CRISPR-Cas9 & Gene Editing

EXAM STATUS CRISPR-Cas9 was asked in 2020 Q6 (UNSLOTTED). Gene therapy for hemoglobinopathies was asked in 2021 Q3 (Slot 3). Per exhaustion principle → LOW repeat probability as standalone. BUT: could appear as part of a hematology (Slot 6), wild card (Slot 10), or as a component within CAR-T / gene therapy answer. The FDA approval of Casgevy (Dec 2023) = major new milestone since last asked → fresh angle possible.

1 THE CRISPR-Cas9 SYSTEM

Core Mechanism

CRISPR = Clustered Regularly Interspaced Short Palindromic Repeats (bacterial adaptive immune system vs phage DNA).
Cas9 = CRISPR-associated protein 9, an RNA-guided endonuclease.

sgRNA (single-guide RNA) directs Cas9 to target DNA → generates double-strand break (DSB) → repaired by:
① NHEJ (non-homologous end joining) — error-prone → insertions/deletions → gene knockout
② HDR (homology-directed repair) — precise → needs DNA template → gene correction/insertion

Evolution of Gene Editing Tools

Generation	Tool	Mechanism	Limitation
1st	ZFNs	Zinc finger + FokI nuclease	Complex design, expensive
1st	TALENs	TAL effector + FokI nuclease	Large size, time-consuming
2nd	CRISPR-Cas9	sgRNA-guided endonuclease	Off-target DSBs, PAM dependency
3rd	Base Editors	nCas9 + deaminase (NO DSBs)	Only single-base changes
4th	Prime Editors	nCas9 + reverse transcriptase (NO DSBs)	Low efficiency, size constraints

Base Editors — Key Details

Type	Conversion	Components	Clinical Use
CBE	C:G → T:A	nCas9 + cytidine deaminase	Exon skipping (DMD)
ABE	A:T → G:C	nCas9 + adenine deaminase	BEAM-101 (SCD) — in clinical trial
GBE	C → G (glycosylase)	nCas9 + glycosylase	Preclinical

~60% of pathogenic point mutations could theoretically be corrected by base editors.

Other CRISPR Modalities (Exam Awareness)

System	What It Does
CRISPRi	dCas9 + repressor (KRAB domain) → transcriptional silencing without cutting DNA
CRISPRa	dCas9 + activator (VP64) → transcriptional activation → upregulate target gene
Epigenome editing	dCas9 + DNMT/TET → alter DNA methylation; + P300/HDAC → alter histone marks
PASTE	Prime editor + Bxb1 recombinase → integrate DNA up to 35 Kb

2 FDA-APPROVED CRISPR THERAPIES (Dec 2023)

CASGEVY (Exagamglogene autotemcel)

First-ever CRISPR/Cas9 FDA-approved therapy
Vertex Pharmaceuticals + CRISPR Therapeutics
Approved: Dec 8, 2023 (SCD) + early 2024 (TDT)
Age: ≥12 years with recurrent VOC
Mechanism: Ex vivo CRISPR editing of autologous HSPCs → disrupts BCL11A erythroid-specific enhancer (+58 kb DHS) → derepresses HbF → HbF inhibits HbS polymerization
Result: 94% freedom from VOC ≥1 year
HbF in ~46% of blood; 99.7% of RBCs carry HbF
BM biopsy: >81% cells with desired edit
TDT: 42/44 patients transfusion-free
Cost: ~$2.2 million/treatment

LYFGENIA (Lovotibeglogene autotemcel)

NOT CRISPR — lentiviral vector gene addition
Bluebird Bio Inc.
Approved: Same day (Dec 8, 2023) for SCD
Mechanism: Lentiviral transduction of autologous HSCs with anti-sickling β-globin transgene (βA-T87Q) → modified globin inhibits sickling
Risk: Insertional mutagenesis (2 patients developed blood malignancies 3–5.5 yrs post-transplant)
Black box warning for hematologic malignancy

SCD Pathology — One-Liner

β6 Glu→Val (GAG→GTG) in HBB gene → HbS polymerization on deoxygenation → sickle cells → vaso-occlusion → ischemic damage + hemolytic anemia. HbF is protective (γ-globin replaces β-globin function + antisickling properties).

Other SCD Therapies (Comparator Context)

Drug	Mechanism	Status
Hydroxyurea	↑ HbF via stress erythropoiesis	FDA — standard of care
L-glutamine	Reduces oxidative stress in RBCs	FDA 2017
Crizanlizumab	Anti-P-selectin mAb → ↓ adhesion/VOC	FDA 2019
Voxelotor	HbS polymerization inhibitor	FDA 2019 (withdrawn 2024)
Allogeneic HSCT	Donor HSCs replace sickle marrow	5-yr OS 92–94%, curative but donor-limited

3 IN VIVO CRISPR — Next Wave (Phase 2/3)

Therapy	Disease	Mechanism	Key Result	Phase
NTLA-2001	ATTR cardiomyopathy (TTR amyloidosis)	IV LNP delivery → CRISPR knockout of TTR gene in liver	>90% reduction in pathogenic TTR protein	Phase 3
NTLA-2002	Hereditary angioedema (HAE)	IV LNP → CRISPR knockout of KLKB1 (kallikrein B1) in liver	91–97% reduction in attack rate; patients attack-free	Phase 3
BEAM-101	SCD (next-gen)	ABE (base editor) → A-to-G edit in HBG1/HBG2 promoters → ↑ HbF	DSB-free; mimics HPFH mutations	Phase 1/2
EDIT-101	Leber congenital amaurosis 10 (LCA10)	Subretinal AAV5 → corrects CEP290 IVS26 splice mutation	First in vivo CRISPR trial (eye)	Phase 1/2

NTLA-2001 is the first in vivo CRISPR therapy to reach Phase 3. Uses lipid nanoparticle delivery (no viral vector).

