Introduction
A growing body of evidence implicates cerebral folate deficiency (CFD) and immune dysregulation in the etiology of a subset of Autism Spectrum Disorder (ASD) cases. A key mechanistic link in this pathway is the presence of autoantibodies that target the folate receptor alpha (FRα). These folate receptor autoantibodies (FRAAs) are hypothesized to disrupt the transport of folate across the blood-brain barrier (BBB) and into the choroid plexus, leading to decreased folate concentrations in the cerebrospinal fluid (CSF) despite normal systemic folate levels. This has become an emerging and fascinating science, with significant implications for the improvement of people’s health. It is important to understand the connection between folate, folate receptor autoantibodies and cerebral folate deficiency syndrome, and its tie into autism spectrum disorders.
Folate and Brain Development: Folate (vitamin B9) is an essential cofactor in numerous biochemical pathways critical for neurodevelopment and neurological function. These include:
- Nucleotide Synthesis: Essential for DNA and RNA synthesis, supporting neurogenesis and myelination.
- Methylation Reactions: As 5-methyltetrahydrofolate (5-MTHF), folate is the primary methyl donor for the synthesis of S-adenosylmethionine (SAMe). SAMe is crucial for the methylation of DNA, histones, proteins, and phospholipids, processes that regulate gene expression, synaptic plasticity, and myelin maintenance.
- Neurotransmitter Synthesis: Involved in the metabolism of serotonin, dopamine, and norepinephrine. These are all critical neurotransmitters and important for proper brain function.
The Folate Receptor Alpha (FRα) and Cerebral Folate Transport: Unlike most tissues, the central nervous system (CNS) relies on a specific transport mechanism to acquire folate from the blood. The primary form of folate in circulation, 5-MTHF, does not readily diffuse across the BBB. Its transport is mediated by the high-affinity Folate Receptor Alpha (FRα), which is highly expressed on the basolateral membrane of the choroid plexus epithelial cells. FRα binds 5-MTHF, internalizes it via endocytosis, and releases it into the CSF, from where it diffuses to neurons and glial cells. Proper function of this folate receptor is critical for healthy cells, especially those connected with the brain.
The Autoimmune Hypothesis: Folate Receptor Autoantibodies (FRAAs)
FRAAs are immunoglobulin G (IgG) antibodies that mistakenly target and bind to the body’s own FRα. Two primary types have been identified:
- Blocking Autoantibodies: Impede the binding of 5-MTHF to the FRα.
- Binding Autoantibodies: Bind to the FRα without necessarily blocking folate binding, but may cause internalization and degradation of the receptor complex.
The net effect of both types is a functional blockade of folate transport into the CSF, leading to Cerebral Folate Deficiency Syndrome (CFDS)—characterized by lower levels of 5-MTHF in the CSF, and in the presence of normal systemic folate levels.
Prevalence of FRAAs in Autism Spectrum Disorders
The prevalence of FRAAs in ASD has been investigated in multiple clinical cohorts, primarily through the use of the FRAT® test.
Key Epidemiological Studies:
- Ramaekers et al. (2007, 2013): In seminal studies, this group first reported a high prevalence of FRα autoantibodies in children with ASD and neurological symptoms. Their initial and follow-up work found that approximately 70-75% of children with ASD tested positive for either blocking or binding FRAAs. This was a groundbreaking finding that established a strong correlation.
- Frye et al. (2013): In a large US-based study of 93 children with ASD, 60% were found to be positive for FRAAs. This study confirmed the high prevalence in a different demographic and further characterized the clinical subphenotype.
- Meta-Analyses and Systematic Reviews: A meta-analysis by Xi et al. (2021) pooled data from several studies. While the reported prevalence rates varied from 47% to 76% across individual studies, the pooled estimate indicated that a significant majority of children with ASD are FRAA-positive, substantially higher than the rate observed in typically developing (TD) controls.
- Prevalence in Controls: The prevalence of FRAAs in typically developing children and healthy adult populations is consistently low, generally reported to be between 3% to 7%. This stark contrast strengthens the argument for a specific association with ASD pathology rather than a general, non-specific autoimmune phenomenon.
Summary of Prevalence Data: Based on a synthesis of the available literature, it is scientifically reasonable to conclude that the prevalence of FRAAs in children with ASD is between 60% and 75%. This identifies a substantial and mechanistically distinct biological subgroup within the heterogeneous ASD population.
Pathophysiological Mechanism Linking FRAAs to ASD Symptoms:
The proposed pathway from FRAAs presence to ASD symptomatology is as follows:
- Autoantibody Production: The initial trigger for FRAA production remains under investigation. Hypotheses include molecular mimicry following a negative immune response, or an immature or dysregulated gut-immune axis allowing for exposure to dietary folate-binding proteins (bovine, or other animal milks), or a genetic predisposition to autoimmunity.
- Impaired Folate Transport: Circulating FRAAs cross the blood-CSF barrier in the choroid plexus and bind to FRα.
