) in 2000 to 114/10,000 (1.14%) in 2008
6
[5]. Moreover, the same network found in the 2010 monitoring of 8 years old children
7
at 11 sites in the US a further increase of ASD prevalence to 147 per 10,000 children,
8
raising the rate to 1.47% of children. They also found different prevalence of ASD
9
among the different participating sites, ranging from 57 to 219 per ten thousand
10
children! [6]. Significant ethnic differences were also found, with the highest rate
11
among non-Hispanic white children and the lowest among Hispanic children. The
12
reason for this wide geographic and ethnic variation in the rate of ASD is not clear,
13
but could be explained by ascertainment variability due to different diagnostic
14
practices, socioeconomic disparities and differences in the access to medical services.
15
Another study, looking at the prevalence of ASD in metropolitan New Jersey in 2006
16
in comparison to 2002, found an increase from 106/10,000 in 2002 to 174/10,000 in
18 19 20 21
cr
us
an
M
d
te
Ac ce p
17
ip t
1
2006, with boys outnumbering girls by nearly 5:1 [7]. In Israel, judging from the number of children who received childhood disability benefits by the Israeli Insurance Institute because of ASD, the cumulative incidence at 8 years of age at 2011 has increased 10 folds from 1991 and reached 0.49% with a ratio of males/females of about 5 [8]. In a different study on children that receive
22
medical services from Maccabi health services the rate in 2010 was found to be
23
0.48% [9]; of 423,524 children aged 1-12 years, 2,034 children were diagnosed as
24
having ASD, with a ratio of males to females of 4.9.
6
Page 6 of 55
The more recent global prevalence of autism was estimated to be 0.62% [5, 10]. In
2
spite of a wide variation in prevalence between the studies, the authors conclude that
3
there is no evidence of a significant impact of ethnic or socioeconomic factors on the
4
rate of ASD.
5
The increasing rate of ASD is mostly attributed to the changes in the clinical
6
definition in both the Diagnostic Statistical Manual of Mental Disorders (DSM) and
7
International Classification of Diseases (ICD) which is the internationally used
8
classification of all diseases, congenital malformations and syndromes. The DSM 4
9
definition of ASD was more general, including additional clinical symptoms
10
compared to earlier versions of the DSM. Moreover, the increase in awareness,
11
increase in available services for children with ASD and the decrease in the age of
12
diagnosis are very important contributors to the elevated prevalence of ASD [5]. It
13
was also shown that many of the children and adults who were previously diagnosed
14
as having mental impairment have severe presentations of ASD according to the
15
diagnostic criteria of DSM 4 [10, 11].The wide variation in the prevalence of ASD in
16
the different US States that have similar populations [5, 6] is an additional proof that
18 19 20 21
cr
us
an
M
d
te
Ac ce p
17
ip t
1
the increased prevalence of ASD mainly stems from better ascertainment following the clinical definitions of ASD [5]. Hence, it is expected that the changes in the definition of ASD symptoms, as recently defined in the DSM 5, will have some influence on the prevalence of ASD, although exact figures demonstrating these changes are still lacking [12, 13]. Indeed, from an analysis of 14 studies where ASD
22
was diagnosed on the basis of DSM 4 criteria, applying the DSM 5 criteria
23
significantly reduced the rate of ASD, especially of the formerly group of Pervasive
24
Developmental Disorder not Otherwise Specified (PDD NOS) [14].
7
Page 7 of 55
In spite of the fact that most of the increase in the prevalence of ASD can be
2
explained by better ascertainment, studies on the incidence (new cases per year) of
3
ASD also showed, in spite of only few changes in the available services, a steady
4
increase in the last years, implying that some of the heightened prevalence may result
5
from a true increase in ASD rate. For example, in the UK in 1988-92 the incidence
6
was 0.40/10,000 person years and it raised to 2.98/10,000 person years in 2000-2001
7
[15]. The authors attributed this increase in incidence to better ascertainment but
8
could not exclude a real increased incidence. An increased incidence was also
9
reported in Israel where diagnostic and treatment methods are well developed for the
10
last 10-15 years [9]. If this is correct, it may largely result from prenatal
11
environmental causes because genetic, ethnic, socioeconomic and geographic factors
12
did not change significantly during that time.
13
1c: Etiology of ASD
14
The main cause of ASD is genetic. Twin studies have shown a high concordance
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among homozygous twins which is much lower in discordant twins [3]. However, in
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search for specific genes responsible for ASD it became obvious that there are
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numerous candidate genes on many chromosomes, and only rare cases of ASD can be related to specific genes. Thus, ASD seems to result from the interaction of genetic factors and the prenatal and postnatal environment [3]. The prenatal causes of ASD can be divided into environmental chemicals (i.e. drugs such as valproic acid, thalidomide, misoprostol; alcohol, cocaine and toxic metals
22
taken by the mother during pregnancy), exposure to particulate matter (PM) air
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pollution of up to 2.5 micron in diameter (PM2.5), maternal infections during
24
pregnancy (i.e. rubella, CMV), maternal and fetal inflammation [16] and maternal
25
diseases (i.e. diabetes mellitus), including allergic diseases such as asthma [17].
