Week 4 - L6 - Genetic Engineering Using Knockout and Knock-in Models

Knockout organisms are primary tools for investigating the genetic basis of conditions and the physiological functions of specific genes.
A prominent application is the study of obesity and satiation. To understand why certain individuals gain weight rapidly or struggle with hunger, researchers identify gene variants in human populations and look for a homologous gene in model organisms, such as mice.
The $LEP$ gene in mice serves as a model for studying these traits. By creating a double knockout of this gene, researchers can observe the resulting phenotype to determine the gene's function.
A comparison between a wild-type mouse ( $LEP^{+/+}$ ) and a double knockout mouse ( $LEP^{-/-}$ ) reveals significant differences:
- The double knockout mouse cannot achieve satiation.
- It eats significantly more than the wild-type counterpart.
- It develops extreme obesity.
The biochemical underpinning of this phenotype is the loss of leptin production. Leptin is a hormone that regulates the hunger response and promotes a feeling of satisfaction after eating.
Variants in the human homologue of the leptin gene contribute to a predisposition for obesity, demonstrating how knockout models can mimic human physiological conditions.

A significant challenge in knockout studies is that many genes are essential for life.
Complete knockouts, where both copies of a gene are disabled, are frequently lethal to the organism.
Death often occurs at the early embryonic stage because the genes involved are critical for developmental processes.
To bypass this lethality, researchers utilize alternative methods such as random mutagenesis or conditional knockouts to fully characterize the roles of essential genes.

While knockout organisms demonstrate what happens when a gene is deactivated, knock-in organisms are used to study novel functions, expression patterns, or locations.
There are several strategies for creating knock-in organisms:
- Insertion of an entirely new gene into the genome to observe its systemic impact.
- Adding a novel component to an existing gene, such as placing a new regulatory element in front of it to observe changes in expression and phenotype.
- Gene tagging to visualize the location of expression within specific cells or tissues.
- Base pair editing to introduce single nucleotide variants (SNVs), allowing researchers to test if a specific change in one or more nucleotides results in a novel gene function.

The Green Fluorescent Protein ( $GFP$ ) is often used as a marker. Through homologous recombination, the $GFP$ locus can be introduced under the control of a specific regulator.
- If expressed in the skin or constitutively throughout the body, the mice will fluoresce green under $UV$ light.
- This effect is primarily visible when the mice are hairless.
- Researchers can also attach the $GFP$ marker to the end of a specific protein product to visualize exactly where that protein is expressed at a cellular level.
Studies involving the rat growth hormone demonstrate the effects of over-expression or heterologous expression:
- A transgene for rat growth hormone is inserted into a mouse genome under a zinc-responsive promoter.
- When the mouse is fed zinc, the promoter activates the gene.
- The resulting phenotype includes significantly increased musculature and a much larger body size compared to mice not provided with the zinc stimulus.
This approach is used for human health research by inserting human versions of genes or their homologs into model organisms to observe the functional impact of their expression.

Strengths of utilizing homologous recombination in genetic engineering include:
- Direct targeting: It enables site-directed or targeted mutagenesis rather than relying on random insertion.
- Versatility: It allows for the modification of any gene or genomic region provided there is a homologous sequence available.
- Efficiency in specific organisms: It is highly efficient in simple eukaryotes like $S. \, cerevisiae$ and in mice, which possess high natural rates of homologous recombination in their cells.
- Economic viability: The initial setup for the process is relatively inexpensive.
Weaknesses and challenges include:
- Reliance on random biological events: The process depends on the natural, chance occurrence of homologous recombination within a cell at the specific target location.
- Screening difficulty: While selection methods exist, identifying successful mutants can be time-consuming.
- Species limitations: In most higher-order eukaryotes besides the mouse, the rate of successful recombination is extremely low, making the process sparingly used or requiring alternative mutagenesis techniques.
- Off-target effects: There is a risk of the gene construct inserting randomly into different parts of the chromosome or undergoing unintended structural rearrangements.