New evidence that Candida albicans possesses additional ATP-binding cassette MDR-like genes: Implications for antifungal azole resistance

Emergence of resistance of Candida albicans to antifungal triazoles is increasingly recognized as an important cause of refractory mucosal candidiasis in HIV-infected patients. Recently, CDR1, which is thought to be analogous to the human MDR-1 P-glycoprotein, has been cloned in C. albicans. It has been proposed that its expression is partially responsible for fluconazole resistance in C. albicans. This gene is characterized by the presence of an ATP binding cassette (ABC) region and is distinct from the BENr gene which does not encode such a functional domain. As the molecular basis for fluconazole resistance appears to be multifactorial, we considered that there may be other ATP binding cassette-containing MDR genes that may potentially contribute to antifungal azole resistance in C. albicans. We therefore sought to identify potential target sequences that may be derived from candidate genes that share homology with the ATP binding cassette region of the human MDR-1 P-glycoprotein. Degenerate oligonucleotide primers based on the known sequence from the ATP binding cassette region of the human MDR-1 P-glycoprotein were used to amplify PCR products within the range of 100 bp in length from C. albicans isolates (3 fluconazole-susceptible and 3 fluconazole-resistant). Sequence analysis of individually subcloned PCR products, derived from the six isolates revealed 34 sequences in total. The results of our study identified 14 clones (with at least one per isolate) with a high degree of homology to the ATP binding cassette of the human MDR-1 P-glycoprotein. The BLAST search did not disclose homology of these new sequences to the C. albicans CDR1 gene, suggesting that C. albicans may possess more than one MDR-like gene. We conclude that C. albicans may possess one or more additional genes encoding ATP binding cassette MDR-like proteins that are distinct from CDR1 and which could participate in the development of fluconazole resistance.