Snapshot: What is recessive ataxia?

What is a recessive disorder?

A recessive disorder is one that has a specific disease mechanism. For a recessive disorder to occur, both copies of the causative gene must be mutated for a patient to show symptoms.  Ataxias that follow this disease mechanism are known as recessive ataxia. However, having a mutation in only one copy of the gene does not lead to a disorder. As people with only one mutated copy of the gene can pass on the defective gene, these people are known as an unaffected carrier. Recessive ataxias range in symptoms and severity but are linked by their disease mechanism. While none of the Spinocerebellar Ataxias (SCAs) are recessive, there are many types of recessive ataxias, including Autosomal Recessive Cerebellar Ataxia Type 1 and 2 (ARCA1 and ARCA2), Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay (ARSACS), Friedreich’s Ataxia, and Ataxia Telangiectasia. For example, Friedreich’s Ataxia is caused by a trinucleotide repeat expansion in the frataxin (FXN) gene. People with only one expanded copy of the FXN gene do not show any symptoms, while people with two expanded copies of the FXN gene are affected by Friedreich’s Ataxia.

How are recessive ataxias inherited?

For every gene in our body, we have two copies, one that is inherited from our mother and one from our father. Both parents of an affected individual have to have at least one copy of the mutation for a child to be born with a recessive disorder. If both parents are unaffected carriers, each child will have a 1 in 4 chance of getting the disorder.

For a patient affected with a recessive ataxia, the chances of having a child affected by the same disorder are low. For a patient to pass on the disease, their spouse must have at least one mutated copy of the causative gene. In the case where a patient’s spouse is a carrier, children have an equal chance of being an unaffected carrier or being affected by the disease. However, carrier rates for ataxias are low in the population, which makes it unlikely that a patient’s spouse is also a carrier for the ataxia mutation.

Map showing the statistical chance of two unnafected carrier parents passing on a mutated gene (25% unaffected child, 50% carrier child, 25% affected child) or an affected parrent and unaffected carrier (50% carrier child, 50% affected child)
How recessive disorders are inherited. Image by Eder Xhako, created with BioRender

How can a patient prevent passing on a recessive disorder to their children?

Generally, when a patient with recessive ataxia passes on the disorder to their children, their spouse is an unaffected carrier. If you are a patient with a form of recessive ataxia and are thinking about having children, your spouse can undergo carrier testing to find out if they are a carrier for the same recessive ataxia. This will determine the likelihood that the recessive ataxia is passed on to your children. If it is determined that the spouse is a carrier, options like IVF with embryo screening can help patients prevent passing on recessive ataxia to their children.

If you would like to know more trinucleotide repeat expansions, you can look at our past Snapshot on Polyglutamine Expansion.

If you would like to learn more about carrier and embryo screening, take a look at these resources by the American College of Obstetricians & Gynecologists and Integrated Genetics.

Snapshot written by Eder Xhako and edited by Larissa Nitschke.

Hunting for a needle in a haystack: Scientists identify the gene that causes ARSACS

Written by Dr. Sriram Jayabal Edited by Dr. Brenda Toscano-Marquez

Scientists uncover SACS, a gene containing the largest exon identified in vertebrates, which leads to ARSACS when mutated.

What is your morning routine? Coffee first, right? Now, try to think of all the diverse movements you need to make to accomplish this routine. For instance, just to get a cup of coffee, you have to complete a sequence of motor tasks: you start by pulling the pot out of the machine, then you walk to the tap, fill the pot with water, walk back, pour the water into the machine, put your coffee in, and then finally turn on the machine.

needle in haystack
Picture courtesy of Pixabay

To perform any of these movements, your brain needs to communicate with dozens of muscles in your body. Unfortunately, in people who are affected by hereditary ataxias, the brain loses the ability to coordinate these precise movements. These diseases primarily affect the way patients walk (the symptom that defines “ataxia”), eventually forcing them to use a wheelchair for the rest of their lives.

Hereditary ataxias can be broadly classified as either dominant or recessive. Dominantly-inherited ataxias can be passed down even if only one of the parents is affected; therefore, the disease does not skip generations. Recessive ataxias are inherited from parents who are both carriers of the disease mutation (and who do not usually show any symptoms). Therefore, recessive ataxias can skip generations.

One such recessive ataxia is called Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay (ARSACS). It was first discovered in people from the Charlevoix-Saguenay-Lac-Saint-Jean region of Quebec, Canada [1]. In this region, it is estimated that one out of every 22 individuals is a carrier for the disease mutation [2]. Though prevalent in this specific area of Canada, ARSACS has now been identified all across the world. Symptoms usually start in early childhood when toddlers are learning to walk. These children experience stiffness in the legs (spasticity) and incoordination in their gait (ataxia), leading them to fall more often. They also have difficulties writing, speaking, and performing tasks that require manual dexterity (usually actions that involve hand movements, like reaching for and grasping an object). They continue to experience worsening gait as they age, often needing a cane or handrail to move around by the time they reach adolescence. Around this time, many patients also experience retinal hypermyelination (an eye abnormality) and peripheral neuropathy (damage to the nerves throughout the body). By their thirties, they become dependent on a wheelchair. There is currently no cure for ARSACS, so it is imperative to study this disease’s underlying causes to identify effective treatments.

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