A novel KIAA0196 (SPG8) mutation in a Chinese family with spastic paraplegia

Hereditary spastic paraplegia (HSP) is one of the most heterogeneous genetic neurodegenerative diseases, caused by mutations in more than 50 different genes. The eighth HSP locus, SPG8, is on chromosome 8p24.13. SPG8 is a rare autosomal dominant-HSP (AD-HSP) caused by mutations in the KIAA0196 gene, with only seven SPG8 families described to date.1 Here, we described the clinical characteristics of AD-HSP caused by a novel mutation in the KIAA0196 gene in a Chinese family. METHODS The proband was a 24-year-old woman who was admitted with a complaint of weakness and stiffness of both legs for about 7 years. In her family, there are 5 other affected individuals with spastic gait and lower-limb stiffness. Brain and spinal cord MRI was performed on the proband. The clinical data of the proband and her affected relatives were reviewed. We obtained written informed consent from the family. Genomic DNA was extracted from peripheral blood leukocytes obtained from the family by routine phenol/chloroform extraction methods. Exons and flanking intron-exon boundaries of KIAA0196 gene were amplified by the polymerase chain reaction (PCR) and sequenced. Forward primer is CATCCACAGTAGCAATAAGGG. Reverse primer is GTCCAGGGAGCACAGTTTAA. RESULTS Clinical information and family details The proband was born without neonatal problems in a non-consanguineous Han Chinese family. Seven years ago, she began to suffer from weakness and stiffness of both legs. The onset of symptoms was subtle, with slow development of leg stiffness and weakness. She showed spastic gait, but was able to walk without assistance. She denied urinary urgency. Physical examination at admission was as follows: alert and cooperative, normal muscular force, active tendon reflex of 4 limbs, positive Hoffman sign of the left upper limb, positive bilateral Babinski sign, decreased vibration sense in the lower legs, and normal coordinate movement. Examination of the fundus oculi was normal. Detailed psychological tests showed mild memory decline: WAIS-RC (Chinese revision of the Wechsler Adult Intelligence Scale): VIQ 85, PIQ 70, FIQ 77 (normal range: 90-109). Visual evoked response (VEP) and brain-stem auditory evoked potential (BAEP) examinations were normal. Electromyogram (EMG) and nerve conduction velocity examination was normal. The brain MRI of the proband showed multiple small hyperintense areas involving the white matter in an asymmetric manner (Figure 1A and 1B). The spinal cord MRI showed atrophy of the thoracic spinal cord (Figure 1C and 1D). Proton magnetic resonance spectroscopy (MRS) revealed decreased N-acetyl aspartate (NAA) peaks with respect to choline and creatine in the right basal ganglia area (Figure 1E and 1F).Figure 1.: Brain MRI in proband showed multiple small hyperintense areas involving the white matter of right hemisphere (A and B). Spinal MRI showed atrophy of thoracic spinal cord (C and D). Proton MRS revealed decreased N-acetyl aspartate (NAA) peaks with respect to choline and creatine in the right basal ganglia area (E and F).Five relatives spanning 3 generations, including the proband's mother, were affected (Figure 2A). Clinically, a rather pure spastic paraplegia was found in all 6 patients (5 women, 1 man) with a mean age at onset of (26.80±11.30) years (range 17-43 years) (Table 1). The core phenotype consists of pyramidal signs and symptoms limited to the legs, with hyperreflexia and spastic gait. Additional notable signs and symptoms were an impaired sense of vibration in the lower legs in all 6 patients and urinary urgency in 5 patients. Only 1 woman used a wheelchair. The clinical characteristics are presented in Table 1.Figure 2.: Pedigrees for the family with KIAA0196 mutations (A). KIAA0196 mutation was pointed with star mark. Sequences showed complementary strand (B). Position of the mutation within a highly conserved region indicated with arrow (C).Table 1: Clinical characteristics of 6 affected subjects with the KIAA0196 mutation (patient numbers refer to Figure 2)Genetic analysis results Gene analysis excluded SPG3, SPG4, and SPG6 as the candidate