Cre-Lox recombination is a site-specific recombinase technology, used to carry out deletions, insertions, translocations and inversions at specific sites in the DNA of cells. It allows the DNA modification to be targeted to a specific cell type or be triggered by a specific external stimulus. It is implemented both in eukaryotic and prokaryotic systems. The Cre-lox recombination system has been particularly useful to help neuroscientists to study the brain in which complex cell types and neural circuits come together to generate cognition and behaviors. NIH Blueprint for Neuroscience Research has created several hundreds of Cre driver mouse lines which are currently used by the worldwide neuroscience community.
An important application of the Cre-lox system is excision of selectable markers in gene replacement. Commonly used gene replacement strategies introduce selectable markers into the genome to facilitate selection of genetic mutations that may cause growth retardation. However, marker expression can have polar effects on the expression of upstream and downstream genes. Removal of selectable markers from the genome by Cre-lox recombination is an elegant and efficient way to circumvent this problem and is therefore widely used in plants, mouse cell lines, yeast, etc.
The system consists of a single enzyme, Cre recombinase, that recombines a pair of short target sequences called the Lox sequences. This system can be implemented without inserting any extra supporting proteins or sequences. The Cre enzyme and the original Lox site called the LoxP sequence are derived from bacteriophage P1.
Placing Lox sequences appropriately allows genes to be activated, repressed, or exchanged for other genes. At a DNA level many types of manipulations can be carried out. The activity of the Cre enzyme can be controlled so that it is expressed in a particular cell type or triggered by an external stimulus like a chemical signal or a heat shock. These targeted DNA changes are useful in cell lineage tracing and when mutants are lethal if expressed globally.
The Cre-Lox system is very similar in action and in usage to the FLP-FRT recombination system.
History Cre-Lox recombination is a special type of site-specific recombination developed by Dr. Brian Sauer and patented by DuPont that operated in dividing mitotic cells, and was initially used in activating gene expression in mammalian cell lines. Subsequently, researchers in the laboratory of Dr. Jamey Marth demonstrated that Cre-Lox recombination could be used to delete loxP-flanked chromosomal DNA sequences at high efficiency in specific developing T-cells of transgenic animals, with the authors proposing that this approach could be used to define endogenous gene function in specific cell types, indelibly mark progenitors in cell fate determination studies, induce specific chromosomal rearrangements for biological and disease modeling, and determine the roles of early genetic lesions in disease (and phenotype) maintenance.
Shortly thereafter, researchers in the laboratory of Prof. Klaus Rajewsky reported the production of pluripotent embryonic stem cells bearing a targeted loxP-flanked (floxed) DNA polymerase gene. Combining these advances in collaboration, the laboratories of Drs. Marth and Rajewsky reported in 1994 that Cre-lox recombination could be used for conditional gene targeting. They observed ≈50% of the DNA polymerase beta gene was deleted in T cells based on DNA blotting. It was unclear whether only one allele in each T-cell or 50% of T cells had 100% deletion in both alleles. Researchers have since reported more efficient Cre-Lox conditional gene mutagenesis in the developing T cells by the Marth laboratory in 1995. Incomplete deletion by Cre recombinase is not uncommon in cells when two copies of floxed sequences exist, and allows the formation and study of chimeric tissues. All cell types tested in mice have been shown to undergo transgenic Cre recombination. However, because DNA recombination only occurs in the dividing cells as typically taught in the classrooms and written in the biology textbooks, it is not sure whether Cre-lox-mediated DNA recombination would work in the non-dividing post-mitotic cells.
Independently, Joe Z. Tsien at MIT has pioneered the use of Cre-loxP system for cell type- and region-specific gene manipulation in the adult brain where hundreds of distinct neuron types may exist and nearly all neurons in the adult brain are known to be post-mitotic. Tsien and his colleagues demonstrated Cre-mediated recombination can occur in the post-mitotic pyramidal neurons in the adult mouse forebrain.
These developments have led to a widespread use of conditional mutagenesis in biomedical research, spanning many disciplines in which it becomes a powerful platform for determining gene function in specific cell types and at specific developmental times. In particular, the clear demonstration of its usefulness in precisely defining the complex relationship between specific cells/circuits and behaviors for brain research, has promoted the NIH to initiate the NIH Blueprint for Neuroscience Research Cre-driver mouse projects in early 2000.NIH blueprint for neuroscience: Cre driver network. https://www.ninds.nih.gov/current-research/trans-agency-activities/nih-blueprint-neuroscience-researchGENSAT brain project, http://www.gensat.org/index.html To date, NIH Blueprint for Neuroscience Research Cre projects have created several hundreds of Cre driver mouse lines which are currently used by the worldwide neuroscience community.
Overview Cre-Lox recombination involves the targeting of a specific sequence of DNA and splicing it with the help of an enzyme called Cre recombinase. Cre-Lox recombination is commonly used to circumvent embryonic lethality caused by systemic inactivation of many genes. As of February 2019, Cre–Lox recombination is a powerful tool and is used in transgenic animal modeling to link genotypes to phenotypes.
