**4.1 Prenatal factors**

The association between prenatal insults and the pathogenesis of autism has been reported recent decades. Early in 2005, Beversdorf *et al*. conducted surveys regarding incidence and timing of prenatal stressors. They found a higher incidence of prenatal stressors in autism at 21-32 weeks gestation, which peaks at 25-28 weeks. Their finding supported the hypothesis of prenatal stressors as a potential contributor to autism, and the timing was consistent with the embryological age suggested by neuroanatomical findings seen in the cerebellum in autism [231]. More specifically, Meyer *et al* demonstrate that the effects of maternal immune challenge between middle and late gestation periods in mice are dissociable in terms of several neuropsychiatric disorders including autism [232]. However, this conclusion was challenged by another group of scientists. Ploeger *et al*. proposed pleiotropic effects during a very early and specic stage of embryonic development, namely early organogenesis (day 20 to day 40 after fertilization) in order to explain the effect of uterine disturbances to the development of autism [233]. They provided ample evidence from literature for the association between autism and many different kinds of physical anomalies such as limb deformities, craniofacial malformations, brain pathology, and anomalies in other organs, which agrees with the hypothesis that pleiotropic effects are involved in the development of autism.

Drugs are the most important prenatal factors affecting embryo and fetal development. Cumulating data support the relationships between maternal medication and fetogeneous diseases including autism. The obnoxious drug thalidomide turned out not only to relate to fetal abnormality but also to autism. Stromland group retrospectively investigated 100 Swedish thalidomide embryopathy cases and found possible association of thalidomide embryopathy with autism [234]. Another example of drug relating to autism is valproate. Williams *et al* reported six cases whose clinical phenotype was compatible with both fetal valproate syndrome (FVS) and autism. Although the sample size is small, the authors claimed the association between this known teratogen and autism had both clinical and research implications [235]. Similarly, Rasalam group provided another line of evidence that prenatal exposure to sodium valproate is a risk factor for the development of an ASD [236].

Another prenatal factor is intrauterine inflammation. Kannan *et al* conducted an animal study to demonstrate intrauterine inflammation results in alterations in cortical serotonin and disruption of serotonin-regulated thalamocortical development in the newborn brain therefore resulting in impairment of the somatosensory system, such as autism [237]. More persuasive evidence comes from Girard's report. According to their results, end of gestation exposure of pregnant rats to systemic microbial product such as lipopolysacharide (LPS) is an independent risk factor for neurodevelopmental diseases such as cerebral palsy, mental deficiency, and autism. And coadministration of IL-1 receptor antagonist with LPS alleviated the detrimental effects caused by LPS [238].

Autoimmune Disorder and Autism 531

the previous results are hardly conclusive. Further research is needed to explore the possible

Mouse models provide a powerful strategy to explore experimentally candidate genes for autism susceptibility, and to use environmental challenges to induce gene mutations and cell pathology early in development. Mouse models have also been used to investigate the effects of alterations in signaling pathways on neuronal migration, neurotransmission and brain anatomy, which are relevant to findings in autistic subjects [250]. These models have

There are currently several mouse models for autism research, most of which are primarily developed by knocking out different candidate genes for other neuropsychiatric diseases such as fragile X syndrome [250, 251], Rett syndrome [252], but now are used as autistic models because of their autistic-like behaviors. Other examples include *Engrailed 1&2* and *PTEN* genetic mice [253, 254]. In addition, there is another group of models constructed by surgical or toxic treatments of candidate regions in the brain, in general during development [255]. Some other reports regarding autistic-like behaviors in BALB/c and A/J mice have

Here the author would like to stress an inbred mouse strain for autistic research. BTBR T(+)*tf/J* mouse, also named as BTBR mouse, is an inbred strain with black top coat and blond undercoat. Anatomically BTBR mice get total absence of the corpus callosum, and severely reduced hippocampal commissure, which are also attributed to their phenotypes [259-262]. Although primarily used as type 2 diabetes model [263-268] and phenylketonuria (PKU) model [269-274], BTBR mice were recently found to be a promising mice model for autism research because they exhibited the three core symptoms for diagnosing autism [275-282]. Using this strain, several groups have begun to explore the pathogenesis of autism. It was well documented that circulating corticosterone is higher in the BTBR than in B6. And higher basal glucocorticoid receptor mRNA and higher oxytocin peptide levels were detected in the brains of BTBR as compared to B6, although their relationship to autism remain disputable [283, 284]. In the meanwhile, potential treatments for autism have been proposed based on the experimental results using BTBR mice. Two independent groups confirm the efficacy of the SERT blocker, fluoxetine for enhancement of social interactions [285, 286]. Another experiment reported repetitive self-grooming behavior in the BTBR mouse model of autism was blocked by the mGluR5 antagonist Methyl-6-phenylethynylpyridine (MPEP) [287]. Behavioral therapies offer another option for autism treatment, Young group reported social peers rescued autism-relevant sociability deficits in adolescent

However, the tools to analyze these animals are not yet standardized, and an important effort needs to be made. Crawley *et al* proposed three standards to evaluate animal model, namely face validity (i.e. resemblance to the human symptoms), construct validity (i.e. similarity to the underlying causes of the disease) and predictive validity (i.e. expected responses to treatments that are effective in the human disease) [290]. Using these standards,

Autism spectrum disorders (ASD) is a common neurodevelopment disorder. Diagnosed before three years old, autistic children present significant language delays, social and

newly developed tests are used to screen more animal models for autism research.

elucidated neuropathology that might underlie the autism phenotype.

mechanisms underlying these results.

also been seen [250, 256-258].

BTBR mice, but not cross-fostering [288, 289].

