Testing a new therapy for SPG4 Hereditary Spastic Paraplegia

SUMMARY/ABSTRACT Hereditary Spastic Paraplegia 4 (SPG4-HSP) is an underdiagnosed neurodegenerative disorder characterized by progressive weakness and spasticity in both legs that escalate into wheelchair dependence. Symptoms result mainly from dying-back degeneration of corticospinal tracts. The disease is caused by mutations in the SPAST gene, which encodes spastin, a microtubule-severing protein with membrane-related properties. Recent studies indicate that that disease pathology is primarily driven by the M1 isoform of spastin, which, when mutated, becomes toxic. A logical therapeutic strategy would be to reduce mutant M1 levels and restore axonal integrity, with the goal of halting and potentially reversing disease progression. Mutant M1 becomes not only toxic but also resistant to degradation, causing it to accumulate. A multi-PI team at Drexel University has developed three recombinant monoclonal antibodies with high specificity for M1. These antibodies were then engineered into intrabody vectors encoding their variable regions for intracellular expression in affected neurons. These intrabodies include a lysosome-targeting sequence to direct the antibody–antigen complex for degradation. This strategy, previously validated for other mutant proteins, is well-suited for SPG4-HSP and could represent a breakthrough therapy. Key questions remain: Can mutant M1 be effectively and sustainably depleted without off-target toxicity? If wildtype M1 is also affected, can any ill effects of this be mitigated? The multi-PI team has developed human induced pluripotent stem cell (hiPSC)-based platforms for studying SPG4-HSP. To test the intrabody approach, the multi-PI team has developed two isogenic hiPSC lines, each with a distinct SPAST mutations, and five SPG4-HSP patient-derived hiPSC lines with their mutation-negative familial controls. These cells are differentiated into motor cortical organoids (MCOs), which are forebrain organoids enriched for corticospinal motor neurons, the neuronal population most affected in SPG4-HSP. Across diverse mutations, MCOs consistently recapitulate disease-relevant phenotypes, including elevated HDAC6 activity, reduced microtubule acetylation, and enhanced neurodegenerative phenotypes. By introducing intrabody vectors into MCOs, it will be determined whether degrading mutant M1 restores cellular homeostasis and reverses neurodegeneration. By integrating precision intrabody engineering with patient-specific hiPSC-derived MCOs, this proposal seeks to mechanistically validate and therapeutically correct the pathogenic accumulation of mutant M1, establishing a novel, targeted approach with strong potential for clinical translation in SPG4-HSP treatment. Project Number: 1R21NS149629-01 | Fiscal Year: 2026 | NIH Institute/Center: National Institute of Neurological Disorders and Stroke (NINDS) | Principal Investigator: PETER BAAS (+1 co-PI) | Institution: DREXEL UNIVERSITY, PHILADELPHIA, PA | Award Amount: $228,688 | Activity Code: R21 | Study Section: Special Emphasis Panel[ZRG1 AN-E (55)] View on NIH RePORTER: https://reporter.nih.gov/project-details/11388925