The role of candidate genes in unraveling the genetics of autism

Why are autism spectrum conditions more prevalent in males?

Autism Spectrum Conditions (ASC) are much more common in males, a bias that may offer clues to the etiology of this condition. Although the cause of this bias remains a mystery, we argue that it occurs because ASC is an extreme manifestation of the male brain. The extreme male brain (EMB) theory, first proposed in 1997, is an extension of the Empathizing-Systemizing (E-S) theory of typical sex differences that proposes that females on average have a stronger drive to empathize while males on average have a stronger drive to systemize. In this first major update since 2005, we describe some of the evidence relating to the EMB theory of ASC and consider how typical sex differences in brain structure may be relevant to ASC. One possible biological mechanism to account for the male bias is the effect of fetal testosterone (fT). We also consider alternative biological theories, the X and Y chromosome theories, and the reduced autosomal penetrance theory. None of these theories has yet been fully confirmed or refuted, though the weight of evidence in favor of the fT theory is growing from converging sources (longitudinal amniocentesis studies from pregnancy to age 10 years old, current hormone studies, and genetic association studies of SNPs in the sex steroid pathways). Ultimately, as these theories are not mutually exclusive and ASC is multi-factorial, they may help explain the male prevalence of ASC.

The children's Empathy Quotient and Systemizing Quotient: sex differences in typical development and in autism spectrum conditions

Children's versions of the Empathy Quotient (EQ-C) and Systemizing Quotient (SQ-C) were developed and administered to n = 1,256 parents of typically developing children, aged 4-11 years. Both measures showed good test-retest reliability and high internal consistency. As predicted, girls scored significantly higher on the EQ-C, and boys scored significantly higher on the SQ-C. A further sample of n = 265 children with Autism Spectrum Conditions (ASC) scored significantly lower on the EQ-C, and significantly higher on the SQ-C, compared to typical boys. Empathy and systemizing in children show similar patterns of sex differences to those observed in adults. Children with ASC tend towards a 'hyper-masculinized' profile, irrespective of sex.

Fetal testosterone and autistic traits

Studies of amniotic testosterone in humans suggest that fetal testosterone (fT) is related to specific (but not all) sexually dimorphic aspects of cognition and behaviour. It has also been suggested that autism may be an extreme manifestation of some male-typical traits, both in terms of cognition and neuroanatomy. In this paper, we examine the possibility of a link between autistic traits and fT levels measured in amniotic fluid during routine amniocentesis. Two instruments measuring number of autistic traits (the Childhood Autism Spectrum Test (CAST) and the Child Autism Spectrum Quotient (AQ-Child)) were completed by these women about their children (N=235), ages 6-10 years. Intelligence Quotient (IQ) was measured in a subset of these children (N=74). fT levels were positively associated with higher scores on the CAST and AQ-Child. This relationship was seen within sex as well as when the sexes were combined, suggesting this is an effect of fT rather than of sex per se. No relationships were found between overall IQ and the predictor variables, or between IQ and CAST or AQ-Child. These findings are consistent with the hypothesis that prenatal androgen exposure is related to children exhibiting more autistic traits. These results need to be followed up in a much larger sample to test if clinical cases of ASC have elevated fT.

An Exploration of Possible Pre- and Postnatal Correlates of Autism: A Pilot Survey

Biological mothers of children, diagnosed with autism or pervasive developmental disorder, and biological mothers of children without developmental delays and matched on age and sex, were surveyed about a number of possible pre- and postnatal correlates of autism or pervasive developmental disorder. A regression analysis for boys showed the mother not vomiting in the first trimester, not having smell aversions, not craving sweets, the mother reporting fewer food aversions during the pregnancy, and having an infection while pregnant significantly predicted a later diagnosis of autism or pervasive developmental disorder. Not vomiting in the first trimester was the only significant predictor of a diagnosis of autism or a pervasive developmental disorder in girls.

Fetal testosterone and sex differences in typical social development and in autism

Experiments in animals leave no doubt that androgens, including testosterone, produced by the testes in fetal and/or neonatal life act on the brain to induce sex differences in neural structure and function. In human beings, there is evidence supporting a female superiority in the ability to read nonverbal signals, specific language-related skills, and theory of mind. Even more striking than the sex differences seen in the typical population is the elevated occurrence of social and communicative difficulties in human males. One such condition, autism, occurs four times more frequently in boys than in girls. Recently, a novel theory known as the "extreme male brain" has been proposed. It suggests that the behaviors seen in autism are an exaggeration of typical sex differences and that exposure to high levels of prenatal testosterone might be a risk factor. In this article, we argue that prenatal and neonatal testosterone exposures are strong candidates for having a causal role in sexual dimorphism in human behavior, including social development, and as risk factors for conditions characterized by social impairments, particularly autism spectrum conditions.

Foetal testosterone, social relationships, and restricted interests in children

BACKGROUND

Sex-differences exist in some areas of human social behaviour. In animals, foetal testosterone (fT) plays a central role in organising the brain and in later social behaviour. fT has also been implicated in language development, eye-contact, and spatial ability in humans.

