Identification of a Novel Pathomechanism Underlying a Congenital Disorder of Glycosylation: Crosstalk between Hexosamine Biosynthetic Pathway, PI3K/AKT Signaling, and Protein Glycosylation

Abstract

Phosphoglucomutase 1 (PGM1) deficiency is a recently identified subtype of congenital disorder of glycosylation that can be treated by dietary modification. PGM1 is required for the interconversion of glucose 1‐phosphate and glucose 6‐phosphate, and is a key enzyme in glycogen metabolism. Although the exact mechanism remains unclear, PGM1 deficiency has been associated with abnormal intracellular glucose and galactose metabolites, as well as reduced N‐linked glycosylation in the endoplasmic reticulum and Golgi. These metabolic disturbances are thought to be responsible for both missing and truncated glycans that have been detected in patient serum transferrin. A recent landmark study demonstrated that oral D‐galactose supplementation improved cellular glycosylation in six patients and resolved hypogonadotropic hypogonadism in two patients. While the clinical benefits of galactose supplementation in PGM1 deficiency are clear, its biological effect on the biochemical and cellular level has not been characterized in detail. It was assumed that dietary galactose supplementation improves galactosylation by increasing intracellular UDP‐galactose relative to UDP‐glucose; however, this has not been confirmed experimentally, nor has an alternative mechanism of action been explored. Here, we report no difference in the ratio of intracellular UDP‐glucose to UDP‐galactose between patient and control cells, despite significant improvement in N‐linked and O‐linked glycosylation in patient cells following galactose supplementation. We found no evidence to support galactose acting as a pharmacological chaperone for PGM1 missense variants. Surprisingly, we discovered disruptions in the hexosamine biosynthetic pathway (HBP) at the metabolic and transcriptional level, as well as downregulation of PI3K/AKT signaling in PGM1 deficient patient cells. Interestingly, galactose supplementation appears to rescue HBP and PI3K/AKT signaling. Collectively, our preliminary studies suggest a hitherto unexplored crosstalk between HBP, PI3K/AKT signaling, and protein glycosylation.Support or Funding InformationThis study is supported in part by the Hayward Foundation and by 1 U54 GM104940 from the National Institute of General Medical Sciences of the National Institutes of Health, which funds the Louisiana Clinical and Translational Science Center.