Abstract
ST3GAL5-CDG is a rare syndrome which is caused by variant GM3 synthases, the enzyme involved in the biosynthesis of a-b-c-series gangliosides. Here we report a novel homozygous ST3GAL5 variant, p.Gly342Ser, in a patient suffering from failure to thrive, severe hearing, visual, motor, and cognitive impairment, and respiratory chain dysfunction. A GM3 synthase assay towards the natural acceptor substrate lactosylceramide was performed upon transfection in HEK-293T cells of expression plasmids carrying wild type and mutated ST3GAL5 cDNAs. The assay revealed a complete loss of enzyme activity. Identical results were obtained with the other four ST3GAL5 variants which have been reported to be pathogenic. HEK-293T clones permanently expressing HaloTag-ST3GAL5 carrying each of the five variants were assessed by quantitative PCR, flow cytometry, western blotting and confocal microscopy. The results indicated that transcription, translation, stability and intracellular localization of the tagged protein were identical to those of the wild type construct. Compared with the very mild phenotype of st3gal5 KO mouse models, the results suggest that unknown mechanisms, in addition to the lack of a-b-c-series gangliosides, contribute to the syndrome. Direct enzyme assay upon transfection in model cells appears to be an effective tool for characterizing variants of glycosyltransferases involved in glycosphingolipid biosynthesis.
Highlights
In all the reported cases, the consequences of the variant were characterized by mass spectrometric analysis of gangliosides from blood or fibroblasts samples from the affected children
Deafness was recognized at an older age. This symptom is related to the lack of GM3 and other ganglio-series gangliosides which have a role in the epithelium of the organ of Corti (Yoshikawa et al 2015, Inokuchi et al 2017)
A mild elevation of blood lactate and signs of respiratory chain dysfunction are emerging as distinctive symptoms, since these have been found in all tested patients
Summary
In all the reported cases, the consequences of the variant were characterized by mass spectrometric analysis of gangliosides from blood or fibroblasts samples from the affected children. No information is currently available on the enzyme activity in vitro, nor on the stability and subcellular localization of these ST3GAL5 variants. Of the two st3gal knock-out mouse models generated in the past, the earlier study (Yamashita et al 2003) revealed a phenotype that was generally normal except for increased insulin sensitivity. In the latter study (Yoshikawa et al 2009), the mice displayed hearing loss which resulted from cell degeneration in the organ of Corti in the cochlea
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