Introduction to CRISPR/Cas9 Therapy

Sickle Cell Disease (SCD) results from a transversion mutation (adenine to thymine) in the sixth codon of the beta-globin (HBB) gene on chromosome 11, substituting valine for glutamic acid.
Traditional allogeneic Hematopoietic Stem Cell Transplantation (HSCT) carries severe risks of Graft-Versus-Host Disease (GVHD) and relies on limited matched donors.
CRISPR-Cas9 utilizes autologous stem cells, eliminating immune rejection and GVHD risks to provide a functional cure.

Pathophysiological Mechanism: The Fetal Hemoglobin Switch

Therapy does not repair the mutated beta-globin gene directly.
It utilizes targeted gene editing to reactivate Fetal Hemoglobin (HbF) production.

Mechanism Stage	Physiological Action
Normal Pathophysiology	Transcription factor BCL11A binds to gamma-globin promoter regions shortly after birth, silencing HbF production.
CRISPR Action	Cas9 endonuclease guided by single-guide RNA (sgRNA) creates a precise double-strand DNA break at the BCL11A erythroid-specific enhancer.
Cellular Repair	Cells utilize error-prone Non-Homologous End Joining (NHEJ) repair, introducing insertions/deletions (indels) that destroy the enhancer sequence.
Therapeutic Result	BCL11A expression is drastically reduced in the red blood cell lineage, reactivating gamma-globin genes. Pancellular HbF production resumes, preventing HbS polymerization.

Exagamglogene autotemcel (Casgevy) represents the first approved CRISPR/Cas9 therapy.

Procedural Phase	Clinical Execution
Mobilization	Patient undergoes stem cell mobilization using Plerixafor; G-CSF is contraindicated due to Vaso-Occlusive Crisis (VOC) risk. CD34+ cells are collected via apheresis.
Manufacturing	Cells undergo ex vivo electroporation with Cas9 protein and sgRNA, followed by quality checks and cryopreservation.
Conditioning	Patient requires admission for myeloablative chemotherapy utilizing high-dose Busulfan.
Infusion	Edited CD34+ cells are infused via central venous catheter. Requires 4 to 6 weeks of strict isolation and transfusion support until engraftment.

Initial approvals covered patients aged 12 years and older.
Clinical expansion to children aged 5 to 11 years initiated following the Phase 3 CLIMB-151 trial.
Trial data demonstrates 97% of adolescents/adults and 100% of pediatric patients remain completely free from severe VOCs for at least 12 months.
Mean total hemoglobin stabilizes at ≥11.9 g/dL, with HbF constituting >40% of total hemoglobin.

Toxicity is primarily driven by Busulfan conditioning.
Risks include severe mucositis, opportunistic infections, and Hepatic Veno-Occlusive Disease (VOD).
High-dose alkylating agents induce near-certain gonadal failure, mandating intensive fertility preservation prior to therapy.

National Sickle Cell Anaemia Elimination Mission (NSCAEM) targets disease elimination by 2047.
Development of "BIRSA 101" (May 2026) marks India's first indigenous, affordable CRISPR-based gene therapy, pioneered by CSIR-IGIB and transferred for scaled manufacturing.

Global research focuses on non-genotoxic conditioning regimens utilizing targeted monoclonal antibodies (anti-CD117) to clear bone marrow without causing systemic toxicity or permanent infertility.
Pre-clinical advancement of in vivo gene editing using lipid nanoparticles for direct bloodstream delivery.