Morquio-like dysostosis multiplex presenting with neuronopathic features is a distinct GLB1-related phenotype

Morquio B disease: a case report

Mucopolysaccharidosis IV type B, or Morquio B disease (MBD), is an autosomal recessive disorder caused by a genetic mutation in GLB1 gene encoding for β-galactosidase on chromosome 3p22.33. β-galactosidase deficiency can result in two different conditions, GM1 gangliosidosis and MBD, of which MBD has a milder phenotype and presents later in life with keratan sulfate accumulation in the retina and cartilage. In this case report, we present a patient diagnosed with MBD at the age of 5 after initially presenting with Morquio dysostosis multiplex and characteristic radiographic findings. Genetic testing confirmed that the patient has β-galactosidase deficiency due to mutation W273l/N484K on GLB1 gene. The patient exhibited elevated mucopolysaccharide levels in urine at 18 mg/mmol and demonstrated an abnormal band pattern of urine oligosaccharides on electrophoresis. The activity of β-galactosidase in his white blood cells was reduced to 12.3 nmol/h/mg protein. At the time of diagnosis, the patient did not present with gait and ambulation issues, but his ability to walk progressively deteriorated in his adolescence as a result of instability and pain in the ankle, knee, and hip joints, accompanied by a global decrease in muscle strength. This case report is the first in the literature to provide an in-depth exploration of the orthopedic treatment and follow-up received by a young adolescent with MBD to provide symptom relief and improve walking ability.

Processed pseudogene insertion in GLB1 causes Morquio B disease by altering intronic splicing regulatory landscape

Morquio B disease (MBD) is an ultra-rare lysosomal storage disease, which represents the relatively mild form of GLB1-associated disorders. In this article, we present the unique case of “pure” MBD associated with an insertion of the mobile genetic element from the class of retrotransposons. Using whole-genome sequencing (WGS), we identified an integration of the processed pseudogene NPM1 deep in the intron 5 of GLB1. The patient’s mRNA analysis and the detailed functional analysis revealed the underlying molecular genetic mechanism of pathogenesis, which is an alteration of the GLB1 normal splicing. By co-expression of minigenes and antisense splice-modulating oligonucleotides (ASMOs), we demonstrated that pseudogene-derived splicing regulatory motifs contributed to an activation of the cryptic exon located 36 bp upstream of the integration site. Blocking the cryptic exon with ASMOs incorporated in the modified U7 small nuclear RNA (modU7snRNA) almost completely restored the wild-type splicing in the model cell line, that could be further extended toward the personalized genetic therapy. To our knowledge, this is the second reported case of the processed pseudogene insertion for monogenic disorders. Our data emphasizes the unique role of WGS in identification of such rare and probably underrepresented in literature types of disease-associated genetic variants.