Dihydropteroate synthase gene mutations in Pneumocystis and sulfa resistance

Emerging Infectious Diseases, Oct, 2004 by Laurence Huang, Kristina Crothers, Chiara Atzori, Thomas Benfield, Robert Miller, Meja Rabodonirina, Jannik Helweg-Larsen

Of note, in all nine studies, DHPS mutations were observed in PCP patients who were not currently receiving TMP-SMX or dapsone. Whether these patients who failed to meet the defined criteria for TMP-SMX or dapsone prophylaxis had ever received one of these medications for prophylaxis or had received TMP-SMX for a reason other than PCP prophylaxis at some point during their lives was difficult to assess with any degree of confidence. Nevertheless, most of the studies found that only a minority of PCP patients who had not been prescribed TMPSMX or dapsone for PCP prophylaxis had Pneumocystis that contained DHPS mutations. The study that reported the highest proportion (48%) used both chart abstraction and patient interview as sources of clinical information regarding PCP prophylaxis (23). This study also used a broad definition of PCP prophylaxis, including patient report of TMP-SMX use for prophylaxis at any time in life. Thus, despite rigorous attempts to document TMP-SMX or dapsone use for PCP prophylaxis and with the broadest definition of prophylaxis applied, nearly half of the patients without TMP-SMX or dapsone use had evidence of DHPS mutations on their clinical PCP specimen. Among the 26 patients with a new diagnosis of HIV infection at the time PCP was diagnosed and who thus had never received PCP prophylaxis, 14 (54%) had Pneumocystis that contained DHPS gene mutations. The specific city of residence was also an independent predictor associated with the risk for Pneumocystis that contained DHPS gene mutations. Patients who lived in San Francisco were five times more likely, and patients who lived in Seattle were more than three times as likely to have mutant DHPS than patients who resided in Atlanta, even when factors including sulfonamide or dapsone PCP prophylaxis and prior PCP were controlled for. The presence of DHPS mutations in patients without prior TMPSMX or dapsone use for PCP prophylaxis, the absence of similar mutations in Pneumocystis isolated from other mammalian species, and the impact of geography on DHPS genotype have substantial implications for disease transmission (i.e., person-to-person transmission) that are beyond the scope of this review (32 35).

Lack of Association of Trimethoprim with DHFR Gene Mutations

Trimethoprim inhibits another of the integral enzymes in folate synthesis, DHFR (Figure). In other microorganisms, point mutations in the DHFR gene are an important mechanism of drug resistance. This finding has led researchers to examine the DHFR gene of P. jirovecii to evaluate whether DHFR mutations contribute to TMP-SMX resistance. To date, two studies have failed to demonstrate an association between the use of TMP-SMX for PCP prophylaxis in HIV-infected persons and the presence of DHFR gene mutations (21,36). In one study, 36 of 37 specimens (from 35 patients, 26 of whom were HIV-infected) demonstrated identical DHFR sequences, with a single specimen showing one synonymous nucleotide change (21). In the second study, 16 (59%) of 27 specimens (from 27 patients, 19 of whom were HIV-infected) had DHFR gene mutations, 14 had synonymous changes, and 2 had nonsynonymous changes (36). Neither of the two patients whose PCP specimen had nonsynonymous DHFR changes had prior exposure to DHFR inhibitors, yet both patients were treated successfully for PCP with TMP-SMX. In addition, this study aligned the Pneumocystis DHFR sequences with those of E. coli, Staphylococcus aureus, Streptococcus pneumoniae, and Plasmodium falciparum and reported that the observed nonsynonymous changes in Pneumocystis DHFR were not in the highly conserved regions of the enzyme as are the amino acid substitutions that confer resistance to TMP (or pyrimethamine) in these other organisms. Thus, the presence and association of DHPS, but not DHFR, gene mutations with the use of specific PCP prophylaxis regimens argue strongly both for the importance of SMX and dapsone against Pneumocystis and the central role of DHPS mutations in the potential development of TMP-SMX or dapsone resistance.