The laboratory diagnosis of mucopolysaccharidosis III (Sanfilippo syndrome): A changing landscape.

Mucopolysaccharidosis type III (MPS III) is characterized by progressive neurological deterioration, behavioral abnormalities, a relatively mild somatic phenotype, and early mortality. Because of the paucity of somatic manifestations and the rarity of the disease, early diagnosis is often difficult. Therapy targeting the underlying disease pathophysiology may offer the greatest clinical benefit when started prior to the onset of significant neurologic sequelae. Here we review current practices in the laboratory diagnosis of MPS III in order to facilitate earlier patient identification and diagnosis. When clinical suspicion of MPS III arises, the first step is to order a quantitative assay that screens urine for the presence of glycosaminoglycan biomarkers using a spectrophotometric compound (e.g., dimethylmethylene blue). We recommend testing all patients with developmental delay and/or behavioral abnormalities as part of the diagnostic work-up because quantitative urine screening is inexpensive and non-invasive. Semi-quantitative urine screening assays using cationic dyes on filter paper (e.g., spot tests) have relatively high rates of false-positives and false-negatives and are obsolete. Of note, a negative urinary glycosaminoglycan assay does not necessarily rule out MPS because, in some patients, an overlap in excretion levels with healthy controls may occur. All urine samples that test positive for glycosaminoglycans with a quantitative assay should be confirmed by electrophoresis, thin layer chromatography, or tandem mass spectrometry, which further improves the sensitivity and specificity. The gold standard for diagnosis remains the enzyme activity assay in cultured skin fibroblasts, leukocytes, plasma, or serum, which can be used as a first-line diagnostic test in some regions. Molecular genetic analysis should be offered to all families of patients to allow genetic counseling for informed family planning. For a small number of variants, genotype-phenotype correlations are available and can offer prognostic value. Prenatal testing via enzyme activity assay in chorionic villi or amniotic fluid cells is available at a limited number of centers worldwide, but whenever possible, a molecular genetic analysis is preferred for prenatal diagnosis. To conclude, we discuss the development of newborn screening assays in dried blood spots and high-throughput methods for sequencing the protein-coding regions of the genome (whole exome sequencing) and their relevance to future changes in the MPS III diagnostic landscape.