SCAR9 Fact Sheet
Ataxia:
SCAR9 (Spinocerebellar Ataxia Autosomal Recessive Type 9)
RELATED GENES:
ADCK3 (COQ8A)
MUTATION TYPE:
ADCK3 -> Pathogenic variants
LOCATION:
Chromosome 1 (1q42.13)
HERITAGE:
Autosomal Recessive
LAST UPDATE:
September 23, 2025 by Marcio Galvão
Content generated with the support of Generative AI, reviewed by the author.
1. ABOUT SCAR9
Spinocerebellar Ataxia Autosomal Recessive Type 9 (SCAR9) , also known as Autosomal Recessive Cerebellar Ataxia Type 2 (ARCA2), is a rare neurological disorder characterized by primary coenzyme Q10 (CoQ10) deficiency . This condition results from mutations in the COQ8A gene (also referred to as ADCK3), located on chromosome 1 (1q42.13), which encodes an atypical kinase involved in the biosynthesis of CoQ10.
Note 1 - In this file, we will refer to SCAR9 ataxia, but the expression "primary coenzyme Coq10 deficiency" may refer to other conditions associated with reduced Coq10 levels in tissues or cells, caused by mutations (pathogenic variants) in any of the ten genes involved in the biosynthesis of coenzyme Coq10, producing different clinical manifestations (COQ2, COQ4, COQ5, COQ6, COQ8A (ADCKE), COQ8B, COQ9, PDSS1, PDSS2). For a general review of all genetic diseases that produce reduced Coq10 levels, and their varied symptoms, see [1] in References .
Coenzyme CoQ10 may also be called Ubiquinone, Ubidecarenone, CoQ or just Q10.
Figure 1 – Image generated by the author with support from Artificial Intelligence
Cellular Respiration
To better understand the relationship between the ADCK3 gene, Coenzyme Q10 (CoQ10), and the effects of the genetic mutation, we present below a simplified explanation of the cellular respiration process. In short, cellular respiration is the third stage (*) of a highly complex process called the respiratory chain, in which cells produce energy from glucose in the presence of oxygen [2]. More precisely, this process occurs in the inner membrane of the mitochondria, organelles found inside cells. In this stage, electrons are removed from hydrogen atoms and transported along the respiratory chain (also known as the electron transport chain). As they are transported, the electrons lose energy, and this released energy enables the production of ATP (adenosine triphosphate)—the main molecule responsible for supplying the chemical energy necessary for the essential reactions that sustain life within the cells.
(*) The first stage of the respiratory chain is glycolysis, and the second is the Krebs cycle.
The Role of Coenzyme Q10 in Cellular Respiration
The name "electron transport chain" is appropriate because electrons are transferred between each step of the chain—and one of the molecules responsible for transporting these electrons is Coenzyme Q10. Therefore, if the biosynthesis of CoQ10 is impaired (due to genetic mutations), cellular energy production will be negatively affected, potentially resulting in diseases related to primary CoQ10 deficiency.
One such disease is SCAR9 ataxia, which manifests in childhood (see Section 3 – Onset). The most visible consequences in children affected by SCAR9 include progressive cerebellar ataxia (leading to difficulties with walking, motor coordination, and balance) and difficulty performing physical activities, due to fatigue (“low energy”) caused by mitochondrial dysfunction (see Section 2 – Typical Symptoms) [3].
2. TYPICAL SYMPTOMS
SCAR9 typically manifests in childhood with progressive ataxia and cerebellar atrophy. In addition to cerebellar dysfunction, patients may experience a variety of neurological and systemic symptoms. SCAR9 symptoms can vary from person to person, even within the same family. Some individuals may develop more symptoms than others, and when they do occur, symptoms can be mild, moderate, or severe.
The following list of symptoms in SCAR9 is for reference only.
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Progressive ataxia caused by cerebellar atrophy
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Muscle weakness.
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Muscle hypotonia (reduced muscle tone, or resistance to movement in a muscle) may occur.
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Fatigue, low exercise tolerance (considering the normal activity level for age and gender).
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In some patients, mental retardation (mild to moderate) may occur.
