Sialidosis type II: Expansion of phenotypic spectrum and identification of a common mutation in seven patients

Sialidosis, an autosomal recessive disorder, is characterized by progressive lysosomal storage of sialylated glycopeptides and oligosaccharides. It occurs as a result of biallelic mutations in the NEU1 gene. Sialidosis is traditionally classified as a milder, late-onset type I and a severe early-onset type II disease. The presence of a cherry-red spot is a well-established ophthalmological clue to the disorder. We present a clinical-radiological report of seven unrelated patients with molecularly confirmed sialidosis type II. To the best of our knowledge, This is the largest reported series of patients with Sialidosis type II. A novel, previously unreported ophthalmic phenotype of bulls-eye maculopathy, is described. All seven phenotypically heterogeneous patients had the same pathogenic variant (c.679G > A; p.Gly227Arg) at a homozygous level in the NEU1 gene. We propose that this is a common mutation in north Indians for this rare disorder. We also observed an overlap of symptoms and a continuum of phenotypes in type I and II Sialidosis.