Autism

Research into the genetic risk for autism has mainly focused on how mutations that arise spontaneously in an individual’s genome—rather than being inherited from a parent—disrupt protein-coding regions and lead to the condition.

What is Autism? Symptoms, Causes, & Types

Researchers have known that genes contribute to autism since the 1970s, when a team found that identical twins often share the condition. Since then, scientists have been racking up potential genetic culprits in autism, a process that DNA-decoding technologies have accelerated in the past decade.As this work has progressed, scientists have unearthed a variety of types of genetic changes that can underlie autism. The more scientists dig into DNA, the more intricate its contribution to autism seems to be.<h2>How do researchers know genes contribute to autism?</h2>Since the first autism twin study in 1977, several teams have compared autism rates in twins and shown that autism is highly heritable. When one identical twin has autism, there is about an 80 percent chance that the other twin has it too. The corresponding rate for fraternal twins is around 40 percent.However, genetics clearly does not account for all autism risk. Environmental factors also contribute to the condition — although researchers disagree on the relative contributions of genes and environment. Some environmental risk factors for autism, such as exposure to a maternal immune response in the womb or complications during birth, may work with genetic factors to produce autism or intensify its features.<h2>Is there such a thing as an autism gene?</h2>Not really. There are several conditions associated with autism that stem from mutations in a single gene, including fragile X and Rett syndromes. But less than 1 percent of non-syndromic cases of autism stem from mutations in any single gene. So far, at least, there is no such thing as an ‘autism gene’ — meaning that no gene is consistently mutated in every person with autism. There also does not seem to be any gene that causes autism every time it is mutated."Still, the list of genes implicated in autism is growing. Researchers have tallied 65 genes they consider strongly linked to autism, and more than 200 others that have weaker ties. Many of these genes are important for communication between neurons or control the expression of other genes.<h2>How do these genes contribute to autism?</h2>Changes, or mutations, in the DNA of these genes can lead to autism. Some mutations affect a single DNA base pair, or ‘letter.’ In fact, everyone has thousands of these genetic variants. A variant that is found in 1 percent or more of the population is considered ‘common’ and is called a single nucleotide polymorphism, or SNP.Common variants typically have subtle effects and may work together to contribute to autism. ‘Rare’ variants, which are found in less than 1 percent of people, tend to have stronger effects. Many of the mutations linked to autism so far have been rare. It is significantly more difficult to find common variants for autism risk, although some studies are underway.Other changes, known as copy number variations (CNVs), show up as deletions or duplications of long stretches of DNA and often include many genes.But mutations that contribute to autism are probably not all in genes, which make up less than 2 percent of the genome. Researchers are trying to wade into the remaining 98 percent of the genome to look for irregularities associated with autism. So far, these regions are poorly understood.<h2>Are all mutations equally harmful?</h2>No. At the molecular level, the effects of mutations may differ, even among SNPs. Mutations can be either harmful or benign, depending on how much they alter the corresponding protein’s function. A missense mutation, for example, swaps one amino acid in the protein for another. If the substitution doesn’t significantly change the protein, it is likely to be benign. A nonsense mutation, on the other hand, inserts a ‘stop’ sign within a gene, causing protein production to halt prematurely. The resulting protein is too short and functions poorly, if at all.<h2>How do people acquire mutations?</h2>Most mutations are inherited from parents, and they can be common or rare. Mutations can also arise spontaneously in an egg or sperm, and so are found only in the child and not in her parents. Researchers can find these rare ‘de novo’ mutations by comparing the DNA sequences of people who have autism with those of their unaffected family members. Spontaneous mutations that arise after conception are usually ‘mosaic,’ meaning they affect only some of the cells in the body.&nbsp;<h2>Can genetics explain why boys are more likely than girls to have autism?</h2>Perhaps. Girls with autism seem to have more mutations than do boys with the condition. And boys with autism sometimes inherit their mutations from unaffected mothers. Together, these results suggest that girls may be somehow resistant to mutations that contribute to autism and need a bigger genetic hit to have the condition.<h2>Is there a way to test for mutations before a child is born?</h2>Clinicians routinely screen the chromosomes of a developing baby to identify large chromosomal abnormalities, including CNVs. There are prenatal genetic tests for some syndromes associated with autism, such as fragile X syndrome. But even if a developing baby has these rare mutations, there is no way to know for sure whether he will later be diagnosed with autism.Reference: https://www.spectrumnews.org/news/autism-genetics-explained/

As this work has progressed, scientists have unearthed a variety of types of genetic changes that can underlie autism. The more scientists dig into DNA, the more intricate its contribution to autism seems to be.

