New insights on biomarkers reflecting the genetic deficiency of folate cycle in autism spectrum disorder

AUTORI

ABSTRACT

Autism (OMIM®: 209850) is a pervasive, heterogeneous, and heritable neurodevelopmental disability with a multifactorial origin and pathogenesis. One decade ago, the term autism was replaced with autism spectrum disorder (ASD), an umbrella term embracing a broad spectrum of persistent impairments in social interactions with stereotyped and repetitive patterns of behaviors, interests, or activities (1). Over the last three decades, the prevalence of ASD has considerably increased worldwide, although with different estimates, depending on the method of ascertainment and the definition of ASD used in various studies (2). Approximately one in 100 children are diagnosed with ASD worldwide, with a median male-to-female ratio of 4.2 and 33% of cases with co-occurring intellectual disability (3). From 2000 to 2016, among 8-year-olds residing in the New York-New Jersey Metropolitan Area, ASD with intellectual disability increased by 200%, and ASD without intellectual disability increased by 500% (4). A very recent report from the U.S. Centers for Disease Control and Prevention (CDC) revealed a prevalence of ASD in 2020 equal to one in 36 and one in 47 among 8-year-olds and 4-year-olds U.S. children, respectively (5).
Multiple genetic and non-genetic prenatal, perinatal and postnatal factors are involved in the etiology and pathogenesis of ASD. Frequently, the association between two or more factors positively correlates with the severity of the disease. In addition to autism core symptoms, affected individuals have a higher burden of comorbidities, such as inborn errors of metabolism, immune and gastrointestinal diseases, epilepsy, sleep disorders, obesity, and many others (6). Thus, many ASD children exhibit various metabolic abnormalities, including perturbations in mitochondrial, neurotransmitters, redox, cobalamin, tetrahydrobiopterin (BH4), carnitine, and folate metabolism. In particular, folate metabolism is one of the most effective models of genome-environment interactions.
The term “folates” refers to as many as 150 distinct molecular species (less than 50 species are detectable in natural animal and plant sources) consisting of a 2-amino-4-hydroxy-pteridine ring linked by a methylene group to a p-aminobenzoyl moiety. The latter is linked through an amide bond to the α-amino group of a monoglutamate or poly-g-glutamate. The synthetic form containing the fully oxidized pteridine ring and no one carbon (1C) substitution is commonly termed folic acid.
Folate is an essential water-soluble B vitamin (vitamin B9) that cannot be produced de novo by the human body. Thus, folate must be derived from the diet. Interestingly, enteric Bifidobacteria, Prevotella, S. thermophilus, and certain Lactobacilli spp. are able to synthesize folate (7). Folate plays a key role in developing neurological pathways and the growth of major cell types within the central nervous system. Preventing neural tube defects by folic acid and other cofactors (i.e., vitamin B12) supplementation during the first trimester of pregnancy and beyond has been a well-known notion for a long time (8).
Folate from the diet is enzymatically converted to the bioactive form tetrahydrofolate (THF), the backbone for 1C reactions; the main circulating blood form of folate is 5-methyltetrahydrofolate (5-MTHF). Multiple physiological processes, such as amino acid homeostasis (methionine, glycine, and serine), nucleotide biosynthesis (purines, thymidine), epigenetic maintenance, and redox defense, are supported by 1C metabolism mediated by folate cofactors (9). Therefore, the folate cycle is closely linked to the methionine-homocysteine cycle, essential for DNA methylation and the transsulphuration pathway, leading to cysteine and glutathione synthesis.
Inherited folate metabolism and transport disorders, such as polymorphisms in the methylenetetrahydrofolate reductase (MTHFR) gene, and factors such as autoantibodies anti-folate receptor (the transporter of 5-MTHF into the central nervous system), gut dysbiosis, and food selectivity cause folate depletion, both in pregnant women and their offspring. Although several studies suggest a significant association between these factors and the risk of autism, the landscape of biochemical abnormalities associated with genetic deficiency of the folate cycle remains to be elucidated. The study of Maltsev et al. (10), published in the current issue of Biochimica Clinica, clarifies the biochemical changes associated with genetic deficiency of the folate cycle in autism. The authors measured eight serum biomarkers in 225 ASD children with genetic folate deficiency and 51 neurotypical children without folate depletion.
Overall, comparison between groups revealed a significant increase in serum levels of creatine kinase (nearly 6-folds), homocysteine (nearly 3-folds), creatinine and lactate dehydrogenase (nearly 2-folds) and a substantial decrease in folic acid (nearly 6-folds), vitamins B3 and B6 (nearly 3-folds), and vitamin D3 (nearly 2.5-folds) in the group of autistic children with genetic deficiency of folate. The most frequent metabolic perturbation among ASD children with genetic folate deficiency was hyperhomocysteinemia.
The association between the genotype of folate deficiency and alterations in the panel of biomarkers was the most challenging investigation. The authors evaluated eight genotypes, including the single nucleotide polymorphism cytosine to thymidine switch at nucleotide 677 in MTHFR (MTHFR C677T) and seven multiple polymorphisms. They found specific changes in the biomarkers panel associated with each genotype of folate deficiency. Notably, the severity of biochemical perturbations depended on the genotype, ranging from the mildest, associated with MTHFR C677T, to the most severe, associated with the genotype MTHFR C677T + MTHFR A1298C + MTR A2756G + MTRR A66G.
The paper of Maltsev et al. (10) confirms the close relationship between genetics and metabolism and the need to investigate human health and disease by the system biology approach. In this context, identifying and quantifying metabolites in serum and other biological fluids, namely metabolomics, offers the opportunity to characterize the individual molecular phenotype, closely dependent on all the variables involved in a given health or disease status (e.g., genome, environment, microbiome) (11). The complexity of ASD, its frequent association with comorbidities and congenital disorders, and the role of environmental factors, including diet, microbiome, and toxicants, contribute to the very high clinical and molecular heterogeneity of autism (12). Consequently, no two autistic people are exactly alike, no stereotyped model can strictly be associated with a given phenotype (clinical and molecular), no single biomarker can be effectively used for the diagnosis and management of the disease, and no single treatment is ever sufficient and adequate for all individuals with autism (13). Hence, any therapeutic treatment should be individualized based on the individual phenotype. Assessing the individual metabolic phenotype, also called metabotype, could represent an extraordinary tool for developing a neonatal screening for autism, especially in babies with no familiar history of genetic ASD or with no evident risk factor.

