pncA Mutations as a Major Mechanism of Pyrazinamide Resistance in Mycobacterium tuberculosis : Spread of a Monoresistant Strain in Quebec, Canada

Pyrazinamide (PZA) is an important first-line tuberculosis drug that is part of the currently used short-course tuberculosis chemotherapy. PZA is a prodrug that has to be converted to the active form pyrazinoic acid by pyrazinamidase (PZase) activity, encoded by thepncA gene ofMycobacterium tuberculosis , and loss of PZase activity is associated with PZA resistance. To further define the genetic basis of PZA resistance and determine the frequency of PZA-resistant strains havingpncA mutations, we sequenced thepncA gene from a panel of 59 PZA-resistant clinical isolates from Canada, the United States, and Korea. Two strains that did not containpncA mutations and had positive PZase turned out to be falsely resistant. Three PZase-negative strains (MIC, >900 μg of PZA per ml) and one PZase-positive strain (strain 9739) (MIC, >300 μg of PZA per ml) did not havepncA mutations. The remaining 53 of the 57 PZA-resistant isolates hadpncA mutations, confirming thatpncA mutation is the major mechanism of PZA resistance. Various new and diverse mutations were found in thepncA gene. Interestingly, 20 PZA-monoresistant strains and 1 multidrug-resistant isolate from Quebec, Canada, all had the samepncA mutation profile, consisting of an 8-nucleotide deletion and an amino acid substitution of Arg140→Ser. Strain typing indicated that these strains are highly related and share almost identical IS6110 patterns. These data strongly suggest the spread of a PZA-monoresistant strain, which has not previously been described.