Autism and Asperger syndrome are autism spectrum disorders (ASD) characterised by a neurodevelopmental pathology exhibiting as impaired social and communicative behavioural patterns that range from mild to severe.[1,2]
The pathogenesis of ASD is a topic of ongoing research, however what is currently understood is that a combination of multiple genetic and environmental factors are aetiological antecedents.[1,2]
Over recent years, evidence suggests there is a subset of ASD individuals with concomitant mitochondrial dysfunction (MD), which may contribute to ASD onset, progression or severity. However, there is significant complexity in establishing the specific mitochondrial impairments that occur in this population.[1,3-5] This article reviews the current evidence on mitochondrial dysfunction in ASD and nutritional interventions that may have clinically relevant outcomes in this population.
Overview of mitochondria and energy production
The synthesis of energy in the form of ATP involves glycolysis in the cytosol of the cell, the tricarboxylic acid cycle (TCA) in the mitochondrial matrix and the electron transport chain (ETC) in the inner mitochondrial membrane.[1,6]
Following the production of pyruvate from glucose in the glycolytic cycle, acetyl-CoA and the electron carriers flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide (NAD) are formed in the TCA. These intermediates are then transferred to the 5-enzyme complex (I-V) mitochondrial electron transport chain (ETC) to undergo oxidative phosphorylation, which also utilises the electron carriers’ coenzyme Q10 (ubiquinone) and cytochrome c.[1,6]
Along with energy production, mitochondria are involved in coordinating calcium signalling and cellular apoptosis.[6,7]
Mutations in mitochondrial DNA, which are located in both the mitochondria and cell nucleus, can contribute to dysfunctional mitochondrial activity and consequent energy metabolism impairments can occur.[6,7]
Mitochondria and oxidative stress
One of the costs of energy synthesis in the ETC is the formation of reactive oxygen species (ROS) or free radicals. When there are optimal levels of endogenous antioxidants substances and enzymes in the mitochondrial membrane, including glutathione, vitamins C and E and superoxide dismutase (SOD), the detrimental impact of ROS is restricted.[1,2,8]
However, when ROS production is upregulated or endogenous antioxidant levels are depleted, proteins, lipids and cellular structures, such as the mitochondria, are more susceptible to ROS-induced damage, resulting in functional impairments when there is prolonged oxidative stress.[3,4,6,8]
ROS-induced damage and the consequent impaired function of the mitochondria further accelerates ROS production, and body tissues with high rates of energy synthesis such as the brain are particularly susceptible.
Evidence from blood, urine and post-mortem brain samples in ASD subjects have demonstrated that this population commonly presents with elevated levels of chronic oxidative stress or redox regulation abnormalities.[3,4,7]
Specific oxidative marker abnormalities observed in this population include reduced levels of antioxidant enzymes (i.e. SOD, glutathione peroxidase) and ratios of S-adenosylmethionine to S-adenosylhomocysteine and glutathione to oxidised glutathione, as well as elevated levels of plasma malondialdehyde and urinary biomarkers (e.g. 8-isoprostane-F2, 6-keto-prostaglandinF1).[3,4,7]
This endogenous pro-oxidant environment in ASD may be a consequence of MD, with recent evidence observing concomitant mitochondrial dysfunction and oxidative stress in some of the ASD population.
Classification and diagnosis of ASD-MD
Similar to the broader ASD population, the clinical presentation of ASD-MD is heterogeneous in terms of the type and severity of impairment displayed and whether these impairments are associated with central nervous system or systemic dysfunctions. The clinical presentation may also be influenced by whether it is primary or secondary MD.
Primary MD is caused by a genetic mutation in the mitochondrial system, whereas secondary MD is a consequence of indirect genetic, biochemical or metabolic aberrations that adversely affect mitochondrial energy synthesis. Such aberrations can include oxidative stress, environmental toxin-derived metabolites, medications, propionic acid from Clostridia difficile, nutrient deficiencies, abnormal calcium signalling or increased levels of nitric oxide.[1,3,6]
The heterogeneous nature of the clinical presentation of MD means an extensive range of investigations are often required to diagnose it, including clinical, biochemical, histological, enzymatic and genetic.