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In some patients, psychomotor regression (developmental delay in childhood) may occur.
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Seizures (epilepsia partialis continua) may occur.
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In some patients, episodes such as strokes (cerebrovascular accidents) may occur.
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Changes in tendon reflexes (reflexes more active than usual)
Other symptoms that may occur (less frequently) include strabismus, tremors, gynecomastia, and hearing problems. The emergence of new symptoms should be monitored by a pediatrician. Symptom progression is typically slow (several decades).
Sources: [1], [3], [4], [5].
Note 2: This page focuses on the mutation in the ADCK3 (or COQ8A) gene , but depending on the gene involved, kidney problems (genes COQ2, COQ6, COQ8B, COQ9, PDSS1, PDSS2), heart problems such as hypertrophic cardiomyopathy (genes COQ2, COQ4, COQ9), retinopathies and other visual problems (genes COQ2, COQ4, COQ5, PDSS1, PDSS2), hearing problems (genes COQ2, COQ6, PDSS2) and different neurological symptoms may arise. For a more detailed discussion of the great phenotypic variability associated with primary coenzyme Coq10 deficiency depending on the gene involved, see the table " Primary Coenzyme Q10 Deficiency: Genes and Associated Clinical Features " in [1] .
3. ONSET
The age at which SCAR9 ataxia symptoms appear ranges from 5 to 11 years.
SCAR9 ICD10: G11.1 (Early-Onset Spinocerebellar Ataxia)
Note 3: Note that the childhood manifestation refers to SCAR9, associated with the ADCK3 gene. There are other "primary coenzyme CoQ10 deficiency" diseases caused by mutations in other genes, and in some of them (such as the COQ2 gene) the age of onset of symptoms can vary from birth to old age.
Sources: [1], [3]. [4].
4. ANTICIPATION
Anticipation is not observed in SCAR9, given that the disease "skips generations".
5. INHERITANCE
SCAR9 has autosomal recessive inheritance. This means that individuals of both sexes have an equal probability of inheriting the mutation that can cause SCAR9 from their parents. An individual only develops the symptoms of the disease if they inherit two mutated copies (alleles) of the ADCK3 gene (one from the father and one from the mother). If they inherit only one mutated copy of the gene (from either parent), they will not develop the disease but will be a carrier of the mutated gene. In people who are carriers (that is, who have only one mutated allele of the ADCK3 gene), the “normal” copy of the gene takes precedence over the defective copy, and therefore carriers do not develop symptoms. However, in people who inherit two defective copies of the ADCK3 gene, the disease will manifest, usually in childhood (see Section 3. Onset).
Note 4: Although in the case of the ADCK3 gene the inheritance is autosomal recessive, it is worth noting that primary deficiency of coenzyme Coq10 can also be sporadic or caused by variants in other genes, with a case of " de novo " mutation in the COQ4 gene having already been identified. See [1].
Probability of children inheriting the disease
Statistically, if both parents are heterozygous carriers (i.e., each has only one mutant allele):
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The probability of each of the children developing the disease (= inheriting one bad gene from the mother and another from the father) is 25% .
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The probability of each of the children not developing the disease and also becoming a carrier (= inheriting only one bad copy of the gene, either from the father or the mother) is 50% .
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The probability of each child not inheriting any mutant gene (neither from the father nor the mother) and not developing the disease nor becoming an asymptomatic carrier is 25% .
Note 5: "Autosomal" means the gene is located on any chromosome except the X and Y sex chromosomes. Genes, like chromosomes, typically exist in pairs (we have a pair of each gene; one copy of the gene is inherited from the mother, the other from the father). Thus, men and women are equally likely to inherit a mutated gene that can cause hereditary ataxia. "Recessive" means that two mutated copies (alleles) of the gene must be inherited (one from the biological father, the other from the mother) for a person to develop the disease.
Figure 2 - Source: MedlinePlus, US National Library of Medicine .
6. PREVALENCE
The exact prevalence of SCAR9 has not been precisely established in the academic literature, largely due to its extremely rare nature and the possibility of underdiagnosis. The prevalence of SCAR9 is estimated to be less than 1 in 1,000,000 individuals.
