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Repeat DNA and Autism Spectrum Disorder

What is the Repeatome?


The Repeatome makes up approximately half of the human genome. It consists of repetitive DNA sequences and more than a million Tandem Repeats. Tandem-Repeats are sections of DNA where a sequence is replicated many times in tandem. A lot of information about these repetitive sequences is mostly unknown, but we know that they significantly correlate with diseases and disorders due to Tandem Repeat Mutations.


Examples of Tandem-Repeat Mutations


Scientists have identified over 50 diseases as being caused by tandem-repeat sequences in a single gene. Healthy people all have a certain amount of Short tandem repeats (STR). Still, STRs in mutant genes are unstable, causing the number of repetitions to expand into really long DNA sequences that affect a disease's pathology.


Huntington's Disease and Fragile X Syndrome are both the result of single-gene Tandem-Repeat mutation.


For more complex genetic details in polygenic diseases, the role of tandem-repeats is still being further explored. Polygenic diseases are those that are caused by more than one gene. There is evidence that Autism Spectrum Disorder could be an example of a Tandem-Repeat mutation.


Image Courtesy of Nature.com.


What is ASD?



Autism Spectrum Disorder refers to a range of conditions characterized by challenges with social skills, repetitive behaviours, speech and nonverbal communication as well as by unique strengths and differences.

- Autism Speaks, on the symptoms and effects of ASD


The 2018 National Autism Spectrum Disorder Surveillance System (NASS) Report estimates ASD prevalence as 1 in 66 children in Canada, or around 1.5%.


Study by Mitra et al.


To further explore Tandem-repeats concerning polygenic diseases, Mitra et al. used the bioinformatics tool "MonSTR" to examine tandem-repeats in those with ASD. This tool helped find that those with ASD were more likely to have tandem-repeat mutations than their siblings without ASD. Mutations expand DNA regions that affect fetal brain development.


Using another tool bioinformatics tool, "SISR," they analyzed the likelihood that these are mutations harmful. This research led to the discovery that 25 mutations, present in those with ASD, were predicted to be the most harmful. The 25 tandem-repeat mutations were found in genes already known to be associated with ASD, supporting previous research on the condition.


There was a previous study done by Totra et al. selecting people at random with or without ASD. Compared to Trost's research, Mitra's method was to compare siblings, therefore, examining how many mutations were inherited from their parents. They used correlative evidence as opposed to direct evidence of the 25 mutations relating to ASD.

The strength of the study performed by Mitra and colleagues is demonstrated by the evidence observed that tandem repeats play a role in the genetics of ASD.


Future Implications


Along with Mitra et al.'s study, there is a lot of positive news with the progression and development of bioinformatics tools alongside therapeutic tools like CRISPR gene-editing techniques. These tools will be useful for future studies, allowing researchers to understand better and correct mutations for patients' betterment.


References


Autism Speaks. (n.d.). Retrieved February, 2021, from https://www.autismspeaks.ca/about/about-autism/

Hannan, A. (2021, January 13). Repeat dna expands our understanding of autism spectrum disorder. Retrieved February, 2021, from https://www.nature.com/articles/d41586-020-03658-7

Mitra, I., Huang, B., Mousavi, N., Ma, N., Lamkin, M., Yanicky, R., . . . Gymrek, M. (2021, January 13). Patterns of de novo tandem repeat mutations and their role in autism. Retrieved February, 2021, from https://www.nature.com/articles/s41586-020-03078-7



Article Contributors: Mina Chong, Edie Whittington

Article Editor: Olivia Ye