Atypical Molecular Basis for Drug Resistance to Mitochondrial Function Inhibitors in Plasmodium falciparum

The continued emergence of drug-resistant Plasmodium falciparum parasites hinders global attempts to eradicate malaria, emphasizing the need to identify new antimalarial drugs. Attractive targets for chemotherapeutic intervention are the cytochrome (cyt) bc₁ complex, which is an essential component of the mitochondrial electron transport chain (mtETC) required for ubiquinone recycling and mitochondrially localized dihydroorotate dehydrogenase (DHODH) critical for de novo pyrimidine synthesis. Despite the essentiality of this complex, resistance to a novel acridone class of compounds targeting cyt bc₁ was readily attained, resulting in a parasite strain (SB1-A6) that was panresistant to both mtETC and DHODH inhibitors. Here, we describe the molecular mechanism behind the resistance of the SB1-A6 parasite line, which lacks the common cyt bc₁ point mutations characteristic of resistance to mtETC inhibitors. Using Illumina whole-genome sequencing, we have identified both a copy number variation (∼2×) and a single-nucleotide polymorphism (C276F) associated with pfdhodh in SB1-A6. We have characterized the role of both genetic lesions by mimicking the copy number variation via episomal expression of pfdhodh and introducing the identified single nucleotide polymorphism (SNP) using CRISPR-Cas9 and assessed their contributions to drug resistance. Although both of these genetic polymorphisms have been previously identified as contributing to both DSM-1 and atovaquone resistance, SB1-A6 represents a unique genotype in which both alterations are present in a single line, suggesting that the combination contributes to the panresistant phenotype. This novel mechanism of resistance to mtETC inhibition has critical implications for the development of future drugs targeting the bc₁ complex or de novo pyrimidine synthesis that could help guide future antimalarial combination therapies and reduce the rapid development of drug resistance in the field.