Introduction:
Methotrexate (MTX) is the treatment of choice for patients with rheumatoid arthritis (RA). However, it has been found to produce toxicity in some patients. Different polymorphisms can play a part in inter-individual differences in toxicity. Our aim is therefore to determine the influence of gene polymorphisms in the MTX metabolic pathway, such as MTHFR (rs1801133 and rs1801131), MTHFD1 (rs2236225), MTR (rs1805087), and ABCC2 (rs4148396), on toxicity in MTX treatment among Caucasian patients diagnosed with RA.

Material and methods:
Real-time polymnerase chain reaction (PCR) analysis with TaqMan probes was performed on these polymorphisms in 200 patients in a retrospective study.

Results:
Patients with TT genotype and C allele for MTHFR rs1801133 gene polymorphism presented a higher risk of anaemia (p = 0.0304; OR = 3.70; 95% CI: 1.10–12.34) and dizziness (p = 0.0438; OR = 8.15; 95% CI: 1.61– 148.68). Patients with MTHFR rs1801131-C allele or CC genotype presented a higher risk of mucositis (p = 0.0188; OR = 3.02; 95% CI: 1.22–7.91), acneiform rash (p = 0.0322; OR = 5.74; 95% CI: 1.34–39.21), and alopecia (p = 0.0072; OR = 5.11; 95% CI: 1.37–17.70). Patients with MTR rs1805087- CC genotype presented a higher risk of anosmia (p = 0.0038; OR = 98.00; 95% CI: 3.16–infinite); patients with the TT genotype or T allele for MTHFD1 rs2236225 presented a higher risk of liver failure (p = 0.00229; OR = 2.34; 95% CI: 1.14–4.96) and alopecia (p = 0.0248; OR = 3.83; 95% CI: 1.10–13.30), and patients with the TT genotype for ABCC2 rs4148396 polymorphism (p = 0.0466; OR = 2.67; 95% CI: 0.98–6.94) presented a higher risk of headaches.

Conclusions:
Pharmacogenomic analysis of these polymorphisms may facilitate decision-making in relation to MTX treatment toxicity among RA patients.