A1298C MTHFR Single Nucleotide Polymorphism Decreases Dopamine Synthesis

23 Feb 2018 Research

Dopamine is an important neurotransmitter that is heavily dependent upon the methylation metabolic pathway which can be reduced in some people that carry a mutation of the A1298C gene.

Methylation can be described as the transferring of a methyl group from one compound to another. This methylation process is necessary for many biochemical reactions including making neurotransmitters, immune responses, gene regulation, processing chemicals – endogenous and xenobiotic compounds, metabolism of neurotransmitters, processing hormones, DNA and histone synthesis, energy production (ATP), myelination, and building and maintaining cell membranes. (10,11)

With optimal operation of the methylation cycle we can expect the methyl groups to be passed easily from one compound to the next. The compound that receives the methyl group is “methylated”, but then it can quickly turn around and donate that same methyl group to another compound, and so on until the final product is reached.

Many enzymes and co-factors are present to facilitate this reaction, and without the presence of the enzymes and their co-factors, this reaction would happen too slowly to be compatible with life. The methylation cycle that occurs inside the cells happens on a grand scale, in that it’s not just one singular cycle happening at a time, but many methylation reactions happening simultaneously. (10,11)

Why would we need so many methylation reactions in a single cell to happen simultaneously? Well, as NASA would put it , “redundancy” or as a “back-up plan” and for energy efficiency. Should something not work properly such as not having enough cofactors or substrate available, the cell can still survive because there are hundreds of the same type of reactions happening all at once, and surely due to the sheer number, the cell will survive.

Why would the body need to create a separate cell for each methylation reaction to occur? In fact, it doesn’t have to because it is able to provide a safe, compartmentalized space in each cell for each reaction to occur, thereby adhering to the First Law of Thermodynamics which also called the Law of Conservation of energy which can be applied to all thermodynamic systems including the body which is designed to function wasting as little energy as possible.

Dopamine is mostly produced in the central nervous system and to some degree in the peripheral nervous system. It is the precursor to epineprine, and norepinephrine, therefore it essentially regulates nerve transmission throughout the body.

It is produced from the action of tyrosine hydroxylase enzyme, along with the combination of oxygen and TetrahydroBiopterin (BH4) on tyrosine (rate-limiting step) to make DOPA, then further metabolized by a decarboxylation reaction to make dopamine. An impairment in the synthesis of dopamine in certain regions in the central nervous system resulting in decreased dopamine levels can cause problems in motor movements such as seen in Parkinson’s disease (4).

Genetic mutations or single nucleotide polymorphism’s (SNP’s)  are inherited changes in the DNA sequencing of a gene. There are four nucleotides that make up a DNA sequence: adenine (A), thymine(T), cytosine (C), and guanine(G). A SNP may substitute a T for A or G for C, changing the chemical properties of the protein which in turn can affect the properties such as bonding, affinity, or allosterism which brings substrates together, puts them in alignment with one another, and catalyzes the chemical reaction needed for an enzymatic reaction to occur.

Conservative mutations do not affect the overall activity of the gene, however non-conservative mutations do affect the activity of the gene. The effects of a non-conservative mutation on an enzyme can vary depending upon the gene affected and its respective function.

A mutation (SNP) can cause an increase or decrease an enzymes’ activity in a metabolic pathway. The degree to which a SNP affects an enzymes’ activity is partly  dependent upon whether or not it is a homozygous or heterozygous SNP.

We each have two copies of a gene, one from our mother and one from our father. A homozygous SNP (++) is where an individual has inherited a mutated gene from each parent. A heterozygous SNP  (+-) is where an individual has only inherited one mutated gene from one parent, the other gene is normal. We can usually expect a homozygous SNP to have a greater impact on the enzymes ability to work properly, by either up-regulating  gene activity or by down-regulating it.

A SNP in the A1298C gene consists of an A to C transition at the 1298 position of the methylenetetrahydrofolate reductase(MTHFR) protein yielding a glutamate to alanine substitution which has a greater impact on enzymatic activity in homozygous than for heterozygous individuals(1).

