Detection of a novel SPG4?nonsense?mutation in a large German pedigree with pure spastic paraplegia

Abstract

Sirs: Spastic paraplegia type 4 (SPG4) is the most frequent type of autosomal dominant inherited spastic paraplegia. The SPG4 gene encodes spastin, an ATPase belonging to the AAA protein family. Mutations have been reported along all 17 exons of the coding region. We here report on a new truncating SPG4 mutation in a large three generation German kindred with 27 family members. Molecular genetic analysis. The causative region was mapped to chromosome 2p21-p22 which is identical to the SPG4 locus. To avoid sequencing of the complete spastin gene (SPG4) the 17 exons were screened by HPLC technology (WAVE-System, Transgenomics). All exons with flanking intron sequences were amplified by PCR. Products were analysed on the dHPLC-system at at least two different temperatures depending on the melting profiles of the amplified sequences. The clinically affected individuals showed clearly distinguishable peak patterns for exon 5 in comparison with unaffected relatives. Sequencing of this exon revealed a T to G transversion in one allele leading to an early inframe stop codon. On the amino acid level this nonsense mutation causes the p.Leu239X truncation of the SPG4 protein. Clinical data. Clinically, 11 subjects showed symptoms of spastic paraplegia (mean age: 44.6 ± 14.1 years, range: 30–61). In eight of them, there was clear evidence for spasticity and increased tendon reflexes in the lower limbs. Plantar responses were extensor in two subjects. Another three individuals had only mild symptoms with brisker tendon reflexes in the lower limbs than in the upper limbs. Phenotype-genotype correlation analysis confirmed the SPG4 mutation in all of them. Interestingly four gene carriers were personally unaware of symptoms. Among the 10 at-risk individuals with normal neurological examination, another two patients, aged 27 and 36 years, were tested positive for the SPG4 mutation. Additional loss of proprioception was present in 8, impairment of exteroception in 6 patients. Sphincter disturbances were complained of by 6 family members with combined urinary/faecal incontinence in 4 of them. There was no evidence for additional cognitive impairment, peripheral neuropathy, cerebellar symptoms, visual or hearing loss. The age of onset ranged from 6 to 48 years (mean 27.8 ± 18.1 years). Clinical severity was also highly variable with younger patients being more severely affected. The variability of the clinical severity but not of the clinical symptoms appears to be a characteristic finding of SPG4 [1]. It points to unknown biological factors positively modulating the disease in some carriers of the SPG4 mutation. The detection of these additional factors might offer therapeutic strategies in the future.