Ms. T, age 55, presents to her psychiatrist’s clinic with a chief complaint of ongoing symptoms of anhedonia and lethargy related to her diagnosis of major depressive disorder (MDD). She also has a history of peripheral arterial disease, hypothyroidism, and generalized anxiety disorder. Her current antidepressant regimen is duloxetine, 60 mg/d, and mirtazapine, 15 mg at night. She recently elected to undergo pharmacogenetic testing, which showed that she is heterozygous for the methylenetetrahydrofolate reductase (MTHFR) C677T mutation (MTHFR C677T CT carrier). Her test report states that she may have impaired folate metabolism. Her psychiatrist adds L-methylfolate, 15 mg/d, to her current antidepressant regimen.
What is the relationship between folic acid and MTHFR?
Methylenetetrahydrofolate reductase is an intracellular enzyme responsible for one of several steps involved in converting dietary folic acid to its physiologically active form, L-methylfolate.1 Once active, L-methylfolate can be transported into the CNS, where it participates in one-carbon transfer reactions.2,3 Mutations in the MTHFR gene have been associated with decreased activity of the enzyme, which has been shown to result in accumulation of homocysteine and may lead to decreased synthesis of neurotransmitters.2,4Commercial pharmacogenetic testing panels may offer MTHFR genetic testing to assist with prescribing decisions for patients with mental illness. The most well-characterized mutation currently is C677T (rsID1801133), which is a single amino acid base pair change (cytosine [C] to thymine [T]) that leads to increased thermolability and instability of the enzyme.5 Carrying 1 or 2 T alleles can lead to a 35% or 70% reduction in enzyme activity, respectively. The T variant allele is most frequent in Hispanics (20% to 25%), Asians (up to 63%), and Caucasians (8% to 20%); however, it is relatively uncommon in African Americans (<2%).5,6 Another variant, A1289C (rs1801131), has also been associated with decreased enzyme function, particularly when analyzed in combination with C677T. However, carrying the 1289C variant allele does not appear to result in as large of a reduction of enzyme function as the 677T variant.7
What is the relationship between MTHFR C677T and depression?
Some researchers have proposed that the C677T mutation in MTHFR may be associated with depression as a result of decreased neurotransmitter synthesis, but studies have not consistently supported this hypothesis. Several studies suggest an association between MTHFR mutations and MDD8-10:
Jiang et al8 performed a meta-analysis of 13 studies including 1,295 Chinese patients and found that having at least 1 C677T variant allele was significantly associated with an increased risk of depression (for T vs C odds ratio 1.52, 95% confidence interval 1.24 to 1.85). The authors noted a stronger association identified in the Northern Chinese population compared with the Southern Chinese population.8
Bousman et al9 found that American patients with MDD and the 677CC genotype had greater Patient Health Questionnaire-9 (PHQ-9) scores at assessments at 24, 36, and 48 months post-baseline compared with those with the 677TT genotype (P = .024), which was unexpected based on previously reported associations.9
Schiepers et al10 also assessed the association between the MTHFR genotype in a Dutch ambulatory care population over 12 years. There was no association identified between scores on the depression subscale of the Symptom Checklist 90 and C677T diplotype.10
Table 16,8-12 provides summaries of these and other selected studies on MTHFR and MDD. Overall, although a pathophysiological basis for depression and decreased MTHFR function has been proposed, the current body of literature does not indicate a consistent link between MTHFR C677T genetic variants alone and depression.
Continue to: Medication changes based on MTHFR: What is the evidence?