This SNP decreases the activity of the MTHFR enzyme. MTHFR A1298C heterozygous has a 20% loss of function, to where the homozygous has a 40% loss of function (2). The decrease in function of the MTHFR enzyme due to a MTHFR A1298C SNP causes a reduction of the conversion of Dihydropiopterin (BH2) to TetrahydroBiopterin (BH4), a rate-limiting co-factor required in several reactions, including neurotransmitter synthesis (epinephrine, norepinephrine, dopamine, serotonin, and melatonin), nitric oxide production in the urea cycle, the conversion of phenylalanine to tyrosine, and the alternate route for the conversion of homocysteine to methionine.

Tetrahydrobiopterin (BH4) is synthesized in the de novo pathway by GTP cyclohydrolase, 6-pyruvol-tetrahydrobiopterin synthetase (6-PTS), pyruvoyltetrahydropterin reductase (PTP reductase), and sepiapterin reductase enzymes, along with the co-factors Zinc, Magnesium, and NADPH. Once BH4 is used as a co-factor and is converted to BH2, then it can be regenerated back to BH4.

The regeneration of biopterin requires pterin-4a-carbinolamine dehydrase and dihydropterinereductase.(10,11) The DHFR enzyme that makes BH2 is the only pathway for metabolizing synthetic folate and can further inhibit the formation BH4. Synthetic folic acid will compete with BH2 for the DHFR enzyme, so people with an A1298C mutation taking synthetic folate can have an even greater reduction in enzymatic activity by tying up BH2 resulting in less BH4 available for the sequential reactions (3). Co-factors that aid in recycling of BH2 to BH4 supplements containing magnesium, folate, copper, B6, BH4 supplementation, and vitamin C are recommended. (8,9)

The amount of dopamine synthesized is directly proportional to the amount of BH4 that is available, because BH4 is produced upstream of the catecholamine synthesis pathway and is the rate-limiting step for making Dopamine.

BH4 is a cofactor for the hydroxylase enzymes that metabolize the aromatic amino acids such as tryptophan hydroxylase (TPH), phenylalanine hydroxylase (PAH), and tyrosine hydroxylase (TH). All of these hydroxylase enzymes require BH4, iron, and oxygen to exert their actions.

The two brain enzymes, tryptophan hydroxylase and tyrosine hydroxylase have a lower affinity for BH4 (Km30M) where as the hydroxylase enzymes for phenylalanine and nitric oxide is higher (Km2-3µM – 0.02-0.03µM) respectively. Perhaps the higher affinity to phenylalanine is due to the fact that it is more important to metabolize phenylalanine because high levels of unmetabolized phenylalanine is neurotoxic, and the higher affinity to nitric oxide synthase (NOS) is due to its important function to maintain vascular tone.

Arginine does not affect BH4 synthesis, but phenylalanine is found to stimulate BH4 synthesis, which makes sense since phenylalanine needs to make sure that there is enough BH4 available for its’ own metabolism to prevent brain damage and Arginine is involved in the forward moving direction of urea removal that does not have a negative feedback inhibition controlling this cycle in order to ensure the safe removal of ammonia from the body (7). In people with an A1298C SNP, the BH4 that is available is first going to be coupled with the NOS enzyme to make nitric oxide since it has the highest affinity, phenylalanine hydroxylase enzyme is next in line to use up BH4, then the two brain enzymes, TPH and TH get what is left over of the BH4 to make their respective products, serotonin and dopamine.

This is not good news for the conversion of tyrosine and tryptophan in the central nervous system to make neurotransmitters, such as dopamine and serotonin for people with a genetic mutation that results in the lowering of the availability of BH4, as this may have a big impact on neurological function and mood disorders.

