Spontaneous selection of Cryptosporidium drug resistance in a calf model of infection
The intestinal protozoan Cryptosporidium is a leading cause of diarrheal disease and mortality in young children. There is currently no fully effective treatment for cryptosporidiosis, which has stimulated interest in anticryptosporidial development over the last [~]10 years with numerous lead compounds identified including several tRNA synthetase inhibitors. In this study, we report the results of a dairy calf efficacy trial of the methionyl-tRNA (CpMetRS) synthetase inhibitor 2093 and the spontaneous emergence of drug resistance. Dairy calves experimentally infected with Cryptosporidium parvum initially improved with 2093 treatment, but parasite shedding resumed in two of three calves on treatment day five. Parasites shed by each recrudescent calf had different amino acid altering CpMetRS mutations, coding either an aspartate 243 to glutamate (D243E) or a threonine 246 to isoleucine (T246I) mutation. Transgenic parasites engineered to have either the D243E or T246I CpMetRS mutation using CRISPR/Cas9 grew normally but were highly 2093 resistant; the D243E and T246I mutant expressing parasites respectively had 2093 EC50S of 613- or 128-fold that of transgenic parasites with wild-type CpMetRS. In studies using recombinant enzymes, the D243E and T246I mutations shifted the 2093 IC50 by >170-fold. Structural modeling of CpMetRS based on an inhibitor-bound Trypanosoma brucei MetRS crystal structure suggested that the resistance mutations reposition nearby hydrophobic residues, interfering with compound binding while minimally impacting substrate binding. This is the first report of naturally emerging Cryptosporidium drug resistance, highlighting the need to address the potential for anticryptosporidial resistance and establish strategies to limit its occurrence. ImportanceCryptosporidium is a leading protozoan cause of diarrhea in young children with no reliable treatment. We report results of a dairy calf drug efficacy trial and the spontaneous emergence of drug resistance. Cryptosporidium parvum infected calves initially improved with drug treatment, but infection relapsed in two animals. Parasites shed by each recrudescent calf had mutations in the gene encoding the drug target that altered its amino acid sequence. Recapitulation of the drug target mutations by CRISPR/Cas9 genome editing resulted in highly drug-resistant parasites, and recombinant mutant enzymes were resistant to inhibition. This is the first report of naturally emerging Cryptosporidium drug resistance. There is a currently a great opportunity to impact public health with new drugs to treat cryptosporidiosis, and this report highlights the need to address the potential for anticryptosporidial resistance and establish strategies to limit its occurrence in order to realize their full potential. One-sentence summaryDrug-target point mutations mediating anticryptosporidial resistance spontaneously arose in the dairy calf C. parvum infection model.