There are over 100 methylation reactions within  the cell, the majority of which require S-adenosylmethionine (SAM), the production of which is intimately associated with the uptake of dietary methionine, the folate cycle and methylcobalamin. Normal cycling of folate results in the formation of an irreversible reaction in which 5, 10 methylenetetrahydrofolate is converted to 5-methyltetrahydrydrofolate (5-MTHF) by the enzyme 5,10 methyltetrahydrofolate reductase (MTHFR). Of note in this reaction is that MTHFR uses FAD (derived from riboflavin, vitamin B2) as an essential cofactor. Without this cofactor the enzyme, MTHFR will not function. In addition, the reduction that occurs requires input from NADH + H⁺ (derived from nicotinamide, vitamin B3). In the absence of these two vitamins (B2/B3) the enzyme is not functional. The activity of the enzyme can be affected by various mutations in the protein. Mutations that affect function generally involve the binding of FAD to the protein, and are very sensitive to FAD concentration.

In the absence of vitamin B₁₂ (Co(I)-Cbl), dietary 5-MTHF cannot enter the folate cycle and is not polyglutaminated, and cannot accumulate inside the cell, with the result that the levels of available intracellular folate are reduced> In the absence of methylcobalamin, the levels of homocysteine, an end product of the methylation cycle,  are gradually raised, eventually leaving the cell and increases in serum. Elevated homocysteine is associated with an increased risk in cardiovascular disease, as well as regional brain atrophy..

The majority of the methyl groups that are used to make 5MTHF in the cell come from serine, via the enzyme serine hydroxymethyltransferase (SHMT)(See JPG). This is then transferred via MTHFR to the methylation cycle as 5MTHF. In order for the methyl group "acquired" during the folate cycling to enter the methylation cycle, vitamin B12 (Co(I)-Cbl) associated with methionine synthase accepts the methyl group from 5MTHF which then forms both methyl-cobalamin (MeCo(III)Cbl) and tetrahydrofolate (THF). THF can then enter the folate cycle. Methionine synthase then transfers the methyl group from MeCbl to homocysteine, which is converted to methionine, and Co(I)Cbl is reformed.

Mutations in the MTHFR gene.

There many different mutations in the MTHFR gene, and nearly everyone has one or more, variants of the gene, which may or may not affect the function of the MTHFR enzyme. Unfortunately the frequency of such variants is not widely understood in the general population, so much so that a certain amount of web-hysteria has result. As an example, one of the common genetic allels is known as the MTHFRC665T allele (rs1801133). Roughly half the population (or around 3.5 billion people) are heterozygous, CT, for this allele, with 12-20% (depending upon ethnicity) homozygous TT and 30-38% homozygous CC. In persons with some mutations in the MTHFR gene, the enzyme has slightly reduced activity and so the speed at which 5MTHF is produced from the folate cycle is reduced, and so the 5MTHF + Co(I)Cbl <=> THF + MeCbl reaction operates slightly less efficiently. As a result new/incoming supplies of MeCbl must be used for processing homocysteine, leading to a more rapid reduction in vitamin B12. This reduction is most rapid in conditions of low functional vitamin B2 (FAD).  (The extent of reduction is dependent upon how many and which types of mutations occur in the MTHFR and other methylation associated genes).

Every methyl group used in synthesis must be supplied by MeCbl. , either acquired by supplementation or from 5MTHF. In the absence of sufficient 5MTHF, there is  rapid depletion of VB12 stores. An alternative solution is to supply dietary folate as 5MTHF (See Fig2), or increase the amount of folate that is cycling via a folic acid supplement (See Fig3)  and Co(I)+Cbl, but as this also is not recycled a new molecule of 5MTHF must always be supplied or the newly synthesized Co(I)-Cbl also will run out. Thus in these individuals they need a constant supply of both MeCbl AND 5MTHF. An alternative solution is to increase the amount of vitamin B2, which will increase the activity of MTHFR. Other genetic mutations which affect these reactions are mutations in the genes for methionine synthase, methionine synthase reductase, SHMT and cystathione synthase.