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Advanced maternal and paternal age was also found to increase the risk of ASD [18,
2
19]. An association of ASD with prematurity was also reported by several
3
investigators [20]. In addition, maternal depression and/or emotional strain also seem
4
to play a role. Many studies pointed to different additional possibilities, but some of
5
these possibilities are rare and difficult to prove, and some have no biological
6
plausibility [3, 19, 21-24]
7
1d: Pathogenesis of ASD
8
Numerous mechanisms for ASD have been offered, based on human studies as well as
9
experimental animal models. In addition to complex genetic susceptibility, epigenetic
10
changes have also been proposed [2]. In the recent years it is proposed that ASD is a
11
result from general immune and metabolic disturbances that affect the brain. One of
12
the mechanisms is immune dysregulation that includes abnormal levels of cytokines
13
and growth factors, fetal and maternal antibodies to brain tissue [16, 17], microglial
14
activation, abnormal numbers of CD4 and CC8 cells and a variety of antibodies [21].
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Additional proposed mechanisms are increased oxidative stress, with or without
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mitochondrial dysfunction. Frye et al found [24] that at least one third of children
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with ASD meet partial or full criteria for mitochondrial dysfunction as a sign of a systemic metabolic problem in ASD. These children also have a variety of markers for increased oxidative stress, and many also have specific mitochondrial DNA abnormalities.
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Other mechanisms, more related to the brain, include abnormal white matter
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connectivity and altered synapses, as demonstrated by different imaging techniques
23
[25]. For example, by using Diffusion Tensor Imaging- an acceptable method for the
24
demonstration of white matter structure, in 22 young children with ASD, Billeci et al
25
[25] found increased fiber length in some areas of the corpus callosum, increased 9
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fractional anisotropy of the fibers and a variety of connectivity modifications. Some
2
investigators found abnormalities in brain serotonin, zinc deficiency or alterations in
3
the gut microbiome – brain –interactions, which might lead to anxiety, reduced
4
sociability and social cognition deficits as demonstrated in mice [21, 23, 26].
5
The purpose of the present review is to summarize the data associating antenatal
6
exposure to different injurious agents in the etiology of ASD. In addition, some
7
animal studies will also be presented as they often support the findings in humans
8
(e.g. prenatal exposure to valproic acid in rats and mice, and exposure
9
of zebrafish embryo/larva). It should be emphasized that the possibility to reproduce
10
in pregnant animals the data observed in human strengthens the epidemiological and
11
pathogenetic data. However, the lack of capability to reproduce the human data in
12
animals does not necessarily decrease the relevance of the human studies.
13
2. Maternal diseases and ASD in the offspring:
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2a. ASD in offspring of diabetic mothers
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Diabetes Mellitus, during pregnancy has been associated with an increased rate of
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ASD in the offspring. Pregestational diabetes Mellitus (PGDM) and gestational
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diabetes (GDM) are associated with a large number of pregnancy complications. PGDM may increase the rate of congenital anomalies in the offspring, affect fetal well-being and growth. GDM, which usually develops in the second half of pregnancy, is mainly associated with disturbed fetal growth and increased rate of a variety of pregnancy complications. Both PGDM and GDM are associated with slight
22
disturbances in postnatal growth and development, also affecting fine and gross motor
23
development and increasing the rate of learning difficulties and of Attention Deficit
24
Hyperactivity Disorder (ADHD), a common comorbid neurobehavioral problem in
25
ASD [27]. 10
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The negative effects of maternal diabetes on the brain may result from intrauterine
2
increased fetal oxidative stress, epigenetic changes in the expression of several genes
3
and other unknown causes. It was shown repeatedly that good control of diabetes in
4
pregnancy may reduce these complications but apparently will not completely prevent
5
them.
6
It is not surprising that in the search of antenatal factors which might be associated
7
with ASD, maternal diabetes, because of its rising incidence, was an obvious
8
candidate. Lyall et al [28] assessed the possible association of maternal diabetes and
9
ASD in 793 children with ASD from a cohort of 66,445 pregnancies. The highest
10
association was found with maternal GDM, as the odds ratio (OR) of ASD among
11
children born to mothers with GD was 1.76. This is in slight contrast to other studies
12
that found that the risk for ASD is higher among offspring of mothers with PGDM
13
compared to mothers with GDM. Gardener et al [19] assessed the possible association
14
of a variety of antenatal maternal factors with ASD in the offspring. The highest
15
association was with advanced parental age but maternal diabetes was among the
16
other leading factors associated with ASD, with an OR of 2.07 (95% confidence
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interval (CI) 1.24-3.47). In recent years more studies have been published showing a possible association with ASD [29]. However there are also studies that could not demonstrate such associations. For example, Hultman et al [30] in their case control study on 408 Swedish children with ASD compared to 2,040 matched controls, found an association of ASD with being born small for gestational age (SGA), low Apgar
22
score or congenital anomalies, but not with maternal diabetes. Guinchat et al [31] in
23
their review of 85 studies studying the possible association between prenatal, perinatal
24
and neonatal factors and ASD did not find a strong association of maternal diabetes
25
and ASD. In contrast to this review, Xu and associates [32] culled from 167
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publications twelve; three cohort and nine case control studies. For the cohort studies,
2
the pooled risk of maternal diabetes resulted in a pooled OD of 1.48 (1.25-1.75,
3
95%CI p
