Galactose Supplements Rescue Brain Lipid Defects in Rare Glycosylation Disorder

SLC35A2-CDG is an X-linked congenital disorder of glycosylation caused by loss-of-function variants in SLC35A2, the gene encoding the UDP-galactose (UDP-gal) transporter in the Golgi and endoplasmic reticulum. Affected individuals—mostly females with de novo germline mutations—typically present with developmental and epileptic encephalopathy, intellectual disability, hypotonia, and brain MRI abnormalities. Despite oral galactose supplementation showing clinical benefit in some patients, the precise cellular mechanism remained unclear.

Researchers used patient-derived skin fibroblasts from eight SLC35A2-CDG individuals, alongside CHO-Lec8 cells as an additional SLC35A2-deficient model, to profile central carbon metabolism, N- and O-linked protein glycosylation, and glycosphingolipid (GSL) synthesis using tracer metabolomics, mass spectrometry, and stable isotope labeling. Contrary to expectations based on the known role of UDP-gal in protein glycosylation, N- and O-glycosylation defects were only minimal in patient fibroblasts. In sharp contrast, lipid glycosylation was profoundly disturbed: glucosylceramide (GlcCer) accumulated markedly, while digalactosylated GSLs and complex gangliosides—including GM1, GD1a, GD1b, and GT1b—were significantly reduced or absent.

Galactose supplementation at a clinically relevant concentration (2 mM, matching serum levels in patients receiving 1 g/kg/day) increased intracellular UDP-galactose pools, enhanced its transport into the Golgi lumen, and directly rescued GSL synthesis. Isotope tracing with U-¹³C galactose confirmed that the supplemented galactose was incorporated directly into the restored GSL species. This effect was consistent across all eight patient fibroblast lines and in CHO-Lec8 cells, suggesting a universal mechanism rather than a variant-specific response. Combination with epalrestat (an aldose reductase inhibitor) or manganese chloride—agents that modulate glycosylation pathways—did not substantially improve outcomes beyond galactose alone.

Serum analysis from SLC35A2-CDG patients identified hydroxylated GSLs, particularly hydroxylated GM3, as potential disease biomarkers, offering a novel diagnostic avenue beyond the currently used transferrin isoelectrofocusing test, which is often normal or normalizes with age in these patients.

The finding that ganglioside synthesis is severely impaired carries significant neurological implications. Gangliosides are highly enriched in the brain, where they regulate synaptic transmission, neuronal survival, myelin formation, and axonal stability. Their deficiency aligns with the hypomyelination and oligodendroglial abnormalities seen in MOGHE—a cortical malformation associated with somatic SLC35A2 variants. The authors propose that beyond galactose supplementation, direct ganglioside supplementation could represent a new therapeutic strategy for this disorder.