loci of the mutations. Genetic analysis of KIAA0196 yielded a novel missense mutation (c.1771T>C; p.Ser591Pro) in the proband (Figure 2B). The genomic location of the variant was chr8:126069902. Subsequently, the mutation was found in 4 of the other affected relatives. The mutation was not detected for the asymptomatic members of the family and it is not present in the dbSNP database (version 139, http://www.ncbi.nlm.nih.gov/projects/SNP/) or the 1000 Genomes Project (http://browser.1000genomes.org/). Mutaion Validation Analysis: Two prediction tools PolyPhen2 and SIFT were used to evaluate the functional effect of this missense variant. Sequence alignment among different species was performed with Clustal Omega. This mutation, never described before, was predicted to be probably damaging by PolyPhen-2 and damaging by SIFT. The S591P variant in KIAA0196 is located in a phylogenetically highly conserved region (Figure 2C). DISCUSSION SPG8 is due to mutations in the KIAA0196 gene, which encodes the protein strumpellin. The function of strumpellin protein is uncertain.1 In the family in our study, a novel mutation in the KIAA0196 gene (c.1771T>C; p.Ser591Pro) was identified. This mutation, never described before, was predicted to be probably damaging by PolyPhen-2 and damaging by SIFT. Proline is a hydrophobic amino acid, while the substituted serine is a hydrophilic amino acid, which suggests that the function of strumpellin protein might be compromised by the mutation. The S591P variant in KIAA0196 is located in a phylogenetically highly conserved region. This variant is a change of an evolutionary conserved amino acid, which supported the pathogenicity of this novel mutation. Clinically, a rather pure spastic paraplegia was found in the proband and her 5 relatives. The phenotype of pure HSP in our patients was similar to that in previous reports of SPG8/KIAA0196.1-3 The main clinical feature is progressive lower limb spasticity due to pyramidal tract defects, with vibration sense disturbances in the lower legs. In previous studies, the SPG8-linked family had a significantly older mean age at onset of symptoms and had significantly more wheelchair-using patients. The age at onset ranged from 20 to above 60 years, with a preference for the third and fourth decades.2,3 In this Chinese family, the age of onset was earlier, with 3 of 6 patients showing first symptoms when they were teenagers. Although the age of onset was earlier, the progression after onset was slower. Only 1 patient in this family used a wheelchair, after 31 years of suffering. In another recent report, a less severe pure adult-onset spastic paraplegia than previously described was reported in a large Dutch family with SPG8.1 These findings indicate inter familial clinical variability for autosomal dominant spastic paraplegia with SPG8. SPG8 is a rare AD-HSP, with only 7 SPG8 families described to date. Brain and spinal cord MRI findings of SPG8 were rarely reported in previous studies. The brain MRI of the proband showed multiple small hyperintense areas involving the white matter asymmetrically. Evidence of atrophy of the corpus callosum was not found in brain MRI. Similar brain lesions were found in one report of SPG5.4 Spinal cord MRI showed atrophy of the thoracic spinal cord in the proband in this Chinese family. This finding is consistent with the earlier evidence of spinal cord atrophy in SPG8.5 Brain MRS in the bilateral basal ganglia area in the proband in this Chinese family revealed decreased N-acetyl aspartate (NAA) peaks with respect to choline and creatine, especially in the right basal ganglia area. The pyramidal tract damage on the left side was more remarkable in this patient. The findings were in accordance with the brain MRS findings. Brain MRS findings in SPG8 were not reported previously. In conclusion, this is the fifth pathogenic KIAA0196 mutation and the first Chinese family reported with SPG 8. The clinical presentation of SPG8 in this Chinese family was a predominantly pure HSP with earlier age of onset and slower progression. As an autosomal dominant spastic paraplegia, SPG8 has an interfamilial clinical variability.