Week 4 - L10 - Precision Genetic Engineering: The Cre-Lox System and CRISPR Integration

Fundamentals of the Cre-Lox Recombination System

• The Cre-Lox system is a specialized genetic tool used primarily for creating conditional knockouts in laboratory mice.

• The system functions by conditionally excising specific genetic material through the action of a site-specific recombinase enzyme.

• This mechanism allows for the targeted removal of a gene of interest exclusively in specific cell types or under specific conditions, rather than a global knockout throughout the entire organism.

Component Requirements: The Floxed Mouse and Cre Mouse

• Executing a conditional knockout requires the generation and mating of two distinct transgenic mouse lines.

• The Floxed Mouse:

  • In this mouse line, the gene of interest is engineered to be surrounded by two sequence elements called LOX P sites.

  • The term "floxed" is a portmanteau for "flanked by LOX P."

  • LOX P sites are specific DNA sequences that must be oriented in the same direction (tandem repeats) to facilitate the excision of the intervening genetic material.

• The Cre Recombinase Mouse:

  • This mouse line contains the Cre recombinase gene, which encodes the enzyme responsible for recognizing and acting upon the LOX P sites.

  • The expression of the Cre gene is placed under the control of a specific enhancer or promoter (designated as Enhancer $X$ or Promoter $X$).

  • The choice of enhancer or promoter dictates precisely where in the mouse (i.e., in which tissues or cell types) the Cre recombinase will be expressed.

The Mechanism of Gene Excision

• To produce a knockout, the floxed mouse is mated with the Cre recombinase mouse to create an $F_1$ generation. Subsequent crossing may be required to achieve a homozygous floxed state.

• In cells where the specific enhancer is active, the Cre recombinase protein is produced.

• The Cre recombinase enzyme binds to the two LOX P sites flanking the gene of interest.

• The interaction of the enzyme with the LOX P sites induces the DNA to form a loop structure.

• Cre mediates a recombination event that effectively merges the two LOX P sites into one, causing the intervening DNA (the gene of interest) to be "looped out" or excised from the chromosome.

• Once the gene is excised in a circular form, it has no mechanism for re-insertion or replication and is subsequentally lost as the cell divides. This renders the gene permanently non-functional in that specific cell lineage.

Liver-Specific Knockout: A Case Study

• Consider a scenario involving Gene $X$, which is hypothesized to have a critical function in the liver but is also essential for embryonic development.

• A global knockout of Gene $X$ would result in embryonic lethality, preventing the study of its function in the adult liver.

• To solve this, a conditional knockout is designed:

  • Cre Source: A mouse is used where Cre recombinase is under the control of the albumin enhancer. Albumin is a protein produced specifically in the liver; therefore, Cre production is restricted to liver cells.

  • Floxed Source: A transgenic mouse is used where Gene $X$ is flanked by LOX P sites on both chromosomes.

• The Breeding Process:

  • The first cross produces offspring that are heterozygous for the floxed Gene $X$ and carry the albumin-Cre transgene.

  • In the non-liver tissues of these mice, the Cre protein is not produced, and the cells maintain two functional copies of Gene $X$.

  • In the liver, the albumin enhancer triggers Cre production, which excises one copy of Gene $X$. This creates a liver-specific heterozygote.

• Achieving Complete Knockout:

  • To observe a mutant phenotype, both copies of Gene $X$ must be removed. This is achieved by crossing the heterozygotes with another floxed mouse to obtain progeny that possess the Cre transgene and are homozygous for the floxed Gene $X$.

  • In these specific mice, the Cre recombinase excise both copies of the gene within the liver, allowing researchers to observe the mutant phenotype in a living adult mouse.

Temporal Control via Inducible Cre-Lox Systems

• Beyond spatial (tissue-specific) control, the system can be modified to provide temporal (time-specific) control.

• This is achieved through an inducible version of Cre, such as a Cre recombinase fused to a specialized receptor.

• In this system, Cre is expressed in the target cells (e.g., liver) but remains sequestered or inactive and cannot bind to LOX P sites.

• Functionality is only triggered by the administration of an exogenous agonist, such as tamoxifen.

• When tamoxifen is introduced, it binds to the receptor attached to Cre, allowing the Cre recombinase to become functional, bind to LOX P sites, and excise the target gene.

• This allows researchers to choose the exact developmental stage or age at which the gene knockout occurs.

Integration with CRISPR Cas Technology

• The CRISPR Cas system provides a highly efficient method for generating the various components needed for Cre-Lox studies using a single RNA molecule and the Cas protein.

• Indel Mutations: Without providing homologous DNA for repair, CRISPR Cas can create double-strand breaks that are repaired via non-homologous end joining (NHEJ), resulting in insertions or deletions (indels) that create standard knockout lines.

• Precise Substitutions: By providing a target DNA sequence for homologous repair (Homology Directed Repair/HDR), CRISPR Cas can be used to introduce specific single amino acid substitutions to model pathogenic variants found in human homologs.

• Generating Floxed Alleles: CRISPR Cas can be used to facilitate the creation of the floxed mouse itself. By using guide and tracer RNAs to target specific introns or exons, researchers can provide a repair template that contains an exon flanked by LOX P sites. Once the Cas protein cuts the genomic DNA, the repair process integrates the LOX P sites, efficiently "floxing" the gene for subsequent use with Cre recombinase.