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CRISPR gene editing

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CRISPR-Cas9 Gene Editing

CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats. It is a genome-editing technology adapted from a natural bacterial defense system against bacteriophages and plasmids.

Principle and normal bacterial function

In bacteria, fragments of DNA from prior invading phages are incorporated into the bacterial genome as CRISPR sequences. These sequences are transcribed into RNA guides. The guide RNA directs a CRISPR-associated nuclease, commonly Cas9, to complementary foreign DNA, which is then cleaved and inactivated.

Components used for gene editing

  1. Cas9 nuclease: An enzyme that cuts double-stranded DNA.
  2. Guide RNA (gRNA): An artificially designed RNA, approximately 20 bases in its variable targeting sequence, complementary to the DNA sequence to be edited.
  3. Target DNA: The specific genomic sequence selected for modification.
  4. Optional donor DNA template: Used when a precise correction or insertion is intended.

Mechanism

  1. A synthetic guide RNA is designed to match the target DNA sequence.
  2. The gRNA binds to Cas9 and guides it to the homologous genomic locus.
  3. Cas9 produces a double-stranded DNA break at that site.
  4. The cell repairs this break by one of two pathways:
DNA repair pathwayResultUse
Nonhomologous end joining (NHEJ)Error-prone repair, commonly produces small insertions or deletions (indels)Gene disruption or knockout
Homology-directed repair (HDR)Uses homologous donor DNA as a template to produce a specified sequence changePrecise mutation correction, insertion, or replacement
NHEJ is more efficient but may create random disruptive mutations. HDR permits precise sequence editing but is relatively less efficient.

Applications

  • Creation of disease models and transgenic animals.
  • Identification of gene function through gene knockout.
  • Introduction of specific mutations into cells to model cancers and inherited diseases.
  • Potential correction of pathogenic mutations in monogenic inherited disorders, such as hemoglobinopathies.
  • Research in cancer biology, regenerative medicine, and targeted molecular therapy.

Advantages

  • High target specificity.
  • Easy and rapid design of guide RNA.
  • Can edit genes in many cell types and animal models.
  • More flexible and simpler than earlier nuclease platforms, such as zinc-finger nucleases and TALENs.
  • Enables both gene disruption and precise sequence correction.

Limitations and concerns

  • Off-target editing: Unintended cleavage at similar DNA sequences may cause harmful mutations.
  • Incomplete editing and mosaicism, particularly relevant in embryos.
  • Difficulties with efficient delivery of Cas9 and gRNA into target cells/tissues.
  • Potential immune responses to Cas proteins.
  • HDR is inefficient in many non-dividing cells.
  • Ethical concerns, especially regarding germline editing, embryo manipulation, and non-therapeutic enhancement of traits.

Pathology relevance

CRISPR is important in pathology because it permits:
  • Experimental recreation of driver mutations in cancer.
  • Functional testing of suspected disease-causing variants.
  • Development of cellular and animal models of inherited and acquired diseases.
  • Potential future correction of mutations responsible for genetic disorders.

Conclusion

CRISPR-Cas9 is a programmable gene-editing system in which a guide RNA directs Cas9 to a specific DNA sequence. Cas9-induced double-strand breaks are repaired by NHEJ, causing gene disruption, or by HDR, allowing precise DNA correction. Its major promise is in disease modeling and therapy for genetic disorders, but safety and ethical issues remain important.
Robbins-based source: Robbins & Kumar Basic Pathology, Gene Editing, pp. 105-106. Robbins describes CRISPR-Cas9 as a bacterial acquired-immunity system repurposed for highly specific genome editing, with NHEJ producing disruptive indels and HDR enabling precise sequence changes.
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