A complement C4-derived glycopeptide is a biomarker for PMM2-CDG

Area of Science:

• Biochemistry and Molecular Biology
• Clinical Diagnostics
• Glycomics

Background:

• Diagnosis of PMM2-congenital disorder of glycosylation (PMM2-CDG), the most common CDG, typically relies on carbohydrate-deficient transferrin (CDT) and genetic testing.
• Current diagnostic methods like CDT testing have limitations, including false negatives and normalization with age, necessitating more sensitive biomarkers.
• Site-specific N-glycosylation alterations in sera of PMM2-CDG patients have not been previously reported.

Purpose of the Study:

• To discover and validate novel glycopeptide alterations in sera of PMM2-CDG patients using advanced mass spectrometry.
• To identify sensitive and specific biomarkers for PMM2-CDG diagnosis, potentially overcoming limitations of existing methods.
• To explore the utility of identified biomarkers for therapeutic monitoring in PMM2-CDG and other CDGs.

Main Methods:

• Multistep mass spectrometry-based N-glycoproteomics was employed to analyze sera from 72 individuals (patients and controls).
• Comprehensive tandem mass tag (TMT)-based discovery experiments were performed, followed by method development for rapid profiling.
• Targeted mass spectrometry was utilized for blinded validation in an independent sample set.

Main Results:

• Analysis of 3,342 N-glycopeptides revealed decreased complex-type N-glycans and increased truncated, mannose-rich, and hybrid species in PMM2-CDG patients.
• A novel glycopeptide from complement C4 (C4) carrying Man5GlcNAc2 was identified in 5 patients with normal CDT results, including those with mild disease or post-transplant.
• This C4-derived Man5GlcNAc2 glycopeptide demonstrated greater sensitivity than CDT and was detected in individuals with variants of uncertain significance in PMM2.

Conclusions:

• The complement C4-derived Man5GlcNAc2 glycopeptide is a promising biomarker for accurate diagnosis of PMM2-CDG.
• This glycopeptide biomarker exhibits higher sensitivity compared to CDT testing, particularly in cases with normal CDT results or mild disease.
• The identified biomarker holds potential for accurate diagnosis and therapeutic monitoring in patients with PMM2-CDG and potentially other congenital disorders of glycosylation.