Gene Therapy & Antisense Oligonucleotides in Motor Neuron Disease

Gene-targeted therapy has moved from theoretical to real in motor neuron disease. In 2023, the FDA approved the first ASO therapy specifically for a genetic form of ALS — a landmark that demonstrated the concept is viable. Other ASO programs are in active trials targeting FUS, C9orf72, and TDP-43 pathways. CRISPR gene editing is in early preclinical stages. For people with PLS, the honest situation is complex: most of these therapies are not applicable to sporadic adult-onset PLS because no causative gene has been identified. But understanding why, and where exceptions exist, matters.

Why gene-targeted therapy makes sense in MND — in principle

Motor neuron disease involves the selective death of motor neurons — the nerve cells that control voluntary movement. In some forms of the disease, a specific gene mutation drives this process. If you can identify the mutation and switch off or correct its effects, you have a chance at actually modifying the disease course, not just managing symptoms.

This is the rationale for gene-targeted therapy. It is not new in concept — researchers have understood for decades that genetic forms of ALS could, in principle, be targeted at the gene level. What changed was technology: the development of effective delivery mechanisms for gene-silencing tools, improved ASO chemistry that enabled CNS penetration via intrathecal injection, and the FDA's willingness to accelerate approvals based on biomarker endpoints when clinical endpoints are hard to measure in small populations.

The challenge is that genetic forms account for only about 10–15% of all ALS cases, and an even smaller fraction of PLS cases. For the majority of patients with sporadic disease — no identified causative mutation — gene-targeted approaches are either not applicable or require a different strategy: targeting the downstream consequences of protein pathology rather than a single gene.

How antisense oligonucleotides work

Antisense oligonucleotides are short, synthetic strands of nucleic acid — typically 18–20 nucleotides long — designed to bind to a specific complementary messenger RNA (mRNA) sequence in a cell. When an ASO binds its target mRNA, it triggers the cell's own machinery (an enzyme called RNase H) to degrade that mRNA, preventing it from being translated into protein. The result is a reduction in the amount of a specific protein produced by the cell.

In motor neuron disease, this is used in two main ways:

• To reduce a toxic protein. If a mutated gene produces a protein that is toxic to motor neurons, an ASO targeting that gene's mRNA reduces the production of the toxic protein. This is how tofersen (targeting SOD1 mRNA) and ulefnersen (targeting FUS mRNA) work.
• To correct aberrant splicing. In TDP-43 pathology, the normal splicing of certain mRNAs (including stathmin-2) is disrupted, producing abnormal protein products. Some ASOs are designed to correct this splicing defect and restore normal protein expression. QRL-201, targeting stathmin-2, uses this approach.

ASOs do not cross the blood-brain barrier well when given systemically, so they are delivered by intrathecal injection — directly into the cerebrospinal fluid surrounding the spinal cord and brain, allowing the drug to distribute throughout the CNS. This requires a lumbar puncture procedure, typically performed monthly or at longer intervals as the drug is maintained. Lumbar puncture-related adverse events (headache, back pain) are common side effects.

Modern ASO chemistry has significantly improved stability, target engagement, and tolerability. Over 30 genes have been linked to ALS pathogenesis; in principle, each represents a potential ASO target. In practice, clinical development has been concentrated in the genes with the largest patient populations and strongest evidence of pathogenicity.

Tofersen (Qalsody) — The first approved ASO for ALS

Target: SOD1 gene mutations  |  Approved: FDA 2023 (accelerated approval)  |  Applicable to: SOD1-ALS only (approximately 2% of all ALS)  |  Evidence level: Strong (for SOD1-ALS)

Tofersen (brand name Qalsody, developed by Biogen) targets the SOD1 gene, which encodes superoxide dismutase 1. Mutations in SOD1 cause a toxic gain-of-function — the mutant protein misfolds and aggregates, damaging motor neurons. SOD1-ALS accounts for approximately 2% of all ALS cases and is one of the most well-characterized genetic forms.

The Phase 3 VALOR trial (NEJM 2022) randomized 108 SOD1-ALS patients 2:1 to tofersen 100 mg intrathecal or placebo over 24 weeks. The primary endpoint — ALSFRS-R change at week 28 in a faster-progression subgroup — did not reach statistical significance. However, tofersen produced substantial reductions in CSF SOD1 protein and plasma neurofilament light chain (NfL), a biomarker of nerve damage, versus placebo. In the 52-week combined analysis, the early-start cohort showed a 3.5-point ALSFRS-R advantage over delayed-start participants (95% CI 0.4–6.7).

Based on the biomarker signal — particularly the NfL reduction — the FDA granted accelerated approval in 2023. Real-world follow-up data from a December 2025 JAMA Neurology study showed that approximately 25% of participants experienced stabilization or functional improvement over three years, with some improvements in grip strength and respiratory function. A German multicenter real-world cohort (Lancet eClinicalMedicine 2024) showed sustained NfL reduction and slower ALSFRS-R decline over 12 months. The Phase 2 ATLAS trial is ongoing for presymptomatic SOD1 mutation carriers — treating before symptoms appear.

What tofersen means for PLS: No known SOD1 mutations have been identified in adult-onset sporadic PLS. Tofersen is not applicable to PLS. The significance of tofersen is what it demonstrates about the concept: that targeting a specific gene in motor neuron disease can produce meaningful biological change and, in some patients, functional benefit. This proof of concept is what justifies continued investment in the ASO approach.