Sources: [3], [4]
7. ADDITIONAL INFORMATION
Diagnosis of SCAR9
The diagnosis of SCAR9 can be established by a pediatric neurologist through a combination of clinical evaluation, family history, and imaging studies, and may be confirmed through molecular genetic testing. In some cases, biochemical tests may aid in the diagnosis, although they are not conclusive.
Laboratory Findings and Diagnostic Support Tests
Individuals with SCAR9 tend to present with the following:
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Elevated or normal lactic acid levels (increased lactate concentration in the cerebrospinal fluid). Note that normal lactate levels do not exclude the possibility of primary Coenzyme Q10 (CoQ10) deficiency.
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Cerebellar atrophy and T2 hyperintensities (detectable on MRI scans).
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Electromyogram (EMG) is usually normal.
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Reduced levels of CoQ10 in skeletal muscle tissue.
Note 6: Measuring CoQ10 levels in the blood is not sufficient for diagnosis, as blood levels mostly reflect dietary intake, rather than endogenous production of CoQ10. There have also been reports of individuals with genetically confirmed CoQ10 deficiency who have normal CoQ10 levels in muscle or skin tissues. Therefore, a normal CoQ10 level does not conclusively rule out the diagnosis.
Sources: [1], [3]
Differential Diagnosis
Since primary Coenzyme Q10 deficiency is a potentially treatable condition through CoQ10 supplementation, it is important to distinguish it from other conditions such as:
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Mitochondrial encephalomyopathies, which may have clinical manifestations indistinguishable from primary CoQ10 deficiency, especially in more severe cases.
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Steroid-Resistant Nephrotic Syndrome (SRNS), which affects the kidneys and may be caused by pathogenic variants in genes other than those related to CoQ10 deficiency.
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Ataxias with childhood onset, such as Friedreich’s ataxia and others. Note that SCAR9 follows an autosomal recessive inheritance pattern and may be confused with other recessive ataxias. The absence of a known family history does not exclude the diagnosis.
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Retinitis pigmentosa (progressive retinal degeneration) and optic atrophy.
Note 7: In addition to primary Coenzyme Q10 (CoQ10) deficiency caused by mutations in the ADCK3 gene, there are also secondary CoQ10 deficiencies that may result from mutations in other genes not directly involved in CoQ10 biosynthesis, and which lead to distinct diseases, such as Glutaric Aciduria Type IIC, a metabolic disorder caused by mutations in the ETFDH gene, and Ataxia with Oculomotor Apraxia Type 1 (AOA1), caused by mutations in the APTX gene. The only way to definitively distinguish between primary and secondary CoQ10 deficiencies is through molecular genetic testing.
Sources: [1], [5]
8. THERAPIES AND DRUGS BEING TRIALED FOR THIS ATAXIA
Primary CoQ10 deficiency is potentially treatable through high-dose oral supplementation (ranging from 5 to 50 mg/kg/day). Treatment should be started as early as possible, as it can help limit disease progression and reverse some symptoms. However, more severe neurological and renal damage that has already been established cannot be reversed. Response to treatment is highly variable and depends on both patient-specific genetic factors and disease severity, as well as other factors that are still unknown. See "Additional notes on the effectiveness of CoQ10 treatment" in Section 9. Treatments .
Source: GeneReviews [1]
9. TREATMENTS
There is currently no cure for SCAR9 ataxia, but its symptoms can be treated to improve quality of life and provide ongoing support. It is important that SCAR9 patients be monitored by a neurologist and a specialized multidisciplinary medical team, with the gradual inclusion of additional healthcare professionals as needed based on symptoms (geneticist, neurofunctional physiotherapist, occupational physiotherapist, speech therapist, nutritionist, etc.).
Treatment with coenzyme Coq10 supplementation
See the next section for information on treating SCAR9 with coenzyme Q10 supplementation.
General recommendations for managing ataxia symptoms
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Your doctor may prescribe different medications for specific symptoms.
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Neurofunctional exercises and physiotherapy are recommended to improve motor coordination, balance, muscle strength and reduce the risk of falls.
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Occupational physiotherapy can provide patients with greater independence in daily activities.
