C-terminally clustered UGDH hypomorphic variants reveal subtle mechanisms of cellular and developmental disruption

• Jamuar Syndrome is an ultrarare disorder caused by variants in the essential gene UGDH • UGDH variants may not alter protein expression but profoundly impact development • Disrupted allosteric control of UGDH in C-terminal variants blunts glycan production • Three novel UGDH variants of undetermined significance were validated in the study Jamuar syndrome (Developmental and Epileptic Encephalopathy 84, OMIM# 618792 ) is a rare autosomal recessive congenital disorder of glycosylation (CDG), caused by variations in the gene encoding UDP-glucose dehydrogenase (UGDH). Although a number of UGDH variants have been functionally characterized, there is an incomplete catalogue of variants and their impacts on development. Here, we present functional data characterizing new missense variants from three unrelated individuals who were D379N homozygous, Y356D homozygous, and compound heterozygous A436G/R442W, respectively. UGDH activity was low to undetectable in patient-derived fibroblasts bearing either UGDH D379N or UGDH A436G/R442W, relative to WT fibroblasts, despite robust UGDH expression in both. Measurement of nucleotide sugar levels revealed a significant decrease in the UGDH product, UDP-glucuronate, and consequent reductions in hyaluronan production, Notch1 levels, and rate of O-and N-linked glycan synthesis, consistent with loss of UGDH activity. These features support the designation of UGDH D379N and UGDH A436G as causative variants in Jamuar Syndrome. We expressed and purified UGDH D379N, A436G, R442W, R443H, and Y356D variants to examine underlying molecular mechanisms. Kinetic properties and structural stability assays selectively revealed significant changes in conformational dynamics that manifested strong effects on endogenous inhibitor binding and product inhibition. The results suggest that alterations to the C-terminal domain impact activity of UGDH in cells by impairing its cofactor exchange rate and diminishing quaternary association. These effects would be maximized at developmental milestones in which hypoxia drives morphological change, since NADH accumulation would then decrease glycosaminoglycan production, with profound developmental consequences.