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Abstract Objective Expand genetic screening for atypical Type I sialidosis (ST‐1) could address its underdiagnosed in both progressive myoclonic ataxia (PMA) and ataxia patients. To evaluate the potential founder effect of mutation in the population. Methods We enrolled 231 patients with PMA or ataxia from the First Affiliated Hospital of Fujian Medical University. Through Whole Exome Sequencing and Sanger sequencing, we identified the causative gene in patients. Haplotype analysis was employed to explore a potential founder effect of the NEU1 c.544A>G mutation. Results A total of 31 patients from 23 unrelated families were genetically diagnosed with ST‐1. A significant 80.6% of these patients were homozygous for the c.544A>G mutation. We discovered six different NEU1 variants, including two novel mutations: c.951_968del and c.517T>G. The mean age of onset was 18.0 ± 7.1 years. The clinical spectrum of ST‐1 featured ataxia and myoclonus as the most common initial symptoms. Over 40% suffered from controlled generalized tonic–clonic seizures. Mobility and independence varied greatly across the cohort. Cherry‐red spots were rare, occurring in just 9.5% (2/21) of patients. Brain MRIs were typically unremarkable, except for two patients with unusual findings. EEGs showed diffuse paroxysmal activity in 17 patients. The c.544A>G mutation in NEU1 is a founder variant in Fujian, with a unique haplotype prevalent in East Asians. Interpretation ST‐1 should be suspected in patients with PMA or ataxia in Southeast China, even without macular cherry‐red spots and seizures, and the premier test could be a variant screening of the founder variant NEU1 c.544A>G.

Background: Hereditary spastic paraplegia (HSP) caused by mutations in ALDH18A1 have been reported as spastic paraplegia 9 (SPG9), with autosomal dominant and autosomal recessive transmission (SPG9A and SPG9B). SPG9 is rare and has shown phenotypic and genotypic heterogeneity in previous reports.Methods: This study screened ALDH18A1 mutations in autosomal recessive HSP patients using combined whole exome sequencing and RNA splicing analysis. We conducted in silico investigations, co-segregation analysis, and ELISA-based analysis of P5CS (Δ1-pyrroline-5-carboxylate synthetase; encoded by ALDH18A1) concentration to validate the pathogenicity of the detected ALDH18A1 variants. All previously reported bi-allelic ALDH18A1 mutations and cases were reviewed to summarize the genetic and clinical features of ALDH18A1-related HSP.Results: A novel missense mutation c.880T>C, p.S294P and an intronic splicing mutation c.-28-13A>G were both detected in ALDH18A1 in an autosomal recessive family presenting with a complicated form HSP. ELISA assays revealed significantly decreased P5CS concentration in the proband's plasma compared with that in the healthy controls. Moreover, review of previously reported recessive cases showed that SPG9B patients in our cohort presented with milder symptoms, i.e., later age at onset and without cognitive impairment.Conclusion: The present study expands the genetic and clinical spectrum of SPG9B caused by ALDH18A1 mutation. Our work defines new genetic variants to facilitate future diagnoses, in addition to demonstrating the highly informative value of splicing mutation prediction in the characterization of disease-related intronic variants.