What Is Methylation?
Methylation, or one-carbon metabolism, is the process of producing and then donating a methyl group to a substrate. It involves specific enzymes and nutrients, particularly vitamin B2, B6, B12 and B9, and serves several functions such as gene regulation (epigenetics), biotransformation, myelination, synthesis of neurotransmitters, as well as DNA and RNA synthesis. Aberrant one-carbon metabolism processes have been associated with raised homocysteine levels as well as neural tube defects (NTD’s), mood disorders and chronic diseases of lifestyle including certain cancers, and cardiovascular disease (CVD).
The DNA Health methylation panel comprises of SNPs on the following genes; MTHFR, MTR, MTRR, CBS and COMT. Variants in these genes, in combination with inadequate B-vitamins, and other essential methyl group donors, are associated with a decreased methylation index and thus an increased risk for disease. Understanding an individual’s methylation genetic profile will give insight to intervene appropriately, improving outcomes in this area.
Dietary folates are normally ingested as polyglutamates, which require deconjugation to monoglutamates by the enzyme, folyl poly‐γ‐glutamate carboxypeptidase (FGCP), prior to absorption, in order to be transported. Folate monoglutamates, including synthetic folic acid, are absorbed in the duodenum and upper part of the jejunum by the high‐affinity proton‐coupled folate transporter PCFT1, encoded by SLC46A1, where folic acid requires a further reduction reaction by dihydrofolate reductase (DHFR) and the riboflavin dependent enzyme, methylene tetrahydrofolate reductase (MTHFR), to form 5-methyl-tetrahydrofolate (5-MTHF). In the cell, 5-MTHF constitutes the main form of folate. It enters the cell via folate receptor (FR)‐α or reduced folate carrier (RFC).
5-MTHF is an essential cofactor for the remethylation of homocysteine (Hcy) back into methionine. Thus, once MTHFR has reduced 5,10-methyleneTHF to 5-MTHF, the Vitamin B12 dependent methionine synthase, encoded by MTR, uses this reduced folate to convert Hcy back into methionine. Methionine synthase reductase, encoded by MTRR, is responsible for reactivating methionine synthase to its functional form.
Once Hcy has been remethylated into methionine by methionine synthase, S-adenosylmethionine (SAMe/AdoMet) is then synthesised from methionine and ATP, by Methionine adenosyltransferase (MAT). SAMe is considered to be the universal methyl donor for all methylation reactions including donating a methyl group for DNA and RNA, proteins and neurotransmitters.
SAMe reactions result in the production of S‐adenosylhomocysteine (SAH/AdoHcy) where a substrate-specific methyl-transferase enzyme will utilise the methyl group from SAMe, thus producing a methylated substrate and a resulting SAH. SAH is then hydrolyzed to adenosine and Hcy by the enzyme S‐adenosylhomocysteine hydrolase (SAHH). Because the equilibrium of this reversible reaction favours SAH formation, which is a potent methylation inhibitor, Hcy and adenosine need to be metabolized to maintain low SAH levels. Hcy can thus be remethylated again, or can be metabolised to cysteine in the trans-sulfuration pathway by the vitamin B6 dependent enzyme, cystathionine beta synthase (CBS).
DNA methylation can be used as an example of the methylation process, where one-carbon metabolism has a major impact on epigenetic regulation. DNA methylation is a vital process that contributes to the area of epigenetics by regulating gene expression through methyl tags, without changing the DNA sequence. Both hypomethylation and hypermethylation can lead to improper gene expression and are thus associated with certain health disorders, including development of cancer. The two major factors that can affect DNA methylation include genes (non-modifiable) and the environment (modifiable), of which diet is a key factor. Gene variants on genes such as MTHFR encoding methylenetetrahydrofolate reductase, MTR encoding methionine synthase, MTRR encoding methionine synthase reductase and CBS encoding cystathionine-B synthase in the methylation pathway can lead to a decreased ability to create sufficient methyl donor (SAMe) and may lead to disruptions in the SAMe:SAH ratio, thus altering methylation capacity and epigenetic regulation. The other major cause of altered epigenetic regulation is due to diet. A diet that is poor in methyl donors, including, vitamin B2, B6, B12, folate and choline, is associated with aberrant methylation activity and increased risk for raised homocysteine levels.
Animal studies have shown that a diet with too little methyl-donating folate or choline before or just after birth causes certain regions of the genome to be under-methylated for life. In a mouse model of genetically identical, pregnant Agouti mice, large overweight mice that are prone to metabolic syndrome, were fed a bisphenol A (BPA) rich diet. On top of which, one group was fed a methyl rich diet, and the other group, a normal mouse diet. Interestingly, the offspring of the mice who were fed the normal mouse diet together with the BPA rich diet showed expression of the Agouti gene in that the mice were phenotypically also large and yellow, displaying the same risks as their mothers. However, the mice that were fed the BPA rich diet together with the methyl rich diet, did not show the phenotypic expression of the Agouti gene as these mice were small, not overweight, with brown fur. This highlights the importance of diet as a key factor in methylation and epigenetics.
One’s mother’s diet during pregnancy and one’s diet as an infant can affect one’s epigenome in ways that stick with an individual into adulthood. For adults too, a methyl-deficient diet leads to a decrease in DNA methylation, but the changes can be reversible when methyl groups are added back to diet.
The Importance Of Methylation
Studies have shown that there is an association between raised Hcy levels and increased risk for a number of diseases, including CVD, certain cancers, mood disorders and dementia. Variations in genes encoding one-carbon metabolism enzymes have been implicated as risk factors for raised Hcy levels, especially when B-vitamins are inadequate.
Individuals specifically at risk for low B9 (folate) or B12 vitamin status and having high Hcy levels are as follows: Those who have a high alcohol intake, smokers, those with a personal/family history of premature cardiovascular disease, malnutrition/malabsorption syndromes, hypothyroidism, kidney failure, and lupus. Individuals taking certain medications that include nicotinic acid, theophylline, bile acid-binding resins, methotrexate, and L-dopa all interact with folate and may increase risk for folate deficiency. Other medications such as proton pump inhibitors, statins, and metformin, interact with vitamin B12 and may increase risk for deficiency. For these individuals, special care should be taken when assessing genetic risk together with environmental vulnerabilities.
Personalised lifestyle and nutrition interventions for improving methylation and decreasing risk for associated diseases of lifestyle will be given according to genotype. It is, however, important to take the full panel into account to ensure a holistic, personalised plan is provided.
When considering general strategies for improving biomarkers associated with disruptions in methylation, the Mediterranean style diet has been shown to yield positive results. Stronger adherence to the Mediterranean style diet showed improvements in folate status as well as homocysteine levels.
Specific nutrients that have been shown to be beneficial in optimizing methylation status include choline (eggs, liver and peanuts), betaine (spinach, beets, shellfish), vitamin B2 (milk, eggs, almonds), B6 (salmon, turkey, avocado), B12 (clams, mussels, beef) and folate (lentils, asparagus, spinach). The levels required are based on unique needs associated with age, biological sex, and reproductive status (i.e., pregnancy or lactation), particular physiopathological status, as well as genetic variants. In some cases, where dietary intake is insufficient, supplementation may be recommended.
Methylation PowerPoint Presentation
Polymorphisms in 1-Carbon Metabolism, Epigenetics and Folate-Related Pathologies
Homocysteine and DNA methylation: A review of animal and
Mandaviya et al, 2014