BSMS205 · Genetics

CRISPRa Therapy for
SCN2A Haploinsufficiency

Chapter 27 · Part V · Functional Genetics

Today's central question

Can you treat a genetic disease
by simply turning up the volume
on the patient's normal allele?

The therapeutic concept

Don't replace the gene.
Don't edit the mutation.
Boost the remaining normal allele.

This is called dosage restoration therapy.

Roadmap

SCN2A and the haploinsufficiency problem
Loss vs gain of function variants
How CRISPRa works · the mechanism
The Tamura 2025 study · cellular rescue
Behavioral and seizure rescue
Why this matters · cis-regulatory therapy

§ 1

SCN2A and the
Haploinsufficiency Problem

SCN2A · a sodium channel

Encodes Na_V1.2 · voltage-gated sodium channel
Critical for action potential generation
Especially important in early brain development
Highly expressed in excitatory neurons

Haploinsufficiency · one copy is not enough

Normal

Two functional alleles
50% + 50% = 100% protein
Neurons fire normally

Haploinsufficient

One functional · one LoF
50% + 0% = 50% protein
Neurons under-fire

Clinical consequences of SCN2A LoF

Developmental delay
Epilepsy — including paradoxical seizures
Autism spectrum disorder
Intellectual disability

The key insight

The remaining SCN2A allele is perfectly normal.
It just isn't producing enough protein.

Therapeutic question: can we make the good copy work harder?

§ 2

Not All Variants
Are the Same

Two opposite variant classes

Class	Channel	Neuron	Phenotype
Loss of function	Reduced or absent	Hypoexcitable	Developmental delay · ASD
Gain of function	Excessive · prolonged opening	Hyperexcitable	Infantile spasms · early epilepsy

Why the distinction matters for therapy

LoF haploinsufficiency · neurons need more Na_V1.2 → use CRISPRa
GoF · neurons have too much activity → would need CRISPRi or sodium channel blocker
Genetic diagnosis must distinguish the two classes

§ 3

How CRISPRa Works

Standard CRISPR · cuts DNA

Guide RNA targets the protein
Cas9 cuts both strands
Cell repairs the break, often with errors
Result: permanent DNA change

CRISPRa · activates without cutting

dCas9 — catalytically dead · binds DNA but cannot cut
Fused to activator domain (e.g. VPR = VP64 + p65 + Rta)
Guide to gene's promoter → recruits transcription machinery
More mRNA · more protein · DNA unchanged

The volume knob mental model

Standard CRISPR

Permanent DNA edit
Like rewriting the score

CRISPRa

Reversible expression change
Like turning up the volume

§ 4

The Tamura 2025 Study

Experimental design

Generate Scn2a haploinsufficient mice (one WT + one LoF allele)
Design CRISPRa: dCas9-VPR + guide RNA targeting Scn2a promoter
Package into AAV9 · deliver systemically
Treat adolescent mice (P30–P60)
Measure: mRNA · sodium current · firing · seizures · behaviour

Tamura et al. 2025, Nature

Result 1 · expression restored

SCN2A expression and sodium current after CRISPRa — mRNA from **~50%** (Het) → **~80–90%** (Het + CRISPRa) of WT.
Sodium current from ~50% → ~85% of WT.
Tamura et al. 2022 *bioRxiv*. CC BY-NC-ND 4.0.

Result 2 · firing rescued

Action potential firing rescue with CRISPRa — WT (black) fires many spikes · Het (cyan) fires few · Het+CRISPRa (purple) fires **like WT**.
Tamura et al. 2022 *bioRxiv*. CC BY-NC-ND 4.0.

§ 5

Behavior and Seizures

Result 3 · seizure protection

CRISPRa protects against chemoconvulsant-induced seizures — 4-AP induced seizures · 60 min survival.
Het (cyan) ~40% survive · Het+CRISPRa (purple) **~WT** survival.
Tamura et al. 2022 *bioRxiv*. CC BY-NC-ND 4.0.

Result 4 · behavioural improvement

Synaptic transmission · restored in cortical pyramidal cells
Social interaction · improved
Overall phenotype · near-normal
Treatment in adolescence · still works

Also tested in human cells

Human iPSC-derived neurons with SCN2A LoF
CRISPRa boosted SCN2A expression
Sodium currents recovered
Approach is not mouse-specific

§ 6

Why This Matters ·
Cis-Regulatory Therapy

Five advantages of CRISPRa therapy

No DNA cutting — no risk of off-target indels or rearrangements
Endogenous regulation — gene stays under native promoter and enhancers
Tunable and reversible — adjust dose · stop if needed
Mutation-agnostic — works for any LoF as long as one allele is intact
Late intervention possible — adolescent mice still rescued

How CRISPRa compares to alternatives

Strategy	What it does	Trade-off
Gene replacement	Add a new SCN2A copy via AAV	SCN2A is too large for AAV (6 kb)
Base editing	Correct point mutations	Only works for specific mutation types
Small molecules	Modulate channel activity	Non-specific · affects all Na channels
CRISPRa	Boost normal allele	Requires viral delivery · long-term safety TBD

Let's compare CRISPR a to other therapeutic strategies. Gene replacement, the classic approach, packages a fresh S C N two A copy into A A V. The trouble is that S C N two A is about six kilobases, larger than the four point seven kilobase A A V capacity. You need split vectors or trimmed promoters, both of which compromise expression control. Base editing can correct specific point mutations but does not work for all variant types — frameshifts, splice mutations, large deletions are all out of reach. Small molecule sodium channel modulators exist but are not selective for Na V one point two — they also hit the other sodium channel subtypes, with side effects. CRISPR a is the only approach that is both mutation-agnostic and dosage-restoring. The trade-offs are viral delivery and long-term persistence, which still need to be characterised.

The bigger picture · cis-regulatory gene therapy

SCN1A in Dravet syndrome · CRISPRa development active
SIM1 in obesity from haploinsufficiency · CRISPRa works in mice
HTT in Huntington's · CRISPRi to silence repeat expansion
Hundreds of haploinsufficiency disorders are candidates

CRISPR a for S C N two A is part of a broader trend called cis-regulatory gene therapy. Other examples are already in development. CRISPR a for S C N one A in Dravet syndrome — another sodium channelopathy. CRISPR a for S I M one in obesity caused by haploinsufficiency. CRISPR i for the H T T repeat expansion in Huntington's disease, where you want to silence rather than activate. The unifying principle is to adjust gene expression to restore balance, rather than rewrite the genome. Hundreds of haploinsufficiency disorders are candidates for this strategy — Shank three in Phelan-McDermid syndrome, M E C P two in Rett syndrome, F O X P one in intellectual disability, and many others. If CRISPR a for S C N two A reaches the clinic successfully, the door opens for all of them.

§ 7

Summary

What to take away

SCN2A LoF causes haploinsufficiency · neurons under-fire
The remaining allele is perfectly normal · just under-expressed
CRISPRa = dCas9 + activator → boost expression without cutting
Tamura 2025 · cellular firing, sodium current, seizures, behaviour all rescued
Works in adolescent mice and in human iPSC neurons
Foundational case for cis-regulatory gene therapy

Next lecture

How does gene regulation
actually work?

Chapter 28 · Gene Regulation — Same Book, Different Readings