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For balance difficulties when walking due to ataxia, canes, walkers or wheelchairs can be used, depending on the stage of the disease.
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Home modifications are recommended (e.g., grab bars in bathrooms).
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It is advisable to control your weight to avoid even greater difficulties with mobility.
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In the case of dysarthria, specialized speech therapy (speech therapy) is recommended.
See information about medications for ataxia symptoms .
See information about treatments and care for patients .
See information about those who have recently been diagnosed .
See information about Support Groups for patients and caregivers.
Treatment with Coenzyme Q10 Supplementation
As discussed in Section 1 – About SCAR9, Coenzyme Q10 (or CoQ10) functions as an electron transporter in chemical reactions that take place inside our cells—specifically within the mitochondria—where essential energy for life is produced. In this process, CoQ10 is converted into ubiquinone (oxidized form) and ubiquinol (reduced form). In addition to being produced endogenously (naturally by the body), CoQ10 can also be obtained from food and supplements.
CoQ10 is a powerful antioxidant, and although it has not yet been approved by the FDA, it is already being used by many people with mitochondrial diseases and various forms of ataxia—particularly patients with SCAR9 and other disorders caused by primary CoQ10 deficiency.
In addition to aiding in energy production and reducing fatigue (which is very common in patients with ataxia), CoQ10 supplementation—which should always be discussed with a physician—offers several other potential benefits:
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Anti-inflammatory effects
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Helps regulate blood pressure
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Antioxidant protection against oxidative stress
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Skin health and anti-aging effects
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Contributes to the treatment of some cardiac conditions, such as congestive heart failure
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Helps maintain the oxidative stability of LDL cholesterol ("good" cholesterol)
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Aids in the prevention and management of migraine symptoms
It is important to choose a reliable brand and ensure the product is of high quality.
Forms of Coenzyme Q10
As mentioned above, CoQ10 naturally exists in two forms: ubiquinone and ubiquinol. These forms are interconvertible through redox reactions. When ubiquinone loses electrons, it is converted into ubiquinol; when ubiquinol gains electrons, it is oxidized back into ubiquinone. This cycle is fundamental to the role of CoQ10 in the mitochondrial electron transport chain. When purchasing CoQ10 supplements, either of these two forms can be used. Some studies suggest that in certain conditions, the reduced form (ubiquinol) may be more bioavailable, especially in individuals with compromised absorption. However, both forms are effective when administered appropriately.
In addition to ubiquinone and ubiquinol, there are other formulations, such as:
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UbiQsome (a phytosomal form of CoQ10, which is relatively inexpensive and has excellent absorption)
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MaxSolv, a liquid, water-soluble variant that is easier to absorb and can be taken in drops, without needing to be consumed with food. However, MaxSolv is more expensive (see: https://www.maxsolve.com.br/)
The choice between these different formulations should consider product quality, brand reliability, and, most importantly, medical guidance, as efficacy and absorption may vary depending on the technology used and individual needs.
Tips for Better Absorption
One important issue regarding CoQ10 supplementation is absorption:
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Most commercial CoQ10 supplements have low bioavailability—typically around 2–3%. This means that from a 100 mg capsule, only about 2 to 3 mg of CoQ10 may actually be absorbed. Therefore, a daily dose of 400 mg would result in approximately 8 to 12 mg being effectively absorbed. These values may vary depending on the formulation and individual characteristics of each patient. However, even this limited absorption has shown benefits in clinical studies.
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Coenzyme Q10 (in both ubiquinone and ubiquinol forms) is fat-soluble, so it should preferably be taken with meals that contain a small amount of fat (not excessive!), which helps dissolve CoQ10 and enhance its absorption. Olive oil is a good fat source to combine with Q10 intake.
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Daily and continuous use of CoQ10 is required to maintain its benefits, respecting the maximum recommended daily doses as explained below.
Oral Dosage
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Some studies suggest that the minimum effective dose for supplementation generally starts at 100 mg/day, which is a common dose for general benefits such as mitochondrial function support and cardiovascular health.
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The maximum safe dose typically recommended for long-term use is around 400 to 500 mg/day for conventional CoQ10 (either ubiquinone or ubiquinol) (Hidaka et al., 2008).
