MAO-A and MAO-B Gene Variants: Neurotransmitter Metabolism and Dysautonomia

Monoamine oxidase A (MAO-A) and monoamine oxidase B (MAO-B) are enzymes that break down monoamine neurotransmitters — serotonin, dopamine, norepinephrine, and histamine. Variants in the genes encoding these enzymes can significantly affect neurotransmitter levels, autonomic function, and the response to medications used in dysautonomia treatment.

What MAO-A and MAO-B Do

MAO-A primarily metabolizes serotonin, norepinephrine, and dopamine in the brain and peripheral tissues. It is the primary enzyme responsible for breaking down dietary tyramine and histamine in the gut, making it critical for histamine intolerance.

MAO-B primarily metabolizes dopamine and phenylethylamine (PEA) in the brain. It plays a smaller role in histamine metabolism than MAO-A.

Both enzymes are located on the X chromosome (Xp11.23), which means males have only one copy of each gene and are more severely affected by loss-of-function variants.

Key Variants and Their Effects

MAO-A low-activity variants (rs6323, VNTR polymorphism). The MAO-A VNTR (variable number tandem repeat) polymorphism in the promoter region affects gene expression. The 3-repeat allele is associated with lower MAO-A activity than the 4-repeat allele. Low MAO-A activity leads to:

• Higher serotonin and norepinephrine levels
• Increased histamine accumulation (reduced histamine breakdown)
• Greater sensitivity to dietary tyramine and histamine
• Potentially higher anxiety and stress reactivity

MAO-A high-activity variants. High-activity MAO-A variants (3.5-repeat or 4-repeat alleles) break down serotonin and norepinephrine more rapidly, potentially contributing to:

• Lower serotonin levels
• Depression and mood disorders
• Reduced response to SSRIs (which work by blocking serotonin reuptake — if MAO-A is breaking it down faster, SSRIs may be less effective)

MAO-B rs1799836 (A/G polymorphism). This variant affects MAO-B activity and has been associated with differences in dopamine metabolism and Parkinson's disease risk.

Implications for Dysautonomia

Histamine intolerance. Low MAO-A activity is one of the most important genetic contributors to histamine intolerance. Patients with low MAO-A activity variants who also have MCAS may have a particularly severe histamine burden, because they are both producing excess histamine (through mast cell activation) and breaking it down more slowly.

Norepinephrine metabolism. In hyperadrenergic POTS, elevated norepinephrine levels are a defining feature. Low MAO-A activity could contribute to elevated norepinephrine by reducing its breakdown. Conversely, high MAO-A activity could contribute to norepinephrine deficiency in hypovolemic or neuropathic POTS.

Medication responses:

• SNRIs (duloxetine, venlafaxine): These medications increase serotonin and norepinephrine by blocking their reuptake. Patients with low MAO-A activity may have a stronger response (or more side effects) because they already have higher baseline levels.
• MAO inhibitors (phenelzine, tranylcypromine): These medications are contraindicated in patients taking SNRIs, SSRIs, or consuming tyramine-rich foods. Patients with low MAO-A activity variants may be at higher risk for serotonin syndrome if MAO inhibitors are used.
• Dietary tyramine. Patients with low MAO-A activity should be cautious with high-tyramine foods (aged cheeses, fermented foods, cured meats) even without MAO inhibitor use.

Testing

MAO-A and MAO-B variants are included in most consumer genetic testing panels (23andMe, AncestryDNA) and can be analyzed through third-party interpretation services. Clinical genetic testing is available through specialized laboratories.

Plasma MAO-B activity. Platelet MAO-B activity can be measured directly in blood and reflects overall MAO-B function, though this test is primarily used in research settings.

Practical Guidance

Patients with low MAO-A activity variants should:

• Follow a low-histamine diet if histamine intolerance symptoms are present
• Be cautious with high-tyramine foods
• Discuss MAO-A status with their prescribing physician before starting SNRIs or other serotonergic medications
• Consider DAO enzyme supplementation (which breaks down dietary histamine) if histamine intolerance is significant

ChatDys resources: Upload your 23andMe or AncestryDNA raw data to the Genetics tab for a complete SNP analysis. Review our MTHFR guide and our HNMT histamine gene article for related genetic information. Track your histamine reactions and medication responses in the Health Tracker.