Efficient in Vivo Introduction of Point Mutations Using ssODN and a Co-CRISPR Approach

Background: The generation of point mutations is a major tool for evaluating the roles of specific nucleotides or amino acids within the regulatory or functional landscape. However, examination of these mutations in vivo requires the generation of animals carrying only the relevant point mutations at the endogenous genomic loci, which is technically challenging. The CRISPR-Cas9 based genome editing greatly facilitates the generation of such genetically modified animals; however, most of the described methods use double-strand DNA (dsDNA) as the donor template. The dsDNA plasmids frequently undergo undesired integration events into the targeted genomic locus. The use of a single-strand oligodeoxynucleotide (ssODN) as the donor template prevents this complication and is therefore the preferred choice for introducing point mutations, as well as short sequences such as protein tags. Results: We successfully applied the CRISPR-based white co-conversion strategy with a ssODN template, instead of the originally described dsDNA plasmid, to create genetically modified Drosophila melanogaster strains. We used the technique to easily introduce point mutations in two distinct chromosomes. Using the generated flies, we were able to demonstrate the in vivo importance of the respective mutations. For the Nucleoporin107 (Nup107) gene, the 1090G > A mutation was confirmed to affect ovarian development, while for the tinman (tin) gene, the regulatory role of the downstream core promoter element (DPE) was demonstrated within the developing Drosophila melanogaster embryo. Conclusions: The described approach has facilitated the successful generation of point mutations in two different chromosomes, by two different labs. Distinct phenotypes associated with the newly-generated genotype were identified, thus exemplifying the importance of investigating the in vivo role of specific nucleotides. In addition, detailed guidelines, recommendations and crossing schemes are provided in order to support the generation of additional genetically modified animals by the scientific community.