Autism genetics, explained

Autism - or more precisely the autism spectrum disorders (ASDs) - represent a broad group of developmental disorders characterized by impaired social interactions, problems with verbal and nonverbal communication, and repetitive behaviors or severely limited activities and interests.It has become the most commonly diagnosed childhood developmental disorder. According to the Centers for Disease Control Prevention in 2007, autism spectrum disorders now affect 1 in every 150 children in the United States. Statistics from the U.S. Department of Education and other government agencies indicate that autism diagnoses are increasing at the rate of 10 to 17 percent per year. Autism can affect any individual and is not based on ethnic, racial or social background. And, it is four times more common in boys than in girls.Scientists are not certain what causes autism, but it's likely that both genetics and environment play a role. The causes of autism may be divided into 'idiopathic', (of unknown cause) which is the majority of cases, and 'secondary,' in which a chromosome abnormality, single-gene disorder or environmental agent can be identified. Approximately 15 percent of individuals with autism can be diagnosed with secondary autism; the remaining 85 percent have idiopathic autism.Researchers have identified a number of genes associated with autism. Studies of people with autism have found irregularities in several regions of the brain. Other studies suggest that people with autism have abnormal levels of serotonin or other neurotransmitters in the brain. These abnormalities indicate that autism usually results from the disruption of normal brain development early in fetal development caused by defects in genes that control brain growth and that regulate how neurons communicate with each other. These are preliminary findings and require further study.The risk that a brother or sister of an individual who has idiopathic autism will also develop autism is around 4 percent, plus an additional 4 to 6 percent risk for a milder condition that includes language, social or behavioral symptoms. Brothers have a higher risk (about 7 percent) of developing autism, plus the additional 7 percent risk of milder autism spectrum symptoms, over sisters whose risk is only about 1 to 2 percent.When the cause of autism is a chromosome abnormality or a single-gene alteration, the risk that other brothers and sisters will also have autism depends on the specific genetic cause."Reference: https://www.genome.gov/Genetic-Disorders/Autism&nbsp;

Autism - or more precisely the autism spectrum disorders (ASDs) - represent a broad group of developmental disorders characterized by impaired social interactions, problems with verbal and nonverbal communication, and repetitive behaviors or severely limited activities and interests.

About Autism

Researchers from the national psychiatric project iPSYCH have found that autism and ADHD share changes in the same genes. The new knowledge relates directly to the biological causes of the two child psychiatric disorders.In Denmark, approximately one per cent of schoolchildren have autism and two to three per cent are diagnosed with ADHD. Autism and ADHD are different developmental disorders, but they can have certain common symptoms. For example, children with autism can demonstrate violent or aggressive behaviour, be impulsive, and have problems in school and with social relations -- and these same symptoms can be shared by children with ADHD.Researchers from iPSYCH, Denmark's largest research project in the field of psychiatry, have now discovered that the similarities between the two diagnoses can be linked to changes in the same genes. The new study is the largest study to date of rare mutations in the genome of people with ADHD and autism."The very fact that mutations are found to the same extent and in the same genes in children with autism and in children with ADHD, points towards the same biological mechanisms being involved," says Anders Børglum, who is professor at Aarhus University, principal investigator at iPSYCH and one of the leading researchers behind the study. The study was carried out in collaboration with the Broad Institute and Harvard.<h2>First time a gene is linked to ADHD and autism.</h2>The results have just been published in the international journal Nature Neuroscience.“This is the first time that the genome has been mapped so comprehensively for both ADHD and autism, and the discovery that children with ADHD have the same amount of deleterious gene mutations in their DNA as children with autism is both striking and quite surprising," says Anders Børglum.The findings point directly towards which biological causal mechanisms are shared by ADHD and autism."In the study, the gene that is most frequently affected by mutations in people with ADHD or autism is the so-called MAP1A gene. The gene is involved in the formation of the physical structure of nerve cells -- their inner 'skeleton', so to speak -- and is important for the development of the brain," explains Anders Børglum.This is the first time that this gene has been linked to the development of ADHD and autism, and the mutations found by the researchers give a significantly increased risk of developing autism and/or ADHD."We discovered an increased burden of mutations that destroy or severely affect the MAP1A gene in those with ADHD and autism, while very few of the control subjects had such changes in the gene," explains Anders Børglum.This means that the risk of ADHD and autism increases by more than 15 times for people who carry such a mutation. The researchers have analysed the genes of approximately 8,000 people with autism and/or ADHD, and 5,000 people without either of the two disorders, all from Denmark. The study also incorporated additional data from approx. 45,000 international control subjects who did not have any psychiatric disorders."The study shows that many more genes for ADHD and autism can be identified directly by studying more people in a similar way with extensive DNA sequencing, thereby providing a more complete picture of the biological causal mechanisms and possible approaches to medical treatment," explains the researcher.As part of the project, Anders Børglum and his colleagues have had access to samples from the Danish National Biobank at the Statens Serum Institut and to data from the Danish Psychiatric Central Research Register. People born in Denmark between 1981 and 2005 were matched with diagnoses of autism, ADHD and mental retardation.Resource: https://www.sciencedaily.com/releases/2019/11/191126121153.htm&nbsp;

