Restriction digest screening facilitates efficient detection of site-directed mutations introduced by CRISPR in C. albicans UME6

Simplifying Genotyping of Mutants from Genome Editing with a Parallel qPCR-Based iGenotype Index

Targeted genome editing is a powerful tool in reverse genetic studies of gene function in many aspects of biological and pathological processes. The CRISPR/Cas system or engineered endonucleases such as ZFNs and TALENs are the most widely used genome editing tools that are introduced into cells or fertilized eggs to generate double-strand DNA breaks within the targeted region, triggering cellular DNA repair through either homologous recombination or non-homologous end joining (NHEJ). DNA repair through the NHEJ mechanism is usually error-prone, leading to point mutations or indels (insertions and deletions) within the targeted region. Some of the mutations in embryos are germline transmissible, thus providing an effective way to generate model organisms with targeted gene mutations. However, point mutations and short indels are difficult to be effectively genotyped, often requiring time-consuming and costly DNA sequencing to obtain reliable results. Here, we developed a parallel qPCR assay in combination with an iGenotype index to allow simple and reliable genotyping. The genotype-associated iGenotype indexes converged to three simple genotype-specific constant values (1, 0, -1) regardless of allele-specific primers used in the parallel qPCR assays or gene mutations at wide ranges of PCR template concentrations, thus resulting in clear genotype-specific cutoffs, established through statistical analysis, for genotype identification. While we established such a genotyping assay in the Xenopus tropicalis model, the approach should be applicable to genotyping of any organism or cells and can be potentially used for large-scale, automated genotyping.

Characterization of the role of Ume6 C-terminal tail in C. albicans morphological plasticity

C. albicans is an important human fungal pathogen and filamentation is essential for its virulence. Ume6 is a transcription factor critical for filamentation. Ume6 is composed of three domains, a long N terminal domain, Zn-finger domain, and a C-terminal domain. Previously, it was shown that the Zn-finger domain is essential for filamentation, as removal of this domain led to a lack of filamentation. However, the role for the C-terminal domain has not been defined. We find deletion of the C-terminal domain leads to a filamentation defect and the defect is not as severe as removal of the Zn-finger or ume6 deletion. We mutated a number of residues in the C-terminal domain to try to identify specific residues important for filamentation, but all of our mutants displayed wild type filamentation. Alpha fold predictions suggest the C-terminal domain forms a single alpha helix that is predicted to interact with the Zn-finger domain via hydrogen bond. Our data suggests the C-terminal domain binds the Zn-finger domain and through this interaction is important for filamentation.

Competitive PCR with dual fluorescent primers enhances the specificity and reproducibility of genotyping animals generated from genome editing

Targeted genome editing is a powerful tool for studying gene function in almost every aspect of biological and pathological processes. The most widely used genome editing approach is to introduce engineered endonucleases or CRISPR/Cas system into cells or fertilized eggs to generate double-strand DNA breaks within the targeted region, leading to DNA repair through homologous recombination or non-homologous end joining (NHEJ). DNA repair through NHEJ mechanism is an error-prone process that often results in point mutations or stretches of indels (insertions and deletions) within the targeted region. Such mutations in embryos are germline transmissible, thus providing an easy means to generate organisms with gene mutations. However, point mutations and short indels present difficulty for genotyping, often requiring labor intensive sequencing to obtain reliable results. Here, we developed a single-tube competitive PCR assay with dual fluorescent primers that allowed simple and reliable genotyping. While we used Xenopus tropicalis as a model organism, the approach should be applicable to genotyping of any organisms.

CRISPR mediated genome editing, a tool to dissect RNA modification processes

Though over 100 distinct RNA modifications have been identified, the roles for many of these modifications in vivo remain unknown. Genome editing is one tool investigators are using to better understand the roles these modifications play and the consequences of their absence. In this chapter, we describe how CRISPR mediated genome editing can be used to interrogate the process of RNA modification in C. albicans. Furthermore, we discuss how the protocols described can be altered to meet experimental demands. The underlying theory on which these protocols are based are applicable to a variety of model systems. The protocols described utilize the widely used S. pyogenes Cas9, but the field of genome editing is quickly evolving. We discuss the recent developments of more flexible CRISPR systems that can target a greater number of sites in the genome. These and other advancements make CRISPR mediated genome editing a practical methodology to investigate RNA modification.

Stay-on-Task Exercises as a Tool To Maintain Focus during a CRISPR CURE

Course-based undergraduate research experiences (CURE) offer the chance for students to experience authentic research investigation in a classroom setting. Such hands-on experiences afford unique opportunities work on a semi-independent research project in an efficient, structured environment. We have developed a CRISPR CURE in which undergraduate and graduate students use in silico, in vitro, and in vivo techniques to edit a fungal genome. During the development of this course, we have found that the asynchronous nature of the CRISPR CURE activities can be disruptive and lead to unproductive class time. To overcome this challenge, we have developed stay-on-task exercises (SOTEs). These short low-stakes assessments provide structured activities that are performed during these asynchronous incubation periods. SOTE activities leverage potentially unproductive class time and complement the CURE learning objectives. We have found SOTEs to be one method of maintaining classroom structure during a CURE. Furthermore, SOTE complexity, length, and subject can be easily modified to match course learning objectives.

Pseudouridine synthase 7 impacts Candida albicans rRNA processing and morphological plasticity

RNA can be modified in over 100 distinct ways, and these modifications are critical for function. Pseudouridine synthases catalyse pseudouridylation, one of the most prevalent RNA modifications. Pseudouridine synthase 7 modifies a variety of substrates in Saccharomyces cerevisiae including tRNA, rRNA, snRNA, and mRNA, but the substrates for other budding yeast Pus7 homologues are not known. We used CRISPR‐mediated genome editing to disrupt Candida albicansPUS7 and find absence leads to defects in rRNA processing and a decrease in cell surface hydrophobicity. Furthermore, C. albicans Pus7 absence causes temperature sensitivity, defects in filamentation, altered sensitivity to antifungal drugs, and decreased virulence in a wax moth model. In addition, we find C. albicans Pus7 modifies tRNA residues, but does not modify a number of other S. cerevisiae Pus7 substrates. Our data suggests C. albicans Pus7 is important for fungal vigour and may play distinct biological roles than those ascribed to S. cerevisiae Pus7.

CRISPR-mediated Genome Editing of the Human Fungal Pathogen Candida albicans

This method describes the efficient CRISPR mediated genome editing of the diploid human fungal pathogen Candida albicans. CRISPR-mediated genome editing in C. albicans requires Cas9, guide RNA, and repair template. A plasmid expressing a yeast codon optimized Cas9 (CaCas9) has been generated. Guide sequences directly upstream from a PAM site (NGG) are cloned into the Cas9 expression vector. A repair template is then made by primer extension in vitro. Cotransformation of the repair template and vector into C. albicans leads to genome editing. Depending on the repair template used, the investigator can introduce nucleotide changes, insertions, or deletions. As C. albicans is a diploid, mutations are made in both alleles of a gene, provided that the A and B alleles do not harbor SNPs that interfere with guide targeting or repair template incorporation. Multimember gene families can be edited in parallel if suitable conserved sequences exist in all family members. The C. albicans CRISPR system described is flanked by FRT sites and encodes flippase. Upon induction of flippase, the antibiotic marker (CaCas9) and guide RNA are removed from the genome. This allows the investigator to perform subsequent edits to the genome. C. albicans CRISPR is a powerful fungal genetic engineering tool, and minor alterations to the described protocols permit the modification of other fungal species including C. glabrata, N. castellii, and S. cerevisiae.