Is autism a problem of too many connections in the brain?

Autism is a disorder characterized by impairments in social relationships, trouble communicating, behavioural problems, dependence upon a routine and others. Autism is not a single disease (like say cystic fibrosis) but instead is made up of a variety of disorders, like Asperger’s syndrome or Pervasive development disorder, which are collectively termed Autism Spectrum Disorder (ASD). ASD is thought to affect nearly 1 in every 68 children with boys being 5 times more likely to develop ASD. ASD is thought to be caused by developmental alterations in the neurons that make up the brain and while there is no current known cause of autism (certainly not vaccines) there have been some recent advancements looking into the genetic factors associated with ASD. One such study, published in Neuron in August indicates that people with ASD may have too many connections within their brain. Having larger brains is often associated with greater intelligence (at least according to humans), so why would too many connections within the brain be a bad thing? To answer this we need to understand some basics about neurons. 

Neurons are the work horse of the brain responsible for transmitting information around the brain and the body and humans are estimated to have around 86 billion neurons in the adult brain. Neurons are composed of a cell body, which contains all the normal things found within a cell including the DNA and can be thought of as the processing centre for information received by the neuron, dendrites, which bring electrical signals from other neurons in the brain to the cell body for processing, and axons, which send information to other neurons or organs from the cell body. Axons allow neurons to communicate to other neurons or cells in the body through what are called synapses. Synapses are small gaps across which molecules called neurotransmitters can flow to signal other neurons or cells. In adult human brains, we have about 100 trillion synapses. In young human brains (before the age of 3) we have about 1 quadrillion (or 1000 trillion) synapses. So why do we lose synapses? The brain actively “prunes” weak or unused synapses out as we get older and learn. In fact it is thought that learning is the process by which the brain gets rid of old useless connections and strengthens others. The reason the brain does this is similar to the reason it’s difficult to take directions on a task from too many people. Picture this, you are asked to draw a picture by listening to descriptions of the scene from 100 people. Some of those people will give detailed descriptions, some will not and some may even give you wrong descriptions. If you listened to all 100 people you would have a hard time drawing the picture and would likely get frustrated and give up. However, if you only listened to the people who were giving good descriptions or who had proven experience with art, you would probably have an easier time drawing the picture and would be less likely to get frustrated. This is similar to your brain. If your brain listened to all 1 quadrillion of its connections it would have hard time organizing itself. However, if it got rid of connections that are rarely used and listened to the signals from those connections that get used frequently, it would be able to complete a task much quicker. As the saying goes, too many cooks in the kitchen spoils the broth.

So what does the research show? The researchers from New York saw that while children with ASD had the same number of synapses at birth as the non-ASD children, the non-ASD children pruned their synapses faster than the ASD children so that by the age of 20, the ASD children had nearly twice the number of synapses as the non-ASD children. This has been supported by previous research showing that mice who have a defect in synapse pruning, and therefore too many synapses, show behaviours similar to people with ASD including deficits in social interaction and increased repetitive-behaviours. The researchers were also able to show that an overactive protein, called mTOR, was responsible to the loss of normal pruning. When the researchers targeted this protein with a drug, rapamycin, the defective pruning was fixed and the ASD-like behaviours in the mice were corrected. This was exciting because it is a targetable protein that has an effect on ASD like behaviours in mice. This drug is currently used to help prevent transplant rejections but does have a number of side effects that would not warrant its use in ASD treatment. However, future research could work to identify a drug that could target mTOR in the brain of children identified early to be at risk for ASD. If that happens, it would represent one of the first drugs to directly target what may be one of the causes of ASD in people. Cautious optimism should be used however, since that is probably still a ways off. Research should also be focused on whether there is a genetic variant in the people with ASD that causes their mTOR to be more active than normal. Already some proteins that affect mTOR function have been associated with ASD like disorders, further suggesting that ASD is genetically related. This research is a good step forward in understanding the cause of ASD as well as offering up a potential target for treatment or prevention. Hopefully we can start focusing our funding on important research like this and not on debunking the current myths and garbage fears surrounding autism and ASD.

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3 thoughts on “Is autism a problem of too many connections in the brain?

  1. Pingback: Pollution, pregnancy and the link to autism | Science Translation

  2. Pingback: Excessive trimming of the brains connections may underlie schizophrenia | Science Translation

  3. Pingback: Autism is associated with an increase in inflammation during second trimester. | Science Translation

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