Genetic Modification in Animals: An Overview

Genetic Modification in Animals: An Overview

This document provides comprehensive notes on genetic modification in animals, covering its historical development, key techniques, diverse applications, ethical considerations, and future insights.

Introduction to Gene Modification

Genetic modification is the result of a gradual discovery process that deepened the understanding of DNA's role in transmitting genetic traits, its chemical and 3D structure, recombinant DNA technologies, cloning, sequencing, and the development of new animal models with both random and directed mutations. This extensive scientific journey led to the birth of modern genetic modifications.

Historical Milestones:

• 1974: The first transgenic mice were observed with directed gene mutation, a breakthrough attributed to Rudolph Jaenisch and Beatrice Mitz.

• 1981: Gordon and Ruddle successfully created a genetically modified mouse through the microinjection of purified DNA into single-cell mouse embryos. This pivotal achievement marked the beginning of transgenesis and enabled controlled genetic alteration in animal models.

Techniques for Altering DNA

Altering DNA in animals involves three fundamental steps:

1. Gene Construct Production: Creating a functional gene construct in vitro that aligns with the hypothesis of the proposed study.

2. DNA Delivery: Facilitating the passage of the required DNA across the plasma membrane, through the cytoplasm, and into the nucleus of the host cell.

3. Genome Integration and Inheritance: Ensuring the exogenous DNA integrates into the host cell's genome and is passed on unaltered to subsequent generations.

1. Gene Construct

A gene construct typically includes:

• A promoter: A DNA sequence that initiates gene transcription.

• A site of transcription initiation: The specific point where RNA synthesis begins.

• A site of polyadenylation: A sequence that signals the addition of a poly-A tail to mRNA.

• A site of transcription termination: A sequence that signals the end of gene transcription.

Types of Transgenesis based on Insertion:

• Transgenesis by addition: Utilizes a construct designed for random insertion into the animal's genome.

• Transgenesis by homologous recombination: Employs a construct engineered for insertion into a specific, targeted site within the genome.

2. Insertion of the DNA of Interest into Cells

Various methods are used to deliver DNA into host cells:

• Calcium Precipitation: DNA is precipitated with calcium salts and incorporated into endosomes. This method is reported to be inefficient due to DNA degradation within the endosome.

• Electroporation: Cells are mixed with DNA in a solution and exposed to a sudden, powerful electric current. This creates temporary pores in the plasma membrane, allowing DNA to enter the cell. It is considered the most efficient physical method and is widely used across most cell types.

• Lipid Micelles or Lipoplexes: Cationic lipid molecules form complexes with DNA, facilitating the passage of genetic material through the cell membrane.

• Microinjection: DNA in solution is physically injected directly into the nucleus using fine glass micropipettes. This method results in approximately $4-8\%$ of born animals with the transgene integrated into their genome expressing the desired phenotype.

• Viral Vectors: A recently employed and highly efficient method. DNA is integrated into the genome of a virus, which then infects the target cell, delivering the genetic material. This method achieves high rates of expression efficiency.

3. Integration

Integration of exogenous DNA into the host genome typically occurs at random locations. This event is rare even in successfully transfected cells, except when viral vectors are used for DNA transfer. Random integration can lead to:

• Low expression levels of the transgene.

• Integration at random sites within the genome, potentially disrupting existing genes.

• A mosaic pattern of expression, where not all cells uniformly express the transgene.

Vector Creation or Transgenesis Vector

A transgenesis vector is composed of nucleic acid manipulated using molecular biology techniques. A standard construct for a transgenesis vector includes:

• Promoter: To regulate gene expression.

• Coding sequence (cDNA): The genetic material that encodes the desired protein.

• Termination/polyA signaling sequence: To ensure proper mRNA processing and stability.

Applications of Genetic Modification in Animals

Genetic modification in animals has diverse applications across several fields:

1. Medicine / Pharming: Engineering animals to produce human therapeutic proteins in their milk, blood, or eggs.

   • Example: Transgenic goats producing antithrombin, a blood-clot prevention drug, in their milk.