Ulefnersen (ION363) — ASO for FUS-ALS

Target: FUS gene mutations  |  Stage: Expanded access program; Silence ALS trial ongoing  |  Evidence level: Emerging (for FUS-ALS); Theoretical (for PLS)

FUS (fused in sarcoma) mutations account for approximately 1–2% of all ALS cases and are notable for causing a particularly aggressive, early-onset form of the disease — FUS-ALS can begin in adolescents and young adults, and toxic FUS proteins accumulate in motor neurons.

Ulefnersen (ION363) is designed to silence the FUS gene, reducing production of the toxic mutant protein. It was first administered in 2019 through an expanded access program. Among patients treated, some experienced up to 83% reduction in neurofilament light chain after six months of treatment — a substantial biomarker improvement. One patient recovered the ability to walk unaided and breathe without a ventilator. One asymptomatic patient treated with ulefnersen had not developed symptoms after three years of continuous treatment.

The Silence ALS program, launched in 2022 by Neil Shneider at Columbia/NewYork-Presbyterian and funded by a $15 million NIH URGenT network grant in 2024, is expanding access and conducting systematic research. FUS mutations are not associated with adult-onset sporadic PLS. Ulefnersen is not applicable to PLS.

C9orf72-targeting ASOs

Target: C9orf72 repeat expansions (hexanucleotide G4C2 repeat)  |  Stage: Multiple programs in Phase 1/2  |  Evidence level: Limited

C9orf72 repeat expansions are the most common genetic cause of ALS, accounting for approximately 5–10% of ALS cases and also causing frontotemporal dementia. The mechanism involves both a toxic gain of function (from the expanded repeat RNA and the dipeptide repeat proteins it produces) and a partial loss of function (reduced C9orf72 protein).

Several ASO programs target C9orf72, using different chemical approaches to suppress G4C2-containing transcripts selectively. Mixed-backbone C9orf72-targeting ASOs have been shown in animal models and a single-patient proof-of-concept study to reduce CSF poly(GP) levels — a marker of the toxic dipeptide repeats. LNA and 2'-O-MOE chemistries have also demonstrated dramatic reductions in C9orf72 repeat-containing transcript levels in patient-derived cells and mouse models.

C9orf72 expansions are not established as a cause of adult-onset sporadic PLS, though some patients diagnosed with PLS who later develop lower motor neuron findings may carry this expansion as part of an ALS/FTD phenotype. For the majority of PLS patients, C9orf72-targeting ASOs are not applicable.

CRISPR gene editing in MND

Stage: Preclinical  |  Evidence level: Theoretical

CRISPR-Cas9 and related gene editing tools offer the possibility of permanently altering a disease-causing gene rather than temporarily suppressing its expression (as ASOs do). In principle, CRISPR could be used to correct a pathogenic mutation, disrupt a toxic gene's function, or introduce a protective gene variant.

In ALS research, CRISPR approaches are being explored in cell models and animal models to target SOD1, C9orf72, and other genes. The challenges for human translation are significant: delivery of CRISPR components to neurons in the CNS is technically demanding, off-target editing carries safety risks, and the immune response to CRISPR components needs careful management. No CRISPR approach has entered clinical trials in ALS or PLS as of 2026. This is a genuine long-term direction in the field, not a near-term option.

Why most gene therapies don't apply to PLS

The reason this page contains so much relevant-to-ALS but not-applicable-to-PLS information is straightforward: PLS does not have a known causative gene for the adult-onset sporadic form that most people with PLS have.

Approximately 80–90% of ALS is sporadic, with no identified causative mutation. PLS, which is rarer and has received far less research attention, is understood to be sporadic in the vast majority of cases. The genetic risk factors that have been identified (various variants that increase susceptibility without being causative) are not the kind of clear targets that ASO or CRISPR approaches address.

Without a causative gene, there is no mRNA to silence, no toxic protein to reduce, and no specific genetic defect to correct. The gene therapy approaches that have worked in SOD1-ALS work because there is a single well-defined toxic protein that can be targeted. That situation does not currently exist for PLS.

This does not mean gene therapy will never be relevant to PLS. It means we first need to understand what, at the molecular level, drives upper motor neuron degeneration in PLS — and that understanding is actively developing through the PLS Natural History Study and other research programs.

Juvenile PLS — A different story

Juvenile PLS, also called ALS2 or infantile ascending hereditary spastic paraplegia (IAHSP), is caused by mutations in the ALS2 gene encoding alsin, a GTPase regulatory protein. This is a recessive, early-onset genetic condition, distinct from adult-onset PLS in almost every way except the phenotypic similarity (progressive upper motor neuron syndrome).

Because juvenile PLS has a known causative gene, it is in principle a gene therapy target. Alsin-replacement or ALS2 mRNA-targeting approaches could theoretically be developed. Research into ALS2 pathophysiology is active, particularly because understanding alsin's role in endosomal trafficking and motor neuron biology may illuminate broader mechanisms of UMN disease. No clinical-stage gene therapy program for ALS2/juvenile PLS exists as of 2026, but this is an area to watch.

Broader ASO landscape and what to watch

The ASO field in motor neuron disease is broad, with programs targeting multiple genes and disease mechanisms. The 2024–2025 literature describes ASOs in motor neuron diseases as a "road to cure" with current limitations — the approved track record is still limited to one drug (tofersen) in one specific ALS subtype, and multiple other ASO programs have failed to meet efficacy endpoints in trials.

The programs most worth watching for potential broader MND applicability — including possibly PLS — are those that target consequences of TDP-43 pathology (stathmin-2 restoration via QRL-201) rather than specific causative mutations, since TDP-43 pathology has been documented in a broader spectrum of MND conditions. Whether this ultimately extends to PLS will require PLS-specific investigation that has not yet been done.