4 CRISPR IN ONCOLOGY (CAR-T Link)

CRISPR-Enhanced CAR-T

CRISPR enables multiplex gene editing of T-cells:
① PD-1 knockout → prevent tumor immune evasion
② TCR knockout → universal "off-the-shelf" allogeneic CAR-T (no GvHD)
③ B2M knockout → evade host immune rejection
Clinical trials in hematologic malignancies + solid tumors showing promising results.

CRISPR Diagnostics (Bonus Exam Point)

Platform	Cas Protein	Target	Application
SHERLOCK	Cas13	RNA	Rapid detection — SARS-CoV-2, Zika, Dengue
DETECTR	Cas12	DNA	HPV detection, point-of-care

5 DELIVERY SYSTEMS & CHALLENGES

Method	Route	Pros	Cons	Example
Ex vivo electroporation	Ex vivo → reinfuse HSPCs	Precise, proven	Myeloablation needed, costly	Casgevy
AAV vectors	In vivo (IV, subretinal, IM)	Efficient transduction	Cargo limit (~4.7kb), immunogenicity, high dose toxicity	EDIT-101 (LCA), DMD trials
LNP (lipid nanoparticle)	In vivo IV	Non-viral, transient expression, liver-tropic	Limited tissue targeting beyond liver	NTLA-2001, NTLA-2002
Lentiviral vectors	Ex vivo	Stable integration, large cargo	Insertional mutagenesis risk	Lyfgenia

Key Safety Concerns

Concern	Detail
Off-target effects	Cas9 DSBs at unintended sites → large indels, translocations, chromothripsis
Genotoxicity	2 SCD gene therapy patients developed blood malignancies 3–5.5 yrs post-transplant
AAV high-dose toxicity	Lethal ARDS reported in DMD CRISPRa trial (n-of-1, CRD-TMH-001, AAV9 @ 1×10¹⁴ vg/kg) — death 8 days post-infusion. Investigators attributed to AAV dose, not CRISPR
Immune response	Pre-existing anti-AAV antibodies, anti-Cas9 immunity
Cost/Access	Casgevy ~$2.2M; inaccessible to low-income countries (majority of SCD/TDT burden)

6 CRISPR CLINICAL MILESTONES

2012 Doudna & Charpentier — CRISPR-Cas9 as programmable gene editor (Science)

2013 Zhang — CRISPR applied in human cells (Science)

2016 First CRISPR human trial: PD-1 edited T-cells for lung cancer (China)

2018 CTX001 (Casgevy) clinical trials begin for SCD + TDT

2019 EDIT-101 — first in vivo CRISPR trial (subretinal, LCA10)

2020 Nobel Prize in Chemistry → Doudna & Charpentier

2021 NTLA-2001 Phase 1 results (ATTR) — first systemic in vivo CRISPR

2023 UK MHRA first to approve Casgevy (Nov) → FDA approves Casgevy + Lyfgenia (Dec 8)

2024 Casgevy approved for TDT. NTLA-2001 enters Phase 3. 94 active CRISPR trials.

2025 BEAM-101 (base editing for SCD) in Phase 1/2. Personalized CRISPR therapy for rare diseases emerging.

7 RAPID-FIRE RECALL CARDS

⚡ CRISPR-Cas9: sgRNA guides Cas9 → DSB → repaired by NHEJ (knockout) or HDR (correction)
⚡ Casgevy = First CRISPR drug (Dec 2023). Edits BCL11A enhancer → ↑HbF → 94% VOC-free at 1yr
⚡ Lyfgenia = Lentiviral (NOT CRISPR). Same-day FDA approval. Black box warning: malignancy
⚡ Base editors: NO DSBs. CBE = C→T. ABE = A→G. BEAM-101 = ABE for SCD (trial)
⚡ Prime editor: nCas9 + RT. Insertions up to 44bp. No DSB, no template needed
⚡ NTLA-2001: In vivo CRISPR via LNP for TTR amyloidosis → >90% TTR knockdown (Phase 3)
⚡ NTLA-2002: In vivo CRISPR for HAE → 91–97% attack reduction (Phase 3)
⚡ SHERLOCK (Cas13/RNA) & DETECTR (Cas12/DNA) = CRISPR diagnostics
⚡ Nobel 2020: Doudna + Charpentier for CRISPR-Cas9
⚡ Safety troika: Off-targets, genotoxicity (malignancy in 2 patients), AAV immunotoxicity
⚡ Casgevy cost: ~$2.2 million — equity crisis for LMICs where SCD burden is highest

8 CROSS-SLOT CONNECTIONS

🔗 Slot 3 (Neuro Pharma): Gene therapy for hemoglobinopathies (SCD) — Casgevy + Lyfgenia. CRISPR in DMD (preclinical). EDIT-101 for LCA10 (eye/neuro).
🔗 Slot 6 (Hematology): SCD gene therapy directly. CAR-T → CRISPR-enhanced off-the-shelf CAR-T. Aplastic anemia + MDS risk post gene therapy.
🔗 Slot 5 (Cardiology): NTLA-2001 for ATTR cardiomyopathy — a cardiac indication of CRISPR.
🔗 Slot 10 (Wild Card): CRISPR as "recent advance in medicine" / "AI+genomics" angle. CRISPR diagnostics (SHERLOCK/DETECTR) for infectious diseases.
🔗 Slot 7 (Anti-infective): CRISPR diagnostics for pathogen detection (SARS-CoV-2, TB rapid tests in development).
🔗 Slot 2 (DM/Endocrine): CRISPR-based islet cell engineering for Type 1 DM (preclinical).