2. Biomedical Research: Creating transgenic animals (e.g., mice, rats, zebrafish) as disease models for studying human conditions.

   • Example: The Oncomouse (Harvard, 1980s), engineered with a cancer-promoting gene, is widely used for cancer research.

3. Agriculture: Developing livestock with improved traits such as disease resistance or faster growth.

   • Example: Genetically modified pigs resistant to the Porcine Reproductive and Respiratory Syndrome (PRRS) virus.

4. Environment: Producing animals with enhanced feed efficiency or reduced waste output to lessen environmental impact.

   • Example: Enviropigs, engineered to produce less phosphorus in their manure, leading to reduced water pollution.

Applications can be broadly categorized into:

• Transgenesis

• Gene Editing

• Gene Knockouts

Transgenesis

Transgenesis involves introducing a foreign DNA sequence (a transgene) into an animal's genome. The goal is for this new DNA to be inherited by offspring and drive a desired trait.

• Alpha-1 Antitrypsin (AAT) Transgenesis: The process of introducing a foreign gene for human AAT into an organism (e.g., a sheep) to produce this therapeutic protein in the host's milk or other secretions. This protein can be used to treat conditions like Alpha-1 antitrypsin deficiency (AATD), and potentially human lung conditions such as emphysema and cystic fibrosis.

• Alpha-lactalbumin ($\alpha$ -LA) in Cows: Alpha-lactalbumin is a major whey protein crucial for lactose synthesis and a source of essential amino acids like tryptophan, which supports cognitive function and mood under stress. While present in cow's milk, its concentration is lower than in human milk, leading to its addition to infant formula to improve nutritional profiles.

Methods of Transgenesis:

• DNA Microinjection: Direct delivery of foreign genes into living cells (egg cells, oocytes, embryos) using fine glass micropipettes.

• Embryonic Stem Cell Gene-Mediated Transfer: Embryonic stem (ES) cells are genetically modified in culture (gene insertion, deletion, or alteration). These modified cells are then injected into the inner cell mass of a host blastocyst. The resulting embryo develops into a chimeric animal, which, through breeding, can pass on the genetic modification to offspring, establishing a stable transgenic line.

Gene Editing

Gene editing in animals involves the precise alteration of DNA sequences using molecular tools like CRISPR-Cas9, TALENs, and ZFNs. Unlike traditional transgenesis, which might insert foreign DNA randomly, gene editing allows for highly accurate addition, deletion, or modification of specific genes.

Gene Editing Tools and Examples:

1. CRISPR-Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats):

   • A bacterial immune system-derived tool using a guide RNA (gRNA) to direct the Cas9 protein to a specific DNA sequence, where Cas9 makes a double-strand cut.

   • Problem: Porcine Reproductive and Respiratory Syndrome (PRRS) causes severe illness and economic losses in pigs.

   • Target Chosen: CD163, a cellular receptor required by the PRRS virus to infect pig macrophages.

   • Steps:

     1. Design CRISPR guide(s) to disrupt CD163 (or remove a critical domain).

     2. Deliver Cas9 + guide RNA into one-cell pig embryos (via microinjection or electroporation) or edit somatic cells for cloning approaches.

     3. Implant edited embryos into surrogate mothers to produce founder pigs.

     4. Screen founders for CD163 disruption via sequencing; breed to establish modified lines.

2. TALENs (Transcription Activator-Like Effector Nucleases):

   • Engineered proteins composed of TALE repeats (derived from Xanthomonas plant bacteria) fused to a FokI nuclease. TALENs are programmable to target specific DNA sequences.

   • Problem: Dairy calves are routinely dehorned, a painful procedure, to reduce injury risks, raising animal-welfare concerns.

   • Target Chosen: The naturally occurring POLLED allele, a genetic variant that produces hornless cattle.

   • Steps:

     1. Identify the POLLED allele sequence from naturally hornless cattle.

     2. Use genome editing (TALENs in the cited work) to introduce that allele into cells from horned dairy breeds.

     3. Screen edited somatic cells to identify those carrying the POLLED allele without unwanted off-target changes.

     4. Utilize somatic-cell nuclear transfer (cloning) to create embryos from an edited cell line and implant them into surrogates.

