I-cell disease (also called mucolipidosis type II) is an inherited metabolic disorder that results from mutations in the GNPTAB gene responsible for providing instructions for the formation of the alpha and beta subunits of GlcNAc-1-phosphotransferase enzyme (3). Two alpha and two beta subunits are produced from the GNPTAB gene, whereas and two gamma subunits are produced from the GNPTG gene. GlcNAc-1-phosphotransferase allows for mannose 6-phosphate receptor-dependent transport to lysosomes. Mutations in the GNPTAB gene lead to the absence of the Man 6-P signal for lysosomal localization of particular hydrolases enzymes (1). Most common symptoms of I-cell disease are skeletal malfunction (kyphosis, scoliosis, fused together fingers), abnormal skull and face (long and narrow head, coarse facial features, depressed nasal bridge), growth failure, mental retardation.
I-cell disease becomes apparent either at birth or by 6-10 months.Other examples of metabolic defects in the mannose 6-phosphate-mediated uptake system resulting in the same phenotype are mucolipidosis type II (pseudo-Hurler Polydystrophy), Pompe disease (PD) and Niemann-Pick disease (NP).Just like I-cell disease, pseudo-Hurler Polydystrophy results from mutations in the GNPTAB gene. The symptoms of pseudo-Hurler polydystrophy are not as severe as those of I-cell disease, besides manifestation of the disease usually takes place later in life and the prognosis is much better for patients with pseudo-Hurler disease because they can survive into adulthood. Most distinct symptoms of this disease are stiffness of the hands and shoulders, carpal tunnel syndrome, scoliosis, and deterioration of hip joints.
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The difference between the two disorders is that pseudo-Hurler syndrome doesn’t show mucopolysacchariduria like I-cell disease does (2).Diagnosis of I-cell disease and pseudo-Hurler disease can be made through estimating of the levels of lysosomal enzyme in plasma. Other methods include checking phosphotransferase activity, the ratio of extracellular to intracellular enzyme activities, finally sequencing of the GNPTAB and GNPTG coding regions allow to detect mutations in the majority of cases
