Autism spectrum disorder is a complex, behaviorally defined neurodevelopmental disorder that affects an estimated 1 in 59 children in the United States.
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It was first described in reports from Kanner who observed children with similar symptoms such as improper facilitation of language, obsessive behaviors, and indifference to people and social interactions.
A lot of research since then has been carried out to determine the causes of autisms. There has been a lot of controversy surrounding the interplay between genetic and environmental factors.
What is autism?
Autism is a heterogeneous condition caused by the interaction of inheritable and de novo gene variations. The core symptoms of autisms are related to deficits in social interaction and communication.
In addition to this, stereotypical and repetitive behaviors are commonly seen in children with autism. Cohort and twins studies have recently begun to establish the role of genes in the etiology of autism. Due to the heterogeneity of the condition, research has reported the involvement of thousands of genes associated with autism.
Genetics, not environment
Historically, there is always a dispute revolving around the interaction between nature and nurture and how much each contributes to the development of autism. Recent studies have begun to acknowledge genetics as the predominant factor in the etiology of autism.
A recent study conducted by researchers from Columbia University, New York has looked into the causes of autism. This is the largest study up to this date, with a cohort of 2 million children born in Denmark, Sweden, Finland, Israel, and Western Australia, of which 22 000 developed autism.
Results from this research showed that 80 % of the risk for developing autism was accounted for by genetic factors as compared to environmental influences which accounted for only 19 % of the risk. However, these environmental factors are not yet identified and further research is needed to elucidate this. The remaining 1 % of the risk for developing autism was due to maternal factors.
The results from this large scale research are in line with previous findings from smaller studies, further increasing their validity.
Genes related to autisms
The advancement of genetic research techniques has spurred interest at the genome-wide level. Research has demonstrated that the risk of developing autism was proportional to the percentage of the genome shared with a sibling or a parent who had autism.
Another large genetic sequencing study conducted by researchers from Harvard University and the Translational Genetics Unit at Massachusetts General Hospital identified 102 genes linked to the development of autism. The research looked at 37,269 genetic samples worldwide.
Out of these 102 genes, 47 were more related to intellectual disability and developmental delay than autism. The other 52 genes were more strongly associated with autism. Three of the genes were related to both intellectual disability and autism.
Copy number variations and autism susceptibility
Copy number variations are structural variants in chromosomes. They can be inherited or occur de novo. Many genes can be affected by these changes, not necessarily resulting in the development of a disease.
However, many studies have discovered a higher number of rare, genic copy number variations in individuals with autism, thus implicating the role of copy number variations in the pathology of autism. Current research estimates conclude that 10% of the risk of developing autism is caused by these variations.
Somatic mosaicism and autism risk
Somatic mosaicism in the occurrence of two genetically distinct populations of cells within an individual driven by postzygotic mutation. This process has been implicated in the pathology of many neurodevelopmental diseases including autism.
A large cohort study found that these pathogenic variations were most frequently expressed in the amygdala which is an area of the brain involved in emotional response and social awareness.
The sequencing studies of post-zygotic mutations have confirmed the implication of genes in autism and have established the significant role of somatic mosaicism in the etiology of autism.
The interplay between genes and environment
Even though autism susceptibility is estimated to be 60- 80% genetic, the rest of the risk for developing autism is accounted for by environmental factors.
Many environmental factors can play a role in the development of autism such as maternal complications, increased parent age, infections during pregnancy and prenatal exposure to anticonvulsants which is a medication for epilepsy.
Valproic acid which is constituent in anticonvulsants has been linked to the autistic phenotype. Animal studies have shown that valproic acid causes delayed neurogenesis and an increase in the brain-derived neurotrophic factor which impairs synapse formation.
However, further research is still needed to uncover the exact mechanisms through which the environment modulates the risk of developing autism.
Concluding remarks
The findings from genetic studies on autism help improve our understanding of biology and the inheritance of this condition.
However, there is still grounds for more research to elucidate how environmental factors and genetic predisposition influence the development of autism.
References
Further Reading
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