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Higher doses, such as those above 1,200 mg/day, are used in research settings and specific clinical treatments, but long-term use of such high doses should be medically supervised due to the risk of side effects and drug interactions (Mancuso et al., 2010).
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For patients with ataxias and mitochondrial diseases, recommended doses of CoQ10 tend to be higher, usually ranging from 400 mg to 1,200 mg per day in the ubiquinone form (the more common one). For ubiquinol, the recommended dosage may be half that of ubiquinone (i.e., 200 mg to 600 mg per day), since some studies suggest that ubiquinol may have approximately twice the bioavailability of ubiquinone (Langsjoen et al., 2008).
Injectable CoQ10
There are also injectable forms of CoQ10 available for specific clinical indications. These have significantly greater absorption than oral administration, as they bypass first-pass metabolism in the liver. However, injectable use is restricted to serious clinical conditions, including certain mitochondrial disorders and severe congestive heart failure (Mortensen et al., 2014). It must be administered in specialized clinics under medical prescription and supervision, as there is a risk of overdose.
Additional Notes
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In a webinar, Dr. Susan Perlman (ataxia specialist) suggested starting with a low dose and trying it for 2 to 3 months. If the patient notices improvement in any symptoms, they may continue. Otherwise, treatment can be discontinued.
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It is advisable to research the risks of long-term use, and it is always important to align supplementation with a physician’s guidance, as CoQ10 may interact with other medications.
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The response to CoQ10 supplementation in patients with ataxia has been limited and variable, with significant improvement in some cases, but not in others. For additional information, please see reference [7].
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Overall, CoQ10 supplementation seems to be more effective for non-neurological symptoms (e.g., cardiac issues). In terms of neurological impact, results are still inconclusive. One theory that may explain why CoQ10 does not consistently improve neurological symptoms is the blood–brain barrier, which restricts CoQ10’s access to brain tissue. Ongoing research is exploring new delivery methods to allow CoQ10 to better reach the brain. See reference [2].
For a technical review on the neuroprotective effects of CoQ10 and its potential antioxidant and anti-inflammatory benefits for SCAR9 and other neurological diseases, please see reference [8].
10. REFERENCES
The references below include academic sources and specialized organizations that supported the information in this fact sheet, including peer-reviewed articles, genetic repositories (OMIM), literature summaries (GeneReviews), and informational materials from ataxia foundations. For more information, see the ataxia.info References list .
Ref #1
Source:
Leonardo Salviati, MD, PhD et al
Copyright © GeneReviews. GeneReviews ® is a registered trademark of the University of Washington, Seattle.
Language:
English
Date:
Last Updated: June 8, 2023
Ref #2
Source:
Presented by: Dr Luke Wainwright
YouTube - Copyright © AtaxiaUKonline
Language:
English. You can enable subtitles and configure automatic translation of subtitles into other languages.
Date:
Jan 18, 2018
Ref #3
Source:
NEUROMUSCULAR DISEASE CENTER (Alan Pestronk, MD)
Washington University, St. Louis, MO - USA
Language:
English
Date:
Last Updated: Please see https://neuromuscular.wustl.edu/rev.htm
Ref #4
Source:
GARD - Genetic and Rare Diseases Information Center.
Copyright © National Center for Advancing Translational Sciences - National Institutes of Health (NIH).
Language:
English
Date:
Last Updated: February 2024
Ref #5
Source:
OMIM ® - An Online Catalog of Human Genes and Genetic Disorders.
Copyright © Johns Hopkins University.
Language:
English
Date:
Edit History: carol : carol : 02/02/2024
Ref #6
Source:
Presentation: Dr. Christian Aguiar
YouTube - Copyright © Dr. Christian Aguiar | Functional and Integrative Medicine
Idioma:
Portuguese
Date:
Aug 12, 2023
Ref #7
Source:
Ying Wang, Siegfried Hekimi
PubMed ® PMID: 35985679
Idioma:
English
Date:
Aug 19, 2022
Ref #8
Source:
Shokufeh Bagheri et al
PubMed ® PMID: 37424991
Idioma:
English
Date:
Aug 19, 2022