Urine karatan sulfate (uKS) in Morquio syndrome type A patients measured via LC-MS/MS method: Improved KS detection as compared to dye-based methods and report of age-specific uKS reference ranges

Morquio syndrome type A (mucopolysaccharidosis type IVA, MPS IVA) is a lysosomal disorder caused by insufficient N-acetylgalactosamine-6-sulfatase (GALNS) activity. The lysosomal enzyme GALNS catabolizes the glycosaminoglycans (GAG) keratan sulfate (KS) and chondroitin 6-sulfate (C6S), and deficient GALNS activity causes accumulation of KS and C6S. While demonstration of deficient GALNS enzyme activity is the gold standard for diagnosis of Morquio syndrome type A, detection of elevated urinary GAG (GAG) levels is often used to screen for Morquio syndrome type A. However, standard dye-based analyses of total GAG levels (the quantitative GAG test) have a high false negative rate in the identification of patients with Morquio syndrome type A, in some cases leading to misdiagnoses. False positives are also possible. To address the use of the liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based urine KS (uKS) test as an improved method to measure KS in comparison with dye-based methods, two international data steering committees were organized by BioMarin Pharmaceutical Inc. (Nashville, USA and Bahia, Brazil) in 2014. Case data on age-specific reference ranges for uKS levels in unaffected individuals and patients with Morquio syndrome type A were accumulated and the performance of the dye-based GAG test and LC-MS/MS uKS tests in identifying patients with Morquio syndrome type A were compared. Multiple independent international labs assessed uKS levels by the dye-based GAG test and the LC-MS/MS uKS test in unaffected individuals and patients with Morquio syndrome type A. We confirm previous findings that the levels of uKS decrease with age in unaffected individuals. In particular, creatinine-normalized uKS levels are highest in children less than 1 year of age and then decrease rapidly. While results from the dye-based test and the LC-MS/MS uKS test were linearly correlated, we highlight multiple cases where the superior sensitivity of the LC-MS/MS uKS test resulted in identification of patients with Morquio syndrome type A while the dye-based test did not. Although uKS has, to date, not been shown to be an efficacy biomarker for enzyme replacement therapy, it is an excellent screening biomarker for Morquio syndrome type A and potentially for other disorders. In contrast to dye-based methods, the LC-MS/MS-based uKS test provides a highly sensitive, specific, and quantitative measure of elevated KS. Optimally, LC-MS/MS-based GAG measurements will enable more accurate urine screening for MPS disorders.