CRISPR

Definition And Core Principles

CRISPR constitutes a genome-editing technology derived from the adaptive immune system of prokaryotes.
It enables precise, targeted modifications in genomic DNA to permanently correct monogenic defects.
Unlike conventional gene therapy that adds exogenous genes, CRISPR directly edits the native DNA sequence.

Cas9 nuclease acts as molecular scissors capable of cutting DNA at specific locations.
Guide RNA (gRNA) provides a synthetic scaffold for Cas9 binding and defines the genomic target through complementary base pairing.

The Cas9-gRNA complex identifies a Protospacer Adjacent Motif (PAM) sequence to bind and unwind the DNA.
Cas9 nuclease domains create Double-Strand Breaks (DSB) slightly upstream of the PAM sequence.

Non-Homologous End Joining (NHEJ) represents an error-prone repair mechanism causing small insertions or deletions (indels) that result in gene knockout or disruption.
Homology-Directed Repair (HDR) enables precise correction or sequence insertion utilizing an exogenous homologous donor DNA template.

Base editing utilizes a catalytically impaired Cas9 fused to a deaminase enzyme to convert single DNA bases without creating double-strand breaks.
Prime editing operates as a search-and-replace tool using a pegRNA and Cas9-reverse transcriptase fusion to write new genetic information without double-strand breaks.
CRISPR interference (CRISPRi) and activation (CRISPRa) modify gene expression by targeting promoter regions without altering the underlying DNA sequence.

Target cells, such as Hematopoietic Stem Cells, are harvested directly from the patient.
Cells undergo genetic modification in the laboratory using electroporation or viral vectors before autologous re-infusion.

Editing machinery is delivered systemically or locally directly into the patient's body.
Viral vectors, particularly Adeno-Associated Virus (AAV), offer low immunogenicity for targeting liver, muscle, or central nervous system tissues.
Non-viral vectors, including Lipid Nanoparticles (LNP), facilitate transient and localized delivery, predominantly to the liver.

Sickle Cell Disease and Transfusion-Dependent Thalassemia management involves targeting the BCL11A erythroid enhancer.
Disruption of this enhancer suppresses BCL11A, leading to high levels of Fetal Hemoglobin (HbF) production.
Exagamglogene autotemcel represents the first approved CRISPR therapy providing functional cures for these conditions.

Duchenne Muscular Dystrophy interventions explore skipping mutated exons to restore the dystrophin reading frame.
Severe Combined Immunodeficiency treatments utilize ex vivo correction of IL2RG or ADA genes in stem cells to restore immune function.

Chimeric Antigen Receptor (CAR) T-cell therapy employs CRISPR to knock out TRAC and PD-1 genes, creating allogeneic off-the-shelf cells for pediatric leukemia.
In vivo editing manages metabolic conditions like Hereditary Transthyretin Amyloidosis by knocking out the TTR gene in the liver.

Offers unprecedented precision, multiplex editing capabilities, and endogenous regulation at the native gene locus.
Provides one-time curative potential while avoiding Graft-Versus-Host Disease through autologous transfer protocols.

Off-target effects involve unintended cleavage at genomic sites with sequence homology, creating risks for oncogenic mutations.
Immunogenicity risks arise from pre-existing immunity to bacteria-derived Cas9 or specific delivery vectors.
Delivery barriers limit effective access to brain and muscle tissues.
High therapeutic costs restrict accessibility, particularly in low- and middle-income countries.

Somatic editing alters non-reproductive cells, providing therapy for the individual without transmitting genetic changes to offspring.
Germline editing modifies gametes or pre-implantation embryos, rendering genetic changes heritable to all subsequent generations.
Germline editing remains globally prohibited due to risks of eugenics, designer babies, unpredictable structural variations, and ethical concerns regarding unborn consent.
Pediatric consent models necessitate child assent alongside parental surrogate permission, acknowledging the irreversible nature and long-term unknown risks of genomic therapies.