PLS Genetics Research
In ALS, genetics has been transformative. The discovery of SOD1 mutations in 1993, the C9orf72 repeat expansion in 2011, and a dozen other high-penetrance genes has produced a field where genetic testing has direct clinical consequences — determining eligibility for targeted therapies, informing family risk, and shaping prognosis. In PLS, the genetic story is almost the opposite: a largely negative result. Most adults with PLS carry no identifiable causative variant, and the absence of a PLS genetics field is itself informative, telling us something important about what this disease is and is not.
Why the genetic question matters
Genetics matters in neurological disease for two reasons: clinical and scientific. Clinically, identifying a causative gene changes management — it may determine eligibility for a targeted therapy, it defines familial inheritance risk, and it can refine prognosis. Scientifically, identifying disease genes points toward pathways and mechanisms that can be targeted therapeutically even in non-genetic cases, if those same pathways turn out to be disrupted through other means.
In PLS, both of these payoffs have been largely absent. Most adults with PLS who undergo genetic testing find nothing. But the exceptions — juvenile PLS, the PLS/HSP diagnostic boundary, the rare ALS gene carrier who presents with a UMN phenotype — are clinically significant enough that the genetic question cannot be ignored. And the increasingly comprehensive genetic tools now available may yet reveal architecture in adult PLS that earlier, targeted-panel approaches missed.
Where the evidence comes from
The ALS gene revolution provides the essential context for understanding PLS genetics — and why its absence is a finding rather than an oversight. The SOD1 story began in 1993 with the discovery that mutations in the superoxide dismutase 1 gene cause familial ALS. This was the proof-of-principle: a single-gene cause of motor neuron disease could be found, and a drug targeted to that gene could modify the disease. The C9orf72 hexanucleotide repeat expansion, discovered simultaneously by two teams in 2011, added another layer: a genetic variant so common that it accounts for roughly 40% of familial ALS and 7% of apparently sporadic ALS. Crucially, C9orf72 expansions can produce UMN-predominant phenotypes — rare cases that present more like PLS than typical ALS. This creates a narrow but real overlap: some patients who receive a clinical PLS diagnosis turn out, on genetic testing, to carry a C9orf72 expansion and are more accurately understood as UMN-predominant ALS.
TDP-43 pathology is another ALS genetics story with PLS implications. TDP-43 protein aggregation in motor neurons is the hallmark of approximately 97% of ALS — a nearly universal feature across genetic and sporadic cases. Whether TDP-43 pathology is similarly central to PLS remains an open question. The cases that have come to autopsy suggest that PLS may have a more heterogeneous pathology, with some cases showing TDP-43 inclusions and others not. If TDP-43 is as convergent a feature in PLS as in ALS, that has major therapeutic implications — because TDP-43 biology is one of the most active areas in MND drug development.
Juvenile PLS is a different disease at the genetic level, and it is the one place in PLS genetics where the story is unambiguous. The ALS2/alsin gene, encoding a protein involved in endosomal trafficking and motor neuron survival, is the cause of autosomal recessive juvenile PLS. Both copies of the gene must be non-functional — meaning both parents carry a loss-of-function variant without being affected themselves. Children with two such variants develop progressive spastic diplegia and pseudobulbar palsy beginning in childhood or early adolescence. The clinical phenotype overlaps substantially with adult PLS, but the genetic mechanism is entirely different: this is a rare, inherited, pediatric disease rather than a sporadic adult-onset one. Research on alsin function in motor neurons has generated biological insights into UMN vulnerability, but the relevance to adult PLS remains indirect.
The most comprehensive modern effort to characterize adult PLS genetics is the Manini 2025 whole genome sequencing study from the University of Milan. Earlier genetic studies used targeted panels — sequencing the coding regions of known ALS and HSP genes — and found very little in PLS. Whole genome sequencing does not limit itself to known genes; it captures everything, including non-coding variants, structural rearrangements, and coding variants in genes not yet linked to any motor neuron disease. The Manini study identified some novel variants in PLS patients, but the core conclusion was consistent with prior work: no single dominant genetic cause accounts for adult-onset sporadic PLS. The disease is not driven by a small number of high-penetrance genes the way familial ALS is. What remains possible — and what WGS-based discovery science may eventually reveal — is a polygenic architecture: multiple low-effect variants that together confer susceptibility, combined with aging and environmental factors. That model would explain why targeted panel testing keeps coming up negative.
The one diagnostic context in which genetics is most directly useful in apparent PLS is at the PLS/HSP boundary. Hereditary spastic paraplegia and PLS share progressive spastic gait, UMN signs in the lower limbs, and relatively preserved lower motor neuron function. The clinical distinction can be genuinely difficult, especially in older patients with late-onset HSP, no identified family history, and slow progression. The genetic distinction is crisp: HSP is defined by one of over 70 SPG loci, while PLS lacks a causative gene. If a patient presenting as PLS is found to carry a pathogenic variant in SPG4 (spastin) or SPG7 (paraplegin) — the two most commonly identified — the diagnosis becomes HSP, the family receives a specific inheritance risk estimate, and the diagnostic uncertainty resolves. This is the strongest argument for genetic testing in apparent adult PLS: not to find a PLS gene, but to rule out a genetically defined HSP mimic.
What we know — and what we don't
Adult-onset sporadic PLS does not have a C9orf72 equivalent. It does not have an SOD1 equivalent. Whole genome sequencing in adults with PLS is predominantly negative for causative variants. The most productive genetic testing in apparent adult PLS is the SPG gene panel, aimed at ruling out HSP — not at finding a PLS gene. In pediatric cases, ALS2/alsin testing is appropriate and may be diagnostic.
The absence of strong PLS genetics is itself a finding with implications. If adult PLS were driven by high-penetrance genetic mutations, we would expect to find them — in patient families, in cohort studies, in WGS analyses. The fact that we do not suggests that adult PLS is driven primarily by aging, environmental factors, or sporadic cellular events rather than inherited mutations. That biology is fundamentally different from SOD1-ALS, and it means that the ASO gene therapy approach — the most successful recent innovation in MND drug development — is unlikely to be directly applicable to most PLS patients.
What remains genuinely open is the TDP-43 question. If TDP-43 pathology is as central to PLS as to ALS, then the substantial drug development effort targeting TDP-43 aggregation and dysfunction would be directly relevant to PLS as well. Characterizing PLS pathology at autopsy — building the case series needed to answer whether TDP-43 is a convergent feature — is an important ongoing research need.
Why it matters for you
If you have adult-onset PLS and are wondering whether your children are at risk, the current evidence supports a reassuring answer in most cases: adult sporadic PLS is not a hereditary disease, and your family members are not at the elevated risk that familial ALS families face. The important exception is the small possibility that you carry a C9orf72 expansion or an SPG gene variant — which is why a genetics referral and panel testing is worth considering, particularly if you have any family history of neurological disease. The genetics guide walks through what testing makes sense for different clinical scenarios. The diagnosis page explains how genetics fits into the overall diagnostic workup.
Individual studies in this section
Full details on each study, including methods, findings, and context: