Recent advances in the genetics of spastic paraplegias.

Hereditary spastic paraplegias (HSPs) are genetically heterogeneous mendelian disorders characterized by weakness and spasticity in the lower limbs associated with additional neurologic signs in "complex" or "complicated" forms. Major advances have been made during the past two decades in our understanding of their molecular bases. The mapping of 34 genes (17 of which have been identified) involved in this clinically diverse group of disorders has highlighted their great genetic heterogeneity. From the combined genetic and clinical information obtained, a new classification is now emerging that will help to better diagnose this condition, evaluate disease progression, guide follow-up, and permit genetic counselling. Evidence is now accumulating that at least part of the physiopathology results from abnormal intracellular trafficking, as well as from altered cell recognition and signaling, oligodendroglial dysfunction, mitochondrial defects, and impaired cholesterol and/or neurosteroid metabolism.