Spasticity is a state of increased muscle tone. Paraplegia refers to the legs (quadriplegia refers to both the arms and legs). In hereditary spastic paraplegia there is an insidiously progressive condition characterized by weakness beginning in the feet and lower legs. There is stiffness of the legs in walking due to the spasticity.
Autosomal dominant hereditary spastic paraplegia can result from changes in a number of different genes. Each gene is autosomal (on an nonsex chromosome) and dominant (capable of causing the disease by itself).
Among the 4 genes causing AD-HSP found through 1999, one that is on chromosome 2 accounts for 40-50% of all families with AD-HSP. Researchers have identified a gene that is responsible for this most frequent form of AD-HSP. They named the gene "spastin" (for spasticity). Spastin appears to code for a critically important enzyme (in technical terms, an ATPase involved in the assembly or function of nuclear protein complexes). (Reference: Hazan J et al. Spastin, a new AAA protein, is altered in the most frequent form of autosomal dominant spastic paraplegia. Nature Genetics 23: 296 - 303, 1999.)