Sialidosis type 1 is a rare autosomal recessive lysosomal storage disorder caused by pathogenic variants in the NEU1 gene, which encodes the sialic acid-degrading enzyme α-neuraminidase. Sialidosis type 1 is a milder form with a late-onset phenotype, characterized by progressive myoclonic epilepsy and ataxia with cherry-red spots. Sialidosis type 2 is an early-onset and more severe form presenting with dysmorphic features, hepatosplenomegaly and cognitive delay. Clinical diagnosis is usually supported by increased urinary bound sialic acid excretion and confirmed by genetic analysis or demonstration of α-neuraminidase enzyme deficiency in cultured fibroblasts. The aim of this study was to present a case of type 1 sialidosis, review the literature, and investigate genotype-phenotype correlations, symptom frequencies, and race-specific mutations in patients diagnosed with type 1 sialidosis. We report herein a family of four Turkish siblings affected with sialidosis type 1 associated with a homozygous variant, c.403G>A p. (Asp135Asn), in the NEU1 gene. A systematic literature review on sialidosis type 1 was carried out, by the PubMed database was searched using keywords included sialidosis and/or NEU1 gene. We selected case reports or series that included genetically confirmed type 1 sialidosis from 1996 to 2023. So far, nearly genetically confirmed 80 patients from unrelated 65 families, more than 40 NEU1 disease causing mutations, have been identified in patients with sialidosis type 1. Among the reported mutations, missense variants are the most common, and few nonsense, frameshift, exonic duplications or small deletions have been reported. c.239C>T p. (Pro80Leu) variant in Chinese and Japanese patients, c.649G>A p. (Val217Met) variant in Japanese patients, c.880C>T p. (Arg294Cys) variant in Indian patients, c.629C>T p. (Pro210Leu) variant in Ecuadorian patients, c.982G>A p. (Gly328Ser) variant in Italian patients, and c.403G>A p (Asp135Asn) and c.625del p. (Glu209Serfs*94) variants in Turkish patients were found higher. Race-specific variants were found with higher percentages in certain populations.

Sialidosis type 2 has variants that are both catalytically inactive (severe), while sialidosis type 1 has at least one catalytically active (mild) variant. This study aimed to discuss the structural changes associated with these variants in a newly reported family carrying N-acetyl-α-neuraminidase-1 (NEU1) variants and explore the clinical characteristics of different combinations of variants in sialidosis type 1. First, whole-exome sequencing and detailed clinical examinations were performed on the family. Second, structural analyses, including assessments of energy, flexibility and polar contacts, were conducted for several NEU1 variants, and a sialidase activity assay was performed. Third, previous NEU1 variants were systematically reviewed, and the clinical characteristics of patients in the severe-mild and mild-mild groups with sialidosis type 1 were analyzed. We report a novel family with sialidosis type 1 and the compound heterozygous variants S182G and V143E. The newly identified V143E variant was predicted to be a mild variant through structural analysis and was confirmed by a sialidase activity assay. Cherry-red spots were more prevalent in the severe-mild group, and ataxia was more common in the mild-mild group. Impaired cognition was found only in the severe-mild group. Moreover, patients with cherry-red spots and abnormal electroencephalographies and visual evoked potentials had a relatively early age of onset, whereas patients with myoclonus had a late onset. Changes in flexibility and local polar contacts may be indicators of NEU1 pathogenicity. Sialidosis type 1 can be divided into two subgroups according to the variant combinations, and patients with these two subtypes have different clinical characteristics.