Conflict of interest

None.

BIBLIOGRAFIA

1. American Psychiatric Association. Diagnostic and statistical manual of mental disorders (DSM-5), 5th ed.; American Psychiatric Association: Arlington, VA, USA, 2013
2. Chiarotti F, Venerosi A. Epidemiology of Autism Spectrum Disorders: A review of worldwide prevalence estimates since 2014. Brain Sci 2020;10:274.
3. Zeidan J, Fombonne E, Scorah J, Ibrahim A, Durkin MS, Saxena S, et al. Global prevalence of autism: a systematic review update. Autism Res 2022;15:778-90.
4. Shenouda J, Barrett E, Davidow AL, Sidwell K, Lescott C, Halperin W, et al. Prevalence and disparities in the detection of autism without intellectual disability. Pediatrics 2023;151:e2022056594.
5. Maenner MJ, Warren Z, Williams AR, Amoakohene E, Bakian AV, Bilder DA, et al. Prevalence and characteristics of autism spectrum disorder among children aged 8 Years – autism and developmental disabilities monitoring network, 11 Sites, United States, 2020. MMWR Surveill Summ 2023;72:1-14.
6. Khachadourian V, Mahjani B, Sandin S, Kolevzon A, Buxbaum JD, Reichenberg A, et al. Comorbidities in autism spectrum disorder and their etiologies. Transl Psychiatry 2023;13:71.
7. Engevik MA, Morra CN, Röth D, Engevik K, Spinler JK, Devaraj S, et al. Microbial metabolic capacity for intestinal folate Production and modulation of host folate receptors. Front Microbiol 2019;10:2305.
8. McNulty H, Rollins M, Cassidy T, Caffrey A, Marshall B, Dornan J, et al. Effect of continued folic acid supplementation beyond the first trimester of pregnancy on cognitive performance in the child: a follow-up study from a randomized controlled trial (FASSTT Offspring Trial). BMC Med 2019;17:196.
9. Zheng Y, Cantley LC. Toward a better understanding of folate metabolism in health and disease. J Exp Med. 2019;216:253-66
10. Maltsev D, Kurchenko A, Marushko Y, Yuriev S. Biochemical profile of children with autism spectrum disorders associated with genetic deficiency of the folate cycle. Biochim Clin 2023
11. D’Argenio V, Dittfeld L, Lazzeri P, Tomaiuolo R, Tasciotti E. Unraveling the balance between genes, microbes, lifestyle and the environment to improve healthy reproduction. Genes 2021;12:605.
12. Fanos V, Noto A, Mussap M. The juniper bush of autism spectrum disorder (ASD): metabolomics, microbiomics, acetaminophen. What else? J Pediatr Neonat Individual Med 2018;7:e070205.
13. Lord C, Charman T, Havdahl A, Carbone P, Anagnostou E, Boyd B, et al. The Lancet Commission on the future of care and clinical research in autism. Lancet 2022;399:271-334. Erratum in: Lancet 2022;400:1926.