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

A 7-year-old girl of Italian ancestry presen5ng with developmental delay, intellectual disability, microcephalia, and bilateral cataract was found to carry compound heterozygous variants in the B4GALT5 gene (deposited in Decipher: ID 412221) encoding a galactosyltransferase producing lactosylceramide (LacCer), the precursors of the bulk of glycosphingolipids. By LC/MS, an accumula5on of ceramide monohexoside and a relevant loss of LacCer were detected in pa5ent’s serum compared to her parents and age-matched healthy controls. The human B4GALT5 cDNA was cloned in an expression vector and by site directed mutagenesis each of the two pa5ent’s variants were introduced. The constructs were then transfected into HEK-293T cells to measure the enzyme ac5vity in vitro. In contrast to the wild-type construct, cells transfected with constructs carrying each of the variants resulted in an enzyme ac5vity indis5nguishable from the background, sugges5ng almost no residual ac5vity. B4GALT6 has been also reported as LacCer synthase in studies performed using knock-out (KO) mouse models. B4galt6 KO mice were healthy whereas b4galt5 KO mice died at early embryonic age. Condi5onal KO mice lacking b4galt5 only in the brain were also healthy. Double KO mice, condi5onal b4galt5 and null b4galt6, were severely compromised in the central nervous system and died by four weeks of age. Altogether, these data suggested that B4GALT5 is the major LacCer synthase that can be rescued by B4GALT6 at least in the mouse brain. Conversely, our findings indicate that human brain func5ons are impaired by B4GALT5 deficiency. Our working hypothesis is that B4GALT6 rescues B4GALT5 ac5vity, but not enough for rescuing specific human brain func5ons. Although further cases need to be iden5fied to confirm the nature of this new disorder and to define the spectrum of clinical manifesta5ons, our data suggest that we have iden5fied a novel congenital disorder of glycosyla5on, B4GALT5-CDG.