CRISPR-Cas9 precision editing of kinetochore protein phosphosite codons in Leishmania mexicana

Abstract

Leishmania mexicana, like other trypanosomatids, possesses a unique kinetochore--the protein complex crucial for chromosome segregation during mitosis. To investigate the functional significance of specific phosphorylation sites on essential kinetochore proteins, we adapted a selection-free precision editing strategy using CRISPR-Cas9 in Leishmania mexicana promastigotes. Our method targeted genomic DNA with 160-bp double-stranded DNA repair templates and guide RNAs to introduce targeted modifications. We focused on six phosphosites within the kinetochore proteins KKT2, KKT4, and KKT7, generating phosphodeficient, phosphomimetic, and synonymous mutants for each site. Across 18 independent transfections, we achieved a successful editing rate of 27.5% as determined by PCR screening, with 30.4% of clones confirmed as edited by Sanger sequencing. A significant portion of these edited clones (22.1%) were homozygous. Despite these precise genomic modifications, none of the phosphosite mutant clones exhibited any apparent growth defects or cell cycle dysregulation, suggesting these phosphorylation sites individually may not be critical for these processes under standard culture conditions. To facilitate higher-throughput precision editing, we developed a Python script that automates the design of the 160-bp repair templates. This script uses a FASTA file, a codon usage table, and a simple configuration file to design templates with a single nonsynonymous mutation and additional synonymous mutations for screening purposes. It also generates a corresponding synonymous-only repair template and primers for both screening and repair template generation, offering a "ready-to-go" approach. While designed for Leishmania, this powerful tool is adaptable for use with other kinetoplastids.