PracticeUpdate Conference Series - SSIEM 2018

Diagnosis of Mucopolysaccharidosis May Be Simplified Worldwide Expansion of dried blood spot testing allows for use of a seven-enzymemucopolysaccharidosis panel

E xpansion of dried blood spot testing allows for use of a seven-enzyme mucopolysaccharidosis panel that can simplify the diagnosis of muco- polysaccharidosis (MPS) worldwide. This conclusion, based on results of a description of a tandem mass spectrometry evaluation, was presented at SSIEM 2018. T i m o t h y C .

however. Such analysis also requires a relatively large volume of blood. These requirements are problematic in international shipments of specimens and in testing infants. Adaptation of these enzyme assays for use in dried blood spots has ameliorated these issues in a number of lysosomal storage disorders. The development of tandem mass spectrometry substrates has allowed multiple enzyme reactions to be combined in a single assay, increasing efficiency. Enzyme deficiencies result in the accumulation of mucopolysaccharides (glycosaminoglycans) in lysosomes of various tissues and in the excessive excretion of partially degraded glycos- aminoglycans in urine. Specific degradative lysosomal enzyme deficiencies have been identified for all the MPS disorders. Glycosaminoglycans stored and excreted in the urine of the various MPS are dermatan sulfate, heparan sulfate, keratan sulfate, and chondroitin 4/6 sulfates. " The development of tandemmass spectrometry substrates has allowed multiple enzyme reactions to be combined in a single assay, increasing efficiency. " Clinical manifestations of the MPS depend on the specific enzyme deficiency, the end organ affected, and the accumulation of glycosaminoglycans in the affected organs. In diseases in which the brain is not involved, no mental retardation occurs. On the other hand, if the brain is affected and other somatic manifestations are minimal, features characteristic of MPS are not as prominent.

Patients suffer from a wide spectrum of symptoms including skeletal dysplasia (with short stature, dysostosis multiplex, and degenerative joint disease), cardiac valvulopathies, impaired pulmonary function, hepatosplenomegaly, sinusitis, otitis media, hearing loss, sleep apnea, corneal clouding, inguinal or umbilical hernia, and neurologic complications. Management of MPS includes sympto- matic treatment, enzyme replacement therapy (laronidase for MPS I, idursulfase for MPS II, galsulfase for MPS VI), and hematopoietic stem cell transplantation for MPS I and VI. Patients with MPS can suffer from compression of the cervical spinal cord caused by glycosaminoglycan infiltration of the dura. Such compression can lead to spastic paresis if not corrected by neurosurgical intervention. In MPS VI, intellectual deficit is generally absent. Patients may present with cervical spinal cord compression caused by cervical spinal instability, meningeal thickening, or bony stenosis, as well as communicating hydrocephalus and optic nerve atrophy. Carpal tunnel syndrome is a common feature of MPS I, II, and VI. Communicating hydrocephalus is common in severe MPS I and typically progresses slowly over months to years. Progressive cognitive impairment is characteristic of severe MPS, whereas cognition is gener- ally normal in attenuated MPS I. Patients with the severe form of MPS II suffer from behavioral disorders and early psychomotor regression, leading to intellectual deficit, as well as seizures. Dr. Wood concluded that his laboratory can now analyze 18 enzymes in dried blood spots to diagnose 20 different lysosomal storage disorders. Molecular testing from the original dried blood spot is available, limiting the need for additional sampling. Expansion of dried blood spot testing allows for use of a seven-enzyme MPS panel. The expansion may potentially simplify the diagnosis of MPS worldwide.

Wood, PhD, of the Greenwood Genetic Center in Greenwood, South Carolina, and col- leagues reported validation results of a new six-plex assay for diag- nosis of five MPS

Timothy C. Wood, PhD

disorders (MPS II, IIIB, IVA, VI, and VII) and neuronal ceroid lipofuscinosis type 2 in dried blood spots using ultraperformance liquid chromatography-tandem mass spectrometry. Analysis of substrates employed a Waters triple quadrupole ultraperformance liquid chromatography-tandem mass spectrometer. Overall, 99 samples with a confirmed diagnosis of one of the six disorders were used to evaluate clinical sensitivity and specificity. To develop normal ranges, 256 blood spots were obtained from unaffected controls or patients with a confirmed alternative diagnosis. A clear separation between normal and affected patients was noted for all six enzymes, providing 100% clinical sensitivity. Intra- and interday precision was acceptable (<20% coefficient of variation). Additionally, by analyzing multiple enzymes, mucolipidosis types II and III and multiple sulfatase deficiency were shown to be detectable. Dr. Wood explained that enzyme analysis is the gold standard for diagnosis of lysosomal storage disorders. Enzyme analysis in leukocytes requires that whole blood samples arrive at the testing laboratory within 24–48 h of collection,

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PRACTICEUPDATE CONFERENCE SERIES • SSIEM 2018