     5. Produce calves that are naturally born hornless. Confirm genotype and phenotype.

3. ZFNs (Zinc Finger Nucleases):

   • Act like molecular scissors that cut DNA at specific locations. ZFNs have been used to create precise genome modifications with potential therapeutic utility in gene therapy.

   • Problem: Generic recessive diseases in Mus musculus (mice).

   • Target Chosen: Insertion of Green Fluorescent Protein (GFP) into the ROSA26 Locus.

   • Steps:

     1. Design ZFNs specific to the target gene or locus.

     2. Inject ZFN mRNA or DNA into one-cell rat embryos (embryo microinjection).

     3. Allow embryos to develop; screen resulting pups for targeted gene disruptions (sequencing to detect insertions/deletions or indels).

Gene Knockout

Gene knockout involves disabling a specific gene to study its function.

Conventional Gene Knockout

• The targeted gene is permanently disabled in all cells of the organism, starting from its earliest stage of development. Also known as whole-body knockouts.

• Originally obtained by targeted insertion through homologous recombination, a process with an extremely low success rate.

• More recently achieved with higher efficiency using CRISPR-Cas9 technology.

Steps for Conventional Gene Knockout:

1. Vector Construction: Build a DNA vector containing a disrupted copy of the target gene. Homologous DNA arms within the vector guide recombination into the correct genomic locus.

2. Introduction into ES Cells: Deliver the targeting vector into mouse embryonic stem (ES) cells. Through homologous recombination, the wild-type allele is replaced by the disrupted allele.

3. Inject into Blastocyst: Inject the genetically modified ES cells into a fertilized blastocyst. This blastocyst is then transferred into a surrogate female mouse. The offspring born are chimeric mice, meaning they are a mixture of modified and unmodified cells.

4. Germline Transmission: Chimeric mice are bred with normal wild-type mice. Some offspring inherit the knockout allele in their germline, becoming heterozygous ($+ / -$) for the knockout gene.

5. Breeding to Homozygosity: Heterozygous offspring ($+ / -$) are crossed with each other ($+ / - \times + / -$). Following Mendelian inheritance patterns, the resulting offspring will be:

   • $25\%$ wild-type ($+ / +$)

   • $50\%$ heterozygous ($+ / -$)

   • $25\%$ homozygous knockout ($- / -$), where the targeted gene is completely absent in all cells.

Conditional Gene Knockout

• The targeted gene is disabled only in specific tissues of the organism and at a chosen developmental stage. This strategy is particularly useful for deleting genes essential for development without causing embryonic lethality.

• The main strategy employed is the Cre-Lox system.

  • Cre enzyme: A recombinase that mediates recombination of DNA regions flanked by LoxP sites.

  • LoxP flanks: Specific 34\-base pair DNA sequences that, when flanking a gene region, allow the Cre enzyme to excise (remove) the region contained between them.

  • Promoters used (to control Cre expression):

    1. Cell-specific promoters (activate Cre in certain cell types).

    2. Temporal promoters (activate Cre at specific developmental stages).

    3. Inducible promoters (activate Cre in response to an external stimulus).

Steps for Conditional Gene Knockout (Cre-Lox System):

1. Vector Construction (LoxP Mouse): Two loxP sites are inserted on each side of an essential exon of the target gene (e.g., gene X) using homologous recombination. This creates a

