Autism risk factors: genes, environment, and gene-environment interactions Factores de riesgo del autisme: genes, ambiente e interacciones genes-ambiente Facteurs de risque d'autisme: gènes, environnement et interactions gène-environnement

Heritability

The recurrence risk of pervasive developmental disorder in siblings of children with autism is 2% to 8%;⁴ and it rises to 12% to 20% if one takes into account the siblings showing impairment in one or two of the three domains impaired in autism respectively.⁵ Moreover, several twin studies have suggested that this aggregation within families is best explained by shared genes as opposed to shared environment.^6-8 Interestingly, the variation of autistic traits in the general population has been shown to be highly heritable, at a similar level of genetic influence to autism itself, even though the results are heterogeneous (heritability 40% to 80%).^9,10 These results have led to a huge effort in research to try to unravel the genetic factors underlying the disorder. However, two recent twin studies have provided intriguing results. One study showed that monozygotic twins had higher concordance rates than dizygotic twins for ASDs, attention deficit hyperactivity disorder (ADHD), developmental coordination disorder, and tic disorder with differences in cross-disorder effects between monozygotic and dizygotic twins, raising the question of the specificity of the underlying genetic factors.⁸ Another study recently challenged the high heritability model of autism, estimating the heritability of autism to be 55 %.³ This study generated considerable discussion, the main criticisms concerning the very large confidence interval of the odds ratio (9% to 81%) and the low participation rate. However, this study is the largest population-based twin study of autism that used contemporary standards for the diagnosis of autism.

The independent heritability of each of the domains of autistic symptomatology is still a matter of debate. While some argue that different autistic symptoms, to a considerable extent, have separate genetic influences,^11,12 others argue that there is strong evidence in favor of the hypothesis that symptom domains represent correlated behavioral manifestations of a single underlying quantitative neurodevelopmental impairment.¹³

Transmission in simplex and multiplex families

According to two studies, the prevalence of de novo chromosomal rearrangements is higher in subjects from simplex families (one affected individual) compared with subjects from multiplex families,^14,15 which is consistent with the high rate of notable de novo mutations identified in probands from simplex families.¹⁶ This is also consistent with the results of studies which have shown that familial aggregation of subclinical autistic traits may occur only in multiplex families, suggesting differential mechanisms of genetic transmission of autism in the population.^17,18

Genotype/phenotype correlations

One of the most important remaining unsolved issues is the understanding of the relationships between genetic variation and phenotype, given the recent observations that identical mutations may be associated with highly divergent phenotype. Indeed, identical CNVs have been associated with autism and schizophrenia, notably 16p11 rearrangements.^15,40-43 STIANK3 and NRXN1 genes were also suggested to be involved in schizophrenia,^44-46 and genes implicated in autism and/or schizophrenia were significantly enriched in ADHD CNV genes in one study.⁴⁷ A first hypothesis to explain this phenotypic heterogeneity is that a secondary insult is necessary during development to result in the phenotype, as in the “twohit model” proposed in developmental delay.⁴⁸ Several studies recently supported the existence of such a combination of rare variants in some cases.^39,49,50 Another hypothesis is the contribution of both rare and frequent variants. This would be consistent with the observations of broader subthreshold traits in siblings.⁵¹ Although, as we have already mentioned, association studies have not provided clear evidence of the contribution of common variants in autism, a recent analysis of genetic variations associated with ASD suggests that common and rare variants contribute to ASD by perturbation of common neuronal networks.³⁵ The last hypothesis, which is not mutually exclusive with other hypotheses, is the contribution of environmental factors which modify the phenotype.

Sex ratio

Autism affects males four times more than females,⁵² and the cause for this difference is not well understood. Several theories have been proposed, among which the involvement of the sex chromosome in the etiology of ASD, and the role of hormonal influences in utero (for review see ref 53). However, none of these theories has been confirmed yet.

