B4GALT5 deficiency impairs glycosphingolipid biosynthesis: a new congenital disorder of glycosylation?

Lactosylceramide is a glycosphingolipid precursor synthesized by two dedicated galactosyltransferases, B4GALT5 and B4GALT6. The specific roles of B4GALT5 and B4GALT6 in humans have not yet been clearly defined. Here, we report the first human case with bi-allelic loss-of-function variants in B4GALT5, suggesting that intact B4GALT5 activity is indispensable for normal glycosphingolipid biosynthesis and human development. We identified bi-allelic variants in the B4GALT5 gene in a child presenting with microcephaly, mild cognitive impairment, and bilateral cataracts. B4GALT5/6 double KO cells transfected with B4GALT5 carrying either of the variants identified in the patient lacked lactosylceramide synthase activity and failed to produce glycosphingolipids. In silico analyses predicted decreased protein stability and impaired UDP-Gal binding for both B4GALT5 variants. Together, these findings indicate that both variants result in deficient B4GALT5 activity, leaving B4GALT6 as the sole source of lactosylceramide synthase activity. Consistent with this, patient plasma and fibroblasts exhibited an approximately 80% reduction in glycosphingolipid levels compared with healthy controls. Unexpectedly, when expressed in model cells human B4GALT6 displayed lower expression and lower catalytic activity, than human B4GALT5, raising questions about its capacity to compensate for B4GALT5 deficiency. In conclusion, we identified a potential new congenital disorder of glycosylation caused by deficient lactosylceramide synthase activity that may be insufficient to support glycosphingolipids synthesis at levels required for normal brain function.