**6. Summary and conclusions** 

**5. Mouse models for autism research** 

In addition, maternal complications of pregnancies are proved to be associated with autism. One group performed a discriminant analysis to explore perinatal complications as predictors for autism. They found three maternal medical conditions including urinary infection, high temperatures, and depression to be highly significant and contribute to the separation between the autistic and normal subjects [239].

#### **4.2 Postnatal factors**

Heavy metals have also been generally considered to contribute to the pathogenesis of autism. Mercury is one of the most widely studied heavy metals. Palmer *et al* studied the association between environmentally released mercury, special education and autism rates in Texas using data from the Texas Education Department and the United States Environmental Protection Agency, and found there was a significant increase in the rates of special education students and autism rates associated with increases in environmentally released mercury. They reported a 43% increase in the rate of special education services and a 61% increase in the rate of autism [240]. Windham group included 284 children with ASD and 657 controls from the San Francisco Bay area in order to explore possible associations between autism spectrum disorders (ASD) and environmental exposures. Their results suggested a potential association between autism and estimated metal concentrations including mercury, cadmium, nickel [241]. Consistent with previous results, Geier *et al* conducted a prospective study which provided biochemical/genomic evidence for mercury susceptibility/toxicity in ASDs indicating a causal role for mercury [242, 243], and they further explored the threshold effect of mercury in a recent publication [244]. In spite of these different pieces of evidence, disagreement exists. IP *et al* performed a cross-sectional cohort study to compare the hair and blood mercury levels of autistic children and a group of normal children. There was no difference in the mean mercury levels. Thus, they concluded that there is no causal relationship between mercury as an environmental neurotoxin and autism [245].

In addition of mercury, lead is also associated with autism. Very early evidence came from a case report, which explored the interaction and possible casual relationship of an elevated blood-lead and autism, as well as treatment of the behavioral symptoms [246]. Later, Canfield *et al* concluded that blood lead concentrations, even those below 10 microgram per deciliter, were inversely associated with children's IQ scores at three and five years of age, and associated declines in IQ were greater at these concentrations than at higher concentrations [247]. Supporting these results, Yorbik group reported that autism could be associated with significant decrease in excretion rate of lead [248].

Hazardous air pollutants have long been related to the development of autism and more evidences have begun to emerge in recent years. Kalkbrenner *et al* conducted a case-control study to screen perinatal exposure to 35 hazardous air pollutants using 383 children with autism spectrum disorders and, as controls, 2,829 children with speech and language impairment. Although the results were biased by exposure misclassification of air pollutants and the use of an alternate developmental disorder as the control group, they provided evidence based on their analysis that methylene chloride, quinoline, and styrene were the plausible candidate exposures for autism spectrum disorders [249]. In another study conducted by Windham group, trichloroethylene, and vinyl chloride have also been related to autism [241].

However, one should notice that the currently available data are mainly derived from epidemiological studies. Considering the limited sample sizes and the different populations,

In addition, maternal complications of pregnancies are proved to be associated with autism. One group performed a discriminant analysis to explore perinatal complications as predictors for autism. They found three maternal medical conditions including urinary infection, high temperatures, and depression to be highly significant and contribute to the

Heavy metals have also been generally considered to contribute to the pathogenesis of autism. Mercury is one of the most widely studied heavy metals. Palmer *et al* studied the association between environmentally released mercury, special education and autism rates in Texas using data from the Texas Education Department and the United States Environmental Protection Agency, and found there was a significant increase in the rates of special education students and autism rates associated with increases in environmentally released mercury. They reported a 43% increase in the rate of special education services and a 61% increase in the rate of autism [240]. Windham group included 284 children with ASD and 657 controls from the San Francisco Bay area in order to explore possible associations between autism spectrum disorders (ASD) and environmental exposures. Their results suggested a potential association between autism and estimated metal concentrations including mercury, cadmium, nickel [241]. Consistent with previous results, Geier *et al* conducted a prospective study which provided biochemical/genomic evidence for mercury susceptibility/toxicity in ASDs indicating a causal role for mercury [242, 243], and they further explored the threshold effect of mercury in a recent publication [244]. In spite of these different pieces of evidence, disagreement exists. IP *et al* performed a cross-sectional cohort study to compare the hair and blood mercury levels of autistic children and a group of normal children. There was no difference in the mean mercury levels. Thus, they concluded that there is no causal relationship between mercury as an environmental

In addition of mercury, lead is also associated with autism. Very early evidence came from a case report, which explored the interaction and possible casual relationship of an elevated blood-lead and autism, as well as treatment of the behavioral symptoms [246]. Later, Canfield *et al* concluded that blood lead concentrations, even those below 10 microgram per deciliter, were inversely associated with children's IQ scores at three and five years of age, and associated declines in IQ were greater at these concentrations than at higher concentrations [247]. Supporting these results, Yorbik group reported that autism could be

Hazardous air pollutants have long been related to the development of autism and more evidences have begun to emerge in recent years. Kalkbrenner *et al* conducted a case-control study to screen perinatal exposure to 35 hazardous air pollutants using 383 children with autism spectrum disorders and, as controls, 2,829 children with speech and language impairment. Although the results were biased by exposure misclassification of air pollutants and the use of an alternate developmental disorder as the control group, they provided evidence based on their analysis that methylene chloride, quinoline, and styrene were the plausible candidate exposures for autism spectrum disorders [249]. In another study conducted by Windham group, trichloroethylene, and vinyl chloride have also been related

However, one should notice that the currently available data are mainly derived from epidemiological studies. Considering the limited sample sizes and the different populations,

associated with significant decrease in excretion rate of lead [248].

separation between the autistic and normal subjects [239].

**4.2 Postnatal factors** 

neurotoxin and autism [245].

to autism [241].

the previous results are hardly conclusive. Further research is needed to explore the possible mechanisms underlying these results.