Intellectual disability

Intellectual disability (ID) is present in 65% to 75% ol individuals with a strict diagnosis of autistic disorder, and in 30% to 55% if all ASDs are considered.^54,55 Two different models are proposed to explain this overlap. The first model proposes that intellectual disability and ASD share common genetic bases, common genes causing a continuum of developmental disorders that manifest in different ways depending on other genetic or environmental factors. This model is supported by the observation that all recurrent genetic defects reported in autism, including autism without mental retardation, have been causally implicated in intellectual disability,⁵⁶ and that analysis of the genes affected by rare CNVs reveal that they are strongly functionally related to genes previously implicated in intellectual disability.^33,34 Hie second model assumes that in patients with intellectual disability, the general cognitive disability unmasks the limitations in the individual's capacity for social reciprocity.⁵⁷ This model is supported by evidence that indicates a continuous distribution of autistic traits in normal population⁹ and etiological similarity across ASD and autistic traits in the general population.⁵⁸ However it is tempered by the results of a recent large-scale CNV study showing a strong effect of large rare genie de novo CNVs on the presence or absence of an ASD diagnosis, but did not support IQ as a useful predictor for probands carrying these risk variants.³⁹

Prevalence

Prevalence studies of autism spectrum disorders conducted in recent years have been the source of an important debate because of a steady and highly significant increase of estimates of the total prevalence of pervasive developmental disorders. Indeed, while the prevalence was estimated at 6 per 1000 in a population of school children in 2005,⁵² recent studies have gone so far as to estimate the prevalence to be one child in 38.⁵⁹The last prevalence estimates in the United States, released by the Centers for Disease Control recently,⁶⁰ reached 1 in 88 child in 2008, while their previous estimate was one in 110 in 2006. However, most of the studies are not comparable in method or in the populations studied. One hypothesis is that this increase is the result of enlargement of diagnostic criteria, and the growing importance of screening for ASDs. The results of an epidemiological study from England, based on a national sample from 2007, support this hypothesis. Indeed the authors found a rate of about 1% in adults across the entire age range, without a significant reduction in the older part of the sample, as one would expect if the prevalence had increased in recent years.⁶¹ However, another study suggested that diagnostic substitution, especially for the most severe cases, and better ascertainment, especially for children at the less severe end of the spectrum, explain only a part of the linear increase observed in the California registry.⁶²

While the hypothesis of an increased incidence in relation to environmental factors could not be confirmed nor excluded definitely, studies using the same protocol several years apart are required.⁶³ Nevertheless, it seems reasonable to think that there may be both a real increase in the number of cases and an increase in the detection of affected children, and one should not wait for the results of these studies to search for environmental factors increasing risk for autism.

Immune dysfunction

Several lines of evidence support the hypothesis of immune changes in autism. First, several studies have shown abnormalities in the peripheral immune system such as T-cell dysfunction, autoantibody production, increase in the number of activated B cells and NK cells, and increase in proinflammatory cytokines.^64-66 Moreover, a landmark study provided evidence for microglial and astroglial activation in brain of patients with ASD.⁶⁷ The most prominent microglial reaction was observed in the cerebellum and cerebral white matter. The authors also found, in the cerebrospinal fluid of other patients, an increase of proinflammatory and modulatory cytokines. Another study consistently reported microglial activation in the dorsolateral prefrontal cortex in brains of patients with ASD.⁶⁸ This neuroglial response may result from either a primary disturbance of neuroglial function or unknown factors that disturb prenatal or postnatal CNS development.

Transcriptome

The first comprehensive gene-expression analysis of brains of patients with ASD recently reported differences in transcriptome organization between autistic and normal brain.⁶⁹ The measure of messenger RNA levels, using Illumina microarrays, in three regions of post mortem brains from patients with autism and controls, showed 444 genes differentially expressed between the cerebral cortices of the autistic and control brains. Moreover, the authors identified two discrete modules of coexpressed genes associated with autism. While the first module, which is related to synaptic function and neuronal projection, was underexpressed in autism cases, the second module, which was enriched for immune genes and glial markers, was overexpressed. These results are consistent with the findings mentioned above, implicating synaptic dysfunction as well as immune dysregulation in autism. Interestingly, the first module shows a highly significant enrichment for variants genetically associated with autism, further supporting the genetic basis of synaptic dysfunction in ASD. On the contrary, the authors did not find any evidence for a genetic etiology for the upregulation of the genes of the second module, suggesting that it is probably a nongenetic, adaptive, or environmental process.

