At “Overcoming Challenges of Morquio A Diagnosis: The Latest from the Lab and the Clinic,” a special symposium held recently during the 2015 American Society of Human Genetics annual meeting in Baltimore, Maryland, Timothy Wood, PhD, FACMG, director of the clinical biochemical genetics laboratory at Greenwood Genetic Center in Greenwood, South Carolina, provided an overview of the laboratory testing used to diagnose the rare condition Morquio syndrome type A, also called mucopolysaccharidosis IV type A (MPS IVA).
Morquio A is a rare lysosomal storage disorder that may first become suspected by a pediatrician, radiologist, or geneticist when a child is aged 2 to 3 years, based on clinical and/or radiographic evidence. However, numerous examples of the common physical attributes associated with Morquio (eg, short stature, skeletal abnormalities) may not be obvious until much later. Laboratory testing for Morquio A can include urine, blood, and/or molecular testing.
Urinary Glycosaminoglycans (GAGs)
If Morquio A is suspected, a urinalysis for glycosaminoglycans (GAGs) may be conducted.
Dr Wood noted that the screening test may provide both false positives and false negatives, and that it can be difficult to distinguish Morquio A from other MPS disorders.
Urinary screening is based on the excretion of excess GAGs that occurs in Morquio A patients (as well as in other MPS disorders). Urinary GAGs, when normalized to creatinine, are often high in infants and young children with Morquio A.
Both quantitative and qualitative analysis can be performed to measure total GAG levels as well as specific GAGs such as keratan sulfate (KS), and both tests are recommended.
Dr Wood noted that the quantitative measure of total GAG levels has its limits as a diagnostic tool, since GAG levels are abnormal in about 75% of Morquio A patients and are also high in patients with other types of MPS disorders.
Qualitatively, KS may be difficult to detect but it is a more accurate predictor of a patient having Morquio A. While a Morquio A patient’s GAG level may not be elevated, their KS level almost always is. Unfortunately, measuring KS is difficult. Dr Wood noted that his laboratory quantitatively measures urine KS using a method that involves enzymatic digestion of KS into disaccharides, which are measured by LC-MS/MS.
Dr Wood concluded that while urinalysis has its limitations in confirming a diagnosis of Morquio A, measurement of urine KS value is a good biomarker to measure treatment efficacy (ie, it decreases in response to treatment).
Dr Wood noted that the best samples to test are ones obtained early in the morning.
The Gold Standard: Enzyme Activity of GALNS
The gold standard for diagnosing Morquio A is to measure the activity of N-acetylgalactosamine-6-sulfate sulfatase (GALNS). Fibroblasts and leukocytes are the recommended sample types for this analysis.
For these tests, Dr Wood noted that whole blood samples should arrive at the laboratory within 48 hours of the blood draw to reduce false positive results.
GALNS activity assays use either a radioactive substrate (eg, a tritiated disulfated trisaccharide prepared from C6S) or, more often, a fluorescent tagged substrate (eg, 4-
methylumbelliferyl-β-D-galactopyranoside-6-sulfate). Enzyme activity is reported relative to the total amount of protein in the cell lysate.
Low GALNS activity may be due to other condition
Patients with multiple sulfatase deficiency and/or mucolipidosis type II or III (ML II/III) may also have low GALNS activity.
If there is any clinical concern for multiple sulfatase deficiency, a second sulfatase enzyme should be measured.
If the enzyme activity was for a fibroblasts sample, a second enzyme test to rule out ML II/III may be necessary. Measuring a second mannose-6-phosphate–targeted enzyme such as
β-galactosidase, arylsulfatase B, iduronate-2-sulfatase, or β-hexosaminidase (EC 184.108.40.206), or α-iduronidase (EC220.127.116.11) in fibroblasts, can help to differentiate the diagnosis.
Finally, measurement of beta galactosidase activity should also be performed to rule out Morquio syndrome type B.
Molecular Testing for GALNS
There are currently more than 277 reported mutations in the GALNS gene, many of which are “private” and found in only 1 individual or family. Unfortunately, it is currently difficult to determine if a novel mutation is pathogenic.
Genetic testing can also be used for prenatal counseling and carrier studies, as well as to confirm a diagnosis following an enzyme activity test.
Wood TC, Harvey K, Beck M, et al. Diagnosing mucopolysaccharidosis IVA. J Inherit Metab Dis
. 2013;36(2):293-307. DOI 10.1007/s10545-013-9587-1