Genetic Modification in Animals: An Overview This document provides comprehensive notes on genetic modification in animals, covering its historical development, key techniques, diverse applications, ethical considerations, and future insights. ### Introduction to Gene Modification Genetic modification is the result of a gradual discovery process that deepened the understanding of DNA's role in transmitting genetic traits, its chemical and 3D structure, recombinant DNA technologies, cloning, sequencing, and the development of new animal models with both random and directed mutations. This extensive scientific journey led to the birth of modern genetic modifications. Historical Milestones: - 1974: The first transgenic mice were observed with directed gene mutation, a breakthrough attributed to Rudolph Jaenisch and Beatrice Mitz. - 1981: Gordon and Ruddle successfully created a genetically modified mouse through the microinjection of purified DNA into single-cell mouse embryos. This pivotal achievement marked the beginning of transgenesis and enabled controlled genetic alteration in animal models. ### Techniques for Altering DNA Altering DNA in animals involves three fundamental steps: 1. Gene Construct Production: Creating a functional gene construct in vitro that aligns with the hypothesis of the proposed study. 2. DNA Delivery: Facilitating the passage of the required DNA across the plasma membrane, through the cytoplasm, and into the nucleus of the host cell. 3. Genome Integration and Inheritance: Ensuring the exogenous DNA integrates into the host cell's genome and is passed on unaltered to subsequent generations. #### 1. Gene Construct A gene construct typically includes: - A promoter: A DNA sequence that initiates gene transcription. - A site of transcription initiation: The specific point where RNA synthesis begins. - A site of polyadenylation: A sequence that signals the addition of a poly-A tail to mRNA. - A site of transcription termination: A sequence that signals the end of gene transcription. Types of Transgenesis based on Insertion: - Transgenesis by addition: Utilizes a construct designed for random insertion into the animal's genome. - Transgenesis by homologous recombination: Employs a construct engineered for insertion into a specific, targeted site within the genome. #### 2. Insertion of the DNA of Interest into Cells Various methods are used to deliver DNA into host cells: - Calcium Precipitation: DNA is precipitated with calcium salts and incorporated into endosomes. This method is reported to be inefficient due to DNA degradation within the endosome. - Electroporation: Cells are mixed with DNA in a solution and exposed to a sudden, powerful electric current. This creates temporary pores in the plasma membrane, allowing DNA to enter the cell. It is considered the most efficient physical method and is widely used across most cell types. - Lipid Micelles or Lipoplexes: Cationic lipid molecules form complexes with DNA, facilitating the passage of genetic material through the cell membrane. - Microinjection: DNA in solution is physically injected directly into the nucleus using fine glass micropipettes. This method results in approximately 4-8 ext{%} of born animals with the transgene integrated into their genome expressing the desired phenotype. - Viral Vectors: A recently employed and highly efficient method. DNA is integrated into the genome of a virus, which then infects the target cell, delivering the genetic material. This method achieves high rates of expression efficiency. #### 3. Integration Integration of exogenous DNA into the host genome typically occurs at random locations. This event is rare even in successfully transfected cells, except when viral vectors are used for DNA transfer. Random integration can lead to: - Low expression levels of the transgene. - Integration at random sites within the genome, potentially disrupting existing genes. - A mosaic pattern of expression, where not all cells uniformly express the transgene. #### Vector Creation or Transgenesis Vector A transgenesis vector is composed of nucleic acid manipulated using molecular biology techniques. A standard construct for a transgenesis vector includes: - Promoter: To regulate gene expression. - Coding sequence (cDNA): The genetic material that encodes the desired protein. - Termination/polyA signaling sequence: To ensure proper mRNA processing and stability. ### Applications of Genetic Modification in Animals Genetic modification in animals has diverse applications across several fields: 1. Medicine / Pharming: Engineering animals to produce human therapeutic proteins in their milk, blood, or eggs.