A complement C4-derived glycopeptide as a novel biomarker for PMM2-CDG

The most common congenital disorder of glycosylation (CDG) is caused by pathogenic variants in PMM2 (PMM2-CDG) that reduce phosphomannomutase activity and impair N-glycan synthesis. Patients present early in life with multi-system disabilities. Traditional diagnosis relies on measuring carbohydrate-deficient transferrin (CDT) and confirmation by genetic testing. However, tests for CDT can be normal in some patients, or normalize with age. Because patients also show subtle alterations in serum glycan structures, we considered that site-specific glycosylation changes might be diagnostic. We analyzed serum from 35 individuals with PMM2-CDG for discovery and validation of glycoproteomic changes. We carried out mass spectrometry-based discovery studies to profile the glycoproteome in an initial set of well-characterized affected individuals and matched controls. Next, we developed a streamlined method for analyzing additional affected individuals. Finally, targeted mass spectrometry was used to validate selected N-glycopeptides in an independent set of samples in a blinded fashion. Of the 3,342 N-glycopeptides derived from 284 glycoproteins identified in discovery experiments, individuals with PMM2-CDG exhibited a general decrease in complex-type N-glycans and an increase in truncated, mannose-rich and hybrid species. We identified a glycopeptide from complement C4 carrying the glycan Man5GlcNAc2, which was not detected in controls. Notably, this glycopeptide was also detected in five affected individuals with normal CDT, including one individual after liver transplant, two individuals with a known genetic variant associated with mild disease and two individuals with well characterized variants, indicating that this glycopeptide is more sensitive than CDT. Finally, this marker was also detected by targeted analysis in two individuals with variants of uncertain significance (VUS) in PMM2 gene. Overall, we believe that C4-derived Man5GlcNAc2 glycopeptide could serve as a novel next-generation biomarker for more accurate diagnosis and tracking progress of patients in therapeutic trials.