The precise magic of CRISPR.

In this issue of FEBS Open Bio, Shen Li et al., in the laboratory of Hector L. Franco (University of North Carolina), provide a proof-of-principle solution for correcting all copies of a gene in the widely used MCF7 breast cancer cell line. The gene for the FOXA1 pioneer transcription factor is localised on chromosome 14, which is present at least 4-5 times in MCF7 cells. To achieve their goal, the authors used a 'classical' version of the CRISPR/Cas9 system. Both sgRNA and Cas9 components were expressed from a single vector, which also has a puromycin resistance cassette; this is an essential module for the chosen strategy, because it ensures expression of both sgRNA and Cas9 in selected cells. A targeting template in the form of nonlinearised plasmid was shown to have the best efficiency and was used to introduce a substitution at position 295 in the gene encoding FOXA1 to change a codon encoding lysine into a codon encoding glutamine (K295Q). The strategy suggested by Li and co-authors is an important development towards genome editing of multiple copy genes in a polyploid environment like cancer cells. One important application of the technique could be in creating models to study the role of single nucleotide polymorphisms in cancer progression and metastasis. Isogenic cancer lines carrying polymorphic variants of key drug targets could be used to optimise anticancer treatment protocols, laying a foundation for personalised therapy.