Hereditary Spastic Paraplegia type 4 is characterized by gait impairments, progressive spasticity and weakness of the lower limbs, resulting from degeneration of the corticospinal tracts. The disease is caused by mutations of the SPAST gene, which encodes a major isoform of spastin called M87 and a minor isoform called M1. Owing to its N-terminal hydrophobic domain not shared by M87, M1 is the isoform that becomes toxic when mutated. Loss-of-function of either M1 or M87 or both may also play a role in the disease, sensitizing corticospinal motor neurons to the toxicity of mutant M1. Here we pursued silence-and-replace gene therapy, which addresses both gain-of-toxicity and loss-of-function components of the disease. We generated an adeno-associated serotype 9 viral vector containing micro-RNA to stop the expression from the endogenous SPAST gene and cDNA to express healthy human M1 and M87. The vector was introduced by intracerebroventricular injections into newborn pups of SPAST-C448Y, a mouse model of the disease that expresses human mutant spastin and displays adult-onset corticospinal degeneration and gait defects. The treatment successfully replaced both isoforms of endogenous spastin with healthy spastin, at physiological levels, and prevented the onset and progression of corticospinal degeneration and gait defects.