- Cerebral Folate Deficiency Syndrome (CFDS): The functional blockade of FRα leads to a significant reduction in the concentration of 5-MTHF in the CSF. Again, folate levels in the circulating blood may be normal in these instances, whereas folate levels in the CFS may be lower.
- Downstream Neurological Consequences:
- Impaired Methylation: Widespread hypomethylation disrupts epigenetic regulation of genes critical for neurodevelopment, synaptic function, and neuronal connectivity.
- Altered Neurotransmission: Disruption in the synthesis of monoamine neurotransmitters (serotonin, dopamine) can affect mood, sleep, and behavior.
- Oxidative Stress: Folate deficiency can impair the synthesis of glutathione, a key antioxidant, leading to increased neuronal oxidative stress.
- Impaired Myelination and Synaptogenesis: Compromised DNA synthesis and lipid methylation can hinder the development and maintenance of white matter and synaptic structures.
These biochemical disruptions collectively contribute to the core and associated symptoms of ASD, including communication deficits, social impairments, repetitive behaviors, regression, seizures, and sleep disturbances.
Clinical Correlations and Sub-phenotyping:
The presence of FRAAs appears to define a specific clinical sub-phenotype within ASD. Children who are FRAA-positive are more likely to exhibit:
- Neurological Regression: A history of a loss of previously acquired language or social skills.
- Co-occurring Neurological Conditions: Such as epilepsy or motor coordination deficits.
- Specific Biochemical Abnormalities: Including biomarkers of oxidative stress and mitochondrial dysfunction.
Diagnosis: The measurement of FRAAs in serum provides a potential biomarker for identifying this ASD subgroup. This is typically done with the use of the FRAT® test. Diagnosis of CFD can be confirmed by measuring 5-MTHF in the CSF, which is exceedingly invasive. Serum FRAA testing, however, offers a less invasive potential screening tool.
Treatment with Leucovorin (Folinic Acid): Folinic acid (leucovorin calcium) is a reduced folate that can bypass the FRα blockade. It utilizes the reduced folate carrier (RFC) system, which is less efficient in the choroid plexus but can be upregulated with high-dose therapy.
- Clinical Trials: Several open-label and a few randomized controlled trials have shown that high-dose folinic acid supplementation can lead to improvements in verbal communication, social interaction, attention, and repetitive behaviors in FRAA-positive children with ASD.
- Mechanism: By providing an alternative route for folate entry into the CNS, folinic acid restores CSF folate levels, thereby reversing the underlying biochemical deficits.
Conclusion
The scientific evidence robustly indicates a high prevalence (60-75%) of folate receptor autoantibodies in a significant subset of children with ASD. These autoantibodies are a key pathogenic factor in a distinct ASD subphenotype characterized by cerebral folate deficiency syndrome. The identification of FRAAs through FRAT® provides a clear biological mechanism, a viable serum biomarker, and a rationale for a targeted nutritional-metabolic intervention (folinic acid). Future research should focus on large-scale, multi-center randomized controlled trials to solidify the efficacy of folinic acid treatment, standardize FRAA testing protocols, and further elucidate the initial triggers for autoantibody production. Recognizing this subgroup is a critical step towards personalized medicine in autism spectrum disorders.
References
- VT Ramaekers, N Blau, JM Sequeira, MC Nassogne, EV Quadros
Folate receptor autoimmunity and cerebral folate deficiency in low-functioning autism with neurological deficits
Neuropediatrics, 2007 - Ramaekers, V. T., et al. (2013). Clinical recognition and treatment of cerebral folate deficiency in children with autism spectrum disorder. Journal of Pediatric Biochemistry, 3(2), 125-134.
- N Bobrowski-Khoury, VT Ramaekers, JM Sequeira, EV Quadros, Folate receptor alpha autoantibodies in autism spectrum disorders: diagnosis, treatment and prevention , Journal of Personalized Medicine 2021
- Rossignol, D. A., & Frye, R. E. Cerebral Folate Deficiency, Folate Receptor Alpha Autoantibodies and Leucovorin (Folinic Acid) Treatment in Autism Spectrum Disorders: A Systematic Review and Meta-Analysis; Journal of Personalized Medicine 2021
- Jiaxiu Zhou,Aiping Liu,Fusheng He,Ya Jin,Shaoming Zhou,Ruihuan Xu,Hailiang Guo,Wenhao Zhou,Qiufen Wei &Mingbang Wang; High prevalence of serum folate receptor autoantibodies in children with autism spectrum disorders, Biomarkers 2018
- Sequeira, J. M., et al. (2018). The effect of folinic acid on the cerebral folate deficiency caused by folate receptor autoantibodies. Molecular Genetics and Metabolism, 125(1-2), 113-117.
- R. E. Frye, J M Sequeira, E V Quadros, S J James & D A Rossignol; Cerebral folate receptor autoantibodies in autism spectrum disorder. Molecular Psychiatry 2012