- Example: Transgenic goats producing antithrombin, a blood-clot prevention drug, in their milk. 2. Biomedical Research: Creating transgenic animals (e.g., mice, rats, zebrafish) as disease models for studying human conditions.- Example: The Oncomouse (Harvard, 1980s), engineered with a cancer-promoting gene, is widely used for cancer research. 3. Agriculture: Developing livestock with improved traits such as disease resistance or faster growth.- Example: Genetically modified pigs resistant to the Porcine Reproductive and Respiratory Syndrome (PRRS) virus. 4. Environment: Producing animals with enhanced feed efficiency or reduced waste output to lessen environmental impact.- Example: Enviropigs, engineered to produce less phosphorus in their manure, leading to reduced water pollution. Applications can be broadly categorized into: - Transgenesis - Gene Editing - Gene Knockouts #### Transgenesis Transgenesis involves introducing a foreign DNA sequence (a transgene) into an animal's genome. The goal is for this new DNA to be inherited by offspring and drive a desired trait. - Alpha-1 Antitrypsin (AAT) Transgenesis: The process of introducing a foreign gene for human AAT into an organism (e.g., a sheep) to produce this therapeutic protein in the host's milk or other secretions. This protein can be used to treat conditions like Alpha-1 antitrypsin deficiency (AATD), and potentially human lung conditions such as emphysema and cystic fibrosis. - Alpha-lactalbumin ($ext{ extalpha}$-LA) in Cows: Alpha-lactalbumin is a major whey protein crucial for lactose synthesis and a source of essential amino acids like tryptophan, which supports cognitive function and mood under stress. While present in cow's milk, its concentration is lower than in human milk, leading to its addition to infant formula to improve nutritional profiles. Methods of Transgenesis: - DNA Microinjection: Direct delivery of foreign genes into living cells (egg cells, oocytes, embryos) using fine glass micropipettes. - Embryonic Stem Cell Gene-Mediated Transfer: Embryonic stem (ES) cells are genetically modified in culture (gene insertion, deletion, or alteration). These modified cells are then injected into the inner cell mass of a host blastocyst. The resulting embryo develops into a chimeric animal, which, through breeding, can pass on the genetic modification to offspring, establishing a stable transgenic line. #### Gene Editing Gene editing in animals involves the precise alteration of DNA sequences using molecular tools like CRISPR-Cas9, TALENs, and ZFNs. Unlike traditional transgenesis, which might insert foreign DNA randomly, gene editing allows for highly accurate addition, deletion, or modification of specific genes. Gene Editing Tools and Examples: 1. CRISPR-Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats): - A bacterial immune system-derived tool using a guide RNA (gRNA) to direct the Cas9 protein to a specific DNA sequence, where Cas9 makes a double-strand cut. - Problem: Porcine Reproductive and Respiratory Syndrome (PRRS) causes severe illness and economic losses in pigs. - Target Chosen: CD163, a cellular receptor required by the PRRS virus to infect pig macrophages. - Steps:1. Design CRISPR guide(s) to disrupt CD163 (or remove a critical domain). 2. Deliver Cas9 + guide RNA into one-cell pig embryos (via microinjection or electroporation) or edit somatic cells for cloning approaches. 3. Implant edited embryos into surrogate mothers to produce founder pigs. 4. Screen founders for CD163 disruption via sequencing; breed to establish modified lines. 2. TALENs (Transcription Activator-Like Effector Nucleases): - Engineered proteins composed of TALE repeats (derived from Xanthomonas plant bacteria) fused to a FokI nuclease. TALENs are programmable to target specific DNA sequences. - Problem: Dairy calves are routinely dehorned, a painful procedure, to reduce injury risks, raising animal-welfare concerns. - Target Chosen: The naturally occurring POLLED allele, a genetic variant that produces hornless cattle. - Steps:1. Identify the POLLED allele sequence from naturally hornless cattle. 2. Use genome editing (TALENs in the cited work) to introduce that allele into cells from horned dairy breeds. 3. Screen edited somatic cells to identify those carrying the POLLED allele without unwanted off-target changes. 4. Utilize somatic-cell nuclear transfer (cloning) to create embryos from an edited cell line and implant them into surrogates. 5. Produce calves that are naturally born hornless. Confirm genotype and phenotype. 3. ZFNs (Zinc Finger Nucleases): - Act like molecular scissors that cut DNA at specific locations. ZFNs have been used to create precise genome modifications with potential therapeutic utility in gene therapy. - Problem: Generic recessive diseases in Mus musculus (mice). - Target Chosen: Insertion of Green Fluorescent Protein (GFP) into the ROSA26 Locus. - Steps:1. Design ZFNs specific to the target gene or locus. 