Researchers from iPSYCH, Denmark's largest research project in the field of psychiatry, have now discovered that the similarities between the two diagnoses can be linked to changes in the same genes.

Autism and ADHD share genes

Hundreds of gene variations were identified as increasing the risk of autism. Nonetheless, those variants just from the encode part, which directly codes for proteins. Now, a new study probing so-called noncoding DNA has found that alterations in regions that regulate gene activity may also contribute to autism. And surprisingly, these variations tended to be inherited from fathers who aren’t autistic.Research into the genetic risk for autism has mainly focused on how mutations that arise spontaneously in an individual’s genome—rather than being inherited from a parent—disrupt protein-coding regions and lead to the condition. That’s because these sporadic mutations have relatively large effects and studies have shown that such mutations, although individually rare, together contribute to about 25% to 30% of cases. But only about 2% of the genome consists of protein-coding areas. Sebat says the large noncoding portion of our DNA—often previously referred to as “junk DNA”—has so far been ignored in autism research.Sebat’s team was especially interested in the parts of noncoding DNA that regulate gene expression. They looked at whole-genome sequences from 829 families that included autistic individuals, their nonautistic siblings, and their parents.&nbsp;And the team found that mothers passed only half of their structural variants on to their autistic children—a frequency that would be expected by chance alone—suggesting that variants inherited from mothers were not associated with autism. But surprisingly, fathers did pass on substantially more than 50% of their variants. This suggests that autistic children might have inherited risk variants in regulatory regions from their fathers but not their mothers, the researchers reported in Science.To check that this result, Sebat’s team then tested a second, larger sample of 1771 families. Once again, autistic children received more structural variants from their fathers but not mothers—though the size of the effect wasn’t quite as large in this second sample.&nbsp;Based on these results, Sebat proposes a more complex model of how autism arises, in which mothers pass on mutations affecting coding regions, which have large effects that women are protected from, while fathers pass on variants affecting noncoding regions; their effects are much more moderate and may only cause symptoms when combined with risk variants from mothers.Reference: https://www.sciencemag.org/news/2018/04/autistic-children-may-inherit-dna-mutations-their-fathers

Now, a new study probing so-called noncoding DNA has found that alterations in regions that regulate gene activity may also contribute to autism. And surprisingly, these variations tended to be inherited from fathers who aren’t autistic.

Autistic children may inherit DNA mutations from their fathers

Most of the risk for autism comes from genes. Mutations in more than 100 genes are known to lead to the condition. Nonetheless, a genetic test cannot diagnose or detect autism. That’s because it is the combination of myriad genes along with environmental factors that may underlie the condition.For example, only about one in four people missing a stretch of chromosome 16 called 16p11.2 has autism. This and other mutations are also associated with other conditions, such as epilepsy or intellectual disability.<h2 class="ql-align-justify">Autism is primarily a genetic condition</h2>Still, if a test reveals a harmful mutation with known ties to autism, the result could give the autistic person and her family an explanation for the condition. Some families also find emotional and practical support from others dealing with the same mutation.There are no drugs tailored to particular autism mutations. But the mutations are often linked to other health problems, such as epilepsy, kidney problems or obesity, so having the information could help prevent or treat those problems.There are four main types of tests for this risk-finding. The oldest is karyotyping, the inspection of chromosomes under a microscope. The second type is chromosomal microarray analysis, which identifies duplications or deletions of DNA too small to show up on a karyotype. The third way is sequencing or scanning for mutations across single genes. The forth is to sequence all of a person’s protein-coding DNA, or the exome.Clinicians may sequence the exomes of both parents and the child to find mutations present in only the child. These spontaneous mutations are more likely to contribute to autism than are inherited ones.Sequencing the entire genome is the most thorough method. It reveals mutations in any part of a person’s genome, not just the 1 percent that includes genes. However, it is still only a research tool; but as its price falls and becomes more widely available, it could replace the other tests.Reference: https://www.spectrumnews.org/news/genetic-testing-autism-explained/

Some families also find emotional and practical support from others dealing with the same mutation.

Autism

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