The GALNS gene codes for an enzyme called N-acetylgalactosamine 6-sulfatase that is located within lysosomes. This enzyme is involved in the degradation of the glycosaminoglycans (GAGs) or mucopolysaccharides, keratan sulfate, and chondroitin 6-sulfate.
Defects in this enzyme are the cause of Morquio A syndrome, a lysosomal storage disorder characterized by growth retardation; a prominent lower face, kyphoscoliosis, an abnormally short neck, knock knees, flat feet, epiphyses, pectus carinatum. Hearing loss, weakness of the legs, abnormal heart development, abnormal skeletal development and/or additional abnormalities may also occur.
The GALNS gene has 14 coding exons spanning approximately 43 kb of genomic DNA on the long arm of chromosome 16. The encoded enzyme comprises 522 amino acids with a molecular mass of 58 kDa. About 148 different mutations have been found in patients with Morquio A syndrome, and approximately 70% of these are missense mutations.
Qubbaj et al. (2008) performed preimplantation genetic diagnosis (PGD) for a couple with three children affected with Morquio disease, having homozygous W159C mutation in GALNS gene. The couple were first cousins, and had a 14-year-old affected daughter and affected twin sisters. Both twins presented with dysmorphic features, spondyloepiphyseal abnormalities, short stature, and cardiac valvular involvement. One of the twins died at the age of 4-years due to a chest infection. The couple decided to go in for IVF and PGD for a subsequent pregnancy. Performing ovarian stimulation, oocyte retrieval, intracytoplasmic sperm injection procedure, embryo biopsy, cell lysis, and multiple displacement amplification, three embryos out of seven were genotypically normal and two were transferred back to the mother on day 4. Pregnancy ensued and a carrier healthy male infant was delivered. Genetic haplotyping analysis of the infant and the leftover MDA samples showed that embryo 3 was the one that was implanted. The misdiagnosis was explained by allele dropout (ADO) of the mutant allele in embryo 3.
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