Near 100% efficient homology-dependent genome engineering in the human fungal pathogen Cryptococcus neoformans

Abstract

We recently described CRISPR/Cas9-based short homology-dependent genome engineering in the human fungal pathogen Cryptococcus neoformans, a haploid budding yeast that is the most common cause of fungal meningitis and an emerging model organism. This was achieved by electroporation of strains stably expressing a codon-optimized Cas9 with two separate DNA molecules, one encoding a selectable marker flanked by short homology arms and a second encoding a sgRNA under the control of the U6 snRNA promoter. However, the efficiency of desired homology-dependent repair relative to undesired non-homologous end-joining (NHEJ) events can be low and variable. Here, we describe methods and strains enabling extremely efficient ([~]99%) homology-dependent genome editing in C. neoformans. This high-efficiency method requires two manipulations. First, we placed the sgRNA-expressing segment into the marker-containing DNA flanked by targeting homology; thus, only a single DNA molecule is introduced into cells. Second, we used a strain mutant for the non-homologous end-joining factor Ku80 (encoded by YKU80). We also report the engineering of a yku80::amdS mutant strain harboring an insertion mutation that can be removed scarlessly via recombination between direct repeats. This enables the functional restoration of YKU80 after homology- dependent genome editing via selection against the amdS marker using fluoroacetamide. This approach minimizes documented drawbacks of using Ku-defective strains in downstream experiments. Finally, we describe a plasmid series that enables rapid cloning of sgRNA-marker constructs for genomic manipulation of C. neoformans, including gene deletion and C-terminal tagging. These methods, strains, and plasmids accelerate the genomic manipulation of C. neoformans.