Biotechnology: Gene Editing Techniques

Gene editing changes DNA sequences and RNA processing to modulate gene expression and protein function. Since 2013, CRISPR-Cas9 enabled precise edits, with applications in disease treatment, animal models, and agriculture.

Trace the history of gene editing from zinc finger proteins and mega nucleases to CRISPR and Cas9, highlighting HDR and clinical breakthroughs in sickle cell and cancer.

Gene editing delivers a nuclease and guide RNA to target cells in vivo or ex vivo. DNA is cut and repaired to yield altered protein products.

Explore gene editing techniques developed over the last half century, including conventional homologous recombination, chemical systems, protein-based nucleus systems, and RNA-based CRISPR, with advances for clinical and laboratory applications.

Homologous recombination is a DNA repair mechanism used in gene targeting to insert a desired mutation into the homologous locus, enabling genome modification in yeast and mouse studies.

Describe the mechanism of homologous recombination in the eukaryotic nucleus, from exonuclease processing of a double-strand break to Rad52/Rad51 mediated strand invasion and double Holliday junction resolution.

Describe the limitations of homologous recombination, including extreme inefficiency across most cell types. Note its low success probability, about one in a million, and high off-target insertions.

Peptide nucleic acids are DNA/RNA analogues with a peptide-like backbone, allowing stable hybridization with complementary sequences and resistance to enzymatic degradation for gene mutation guidance.

Explain how peptide nucleic acids act as a restriction DNA cutter at sites in DNA via pseudo complementary patterns and Cerium and EDTA-driven excision, noting turnover time and manufacturing challenges.

Examine protein-based nuclease systems for DNA sequence editing, highlighting meganucleases' high specificity and low toxicity, large recognition sites of 12 to 40 base pairs, and limited repertory.

Examine two engineering paths for meganucleases: amino acid variation with selection to modify recognition, and domain fusion to create chimeric meganucleases with new sites.

Meganucleases, derived from chloroplasts of green algae, are homodimeric nucleases that recognize a 22-base-pair DNA site to introduce a double-strand break, repaired by HDR with a provided insert.

Zinc finger nuclease uses engineered zinc finger binding domains to recognize specific DNA sequences and create double-strand breaks repaired by DNA repair machinery, enabling precise genome modification in higher organisms.

Zinc finger nucleases combine a DNA binding domain of zinc finger repeats with a FokI cleavage domain, enabling them to recognize base-pair targets and act as genomic scissors.

Zinc finger nucleases are delivered into cells by transfection or electroporation, enter the nucleus, and create a double-strand break. A repair template guides natural repair to introduce the desired mutation.

Explore zinc finger nuclease strengths such as rapid disruption and heritable mutations across somatic and germ cells, with single-transfection knockouts and knock-ins; weigh off-target risks and immunogenicity.

Explore TALEN, a protein-based genome editing tool using a DNA binding domain and the catalytic nuclease domain to recognize single nucleotides via repeat variable die residue and cut target DNA.

Left and right TALEN monomers recognize DNA with 34‑amino‑acid repeats and residues at 12 and 13; binding induces double‑strand break repaired by NHEJ or HDR, yielding insertions, deletions, or substitutions.

Talent offers site-specific, permanent and heritable mutations through single-nucleotide recognition with fewer off-target effects, yet requires construction of two nuclease sites and faces vector delivery challenges due to repetitive sequences.

Explore CRISPR, an RNA-protein based gene editing system derived from ancient prokaryotic immunity, where CRISPR arrays store viral genomes and Cas proteins cleave invaders to defend the host.

Explore the general mechanism of CRISPR-Cas, including spacer acquisition with Cas1 and Cas2, crRNA biogenesis, and interference that cleaves foreign DNA.

Explore the three types of CRISPR/Cas systems, with a focus on type II and its Cas9 protein, CRISPR RNA, and trans activating CRISPR RNA mechanisms in genome editing.

Describe how type I CRISPR-Cas system uses spacer acquisition with Cas1 and Cas2, pre-crRNA processing in the Cascade by Cas6, and mature crRNA guiding Cas3 to nick and degrade DNA.

Trace the CRISPR-Cas mechanism in three steps: spacer acquisition, crRNA processing, and interference, leading to Cas9 guided cleavage of viral DNA at the PAM site.

Examine how the type iii CRISPR-Cas system processes pre-crRNA with Cas6, matures via Cas10, includes anti-crRNA steps, and uses iii-a for DNA cleavage and iii-b for RNA degradation.

Assess CRISPR-Cas strengths, including simplicity, efficiency, rapid embryo editing, and robust design tools, while recognizing limitations like off-target effects and delivery challenges in mature cells.

Explore ethical concerns surrounding gene editing techniques, including illegal germline mutations, eugenics, designer babies, and potential weaponization, and the need for regulated frameworks, consensus, and expert oversight.

Explore gene editing applications across agriculture, animal husbandry, and human medicine, including disease-resistant crops, abiotic stress tolerance, and somatic editing for HIV and cancer.