METHODS

Fifty-eight children (35 male and 23 female), whose fT was analysed in amniotic fluid, were followed up at age 4. Their mothers completed the Children's Communication Checklist, a questionnaire assessing language, quality of social relationships and restricted interests.

RESULTS

fT was negatively correlated to quality of social relationships, taking sex-differences into account. fT was also positively correlated with restricted interests in boys.

CONCLUSIONS

These findings implicate fT in both social development and attentional focus. They may also have implications for understanding the sex ratio in autism.

Mercury exposure in children with autistic spectrum disorder: case-control study

Although mercury has been proven to be a neurotoxicant, there is a lack of data to evaluate the causal relationship between mercury and autism. We aim to see if there is increased mercury exposure in children with autistic spectrum disorder. We performed a cross-sectional cohort study over a 5-month period in 2000 to compare the hair and blood mercury levels of children with autistic spectrum disorder (n = 82; mean age 7.2 years) and a control group of normal children (n = 55; mean age 7.8 years). There was no difference in the mean mercury levels. The mean blood mercury levels of the autistic and control groups were 19.53 and 17.68 nmol/L, respectively (P = .15), and the mean hair mercury levels of the autistic and control groups were 2.26 and 2.07 ppm, respectively (P = .79). Thus, the results from our cohort study with similar environmental mercury exposure indicate that there is no causal relationship between mercury as an environmental neurotoxin and autism.

Measles-mumps-rubella vaccine and autistic spectrum disorder: report from the New Challenges in Childhood Immunizations Conference convened in Oak Brook, Illinois, June 12-13, 2000

BACKGROUND

Parents and physicians are understandably concerned about the causes and treatment of autism, a devastating disease that affects the entire family. Although much has been learned about autism, there are many gaps in our knowledge about what causes the disorder and how it can be prevented. Autistic symptoms occur along a spectrum, often referred to as autistic spectrum disorder (ASD). Concern has been raised about a possible association between measles-mumps-rubella (MMR) vaccine and inflammatory bowel disease (IBD) and ASD, especially autism with regression. Also, increased requests for educational services related to ASD have raised concerns about possible increases in the incidence of ASD.

METHODS

On June 12-13, 2000, the American Academy of Pediatrics (AAP) convened a conference titled "New Challenges in Childhood Immunizations" in Oak Brook, Illinois. At this conference, parents, practitioners, and scientists presented information and research on MMR vaccine and ASD. Attendees included representatives from select AAP committees and sections as well as federal and other organizations that address related issues. The multidisciplinary panel of experts reviewed data on what is known about the pathogenesis, epidemiology, and genetics of ASD and the available data on hypothesized associations with IBD, measles, and MMR vaccine. Supplemental information was requested from authors who have proposed the hypotheses and other experts in relevant areas.

RESULTS

Autism is a complex disorder of uncertain and probably multiple etiologies. Genetic predisposition to ASD may involve as many as 10 genes. Many experts believe that the abnormal brain development in autism occurs before 30 weeks' gestation in most instances. In utero rubella is a known cause of autism. Animal model data support the biologic plausibility that exposure to yet unrecognized infectious or other environmental agents could cause ASD. Several factors may contribute to apparent increases in incidence of ASD in recent years. Most data indicate increased recognition and reporting as primary factors, but the epidemiologic data are insufficient to determine if there has been a true increase in the incidence of ASD. Increased reporting of ASD in recent years has occurred long after the introduction of MMR vaccine in the United States in 1971 and widespread use of this vaccine in the 1970s for routine immunization of children at 12 to 15 months of age. Appropriate detailed studies are needed to define the true incidence and prevalence of ASD. Epidemiologic studies in Europe indicate no association between MMR vaccine and ASD. Some children with ASD have gastrointestinal symptoms, but an increased rate of any specific gastrointestinal disorder in children with ASD has not been established. Studies to detect evidence of measles virus in intestinal tissue specimens from patients with IBD or autism with gastrointestinal symptoms have not used uniform techniques. Several laboratories have found no evidence of measles viruses in tissue specimens from patients with IBD, but 2 groups have found evidence of measles virus using different techniques. A group that found evidence of measles virus in affected tissue specimens from patients with IBD has also reported detecting portions of measles virus in peripheral blood lymphocytes and intestinal tissue specimens from patients with autism and gastrointestinal disorders. Finding a portion of a virus using molecular techniques does not constitute evidence for a causal relationship, because some viruses persist in unaffected hosts. Additional controlled studies in several laboratories are needed to determine if portions of measles virus persist in intestinal and other tissues of people with and without gastrointestinal disease and/or ASD.

CONCLUSIONS

Although the possible association with MMR vaccine has received much public and political attention and there are many who have derived their own conclusions based on personal experiences, the available evidence does not support the hypothesis that MMR vaccine causes autism or associated disorders or IBD. Separate administration of measles, mumps, and rubella vaccines to children provides no benefit over administration of the combination MMR vaccine and would result in delayed or missed immunizations. Pediatricians need to work with families to ensure that children are protected early in the second year of life from these preventable diseases. Continued scientific efforts need to be directed to the identification of the causes of ASD.