Characteristic histological findings from such investigations in ASD-MD individuals can include skeletal muscle fibre abnormalities, such as fibres that are ragged red or blue, low in cytochrome c oxidase or with excessive concentrations of mitochondria.[1,8]
The primary enzymatic aberrations associated with ASD-MD involve impaired ETC complex activity, particularly an upregulation of this process, which is reflected by abnormal biochemical parameters also observed in this population group.[5,6,8]
At least one abnormal biochemical parameter or ratio is usually observed in individuals with ASD-MD. These biochemical markers can include altered levels of the TCA metabolites and intermediates lactate, pyruvate or alanine due to reduced TCA (aerobic) activity or elevated ammonia as a consequence of reduced TCA and urea cycle functionality; low levels of plasma total and free carnitine due to impaired beta-oxidation resulting in high levels of unprocessed fatty acids depleting free carnitine; and elevated markers of damage in high-energy tissues, such as creatine kinase, alanine aminotransferase or aspartate aminotransferase as a result of mitochondrial dysfunction.[1,8]
Genetic mutations that are estimated to cause MD in approximately 1/200 ASD individuals include mitochondrial DNA deletions, over-replications or decreased expression of mitochondrial or ETC genes.
The heterogenic clinical presentation of ASD-MD was demonstrated in a 2015 systematic review and meta-analysis that identified and compared characteristics of ASD-MD subjects with those exhibited by the general ASD population.
Following an analysis of 68 studies, MD was found to be the most prevalent metabolic abnormality associated with ASD, with 5% of ASD subjects having MD, significantly higher than the level of MD in the general population (0.01%), indicating a likely association between ASD and MD in an ASD population subset. Within this ASD-MD subset, 79% had secondary MD, highlighting the low rate of genetic aetiology.
The authors suggested that this prevalence rate underestimated the actual proportion of ASD-MD, due to the complexity associated with diagnosis. Another factor is the significantly higher proportion of abnormal biochemical markers of MD observed in the ASD population (approximately 50%) compared to the prevalence of diagnosed ASD-MD (5%), suggesting it may present on a spectrum of severity (resulting in milder cases being overlooked).[1,5]
Both the general ASD and ASD-MD groups demonstrated abnormal biochemical values in lactate, pyruvate, lactate-pyruvate ratio, carnitine, ammonia, creatine kinase and aspartate aminotransferase compared to non-ASD subjects, as well as an increased prevalence of mitochondrial DNA deletions. There was also an association observed between some biochemical markers and ASD severity.
Compared with ASD subjects, ASD-MD subjects demonstrated distinct characteristics including developmental regression, seizures, motor delay, gastrointestinal abnormalities and a higher incidence of increased lactate and pyruvate, fatigue and ataxia. The prevalence of such abnormalities were similar to the general non-ASD MD population, indicating ASD-MD may be a specific sub-group of the MD population.
A separate study observed other distinct abnormalities in ASD-MD subjects associated with oxidative, inflammatory and neurodevelopmental parameters. Specific characteristics exhibited by ASD-MD versus ASD-noMD subjects in this study included abnormalities in adaptive behaviour, communication and daily living.
Compared to healthy subjects, both ASD-MD and ASD-noMD subjects were observed to have higher levels of a plasma inflammation biomarker (3-nitrotyrosine [3CT]) and glutathione redox abnormalities, with ASD-MD subjects exhibiting a more favourable glutathione redox status compared with ASD-noMD subjects.
In the ASD-MD group, 3CT levels were higher in younger versus older children, with the authors suggesting that this may be indicative of mitochondrial damage and subsequent permanent mitochondrial dysfunction being induced by inflammation at a young age (e.g. illness).
