Abstract
The human pathogenic parasite Trypanosoma brucei possess both de novo and salvage routes for the biosynthesis of pyrimidine nucleotides. Consequently, they do not require salvageable pyrimidines for growth. Thymidine kinase (TK) catalyzes the formation of dTMP and dUMP and is one of several salvage enzymes that appear redundant to the de novo pathway. Surprisingly, we show through analysis of TK conditional null and RNAi cells that TK is essential for growth and for infectivity in a mouse model, and that a catalytically active enzyme is required for its function. Unlike humans, T. brucei and all other kinetoplastids lack dCMP deaminase (DCTD), which provides an alternative route to dUMP formation. Ectopic expression of human DCTD resulted in full rescue of the RNAi growth phenotype and allowed for selection of viable TK null cells. Metabolite profiling by LC-MS/MS revealed a buildup of deoxypyrimidine nucleosides in TK depleted cells. Knockout of cytidine deaminase (CDA), which converts deoxycytidine to deoxyuridine led to thymidine/deoxyuridine auxotrophy. These unexpected results suggested that T. brucei encodes an unidentified 5'-nucleotidase that converts deoxypyrimidine nucleotides to their corresponding nucleosides, leading to their dead-end buildup in TK depleted cells at the expense of dTTP pools. Bioinformatics analysis identified several potential candidate genes that could encode 5’-nucleotidase activity including an HD-domain protein that we show catalyzes dephosphorylation of deoxyribonucleotide 5’-monophosphates. We conclude that TK is essential for synthesis of thymine nucleotides regardless of whether the nucleoside precursors originate from the de novo pathway or through salvage. Reliance on TK in the absence of DCTD may be a shared vulnerability among trypanosomatids and may provide a unique opportunity to selectively target a diverse group of pathogenic single-celled eukaryotes with a single drug.
Highlights
The parasitic trypanosomatids are vector-borne single-celled eukaryotic pathogens that cause significant disease and mortality in tropical and subtropical countries [1]
Human pathogenic trypanosomatids are responsible for several life threatening diseases, together infecting 20 million people, while treatment is complicated by poor drug therapies
We find that seemingly redundant enzymes thymidine kinase and cytidine deaminase are required, not for their typical role in salvaging exogenous precursors, but are instead essential for de novo synthesis of thymine nucleotides
Summary
The parasitic trypanosomatids are vector-borne single-celled eukaryotic pathogens that cause significant disease and mortality in tropical and subtropical countries [1]. New drugs for the treatment of all three diseases are badly needed. HAT, known as sleeping sickness, is caused by Trypanosoma brucei, an extracellular parasite that replicates in the blood in early stages of infection. It crosses the blood-brain barrier in later stages leading to progressive neurological complications that disrupt the sleep/wake cycle and which eventually progress to coma and death [4]. The WHO estimates that yearly cases have dropped below 10,000 [5]; attempts to fully eliminate the disease face the difficulties of a substantial animal reservoir and problematic drug therapies that have high toxicity or are difficult to administer in a rural setting
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