In this context, variants in the monoamine-degrading enzymes, catechol-O-methyltransferase (COMT) and monoamine oxidase A (MAOA), modulate brain dopamine, norepinephrine, and serotonin levels and functions and have been linked to both depression and obesity [86]. This evidence concerns the gene COMT and obesity due to melanocortin 4 receptor deficiency.