Production of SAM, its relationship to folate and the build up of homocysteine

In normal individuals, MeCbl reacts with homocysteine, to form methionine and Co(I)Cbl is formed. If sufficient folate is present, 5-methyltetrahydrofolate is produced as an alternative product of  the folate cycle. In the presence of Co(I)Cbl this methyl group is transferred from 5MTHF to Co(I)Cbl to form MeCbl and THF is regenerated.

In vitamin B12 deficient individuals, homocysteine accumulates within the cell and cannot be used to  convert 5MTHF to THF. This in itself can lead to functional folate deficiency within the cell. Thus, although serum levels may be normal or elevated. Co(I)Cbl is absent and so the methyl group from dietary 5MTHF cannot be transferred to MeCbl and thence to homocysteine => methionine and hence a deficiency in SAM results. DNA, RNA and proteins are not methylated and this can lead to defective function and cancer.

Even in people who start being vitamin B12 sufficient, the constant demand for methylation, and the lack of 5MTHF, leads to the usage of MeCbl and ultimately leads to vitamin B12 deficiency.

MTHFR mutations have also been correlated with a higher incidence of various conditions including  heart disease, stroke, high blood pressure (hypertension), high blood pressure during pregnancy (preeclampsia),  glaucoma, psychiatric disorders (schizophrenia, depression, bipolar disease), and certain types of cancer. The correlation though is somewhat small as too is the increased incidence (generally less than two-fold).

More recently studies have shown a significant correlation between MTHFR TT mutations, reduced vitamin B12 and Parkinson's Disease. A positive correlation has also been shown between MTHFR mutations and rheumatoid arthritis

More recently it has been shown the many people with Chronic Fatigue Syndrome (CFS/ME)  have methylation mutations

Other important methylation reactions

Apart from the formation of methylated myelin basic protein, the are over 200 methylation reactions in the body including the production of creatine (involved in energy production), CoQ10 (an essential electron shuffle vector in the electron transport chain), carnitine (involved in uptake of fatty acids into mitochondria), the production of phosphatidyl-choline, adrenalin, melatonin (essential for neuronal development, gut health, and sleep),myelin basic protein, methylation of lysine, histidine and arginine, and in the inactivation of histamine, dopamine, nor-adrenalin, and adrenalin, amongst others.

Role of MTHFR and Multiple Sclerosis

Essential in the formation of myelin sheath is its association with myelin basic protein (MBP). In order for MBP to associate with the myelin it must first be post-transationally modified by methylation of arginine. If the arginine is not methylated, the structure of MBP is changed and otherwise buried arginine residues are exposed. These can then act as substrates for nitric oxide synthase (NOS), which converts arginine to citrulline. This then is a different MBP from normal and hence can generate an immune response to MBP and MS may develop.  MTHFR mutations have been correlated with MS, as too has VB₁₂ deficiency.

MTHFR and Pregnancy

Many people with methylation mutations such as MTHFR have trouble getting pregnant or maintaining their pregnancy. The baby born to an MTHFR positive mother and/or father can inherit the condition.  A lack of folate/and or vitamin B12 particularly in these individuals can lead to Spina bifida, anenceaphaly, and Down's syndrome. Elevated homocysteine can lead to low birth weight and premature babies. Previous studies suggested that more than 80% of babies with autism have some kind of methylation mutation. More recent studies performed by out group have contradicted this. Our studies have rather shown that every child with autism has a combination of functional vitamin B2 deficiency and paradoxical B12 deficiency. This functional B2 deficiency leads to reduced activity of the MTHFR enzyme, particularly in those with mutations in the gene, and as such  regeneration of 5MTHF from within the folate cycle is reduced, leading to lower levels of methylation in these individuals.