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"I’ve been working with iTL over the past 5 years in the production of 3 different genetically altered mice. Not only did iTL help in the design of the mice, […]” - Raghu Mirmira, MD, PhD University of Chicago.
Our study of biology has benefited greatly from the ability to create gene constructs and express them at the cell or organism level. While introducing and establishing gene expression constructs in cells may be more straightforward and can be less rigorous in methodology or approach, transgenic animal model production involves some deeper planning and considerations.
With transgenic animal production, model design features relating to the transgenic construct components, integration of the transgenic construct into the genome, and copy number of the transgene become significant considerations that may impact the utility of the animal model and your research in the years to come.
Creating transgenic mice comes down to two types of methodologies: random genomic insertion of the transgene, or specific targeted insertion of the transgene.
While the timeline to producing mice via random transgenic insertion may be shorter, each mouse produced is an independent “founder” which must be genotyped and phenotyped for proper insertion and expression of the transgene. Targeted transgenic insertion involves a longer timeline for mouse production, providing the benefits of reliability, predictability, and robustness. So which is the better option to go with?
As a quick guide for planning your next transgenic mouse model, below are 4 distinguishing reasons that can help you determine whether a targeted transgenic model is necessary.
While random insertion transgenesis may land your transgenic construct anywhere in the mouse genome, a targeted transgenic knockin is designed to insert your construct into one specific location, typically a transcriptionally active “safe harbor” locus such as ROSA26. Genomic locations that are undesirable for transgene integration include genes (resulting in knockout of the interrupted gene) and heterochromatin regions. A safe harbor locus is a specific genomic location that has been validated to be transcriptionally active and permissive for expression of transgenic constructs. Mouse ROSA26 locus is one of the most commonly used insertion sites for creating targeted transgenic knockin mice.
Random transgenic integration tends to involve transgenic construct breakage before the integration, as well as transgene concatemerization (multiple attached copies of the transgene inserted together). Targeted transgenic insertion is the optimal methodology for achieving intact, single-copy transgene integration. Additionally, by using mouse embryonic stem (ES) cells in targeted transgenic mice production, the targeted ES cells can be screened for proper integration of the construct prior to generating the mice, which is more robust and higher throughput compared to screening mice.
ROSA26 is a transcriptionally active locus that has been used to create over 700 targeted transgenic mouse models, as shown in Mouse Genome Informatics. Transgene expression options include using endogenous ROSA26 gene promoter or adding an exogenous promoter into the construct, such as the artificial compound CAG promoter. A conditional expression design is commonly achieved by placing a STOP cassette between the promoter and the cDNA, and the STOP cassette is typically flanked with loxP sites for cassette removal by Cre-lox recombination.
Specific targeting of your transgenic construct into ROSA26 locus means knowing that your transgene will be there generation after generation. Randomly inserted trangenes may be unstable over time, leading to changes in expression and changes in copy number. The variability of random insertion allows transgene integration into genomic regions that may be less stable, and transgene concatemers at an insertion site may also change with time.
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