Higher levels of 3CT were also positively correlated with cognitive function, development and behaviour in the ASD-MD group, which may be due to chronic oxidation resulting in an increase in both concentration and activity of dysfunctional mitochondrial.
The specific biochemical characteristics observed in the ASD-MD group suggest that ASD-MD subjects may be a distinct subgroup of ASD, with further research required to determine the specific interrelationship between ASD-MD and oxidative stress.
Treatment and management of ASD-MD
As the specific aetiology of ASD-MD is yet to be confirmed and there is no currently known cure, treatment is associated with the clinical management of behavioural impairments and addressing exogenous and endogenous factors that contribute to or exacerbate mitochondrial damage and the consequent systemic dysfunctions.
Along with the broader dietary interventions often used in ASD individuals, the primary aim of nutritional interventions in ASD-MD is to attenuate ROS-induced mitochondrial damage and support cellular energy synthesis.
Particular interventions that have shown some benefit in ASD-MD individuals include essential fatty acids, coenzyme Q10, carnitine, carnosine, and vitamins B1, B2, B3, B12 (methylcobalamin), C and folic or folinic acid.[1,9,10]
The studies that have assessed the effect of these nutrients in ASD-MD subjects have used varying combinations of these interventions at different dosages and timeframes, indicating that a multi-nutrient approach may provide the most benefit.[5,9,10]
Particular clinical outcomes observed in such studies includes improvements in muscle tone, intellectual disability, cognition, childhood autism rating scale, clinical global impressions scores, ETC complex I activity, glutathione concentrations and reduced levels of Clostridia-derived metabolites.
Along with the role of nutritional interventions to improve clinical outcomes, further research is required to determine if MD is an aetiological or concomitant factor in ASD pathogenesis, and if ASD-MD is a subset group with specific clinical characteristics or if it exists on a continuum of severity within the broader ASD population. The interrelationship between mitochondrial dysfunction and ASD-associated behavioural, gastrointestinal and neurological impairments also requires further investigation.
- Rossignol DA, Frye RE. Mitochondrial dysfunction in autism spectrum disorders: a systematic review and meta-analysis. Mol Psychiatry 2012 Mar; 17(3):290-314.
- Frye RE, Delatorre R, Taylor H, et al. Redox metabolism abnormalities in autistic children associated with mitochondrial disease. Transl Psychiatry 2013;3:e273.
- Morris G, Berk M. The many roads to mitochondrial dysfunction in neuroimmune and neuropsychiatric disorders. BMC 2015;13:68.
- Gargus JJ, Imtiaz F. Mitochondrial energy-deficient endophenotype in autism. Am J Biochem Biotech 2008;4(2):198-207.
- Delhey LM, Nur Kilinc E, Yin L, et al. The effect of mitochondrial supplements on mitochondrial activity in children with autism spectrum disorder. J Clin Med 2017;6(2):18.
- Siddiqui MF, Elwell C, Johnson MH. Mitochondrial dysfunction in autism spectrum disorders. Autism Open Access; 2016;6(5):1000190.
- Griffiths KK, Levy RJ. Evidence of mitochondrial dysfunction in autism: biochemical links, genetic-based associations and non-energy related mechanisms. Oxid Med Cell Longev 2017;2017:4314025.
- Palmieri L, Persico AM. Mitochondrial dysfunction in autism spectrum disorders: cause or effect? Biochim Biophys Acta 2010;1797 (6-7):1130-1137.
- Guevara-Campos J, Gonzalez-Guevara L, Cauli O. Autism and intellectual disability associated with mitochondrial disease and hyperlactacidemia. Int J Mol Sci 2015;16(2):3870-3884.
- Adams JB, Audhya T, Geis E, et al. Comprehensive nutritional and dietary intervention for autism spectrum disorder – a randomised, controlled 12-month trial. Nutrients. 2018;10(3):369.