When using cell lines or model organisms to study gene function, we usually use a strategy called gain of function / loss of function to learn the specific functionality and regulatory mechanisms of target gene(s) in cells or animals. Gain of function is usually achieved by gene knockin (KI) techniques to promote over-expression of gene, while loss of function is usually achieved via gene knockout (KO). In this paper, we mainly discuss the KI, or over-expression model. Gene knockin (KI) refers to the technology of inserting exogenous functional genes or fragments into the genome by transgenic or homologous recombination technique, enabling their ability to function in the cell.

Due to the random integration of transferred genes in mice prepared by traditional transgenic technology, this leads to problems such as an uncertain copy number across integration sites, making it is necessary to screen the offspring for mice with stable protein expression. In addition, the phenotype of the first-generation-mice is easily to lost during passage due to reasons such as the dilution of copy number and the silencing of surrounding genes caused by the integration site – posing a significant inconvenience to the subsequent establishment of lines and repeated experiments.

Is there a site that allows for the stable, high-expression of a transferred gene and avoids affecting the expression of other genes nearby? To obtain mice with stable gene expression without screening, it is necessary to provide a reliable ‘safe harbor’ for the exogenous gene. Within these ‘safe harbors’, the exogenous gene(s) are stably and efficiently expressed, and cause no side effects on regular functionality of associated cells and tissues.

Researchers have tested numerous different gene insertion sites in the mouse genome and found a few special sites wherein it is very safe and stable to insert any DNA – the ROSA26 ‘safe harbor’ locus is included among these special sites. The identification of ROSA26 is probably one of the most exciting discoveries in the history of genetic engineering, and is regarded as one of the most important breakthroughs in the whole history of genetics.

ROSA26 was first discovered by Friedrich and Soriano when they studied gene mutations in mouse embryonic stem cells (ESCs). The ROSA26 locus is known in the scientific community by the official name: gene trap ROSA 26 [Gt(ROSA)26Sor]. ROSA26 is a non-coding gene composed of three exons on mouse chromosome-6, a region where it is easy to insert genes. There are no known functional proteins encoded by the ROSA26 gene. Additionally, the ROSA26 locus makes it easy to perform homologous recombination (HR), maintains expression levels of the protein from gene constructs inserted into this region, and does not affect the expression or function of other endogenous genes. Given its wide expression across all cell types and developmental stages, the ROSA26 region is often used as a safe site for gene targeting in mouse models.

In general, there are three different types of ROSA26 locus knockin (KI):

• The first type is the original version, wherein the cDNA of the knockin gene is controlled by the ROSA26 promoter and is constitutively expressed in vivo (Figure a). Specifically, the transgene of the splice acceptor (SA) sequence of the cDNA was inserted into the XbA1 restriction enzyme cutting site in the first intron of the ROSA26 gene. A stop cassette, which is consisted of neomycin-resistant gene (NeoR, Positive selective marker) and three polyadenylate (pA), was inserted upstream of the transgene.
• The second strategy is conditional knockin (cKI), a variant of the first strategy which is achieved by inserting loxP sites on both sides of stop cassette (Figure b). In this case, the presence of the termination box can prevent the expression of the transgene. Upon mating with CRE expressing mice, the termination box sequence can be removed by homologous recombination (HR) in select tissues, depending on the type of CRE mouse strains used. However, the moderate strength of the natural ROSA26 promoter may not always achieve the desired transgenic expression levels.
• In order to overcome the limitation of the natural ROSA26 promoter, the third strategy is to introduce an exogenous promoter or enhancer (such as CAG promoter) to drive high transgene expression (Figure c). In addition, an IRES-GFP box with FRT sites on both sides can be cloned in downstream of the transgene to visualize the expression of the transgene in vivo.

Source: Official website of Creative Biolabs

Tbx1 Mice (E1-E2 knockout)

Figure 1. Tbx1 heterozygous mice (HT) exhibited ASD-related behavioral phenotypes
left: frequencies and durations of vocalizations
right: T-maze spontaneous alternation behavior (SAB)[6]

Shank3B Mice (E13-E16 knockout)

Figure 2. Shank3B^-/- mice have reduced social interaction and abnormal social novelty recognition^[7]

Cntnap2 Mice (E1 knockout)

Figure 3. Cntnap2^-/- mice exhibit an ASD phenotype with abnormal communication and social behavior^[8]