Are the TR₄₆/Y121F/T289A Mutations in Azole-Resistant Aspergillosis Patient Acquired or Environmental?

LETTER

Drs. Vazquez and Manavathu report development of azole resistance in a patient with invasive aspergillosis receiving long-term voriconazole treatment (1). Aspergillus fumigatus was cultured from a lung biopsy specimen and was found to be voriconazole susceptible. During therapy, 4 bronchoalveolar lavage (BAL) fluid samples grew A. fumigatus, but no azole susceptibility was reported. The patient died, and at autopsy, a sixth isolate was cultured from the respiratory tract. Analysis of the Cyp51A gene showed no mutations in the first isolate, but the isolate recovered at autopsy harbored the 46-bp tandem repeat (TR₄₆) Y121F and T289A mutations (TR₄₆/Y121F/T289A). The authors concluded without convincing evidence that the susceptible A. fumigatus isolate developed the TR₄₆/Y121F/T289A mutations during therapy while examining only 2 of a total of 6 isolates (1).

Generally two routes of resistance development are recognized in A. fumigatus: through patient therapy and through exposure of the fungus to azole fungicides used in the environment (2). The clinical and mycological characteristics of both routes are very different. Patient-acquired resistance is characterized by initial azole-susceptible disease that evolves into azole-resistant infection during long-term azole therapy (3). These patients commonly have chronic lung diseases, and resistance development is reported exclusively in patients with cavitary pulmonary lesions. It is believed that asexual reproduction within a cavity strongly enhances the probability of resistance selection (4). Azole-resistant A. fumigatus isolates recovered from these patients show a diversity of resistance mutations, including point mutations in the Cyp51A gene (5). Patient-acquired resistance can be proven if the first cultured azole-susceptible isolate and the azole-resistant isolate recovered from the same patient are shown to be isogenic (6, 7).

In regions with environmental A. fumigatus resistance, patients may inhale Aspergillus spores that are already resistant to medical triazoles. As a consequence, any Aspergillus disease may develop due to azole-resistant strains, and most patients with azole-resistant aspergillosis have no history of azole therapy (8, 9). The resistance mechanisms found in isolates recovered from these patients are characterized by multiple combined mutations. Furthermore, microsatellite genotyping of these isolates shows that they cluster together even when recovered from epidemiologically unrelated patients (8, 9).

Until now, TR₄₆/Y121F/T289A was considered to have emerged exclusively through environmental azole fungicide exposure (10). Vazquez and Manavathu state that in their patient the mutation developed during azole therapy. As the clinical presentation and management are different for the two routes of resistance development, it is important to further analyze the origin of TR₄₆/Y121F/T289A in this patient. The two isolates recovered from this patient should be subjected to microsatellite genotyping in order to confirm isogenicity. In the absence of this proof, the statement “emergence of resistance in AF50593 occurred during voriconazole therapy” is merely speculation. Currently, two papers report the TR₄₆/Y121F/T289A mutations in the United States (11). Environmental surveys are warranted to determine the presence of these resistance mutations in the environment and possibly identify niches associated with high burdens of resistant A. fumigatus isolates. These insights will help to improve the management of patients with aspergillosis (12).