α-mannosidosis diagnosis in Brazilian patients with MPS-like symptoms

α-mannosidosis is an inborn error of metabolism caused by the deficiency of the lysosomal enzyme α-mannosidase, which is encoded by the MAN2B1 gene and inherited in an autosomal recessive manner. The impairment of affected individuals is multisystemic and very similar to the observed in some mucopolysaccharidosis (MPS) patients. The aim of this study was to search for α-mannosidosis cases in individuals with clinical suspicion of MPS without a confirmed diagnosis. Biochemical and molecular analysis were standardized by our group for this study. Two hundred and fifty samples from patients with clinical suspicion of MPS, but with inconclusive MPS biochemical and/or molecular analysis, were screened for α-mannosidase activity. Subsequently the MAN2B1 gene was sequenced in samples from 53 patients by the Sanger method.

The measurement of enzymatic activity detected fifty-three samples with abnormal results, suggesting α-mannosidosis. Molecular analysis confirmed three affected families, which presented the nonsense variant p.Ser899Ter. This variant generates a premature stop codon in exon 22, resulting in a truncated protein with no residual enzymatic activity.

In conclusion, this work brings data for the beginning of a genetic characterization of α-mannosidosis in the Brazilian population. It also shows that α-mannosidosis cases may be underdiagnosed due to the clinical similarity to MPS and the lack of information about this ultra-rare disease. Based on our data, we strongly recommend to all screening centers to consider α-mannosidosis testing together with screening for MPS as a tool for diagnosis to MPS-like phenotype individuals, since the phenotype similarity between these diseases poses a significant challenge for clinicians worldwide and often leads to the failure of the correct clinical diagnosis and treatment.