Learning Objectives

• Define the components and origin of the CRISPR/Cas9 system.
• Explain the mechanism of action involving gRNA and the Cas9 endonuclease.
• Identify clinical applications in infectious disease, hematology, and oncology.

1. Mechanism of Action

CRISPR/Cas9 is a revolutionary genome editing tool originally derived from bacteria (where it serves as an adaptive immune system against viruses). The system consists of two primary components:

• Guide RNA (gRNA): A synthetic RNA sequence designed to be complementary to a specific target DNA sequence in the genome.
• Cas9 Endonuclease: An enzyme that acts as “molecular scissors,” creating a single- or double-strand break at the precise location specified by the gRNA.

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2. Outcomes and Repair

Once the DNA is cut, the cell attempts to repair the break, which can be harnessed for two main goals:

• Gene Knock-out: Error-prone repair often leads to frameshift mutations or gene inactivation.
• Gene Editing: A donor DNA template can be provided to replace a disease-causing allele with a healthy variant.

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3. Clinical Applications

The specificity of CRISPR allows for targeted medical interventions:

• Hematology: Replacing alleles for conditions like Sickle Cell Disease or Beta-thalassemia.
• Infectious Disease: Removing virulence factors from resistant pathogens.
• Oncology: Specifically targeting and disrupting genes within tumor cells to inhibit growth or induce apoptosis.

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