Mucopolysaccharidosis (MPS) IVA or Morquio syndrome is a rare lysosomal storage disorder caused by N-acetylgalactosamine-6-sulfatase deficiency. The condition is characterized by intracellular accumulation of the glycosaminoglycans keratan sulfate and chondroitin-6-sulfate, which classically leads to progressive skeletal and joint abnormalities, short stature, cardiorespiratory compromise, impaired vision, hearing loss, and hepatomegaly. The diagnosis is made by the identification of reduced N-acetylgalactosamine-6-sulfatase activity as well as detection of compound heterozygous or homozygous pathogenic variants in GALNS.
We present a case of two sisters with the severe classic phenotype of Morquio syndrome who were born to healthy non-consanguineous parents of East Indian background. Patient 1 first presented at age 1.5 years with trouble walking and abnormal gait with a subsequent skeletal survey suggestive of dysostosis multiplex. She was diagnosed at age 2 years with Morquio syndrome based on reduced leukocyte N-acetylgalactosamine-6-sulfatase activity of 2.8 nmol/17hr/mg protein (reference range 45-443 nmol/17hr/mg protein). Medical history for patient 1 includes normal cognition, truncal shortening, scoliosis, distal joint laxity, cervical arthropathy, gibbus deformity, corneal clouding, and obstructive sleep apnea. Patient 2, the younger sister, was diagnosed with MPS IV at age 3 following a history of frequent falls and identification of genu valgum and early dysostosis. Given the family history, she was tested for Morquio syndrome and N-acetylgalactosamine-6-sulfatase leukocyte activity was <0.062nmol/17hr/mg protein (reference range 45-433 nmol/17hr/mg protein). Patient 2 has a history of cervical C1-C2 instability and occipitocervical stenosis, severe bilateral genu valgum, hip dysplasia, leg length discrepancy, pectus carinatum and corneal clouding. Both children, now at age 11 and 10 years respectively, need to use a wheelchair for any distance outside the home, despite orthopedic procedures.
Prior genetic testing for MPS IVA, which utilized next generation sequencing (NGS), did not identify sequence variants associated with MPS IVA, however it was noted that a large region of the GALNS gene corresponding to exon 9 was not covered in the sequencing data. After establishing care with our center, repeat enzyme testing identified absent N-acetylgalactosamine-6 sulfatase activity for patient 1 and significantly reduced activity of 0.18 nmol/17hr/mg protein for patient 2 (reference range 49-255 nmol/17hr/mg protein; affected range < 7 nmol/17hr/mg protein). Subsequent polymerase chain reaction (PCR) amplification of the coding exons for GALNS produced an appropriate product for all exons with the exception of exon 9 and we hypothesized that a deletion encompassing exon 9 was present in our patients. Therefore, an allele specific PCR assay was designed to confirm the exon 9 deletion and determine the precise deletion breakpoints (c.899-397_1003-18632del) for our patients. The reference mRNA sequence was NM_000512.4 with codon 1 corresponding to the start ATG.
To our knowledge, the pathogenic deletion identified in our patients (c.899-397_1003-18632del) has not been reported in the literature. Various deletions encompassing at least one exon have been reported in GALNS, however none encompassing only exon 9. NGS is increasingly utilized as a first-tier test for lysosomal storage diseases via targeted gene panels or exome/genome sequencing. The inability of this testing to detect large deletions or rearrangements, especially in the heterozygous state, could lead to a delayed or even missed diagnosis. For example, if the deletion described in this case was in cis with missense change in GALNS, NGS would label the patient as a heterozygote rather than affected. Our case highlights the benefit of biochemical analysis for lysosomal storage disorders in cases with equivocal or incomplete molecular analysis. Molecular analysis is useful to confirm a biochemical diagnosis and assist with genetic counseling and future prenatal testing and carrier testing. Recognizing limitations of molecular testing is important to ensure accurate diagnosis and treatment for individuals with Morquio syndrome in a timely manner.