Prenatal and perinatal factors

A recent meta-analysis of prenatal factors, limited to pregnancy-related factors, identified few significant risk factors.⁸¹ The main factors are maternal gestational diabetes, maternal bleeding during pregnancy, and maternal medication. The latter issue will be further discussed later. Moreover, increased risk was also found in this meta-analysis for first-born children compared with children born third or later, and, in Nordic countries, for offspring of mothers born abroad. Exposure to intrauterine infections was associated with a significant increase in risk for autism in the analysis limited to the four studies that controlled for multiple covariates or used sibling controls. Hie association between maternal infection and autism risk is further supported by the results with rodent models of the maternal infection. In these animal models, gestational viral infection is mimicked by systemic administration of Poly I:C, a synthetic doublestranded RNA, which elicits an innate immune response. It seems that gestational viral infections trigger a maternal immune response, which can perturb fetal brain development, at least in part through interleukin-6.⁸²

In another meta-analysis focusing on the perinatal and neonatal period,⁸³ the same authors identified several potential risk factors, the main being fetal presentation, umbilieal-cord complications, fetal distress, birth injury or trauma, multiple birth, maternal hemorrhage, summer birth, low birth weight, small for gestational age, low 5minute Apgar score, meconium aspiration, neonatal anemia, ABO or Rh incompatibility, and hyperbilirubinemia. Feeding difficulties and congenital malformation that are also mentioned should rather be considered as symptoms of an underlying cause of autism. The identification of summer birth as a risk factor is consistent with the results of a recent study showing that maternal infection in the first trimester increases autism risk.⁸⁴

Overall, preterm birth was not associated with the risk of autism. However, a recent study based on rigorous diagnostic assessment using validated instruments suggested an association between preterm birth and risk for ASD consistent with the results of most previous prospective studies.⁸⁵ This study estimated the prevalence to be 5% in adolescents who had a birth weight <2000 g in the US, which is significantly greater than the last national prevalence estimates. Most of these subjects were born preterm (96.7%); however, 32.3% were small for gestational age and the authors did not use multivariate analyses to simultaneously control for birth weight and gestational age. This methodological issue concerns most of the studies on perinatal and neonatal risk factors, which makes it difficult to interpret the results of these studies, since many of the events studied are likely to occur at the same time. Another limitation to the interpretation of the results is that some studies have suggested that increased rates of birth and pregnancy complications are likely secondary to familial factors associated with autism.⁸⁶

Socioeconomic status

Although one study did not find any association between risk of autism and socioeconomic status⁸⁷ including maternal education level, the latter may significantly influence the age of first single words.⁸⁸ Moreover, as already mentioned, autism risk was found to be significantly increased for the offspring of mothers born abroad in a meta-analysis.⁸¹ This risk was further specified in a very recent study⁸⁹ showing that children of migrant parents are at an increased risk of autism with intellectual disability, especially when parents migrated to Sweden from regions with a low human development index, and a decreased risk of high-functioning autism. The risk for low-functioning autism peaked when migration occurred around the time of pregnancy. Different mechanisms can be proposed to explain these results, such as the high level of maternal stress or low immunity regarding common infections.

Drugs and toxic exposure

As previously mentioned, exposure to medication during pregnancy was found to increase autism risk in the most recent meta-analyses.⁸¹ Prenatal exposure to valproate is a recognized risk factor for ASD, especially in the first trimester of pregnancy. Children exposed in utero to valproate have 8-fold increased risk to have ASD.⁹⁰ Interestingly, a downregulation of NLGN3 was observed in hippocampal subregions and the somatosensory cortex of mice prenatally exposed to valproate.⁹¹ Moreover, one ol the major concerns regarding medication exposure during pregnancy concerns the use of antidepressants, since selective serotonin reuptake inhibitor medication during pregnancy increased from 1.5% in 1996 to 6.4% in 2004 and 6.2% in 2005.⁹² It was suggested that antidepressant exposure during pregnancy modestly increases the risk of ASD, especially in the first semester.⁹³ Lastly, exposure in utero to an organophosphate insecticide, chlorpyrifos, was found to increase ASD risk and it was suggested that synthetic chemicals should be far more explored.⁹⁴