Surprisingly, the zebrafish has been helping scientists decode the connection between autism and sensitivity to sound. For the study, Queensland Brain Institute associate professor Ethan Scott and Dr Lena Constantin compared genetic mutations in humans and zebrafish.
They took fish with the same genetic makeup as humans with Fragile X syndrome and autism. When they compared neural networks and the pathways they cause sensitivity to sound, the researchers discovered striking similarities.
“Loud noises often cause sensory overload and anxiety in people with autism and fragile X syndrome; sensitivity to sound is common in both conditions, ”said Dr. Constantin.
“We think the brain is transmitting more auditory information because it is filtered and adjusted differently.”
“Half of the men and a fifth of the women with fragile X syndrome also meet the diagnostic criteria for autism spectrum disorder.”
Dr. Gilles Vanwalleghem and Rebecca Poulsen, also from the Queensland Brain Institute, contributed to study how the zebrafish interprets its world. They wanted to examine how neurons work together to process stimuli and information.
Before continuing, let’s discuss some of the symptoms and characteristics of both Fragile X syndrome and autism. Both involve sensitivity to sound but they have some key differences.
What is fragile X syndrome?
The FMR1 gene, located on the X chromosome, makes a protein called FMR, which helps nerve cells communicate. A child’s brain needs this protein for normal development. Children with fragile X syndrome do not make this protein; if they have another fragile X chromosome disorders, they can still produce some RMF.
What causes it?
People with this condition have too many copies of a segment of DNA known as CGG. In most people without the disease, this segment repeats between 5 and 40 times. In people with fragile X, it recurs more than 200 times. The more times the DNA segment is repeated, the more severe the symptoms.
Children with a mother who carries the FMR1 gene mutation have a 50% chance of developing the condition. A father can only pass Fragile X to his daughters. Boys are more likely to develop Fragile X than girls and tend to have worse symptoms. This occurs because girls have two copies of the X chromosome, so even if one has the genetic change, the other may not.
Children have one X and one Y chromosome. So if the X chromosome carries the gene mutation, they will have Fragile X syndrome. However, some people can inherit the condition without showing symptoms. Called carriers, they can still pass the genetic mutation on to their offspring.
Symptoms of fragile X syndrome
- Difficulty learning developmental skills like sitting, crawling, or walking
- Language and speech difficulties
- Shake hands and avoid eye contact
- Poor impulse control
- Sensitivity to sound, as well as high sensitivity to light.
- Hyperactivity and difficulty paying attention.
- Aggressive and self-destructive behavior in children
Children can also have physical changes to their body, such as an enlarged head, a long, narrow face, and large ears. They may also have a large forehead and chin, loose joints, flat feet, and enlarged testicles (after puberty).
Characteristics and signs of autism
Autism, a developmental disorder that includes communication and behavior problems, also involves sensitivity to sound. Symptoms can range from mild to severe, and children with it may need special learning environments. Autism occurs four times more often in boys than in girls. Most of the time, parents will notice symptoms before the child is 3 years old.
Common symptoms of autism include the following:
- Not making eye contact
- A limited range of interests or an intense interest in certain subjects.
- Do something over and over again, such as repeating words or phrases, rocking back and forth, or moving a lever.
- High sensitivity to sound, as well as to touches, smells, or sights that seem normal to other people.
- Do not look or listen to other people.
- Not looking at things when someone else points them out.
- Rejection of hugs or human caresses.
- Problems understanding or using speech, gestures, facial expressions, or tone of voice.
- Speak in a singsong, flat, or robotic voice
- Problems adapting to changes in routine.
So how can zebrafish help us better understand autism and fragile X syndrome in humans? Experts reveal some fascinating discoveries about the similarities between neurons in humans and zebrafish.
“Fragile X syndrome is caused by altering a gene, so we can alter that gene in zebrafish and see the effects,” said Dr. Vanwalleghem.
“We can study the entire brain of zebrafish larvae under the microscope and see the activity of each brain cell individually.”
Dr. Constantin explained that the team recorded the brain activity of the zebrafish larvae while playing sound clips or showing them movies.
“The movies simulate movement or predators; the reaction to these visual stimuli was the same for fish with fragile X mutations and those without them, ”said Dr. Constantin.
“But when we gave the fish a burst of white noise, there was a dramatic difference in brain activity in the fragile X model fish.”
When they looked at how the sudden sounds affected the fish, the team designed a range of 12 different sound volumes. They found that fish with fragile X characteristics heard more subtle noises than other fish.
“Fish with fragile X mutations had more connections between different regions of their brain, and their responses to sounds were more abundant in the rhombencephalon and thalamus,” said Dr. Constantin.
The thalamus functions as a control center in the brain, sending sensory information from the body to parts of the brain. The rhombencephalon coordinates behavioral responses. Dr. Constantin says that the way neural pathways develop and respond to stimuli shows which parts of the brain are used and how we process sensations.
“With the zebrafish, we have been able to see much more detail and, for the first time, we have seen more activity in the rhombencephalon, which we are eager to explore further.”
“We hope that by discovering critical information about how the brain processes sound, we will learn more about the sensory challenges faced by people with Fragile X syndrome and autism.”