CRISPR/Cas9 has revolutionised genetic research, with seemingly no end to its potential applications. Until recently genome engineering relied on the use of stem cells to target new mutations in mice but now researchers can perform genome editing directly in embryos or specific tissues. As with most projects that use gene editing technology, CRISPR/Cas9 has transformed the efficiency and enhanced the applications of the IMPC.
New CRISPR/Cas9-based techniques are constantly being developed, and existing systems adapted and improved, allowing increasingly sophisticated genetic changes to be made. Two new papers published in the journal BMC Biology explore new advancements and highlight applications for these techniques.
The CRISPR/Cas9 system used in conjunction with single stranded DNA donors is revolutionising our ability to generate targeted mutations directly in the embryo. Whilst short synthetic DNA molecules facilitate this, the use of longer single-stranded DNA donors is a more recent addition to the genome editing toolbox. The two new articles summarised here compare long and short single-stranded donors in a high-throughput setting, both look at conditional knock-out mutants while also presenting advances for the generations of point mutations.
In the first study, led by researchers in Lydia Teboul’s group at the MRC Harwell institute, long single-stranded molecules are utilised to facilitate the generation of conditional alleles. They also apply the system to the introduction of point mutations remote from the recognition site of active Cas9/sgRNA complexes, which up to now has not been possible. This last technique is particularly valuable for human genomic sequencing since it enhances our ability to replicate human mutations in mice.
Alongside this breakthrough the researchers also highlight the unpredictability of this technique. As well as on-target integrations, the system can also produce an array of incorrect alleles. These include unintended point mutations, small or larger sequence rearrangements, and additional donor integrations. Such events are unpredictable by-products and therefore must be omitted in the process of validation of newly established mutant lines.
However, these by-products do not reduce the value of the new system. Instead, they illustrate the importance of a comprehensive validation of new mutants, including sequencing of the locus and copy counting of the number of copies of donor integrations. This process will likely become simplified over time, and issues overcome as new technological advancements are made. In the meantime, although unpredictable, the existing strategies remain efficient at generating desirable mutants.
Author on the paper Lydia Teboul said “An increasing body of evidence is being compiled to indicate that model validation is the newest challenge for the community. After all, the quality and reproducibility of research based on genome editing mutants depends entirely on the thorough characterisation of the mutant in question.”
The aim of the second study was to look at scaling production of conditional null alleles to create IMPC mouse lines. The research, led by Jason Heaney at Baylor College of Medicine, tested the feasibility of using CRISPR/Cas9 gene editing technology to generate conditional knockout mice using Cas9-initiated homology-driven repair (HDR), with both short oligonucleotides and longer single stranded DNA.
The results demonstrate that using pairs of short oligodeoxynucleotides can generate conditional null alleles at many loci; however, at scale there are inefficiencies with this process. On the other hand, long single stranded DNA donors may enable high-throughput production of conditional alleles. Although long single stranded DNA donors are most efficient at generating conditional alleles, pairs of short oligodeoxynucleotides are a viable alternative when use of a long single stranded donor is not feasible due to distance between loxP sites or complexity of sequence between loxP sites. Importantly, in agreement with the Teboul group, random integration of donor DNA and mutagenesis events at the target integration sites were detected when using either type of DNA donor.
Author on the article Jason Heaney said “Single stranded DNA donors are a critical component of the CRISPR/Cas9 genome editing toolbox and are an invaluable resource for producing conditional knockout alleles in mice. But, given the preponderance for random integration and potential for mutagenesis at sites of HDR, new mouse models produced with these donor DNAs must be carefully screened.”
Both studies highlight that it is essential to screen the sequence errors to check for point mutations, rearrangements and additional donor integrations. Researchers involved in genome editing face several challenges when using CRISPR/CAS9 technology and this latest research show it is essential to understand the unpredictability of different systems. Additionally, standards for the validation and documentation of mutants would be extremely beneficial to the field, and help to ensure quality and research reproducibility.