Methotrexate (MTX) has several enzymatic targets in the folate pathway. To better understand the variability in response to MTX, we characterized the interindividual variability of intracellular folate pools in children with juvenile arthritis (JA) and determined clinical and genetic contributors to this variability.
This exploratory single-center cross-sectional study evaluated 93 patients with JA not currently receiving MTX. Whole blood, plasma, and erythrocyte folate concentrations were determined after deconjugation and analyzed through reversed-phase separation and stable isotope dilution tandem mass spectrometry. Folate polyglutamates were measured in red blood cell lysates using an ion-pair reversed phase chromatography tandem mass spectrometry method.
Intracellular concentrations of 5-methyl-tetrahydrofolate (5-CH3-THF) and 5,10-methenyl-tetrahydrofolate varied approximately 20-fold and 80-fold, respectively. The polyglutamated forms of 5-CH3-THF as a percentage of total 5-CH3-THF (5-CH3-THFGlun) were also measured. Hierarchical clustering of 5-CH3-THFGlun revealed two groups, each with two distinct clusters. There was an inverse relationship between 5-CH3-THFGlun chain length and plasma 5-CH3-THF concentrations. A subgroup of patients with a historical intolerance to MTX had significantly lower cellular folate concentrations (P<0.0001). In univariate analyses, clinical variables including sex, age, and folate supplementation in addition to variations in MTHFR, MTR, and SLC25A32 were associated with differential intracellular folate redox concentrations. Multivariate analysis further supported the association of single nucleotide polymorphisms in SLC25A32, MTHFR, and MTR with variability in intracellular 5-CH3-THF and 5,10-methenyl-tetrahydrofolate concentrations, respectively.
Measurement of intracellular folate isoforms may contribute toward a better understanding of individual MTX effects in JA. Clinical variables in addition to genotypic differences beyond MTHFR may additionally explain differential intracellular folate concentrations and variable responses to MTX.