DEVELOPMENT OF A GENE THERAPY TREATMENT FOR HEREDITARY SPASTIC PARAPLEGIA TYPE 52 (SPG52)

Hereditary spastic paraplegia type 52 (SPG52) is an ultra-rare neurodegenerative disorder causing lower-limb weakness and spasticity, thinning of the corpus callosum (CC), and other symptoms. SPG52 results from biallelic loss-of-function mutations in the AP4S1 gene, which encodes a subunit of the adaptor protein complex 4 (AP-4). Mutations in any AP-4 subunit lead to whole-complex degradation and shared pathology. The AP-4 complex sorts and traffics proteins like ATG9A and DAGLB. Gene therapy targeting AP4S1 offers a rational approach since no cure exists. We generated patient-derived hiPSCs to evaluate our approach. SPG52 hiPSC-derived cortical neurons showed AP-4 cargo accumulation at the trans-Golgi, reduced arborization, impaired autophagy, and reduced neuronal activation compared with controls. Overexpressing AP4S1 fully rescued ATG9A accumulation. We developed the first SPG52 mouse model with a complete AP4S1 gene knockout (KO). This model exhibits thinning and reduced white matter in the CC, lower body weight, reduced muscle strength, and shorter lifespan. Neuromuscular tests revealed decreased muscle action potential amplitudes and sensory function in the hindlimbs of KO mice. They also experience corticospinal tract neurodegeneration. KO mice display behavioral differences and learning deficits, but can still perform daily activities. We tested our gene therapy in neonatal and young adult animals with a neurotropic AAV vector carrying the AP4S1 gene. It fully restored hindlimb CMAP and partially improved corpus callosum thinning, motor coordination, and muscle strength, but did not change body weight. These results bring us closer to safety trials and a potential treatment for children with SPG52, who currently lack one.