The Cre-lox system is used as a genetic tool to control site specific recombination events in genomic DNA. This system has allowed researchers to manipulate a variety of genetically modified organisms to control gene expression, delete undesired DNA sequences and modify chromosome architecture.
The Cre protein is a site-specific DNA recombinase that can catalyse the recombination of DNA between specific sites in a DNA molecule. These sites, known as loxP sequences, contain specific binding sites for Cre that surround a directional core sequence where recombination can occur.
When cells that have loxP sites in their genome express Cre, a recombination event can occur between the loxP sites. Cre recombinase proteins bind to the first and last 13 bp regions of a lox site forming a dimer. This dimer then binds to a dimer on another lox site to form a tetramer. Lox sites are directional and the two sites joined by the tetramer are parallel in orientation. The double stranded DNA is cut at both loxP sites by the Cre protein. The strands are then rejoined with DNA ligase in a quick and efficient process. The result of recombination depends on the orientation of the loxP sites. For two lox sites on the same chromosome arm, inverted loxP sites will cause an inversion of the intervening DNA, while a direct repeat of loxP sites will cause a deletion event. If loxP sites are on different chromosomes it is possible for translocation events to be catalysed by Cre induced recombination. Two plasmids can be joined using the variant lox sites 71 and 66.
Cre recombinase Cre recombinase can be synthesized by the corresponding gene under the direction of cell-specific promoters, including promoters under the control of doxycycline. An additional level of control can be achieved by using his Cre recombinase, engineered to reversibly activate in the presence of the estrogen analogue 4-hydroxy tamoxifen. This provides the advantage that the Cre recombinase is active for a short time. This prevents non-specific actions of Cre recombinase. The Cre recombinase can recognize cryptic sites in the host genome and induce unauthorized recombination, damaging host DNA. This tool is suitable for deleting antibiotic resistance genes, but above all it allows conditional knockouts that can be induced at specific times in the cell type of choice. Models thus obtained are more likely to mimic the physiological situation.
The Cre protein (encoded by the locus originally named as "Causes recombination", with "Cyclization recombinase" being found in some references) consists of 4 subunits and two domains: The larger carboxyl (C-terminal) domain, and smaller amino (N-terminal) domain. The total protein has 343 amino acids. The C domain is similar in structure to the domain in the Integrase family of enzymes isolated from lambda phage. This is also the catalytic site of the enzyme.
loxP site
loxP (locus of X-over P1) is a site on the bacteriophage P1 consisting of 34 bp. The site includes an asymmetric 8 bp sequence, variable except for the middle two bases, in between two sets of symmetric, 13 bp sequences. The exact sequence is given below; 'N' indicates bases which may vary, and lowercase letters indicate bases that have been mutated from the wild-type. The 13 bp sequences are palindromic but the 8 bp spacer is not, thus giving the loxP sequence a certain direction. Usually loxP sites come in pairs for genetic manipulation. If the two loxP sites are in the same orientation, the floxed sequence (sequence flanked by two loxP sites) is excised; however if the two loxP sites are in the opposite orientation, the floxed sequence is inverted. If there exists a floxed donor sequence, the donor sequence can be swapped with the original sequence. This technique is called recombinase-mediated cassette exchange and is a very convenient and time-saving way for genetic manipulation. The caveat, however, is that the recombination reaction can happen backwards, rendering cassette exchange inefficient. In addition, sequence excision can happen in trans instead of a in cis cassette exchange event. The loxP mutants are created to avoid these problems. 13bp8bp13bp --
+ Example Alternate loxP Sites Name 13bp Recognition Region 8bp Spacer Region 13bp Recognition Region Wild-Type lox 511 lox 5171 lox 2272 M2 M3 M7 M11 lox 71 lox 66 loxPsym
Holliday junctions and homologous recombination During genetic recombination, a Holliday junction is formed between the two strands of DNA and a double-stranded break in a DNA molecule leaves a 3' OH end exposed. This reaction is aided with the endonuclease activity of an enzyme. 5' Phosphate ends are usually the substrates for this reaction, thus extended 3' regions remain. This 3' OH group is highly unstable, and the strand on which it is present must find its complement. Since homologous recombination occurs after DNA replication, two strands of DNA are available, and thus, the 3' OH group must pair with its complement, and it does so, with an intact strand on the other duplex. Now, one point of crossover has occurred, which is what is called a Holliday intermediate.
The 3' OH end is elongated (that is, bases are added) with the help of DNA Polymerase. The pairing of opposite strands is what constitutes the crossing-over or recombination event, which is common to all living organisms, since the genetic material on one strand of one duplex has paired with one strand of another duplex, and has been elongated by DNA polymerase. Further cleavage of Holliday intermediates results in formation of Hybrid DNA.
This further cleavage or 'resolvation' is done by a special group of enzymes called Resolvases. RuvC is just one of these Resolvases that have been isolated in bacteria and yeast.