Hereditary spastic paraplegia (HSP) is characterized by weakness and spasticity of the lower limbs, owing to degeneration of corticospinal axons. The most common form is due to heterozygous mutations in the SPG4 gene, encoding spastin, a microtubule (MT)‐severing protein. Here, we show that neurite growth in immortalized and primary neurons responds in pleiotropic ways to changes in spastin levels. Spastin depletion alters the development of primary hippocampal neurons leading to abnormal neuron morphology, dystrophic neurites, and axonal growth defects. By live imaging with End‐Binding Protein 3‐Fluorescent Green Protein (EB3‐GFP), a MT plus‐end tracking protein, we ascertained that the assembly rate of MTs is reduced when spastin is down‐regulated. Spastin over‐expression at high levels strongly suppresses neurite maintenance, while slight spastin up‐regulation using an endogenous promoter enhances neurite branching and elongation. Spastin severing activity is exerted preferentially on stable acetylated and detyrosinated MTs. We further show that SPG4 nonsense or splice site mutations found in hereditary spastic paraplegia patients result in reduced spastin levels, supporting haploinsufficiency as the molecular cause of the disease. Our study reveals that SPG4 is a dosage‐sensitive gene, and broadens the understanding of the role of spastin in neurite growth and MT dynamics.

Alternating hemiplegia of childhood (AHC, MIM 104290) is a rare syndrome, characterised by early onset of episodic hemi- or quadriplegia lasting minutes to days. The majority of patients are sporadic. Only a few familial cases are reported in the literature. Here we describe a new familial case from a Greek island with four affected members in two generations, the mother and three out of four children. All patients share a normal karyotype. Due to the partial clinical overlap of familial hemiplegic migraine (FHM) with AHC, we screened the ATP1A2 gene coding for the α2 subunit of the Na,K pump, associated with FHM type 2. We found a novel heterozygous mutation segregating with the disease and causing a threonine to asparagine replacement (T378N). This missense mutation localises to the ATPases phosphorylation site of the hydrolase domain. The affected residue is highly conserved in all the known α subunits of the Na,K and Na,H pumps from vertebrates to invertebrates. Functional data suggest that loss of function of the mutated ATP1A2 isoform is involved in generating the disease phenotype. This is the first mutation associated with AHC identified so far

Mutations in the SPG7 gene encoding a mitochondrial protein termed paraplegin, are responsible for a recessive form of hereditary spastic paraparesis. Only few studies have so far been performed in large groups of hereditary spastic paraplegia (HSP) patients to determine the frequency of SPG7 mutations. Here, we report the result of a mutation screening conducted in a large cohort of 135 Italian HSP patients with the identification of six novel point mutations and one large intragenic deletion. Sequence analysis of the deletion breakpoint, together with secondary structure predictions of the deleted region, indicate that a complex rearrangement, likely caused by extensive secondary structure formation mediated by the short interspersed nuclear element (SINE) retrotransposons, is responsible for the deletion event. Biochemical studies performed on fibroblasts from three mutant patients revealed mild and heterogeneous mitochondrial dysfunctions that would exclude a specific association of a complex I defect with the pathology at the fibroblast level. Overall, our data confirm that SPG7 point mutations are rare causes of HSP, in both sporadic and familial forms, while underlying the puzzling and intriguing aspects of histological and biochemical consequences of paraplegin loss.

Background: Hereditary spastic paraplegia (HSP) is a group of genetically heterogeneous disorders characterized by progressive spasticity of the lower limbs. Mutations in the SPG4 gene, which encodes spastin protein, are responsible for up to 45% of autosomal dominant cases.Objective: To search for disease-causing mutations in a large series of Italian patients with HSP.Design: Samples of DNA were analyzed by direct sequencing of all exons in SPG4. Samples from a subset of patients were also analyzed by direct sequencing of all exons in SPG3A, SPG6, SPG10, and SPG13. Setting: Molecular testing facility in Italy. Patients: Sixty unrelated Italian patients with pure (n=50) and complicated (n = 10) HSP. Main Outcome Measures: Mutations in SPG4, SPG3A, SPG6, SPG10, and SPG13.Results: We identified 12 different mutations, 8 of which were novel, in 13 patients. No mutations of any of the other HSP genes tested were found in 15 patients with sporadic pure HSP who did not have mutations in the SPG4 gene. Conclusions:The overall rate of mutation in the SPG4 gene within our sample was 22%, rising to 26% when only patients with pure HSP were considered. The negative result obtained in 15 patients without mutations in SPG4 in whom 4 other genes were analyzed (SPG3A, SPG6, SPG10, and SPG13) indicate that these genes are not frequently mutated in sporadic pure HSP.

These findings expand the mutation spectrum of SPG11 and suggest that SPG11 mutations may occur more frequently in familial than sporadic forms of cHSP without TCC. This helps to define further clinical and molecular criteria for a correct diagnosis of the SPG11 related form of cHSP. In addition, the intragenic deletion detected here, and the mechanism involved, both provide clues to address the issue of SPG11 missing mutant alleles previously reported.