2. Inject ZFN mRNA or DNA into one-cell rat embryos (embryo microinjection). 3. Allow embryos to develop; screen resulting pups for targeted gene disruptions (sequencing to detect insertions/deletions or indels). #### Gene Knockout Gene knockout involves disabling a specific gene to study its function. ##### Conventional Gene Knockout - The targeted gene is permanently disabled in all cells of the organism, starting from its earliest stage of development. Also known as whole-body knockouts. - Originally obtained by targeted insertion through homologous recombination, a process with an extremely low success rate. - More recently achieved with higher efficiency using CRISPR-Cas9 technology. Steps for Conventional Gene Knockout: 1. Vector Construction: Build a DNA vector containing a disrupted copy of the target gene. Homologous DNA arms within the vector guide recombination into the correct genomic locus. This vector typically includes selectable marker genes (e.g., for antibiotic resistance) to identify successfully modified cells, and often a negative selectable marker to screen against random integration. 2. Introduction into ES Cells: Deliver the targeting vector into mouse embryonic stem (ES) cells (e.g., via electroporation). Through homologous recombination, the wild-type allele is replaced by the disrupted allele. Cells that have undergone successful homologous recombination are selected using genetic markers. 3. Inject into Blastocyst: Inject the genetically modified ES cells into a fertilized blastocyst (an early-stage embryo). This blastocyst is then transferred into a surrogate female mouse. The offspring born are chimeric mice, meaning they are a mixture of modified (from the ES cells) and unmodified (from the host blastocyst) cells. These chimeras can be identified by coat color markers if different ES cells and blastocysts are used. 4. Germline Transmission: Chimeric mice are bred with normal wild-type mice. Some offspring inherit the knockout allele in their germline, becoming heterozygous ($+/- ext{}$) for the knockout gene. This step confirms that the modified ES cells contributed to the germline, allowing for the stable inheritance of the knockout. 5. Breeding to Homozygosity: Heterozygous offspring ($+/- ext{}$) are crossed with each other ($+/- ext{ imes }+/- ext{}$). Following Mendelian inheritance patterns, the resulting offspring will be: - 25 ext{%} wild-type ($+/+ ext{}$) - 50 ext{%} heterozygous ($+/- ext{}$) - 25 ext{%} homozygous knockout ($-/- ext{}$), where the targeted gene is completely absent in all cells. Homozygous knockouts are then analyzed to study the function of the disabled gene. ##### Conditional Gene Knockout - The targeted gene is disabled only in specific tissues of the organism and at a chosen developmental stage. This strategy is particularly useful for deleting genes essential for development without causing embryonic lethality. - The main strategy employed is the Cre-Lox system.- Cre enzyme: A recombinase that mediates recombination of DNA regions flanked by LoxP sites. - LoxP flanks: Specific 34 ext{$}$-base pair DNA sequences that, when flanking a gene region, allow the Cre enzyme to excise (remove) the region contained between them. - Promoters used (to control Cre expression):1. Cell-specific promoters (activate Cre in certain cell types). 2. Temporal promoters (activate Cre at specific developmental stages). 3. Inducible promoters (activate Cre in response to an external stimulus, like a drug). Steps for Conditional Gene Knockout (Cre-Lox System): 1. Vector Construction (LoxP Mouse): Two loxP sites are inserted on each side of an essential exon of the target gene (e.g., gene X) using homologous recombination in ES cells. These modified ES cells are then used to generate mice (similar to conventional knockout steps 2-5, except the resulting mice carry the 'floxed' gene X, denoted as X^{ ext{flox/flox}}$). These mice are phenotypically normal because gene X is still functional. 2. <strong>Cre Transgenic Mouse</strong>: A separate mouse line is created where the Cre recombinase gene is expressed under the control of a specific promoter. This promoter dictates <em>where</em> and <em>when</em> Cre is active (e.g., a liver-specific promoter for liver cell knockout, or a tamoxifen-inducible promoter). These mice are also phenotypically normal. 3. <strong>Breeding Cross</strong>: The$X^{ ext{flox/flox}}$mice are bred with the Cre transgenic mice. The F1 offspring will be heterozygous for both the floxed gene and the Cre recombinase (e.g.,$X^{ ext{flox/+}}; ext{Cre}^{+/-}$$). 4. Generating Conditional Knockout: Further