Folate Receptor Alpha Autoantibodies and Their Implication in Autism Spectrum Disorder

What are folate receptor autoantibodies (FRAAs)? To understand the relevance of folate receptor autoantibodies (FRAAs) and their possible relationship in autism spectrum disorders, we must first understand what folate (vitamin B9) is and its importance in this overall connection as well.

Folate (Vitamin B9), in its biologically active form 5-methyltetrahydrofolate (5-MTHF), is a critical nutrient that serves as a one-carbon donor essential for:

• DNA synthesis and repair
• Cellular methylation cycles (e.g., epigenetic regulation, neurotransmitter synthesis, phospholipid production)
• Mitochondrial function and antioxidant defense

The lipophilic nature of 5-MTHF (which is a reduced folate) prevents efficient diffusion across the blood-brain barrier (BBB) and into systemic cells. Transport of reduced folate is primarily mediated by two mechanisms:

1. Reduced Folate Carrier (RFC): A low-affinity, high-capacity transporter active in many tissues.
2. Folate Receptor Alpha (FRα): A high-affinity, glycosylphosphatidylinositol (GPI)-anchored glycoprotein. FRα is particularly critical in tissues with high folate demand and where RFC activity is low, including:
   • The choroid plexus epithelial cells (the site of the blood-CSF barrier).
   • Developing neurons and glia.
   • Placenta and kidney proximal tubules.

At the BBB, FRα on the apical surface of choroid plexus cells binds circulating 5-MTHF, internalizes it via endocytosis, and releases it into the cerebrospinal fluid (CSF), fromwhere it is taken up by FRα-expressing neurons and glia. This is a most important transport system, critical for proper neurological function.

Now, let’s understand exactly what folate receptor autoantibodies (FRAAs) are, and what role they play.

FRAAs are immunoglobulin G (IgG) class autoantibodies produced by the host’s immune system that mistakenly target and bind to the body’s own FRα.

• They are classified into two functional types based on their effects on the cell and the transport of folate:
  • Blocking Antibodies: Bind to the folate-binding pocket of FRα, directly inhibiting the binding of 5-MTHF. This is considered the most functionally disruptive type.
  • Binding Antibodies: Bind to FRα at an epitope outside the binding pocket. While they do not directly block folate binding, they are thought to cause internalization and degradation of the receptor, effectively reducing its surface availability and disrupting proper folate transport.

The core hypothesis posits that FRAAs disrupt cerebral folate metabolism through a multi-step mechanism beginning with the transport blockage at the blood-csf barrier. Circulating FRAAs, particularly blocking antibodies, bind to FRα on choroid plexus epithelial cells. This competitively inhibits 5-MTHF binding and receptor-mediated transcytosis, leading to a selective cerebral folate deficiency (CFD). In essence, reduced CSF 5-MTHF leads to a condition of CFD, which is biochemically defined by low levels of 5-MTHF in the cerebrospinal fluid (CSF) despite normal or elevated serum folate levels. This creates a critical folate deficit within the central nervous system (CNS) microenvironment.

Folate deficiency can lead to impaired neurodevelopment. Deficiency during critical developmental windows can disrupt proper brain development, these processes. As a key methyl donor for S-adenosylmethionine (SAM), 5-MTHF deficiency limits DNA and histone methylation. This can lead to widespread alterations in gene expression patterns critical for neurodevelopment and synaptic function.

Folate cycle impairment also disrupts the synthesis of glutathione and NADPH, reducing antioxidant capacity. This increases neuronal vulnerability to oxidative stress and can impair mitochondrial energy production. Additionally, folate is involved in the synthesis of serotonin, dopamine, and norepinephrine. CFD may disrupt monoaminergic signaling, relevant to ASD behaviors.

The link between FRAAs and CFD with ASD symptoms is supported by clinical, biochemical, and interventional research, though it is considered a subset-specific mechanism, not universal to all ASD.

Multiple independent studies have reported a significantly higher prevalence of FRAAs in children with ASD (ranging from 44% to 75%) compared to neurotypical controls or children with other neurological disorders (typically < 10%).

Children with ASD and positive FRAAs are more likely to exhibit low CSF 5-MTHF levels, confirming the functional impact of the antibodies. Notably, many do not show hematological signs of systemic folate deficiency. The presence of FRAAs may define a distinct ASD endophenotype. Clinical features associated with this subset often include:

• Neurological regression (loss of language/motor skills) around 18-24 months.
• Co-occurring neurological disorders: epilepsy, motor dyspraxia, sleep disturbances.
• Higher frequency of language delay and irritability.

Polymorphisms in folate-pathway genes (e.g., MTHFR, DHFR) may also increase susceptibility to the effects of FRAAs or interact with them to exacerbate folate metabolic disruption.

The most compelling evidence for a causal link comes from intervention studies. Folinic acid (leucovorin, 5-formyltetrahydrofolate) is a reduced folate that can bypass the FRα blockade. It utilizes the RFC transporter to cross the BBB, thereby replenishing cerebral folate. Several open-label and a few randomized controlled trials (RCTs) have shown that high-dose folinic acid supplementation (2 mg/kg/day) in FRAA-positive children with ASD leads to:

• Significant improvements in communication, social interaction, attention, and stereotypical behaviors in a substantial subset of treated children.
• Correction of low CSF 5-MTHF levels.
• The response is most pronounced in those with both FRAAs and low CSF folate.

It is important to note that FRAAs are not a unitary cause of ASD. FRAAs are one identified biological mechanism contributing to a subset of ASD cases, consistent with the extreme heterogeneity of the disorder. Measurement of serum FRAAs conducted through the FRAT^® test (with specific differentiation of blocking vs. binding activity is moving towards clinical utility as a biomarker for a treatable subset of ASD.

Key questions do remain, however:

• The initial trigger for autoantibody production (molecular mimicry; genetic predisposition).
• The precise neurodevelopmental pathways disrupted by CFD.
• Optimizing treatment protocols and identifying predictive factors for treatment response.

The current body of evidence has led to increased recognition in specialized neurology and developmental medicine practices. It represents a paradigm of translational medicine: identifying a specific autoantibody, understanding its pathophysiological mechanism, and applying a targeted, rational treatment. The positive response of many affected individuals to folinic acid treatment provides strong proof-of-concept, positioning FRAAs as one of the most well-characterized and actionable biological pathways identified in ASD research to date.