Hereditary spastic paraplegia: gain-of-function mechanisms revealed by new transgenic mouse

Abstract

Mutations of the SPAST gene, which encodes the microtubule-severing protein spastin, are the most common cause of hereditary spastic paraplegia (HSP). Haploinsufficiency is the prevalent opinion as to the mechanism of the disease, but gain-of-function toxicity of the mutant proteins is another possibility. Here, we report a new transgenic mouse (termed SPAST C448Y mouse) that is not haploinsufficient but expresses human spastin bearing the HSP pathogenic C448Y mutation. Expression of the mutant spastin was documented from fetus to adult, but gait defects reminiscent of HSP (not observed in spastin knockout mice) were adult onset, as is typical of human patients. Results of histological and tracer studies on the mouse are consistent with progressive dying back of corticospinal axons, which is characteristic of the disease. The C448Y-mutated spastin alters microtubule stability in a manner that is opposite to the expectations of haploinsufficiency. Neurons cultured from the mouse display deficits in organelle transport typical of axonal degenerative diseases, and these deficits were worsened by depletion of endogenous mouse spastin. These results on the SPAST C448Y mouse are consistent with a gain-of-function mechanism underlying HSP, with spastin haploinsufficiency exacerbating the toxicity of the mutant spastin proteins. These findings reveal the need for a different therapeutic approach than indicated by haploinsufficiency alone.

New hypothesis for the etiology of SPAST‐based hereditary spastic paraplegia

“…For our mouse, the full‐length spastin with C448Y mutation (SPAST C448Y ) was inserted into the mouse ROSA26 locus (Qiang et al, ). It was our intent that this animal not be haploinsufficient so that we could specifically test the role of gain‐of‐function toxicity of the mutant spastin protein.…”

“…It was our intent that this animal not be haploinsufficient so that we could specifically test the role of gain‐of‐function toxicity of the mutant spastin protein. Despite having normal levels of endogenous mouse spastin, both heterozygous and homozygous SPAST C448Y mice displayed locomotor phenotypes far more reminiscent of HSP than any other mouse to date, and also displayed corticospinal degeneration (Qiang et al, ). The animals displayed adult‐onset tremor‐like spasticity and gait deficiencies, with homozygotes worse than heterozygotes (Qiang et al, ).…”

“…Despite having normal levels of endogenous mouse spastin, both heterozygous and homozygous SPAST C448Y mice displayed locomotor phenotypes far more reminiscent of HSP than any other mouse to date, and also displayed corticospinal degeneration (Qiang et al, ). The animals displayed adult‐onset tremor‐like spasticity and gait deficiencies, with homozygotes worse than heterozygotes (Qiang et al, ). The corticospinal degeneration was characterized by dying back of axons from their synapses (Qiang et al, ).…”

“…The animals displayed adult‐onset tremor‐like spasticity and gait deficiencies, with homozygotes worse than heterozygotes (Qiang et al, ). The corticospinal degeneration was characterized by dying back of axons from their synapses (Qiang et al, ).…”

“…As a first step toward understanding a potential relationship between these two factors, we conducted studies on cortical neurons cultured from our SPAST C448Y mice and found that lysosomal transport deficiencies in these neurons are worsened by siRNA‐based depletion of endogenous mouse spastin (Qiang et al, ). This result supports the view that haploinsufficiency exacerbates the cellular defects resulting from the gain‐of‐function toxicity of the mutant spastins.…”