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When it comes to lifelong conditions like autism spectrum disorder, we tend to be biased in a way that obscures reality. Recently, two studies came out with seemingly contradictory findings on the cause of autism. One study by Bai et al. They came to the conclusion that 80 percent of the risk of developing an autism spectrum disorder ASD is due to genetics.

The other study, conducted by Abdelli et al. Following from this, their conjecture was that pregnant mothers consuming this common food preservative would be more likely to have ASD children. So, which is it? Is ASD mostly due to genetics or mostly due to environmental factors? ASD occupies a place of power in our psyche — one only has to look at the persistence of pseudoscientific beliefs like the long-debunked theory that vaccines cause autism. In addition, the idea that we have control over whether or not our children are healthy is inherently appealing.

Just tweak your diet and everything will turn out fine. If we were to only read the headlines, in the face of apparently contradictory findings such as these, we might feel tempted to simply select the finding that fits our world view best, but giving into this temptation simply doesn't reflect a mindset that's interested in objective reality.

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So, let's dive into the nitty-gritty. First, the Bai et al. Of this remaining 20 percent, only 1 percent was due to maternal factors, such as weight or diet. The Abdelli et al. PPA plays an important role in the nervous system, modulating cell signaling, but Abdelli et al. To test this, Abdelli et al. Untreated neural stem cells tended to differentiate equally into either neurons or glial cells. The latter are a kind of support cell for neurons; they don't do the "thinking" that neurons do, but they provide structure, supply nutrients, insulate neurons from each other, and destroy old neurons and pathogens.

The stem cells treated with PPA, however, tended to differentiate far more often into glial cells rather than neurons. This is significant because ASD brains have far more glial cells than neurotypical brains. Extrapolating from their laboratory evidence, Abdelli et al. Not really. It's true that Abdelli et al. This does seem to conflict with Bai et al. It's important to remember that Abdelli et al.

PPA's actual activity in the human body could very well be significantly different than its effect in the lab, potentially such that it winds up contributing to that 1 percent. Abdelli et al.

Autism Spectrum Disorder

What's more, Bai et al. The amount of PPA, for instance, that a mother consumes is not something that this study examined directly, and so it may be the case that environmental factors play a larger role than the reported 1 percent of variance. Autism was found to occur more often in families of physicists, engineers and scientists. A study of brothers and parents of autistic boys looked into the phenotype in terms of one current cognitive theory of autism.

The study raised the possibility that the broader autism phenotype may include a "cognitive style" weak central coherence that can confer information-processing advantages. A study in showed a positive correlation between repetitive behaviors in autistic individuals and obsessive-compulsive behaviors in parents.

It found that correlation for social impairment or competence between parents and their children and between spouses is about 0. Out of 64 siblings, 4 6. It has been suggested that the twinning process itself is a risk factor in the development of autism, presumably due to perinatal factors. Twin and family studies show that autism is a highly heritable condition, but they have left many questions for researchers, most notably. Also spontaneous mutations can potentially occur specifically in one twin and not the other after conception.

The most parsimonious explanation for cases of autism where a single child is affected and there is no family history or affected siblings is that a single spontaneous mutation that impacts one or multiple genes is a significant contributing factor. These mutations themselves are characterized by considerable variability in clinical outcome and typically only a subset of mutation carriers meet criteria for autism. For example, carriers of the 16p In this model, autism often arises from a combination of common, functional variants of genes.

Each gene contributes a relatively small effect in increasing the risk of autism. In this model, no single gene directly regulates any core symptom of autism such as social behavior. Instead, each gene encodes a protein that disrupts a cellular process, and the combination of these disruptions, possibly together with environmental influences, [58] affect key developmental processes such as synapse formation. In the low-risk families, sporadic autism is mainly caused by spontaneous mutation with poor penetrance in daughters and high penetrance in sons.

The high-risk families come from mostly female children who carry a new causative mutation but are unaffected and transmit the dominant mutation to grandchildren.

Genetics Significantly Influences Variability of Autism Occurrence

Several epigenetic models of autism have been proposed. An epigenetic model would help explain why standard genetic screening strategies have so much difficulty with autism. Genomic imprinting models have been proposed; one of their strengths is explaining the high male-to-female ratio in ASD. Though autism's genetic factors explain most of autism risk, they do not explain all of it. A common hypothesis is that autism is caused by the interaction of a genetic predisposition and an early environmental insult.

Some of these theories focus on prenatal environmental factors, such as agents that cause birth defects; others focus on the environment after birth, such as children's diets. All known teratogens agents that cause birth defects related to the risk of autism appear to act during the first eight weeks from conception , strong evidence that autism arises very early in development. In many cases the findings are inconclusive, with some studies showing no linkage. Alleles linked so far strongly support the assertion that there is a large number of genotypes that are manifested as the autism phenotype.

