Bi-allelic inactivating variants of lactosylceramide synthase B4GALT5 responsible for a novel congenital disorder of glycosylation involving glycosphingolipids

A 10-year-old girl of Italian ancestry presenting with developmental delay, intellectual disability, microcephalia, and bilateral cataract was found to carry compound heterozygous variants in the B4GALT5 gene encoding a galactosyltransferase producing lactosylceramide, the precursors of the bulk of glycosphingolipids. b4galt6 has been also reported as an alternative lactosylceramide synthase in studies performed using knock-out mouse models. The evidence of such syndrome suggests that B4GALT6 may not rescue for the lack of activity of B4GALT5 in humans. B4GALT5, B4GALT6 and b4galt6 cDNAs were cloned in an expression vector and by site directed mutagenesis each of the two patient’s variants were introduced. Each construct was then transfected into HEK-293T cells to measure galactosyltransferase activity in vitro via a novel procedure based on LC/MS. In contrast to the wild-type B4GALT5, both the variants resulted in an enzyme activity indistinguishable from the background, suggesting almost no residual activity. B4GALT6 activity was also similar to the background, while mouse b4galt6 showed a measurable activity. These data suggest that B4GALT5 can be rescued by mouse b4galt6 but not by human B4GALT6, in fact human brain functions are impaired by B4GALT5 deficiency probably due to the very low activity of B4GALT6, which we are trying to explain analyzing the structures via in silico techniques. Although further cases need to be identified to define the spectrum and the molecular basis of the clinical manifestations, our data suggest that we have identified a novel congenital disorder of glycosylation, B4GALT5-CDG.