Loss-of-function mutations in the ATP13A2/PARK9 gene cause complicated hereditary spastic paraplegia (SPG78).

Hereditary spastic paraplegias are heterogeneous neurodegenerative disorders characterized by progressive spasticity of the lower limbs due to degeneration of the corticospinal motor neurons. In a Bulgarian family with three siblings affected by complicated hereditary spastic paraplegia, we performed whole exome sequencing and homozygosity mapping and identified a homozygous p.Thr512Ile (c.1535C > T) mutation in ATP13A2. Molecular defects in this gene have been causally associated with Kufor-Rakeb syndrome (#606693), an autosomal recessive form of juvenile-onset parkinsonism, and neuronal ceroid lipofuscinosis (#606693), a neurodegenerative disorder characterized by the intracellular accumulation of autofluorescent lipopigments. Further analysis of 795 index cases with hereditary spastic paraplegia and related disorders revealed two additional families carrying truncating biallelic mutations in ATP13A2. ATP13A2 is a lysosomal P5-type transport ATPase, the activity of which critically depends on catalytic autophosphorylation. Our biochemical and immunocytochemical experiments in COS-1 and HeLa cells and patient-derived fibroblasts demonstrated that the hereditary spastic paraplegia-associated mutations, similarly to the ones causing Kufor-Rakeb syndrome and neuronal ceroid lipofuscinosis, cause loss of ATP13A2 function due to transcript or protein instability and abnormal intracellular localization of the mutant proteins, ultimately impairing the lysosomal and mitochondrial function. Moreover, we provide the first biochemical evidence that disease-causing mutations can affect the catalytic autophosphorylation activity of ATP13A2. Our study adds complicated hereditary spastic paraplegia (SPG78) to the clinical continuum of ATP13A2-associated neurological disorders, which are commonly hallmarked by lysosomal and mitochondrial dysfunction. The disease presentation in our patients with hereditary spastic paraplegia was dominated by an adult-onset lower-limb predominant spastic paraparesis. Cognitive impairment was present in most of the cases and ranged from very mild deficits to advanced dementia with fronto-temporal characteristics. Nerve conduction studies revealed involvement of the peripheral motor and sensory nerves. Only one of five patients with hereditary spastic paraplegia showed clinical indication of extrapyramidal involvement in the form of subtle bradykinesia and slight resting tremor. Neuroimaging cranial investigations revealed pronounced vermian and hemispheric cerebellar atrophy. Notably, reduced striatal dopamine was apparent in the brain of one of the patients, who had no clinical signs or symptoms of extrapyramidal involvement.

Clinical Spectrum and Genetic Variability in Bulgarian Patients with Niemann-Pick Disease Type C.

BACKGROUND: Niemann-Pick disease type C (NP-C) is a rare autosomal-recessive lysosomal storage disorder caused by mutations in either the NPC1 (in 95% of cases) or the NPC2 gene. METHODS: In a prospective, observational cohort study, all Bulgarian patients diagnosed with NP-C to date (since 2010) underwent detailed neurological examination and neuro-ophthalmological, neuropsychological and psychiatric evaluations, as well as brain MRI, abdominal ultrasound and hearing tests. Plasma chitotriosidase was also measured, when possible. RESULTS: The Bulgarian national NP-C cohort comprised 11 patients who were diagnosed based on molecular genetic analysis (n = 9) and/or filipin staining of skin fibroblasts (n = 3). The mean age at onset was 14.4 (SD 8.3). Diagnoses were achieved 1-23 years after initial clinical presentation. All patients who underwent genetic mutation analysis were compound heterozygotes: a total of 12 NPC1 mutations were recorded, 5 of which were novel. Two patients had late-infantile onset, 4 had juvenile onset, and the remaining 5 had the adult-onset form of NP-C. Initial symptoms were neurological in 9 patients, visceral in one, and predominantly psychiatric in another. Vertical gaze palsy was present in all patients. Dysarthria, pyramidal involvement, cognitive impairment, and organomegaly with varied severity were observed in 10 of them. Ataxia was present in 9 and dystonia in 7. Four patients had epileptic seizures, and gelastic cataplexy was reported in 5. Brain MRI revealed hyperintense white matter lesions in 5 patients and cortical and/or cerebellar atrophy in 4. CONCLUSIONS: This Bulgarian NP-C cohort showed wide variability in terms of NPC1 mutations and predominant forms of neurological involvement. Diagnosing NP-C is challenging, and it was often delayed in this cohort due to the heterogeneity of patients' clinical signs and symptoms.