At least some of the alleles associated with autism are fairly prevalent in the general population, which indicates they are not rare pathogenic mutations. This also presents some challenges in identifying all the rare allele combinations involved in the etiology of autism. A study compared genes linked with autism to those of other neurological diseases, and found that more than half of known autism genes are implicated in other disorders, suggesting that the other disorders may share molecular mechanisms with autism.

Conciatori et al. Tischfield et al. In Rodier's original work, teratogens are considered to play a part in addition, and that the possibility remains open for a range of teratogens to interact with the mechanisms controlled by these genes unfavourably this has already been demonstrated using valproic acid, a known teratogen, in the mouse model. If the SHANK3 gene is not adequately passed to a child from the parent haploinsufficiency there will possibly be significant neurological changes that are associated with yet another gene, 22q13, which interacts with SHANK3.

Alteration or deletion of either will effect changes in the other. A deletion of a single copy of a gene on chromosome 22q13 has been correlated with global developmental delay, severely delayed speech or social communication disorders and moderate to profound delay of cognitive abilities. Behavior is described as "autistic-like" and includes high tolerance to pain and habitual chewing or mouthing [] see also 22q13 deletion syndrome.

This appears to be connected to the fact that signal transmission between nerve cells is altered with the absence of 22q SHANK3 proteins also interact with neuroligins at the synapses of the brain further complicating the widespread effects of changes at the genetic level and beyond. Mice with a neuroligin-3 mutation exhibit poor social skills but increased intelligence. It is also known to play a key role in both normal and abnormal development, such as cancer metastases.

A mutation of the gene, rendering it less active, has been found to be common amongst children with autism. The objective of the study was to locate specific brain cells involved in autism to find regions in the genome linked to autism susceptibility genes. The focus of the research was copy number variations CNVs , extra or missing parts of genes. Each person does not actually have just an exact copy of genes from each parent. Each person also has occasional multiple copies of one or more genes or some genes are missing altogether. Neurexin 1 is one of the genes that may be involved in communication between nerve cells neurons.

Neurexin 1 and other genes like it are very important in determining how the brain is connected from cell to cell, and in the chemical transmission of information between nerve cells. These genes are particularly active very early in brain development, either in utero or in the first months or couple of years of life. In some families their autistic child had only one copy of the neurexin 1 gene.


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Besides locating another possible genetic influence the findings were statistically insignificant , the research also reinforced the theory that autism involves many forms of genetic variations. A study implicated the neurexin 1 gene in two independent subjects with ASD, and suggested that subtle changes to the gene might contribute to susceptibility to ASD. A Neurexin 1 deletion has been observed occurring spontaneously in an unaffected mother and was passed on to an affected child, suggesting that the mutation has incomplete penetrance.

There is a large number of other candidate loci which either should be looked at or have been shown to be promising. Several genome -wide scans have been performed identifying markers across many chromosomes. A few examples of loci that have been studied are the 17q21 region, [] [] the 3p locus, [] PTEN, [] 15q Several of these genes appeared to be targets of MEF2 , [] [] one of the transcription factors known to be regulated by neuronal activity [] and that itself has also recently been implicated as an autism-related disorder candidate gene.

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Please help by editing the article to make improvements to the overall structure. December Learn how and when to remove this template message. This section needs expansion. You can help by adding to it. February Main article: Epigenetics of autism. Advances in autism genetics: on the threshold of a new neurobiology. Nat Rev Genet. The genetics of autistic disorders and its clinical relevance: a review of the literature. Mol Psychiatry. Autism: the quest for the genes. Expert Rev Mol Med. A broader phenotype of autism: the clinical spectrum in twins.

Genetics of autism: complex aetiology for a heterogeneous disorder. A case-control family history study of autism. J Child Psychol Psychiatry. Searching for ways out of the autism maze: genetic, epigenetic and environmental clues. Trends Neurosci. A review of gene linkage, association and expression studies in autism and an assessment of convergent evidence. Int J Dev Neurosci. Specific genetic disorders and autism: clinical contribution towards their identification. J Autism Dev Disord. The study of autism as a distributed disorder. Autism: highly heritable but not inherited.

Nat Med. Copy-number variations associated with neuropsychiatric conditions. Rare structural variation of synapse and neurotransmission genes in autism. Ethical implications of array comparative genomic hybridization in complex phenotypes: points to consider in research. Genet Med.

CDC Grand Rounds: Autism Spectrum Disorder

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PMC Lay summary : CSHL , Personality characteristics of the parents of autistic individuals. Broader autism phenotype: evidence from a family history study of multiple-incidence autism families. Autism occurs more often in families of physicists, engineers, and mathematicians". Is there a link between engineering and autism? The link between autism and skills such as engineering, maths, physics and computing: a reply to Jarrold and Routh. The Geek Syndrome. Wired Magazine December Retrieved on December 10, Exploring the cognitive phenotype of autism: weak "central coherence" in parents and siblings of children with autism: I.

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Frontiers | Autism Spectrum Disorder: Why Do We Know So Little? | Neurology

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Background

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