Further evidence for a fourth gene causing X-linked pure spastic paraplegia

X-linked hereditary spastic paraplegias (HSPs) present with two distinct phenotypes: pure and complicated. The pure form is characterized by slowly progressive weakness and spasticity of the lower limbs, whereas the complicated forms have additional features (optic neuropathy, retinopathy, extrapyramidal disturbance, dementia, epilepsy, ataxia, ichthyosis, mental retardation, and deafness). Three X-linked loci have been identified for the complicated HSP, while mutations in the proteolipid gene (PLP) (locus SPG2) were implicated in both pure and complicated forms. The absence of identified mutations in the PLP gene in families with both complicated and pure HSP, linked to the SPG2 locus, suggests the existence of another gene in close proximity. We had previously reported a large pedigree with an X-linked form of pure HSP affecting 24 males [Zatz et al., 1976: J Med Genet 13:217-222]. Here, we present the results of linkage analysis in 19 members of this Brazilian family with markers in or near the PLP locus. Positive LOD scores were obtained with markers at the PLP locus (Zmax = 2.41 at Theta = 0); however, no mutation was found in the coding region of PLP, the intron-exon boundaries, or part of the promoter region. The possibility of a duplication of the PLP gene was also excluded. These results suggest either that there is another X-linked gene in close proximity to the PLP gene or that a novel mutation in the noncoding regions of the PLP gene may cause the disease in this family.