The control of Dopamine release by BH4 was studied in the striatum of rats by administering BH4 through the dialysis membrane. This study showed that Dopamine continued to be released even when blocking synthesis and reuptake of Dopamine. However, when the Na+ and Ca++ channels for Dopamine reuptake were blocked along with concurrent administration of BH4, studies showed that Dopamine would not be released. A test that blocked the co-factors and precursors, tyrosine, tyrosine hydroxylase, and NOS, while adminsitering BH4 resulted in a direct release of Dopamine independent of its precursors by acting outside of the neuron. This mechanism is not completely understood. Concurrent administration of Arginine and BH4 also caused a release of dopamine in the rat striatum which was concentration dependent. (7)

In another study, knock-out mice lacking the BH4-biosynthesizing enzyme sepiapterin reductase and 6-pyruvoyl-BH4 enzymes were used to assess the development of the dopamine pathway and its dependency upon BH4 in infant mice. The study compared the dopamine development in normal mice (Spr +/+) compared to mutant mice (Spr -/-) for the first 21 days of development. The mice were dissected and the tissue was homogenized and analyzed by Western blot. The brains of the mice were biochemically analyzed at postnatal days 0,7, and 14. The result of the normal mice, and heterozygous mice did not show a significant difference in BH4 concentration at any point, but the mutant mice had lower BH4 than the normal mice at each point. Next, the monoamine neurotransmitters and their respective metabolites  were measured to reveal how they would be affected by low BH4 levels. As expected, the normal mice (Spr +/+) had an increase of dopamine (3.9  +/- 2.0, 8.0 +/- 1.2, and 14.6 +/- 3.1 pmol/mg of protein at Days 0,7, and 14, respectively). The dopamine level of the mutant mice at Day 0 was not statistically different from normal mice, but the mutant mice did not show an increase at Day 7 or Day 14. The metabolite of Dopamine, homovanillic acid in the brains of the mutant mice compared to normal mice at Day 0 were similar, but then at Day 7 and 14 did not show a significant increase when compared to Day 0. The graph above depicts the results of this study Phenylalanine levels  and tyrosine levels in the brains of the mutant mice deficient in BH4 were measured since we would expect low levels of BH4 to result in high phenlalanine and tryosine, and ultimately resulting in decreased dopamine. The mutant mice showed a marked increase of phenylalanine from Day0 to Day 7. As expected, and tyrosine was reduced in the brains of the mutant mice when compared the normal mice.  This study shows how BH4 deficiency can negatively impact the concentration of Dopamine in the brain in animals, which in turn can affect all neurological processes that are Dopamine dependent(6).

Treatment of BH4 deficiency that results in a lower production of dopamine causing symptoms usually consists of a diet low in phenylalanine  to combat high levels of phenylalanine, L-dopa and 5-hydroxytryptophan (a neurotransmitter precursor) since it can pass the blood brain barrier easier than supplementing with BH4 to help stabilize the neurotransmitter deficiency.(5)

Folinic acid is indicated since a folate deficiency may be present due to the inhibition of the production of 5-MTHFR.  Kuvan(sapropterin) is relatively new a prescription medication often used in patients that have a  biopterin deficiency such as in PKU patients to lower the blood levels of phenylalanine that is being used with success to help with dopamine related dystonias.

Dopamine plays an important role in many brain functions such as mood, memory, and attention as seen in disorders such as Attention Deficit Disorder, Autism, Alzheimer’s Disease, Parkinson’s Disease, and depression. Since dopamine is a precursor to epinephrine and norepinephrine, other symptoms related to blood pressure and movement may become prevalent as well.

The genetic mutation A1298C is an important connection to low dopamine levels in patients. Any patient that presents with dopamine related symptoms should have a genetic workup for this mutation because these patients may not be able to achieve optimal health even with a well balanced diet. However, with genetic information and proper intervention, these individuals would be able to lead a normal and productive life.

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(1) Friedman, G., Goldschmidt, N., Friedlander, Y., Ben-Yehuda, A., Selhub, J., Babaey, S., et al. (n.d.). Journal of Nutrition. A Common Mutation A1298C in Human Methylenetetrahydrofolate Reductase Gene: Association with Plasma Total Homocysteine and Folate Concentrations J. Nutr. September 1, 1999 vol. 129 no. 9 1656-1661

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