Apparent Absence of Selective Pressure on Pneumocystis jirovecii Organisms in Patients with Prior Methotrexate Exposure

Area of Science:

• Medical Mycology
• Molecular Biology
• Pharmacology

Background:

• Pneumocystis jirovecii infections are common in patients receiving methotrexate (MTX) due to its immunosuppressive properties.
• Methotrexate (MTX), a dihydrofolate reductase (DHFR) inhibitor, might also possess anti-P. jirovecii activity, potentially exerting selective pressure on the fungus.

Purpose of the Study:

• To investigate whether methotrexate (MTX) exposure influences the genetic diversity of the dihydrofolate reductase (DHFR) gene in Pneumocystis jirovecii.
• To compare P. jirovecii dhfr gene sequences from patients with and without prior MTX exposure.

Main Methods:

• Collected P. jirovecii isolates from two groups of patients: those with prior MTX exposure (n=21) and a control group without MTX exposure (n=22).
• Sequenced the dhfr gene of P. jirovecii isolates to identify single nucleotide polymorphisms (SNPs).
• Analyzed SNPs to define P. jirovecii dhfr alleles (A, B, C) and compared their frequencies between the two patient groups.

Main Results:

• Three SNPs were identified in the P. jirovecii dhfr gene at positions 278, 312, and 381.
• Three distinct P. jirovecii dhfr alleles (A, B, C) were defined based on these SNPs.
• Allele A was the most frequent in both groups; no significant difference in dhfr gene diversity was observed between MTX-exposed and unexposed patients.

Conclusions:

• Methotrexate (MTX) does not appear to exert significant selective pressure on the Pneumocystis jirovecii dhfr gene.
• The genetic diversity of P. jirovecii dhfr remains largely unchanged despite MTX treatment in patients.