Neurodegenerative upper motor neuron (UMN) syndromes ranging from primary lateral sclerosis (PLS) to pure and complicated types of hereditary spastic paraplegia (HSP) remain challenging to differentiate clinically, especially in the early stages of disease. As they share the hallmark of spastic paraparesis, easily accessible biomarkers are warranted to facilitate an early diagnosis. We examined serum neurofilament light chain (sNfL) and serum glial fibrillary acidic protein (sGFAP) as diagnostic biomarkers to differentiate PLS from HSP, represented by two paradigmatic subtypes: SPG4, the most common type of pure HSP, and adrenomyeloneuropathy (AMN), a common complicated form of HSP. In addition to sNfL and sGFAP raw levels, we used age-adjusted z-scores to account for age-related biomarker level increases. In our cohort of 18 PLS patients, 18 AMN patients, 25 SPG4 patients and 60 controls, sNfL z-scores were higher in PLS than in SPG4 (p < 0.001), AMN (p = 0.03), and controls (p < 0.001). Furthermore, sNfL z-scores allowed distinguishing PLS from SPG4 (AUC 0.82, 95% CI 0.67-0.98) and-slightly less accurate-from AMN (AUC 0.77, 95% CI 0.60-0.95). sGFAP z-scores did not differ significantly between groups. Our study suggests that serum NfL, but not GFAP, is a potential diagnostic biomarker in degenerative UMN diseases and may help to differentiate PLS from pure and complicated forms of HSP. Our results indicate that axonal degeneration-the source of NfL release-is predominant over astrocytic pathology-the source of GFAP release-in PLS, AMN, and SPG4.

Hereditary spastic paraplegia type 5 (SPG5) is an autosomal recessive neurological disorder caused by mutations in the CYP7B1 gene, which encodes cholesterol 7α-hydroxylase, an essential enzyme in cholesterol metabolism. These mutations lead to elevated levels of 25- and 27-hydroxycholesterol, oxysterols known to be neurotoxic and blood-brain-barrier permeable. Their accumulation contributes significantly to SPG5 pathogenesis, resulting in spastic gait disturbance and severely impaired quality of life. Using a Cyp7b1-/- mouse model that mirrors the metabolic phenotype of SPG5, we developed a gene therapy approach to correct oxysterol imbalance. We designed an AAV8-TTR-hCYP7B1 vector to deliver the CYP7B1 gene specifically to the liver. Following intravenous administration, oxysterol levels in blood and liver were rapidly normalized, even at low doses (1E10), with no observed toxicity at the highest tested dose (1E11). Despite these promising peripheral results, oxysterol levels in the brain, particularly 27-hydroxycholesterol, remained only partially corrected six weeks post-treatment. Our findings suggest that while liver-targeted gene therapy is effective at restoring peripheral cholesterol metabolism, a successful therapeutic strategy for SPG5 must also address central nervous system involvement. We conclude that successful treatment of SPG5 would require a novel gene therapeutic approach that also targets the CNS.

FARS2-associated hereditary spastic paraplegia, later onset spastic paraplegia type 77, is a rarely neurodegenerative disease. Here, we reported two affected siblings in an autosomal recessive spastic paraplegia family with a pseudo-homozygous missense variant and Alu-mediated exon 5 deletion in FARS2. Both patients gradually developed altered gaits and weakness in both lower limbs. In our literature review, spastic paraplegia type 77 shows high heterogeneity in clinical manifestations. Our study broadens the scope of pathogenic mechanisms of SPG77 resulting from compound heterozygous mutations in FARS2 and provides strong evidence that deletion in FARS2 due to recombination event mediated by Alu element.

An increasing number of cases of comorbid hereditary spastic paraplegia (HSP) and multiple sclerosis (MS) have been described. We report a patient with the SPG3A form of HSP and features of relapsing-remitting MS (RRMS). We took this opportunity to review the current literature of co-occurring MS and HSP. The patient underwent clinical, laboratory and neuroimaging evaluations. We performed a literature search for cases of HSP and MS. The 2017 McDonalds Criteria for MS were retrospectively applied to the selected cases. A 34-year-old woman, presenting a molecular diagnosis of SPG3A, complained subacute sensory-motor symptoms. Spinal MRI disclosed T2-hyperintense lesions at C2, T6 and T4 level, the latter presenting contrast-enhancement. CSF analysis showed oligoclonal bands. She was treated with intravenous high-dose steroids, with symptom resolution. The literature review yielded 13 papers reporting 20 possible cases of MS and HSP. Nine patients (5 M, median age 34) met the 2017 McDonald criteria. Five (25%) received a diagnosis of RRMS and four (20%) of primary progressive MS. Brain MRI showed multiple WM lesions, mostly periventricular. Six of seven cases (85.7%) had spinal cord involvement. Oligoclonal bands were found in 6/8 (75%) patients. Seven patients (77.7%) improved/stabilized on immunotherapy. This is the first description on the association between SPG3A type of HSP and MS. This report adds to the other reported cases of co-occurring HSPs and MS. Although it remains unclear if this association is casual or causal, clinicians should be aware that an HSP diagnosis does not always exclude a concomitant MS.

Mutations in the gene encoding the microtubule severing ATPase spastin are the most frequent cause of hereditary spastic paraplegia, a genetic condition characterised by length-dependent axonal degeneration. Here, we show that HeLa cells lacking spastin and embryonic fibroblasts from a spastin knock-in mouse model become highly polarised and develop cellular protrusions. In HeLa cells, this phenotype was rescued by wild-type spastin, but not by forms unable to sever microtubules or interact with endosomal ESCRT-III proteins. Cells lacking the spastin-interacting ESCRT-III-associated proteins IST1 or CHMP1B also developed protrusions. The protrusion phenotype required protrudin, a RAB-interacting protein that interacts with spastin and localises to ER-endosome contact sites, where it promotes KIF5-dependent endosomal motility to protrusions. Consistent with this, the protrusion phenotype in cells lacking spastin also required KIF5. Lack or mutation of spastin resulted in functional consequences for receptor traffic of a pathway implicated in HSP, as Bone Morphogenetic Protein receptor distribution became polarised. Our results, therefore, identify a novel role for ESCRT-III proteins and spastin in regulating polarised membrane traffic.