Bis(monoacylglycero)phosphate (BMP) as a circulating biomarker of lysosomal dysfunction in GNE myopathy.

GNE myopathy (GNEM) is a rare neuromuscular disorder caused by pathogenic variants in the GNE gene and traditionally associated with impaired sialic acid biosynthesis. In a previous untargeted lipidomic study, we identified a lipid feature significantly enriched in the serum of patients with GNE myopathy compared with healthy controls. In the present work, we aimed to determine the structural identity of this disease-associated feature and to establish a robust analytical strategy for its quantitative assessment. Analysis of the mass spectrometric data pointed to bis(monoacylglycero)phosphate 18:1_18:1 or BMP (18:1_18:1) as a plausible candidate, based on the consistency of the nominal mass and fragmentation pattern with this lipid class. In the present work, we therefore aimed to determine the structural identity of this disease-associated feature and to establish a robust analytical strategy for quantitative assessment. We therefore performed the chemical synthesis of BMP (18:1_18:1) and, based on its MS/MS fragmentation pattern, developed a selective tandem mass spectrometry method for its quantification. Levels of BMP (18:1_18:1) were measured in serum samples from patients with GNE myopathy, healthy controls, and patients with myotonic dystrophy type 2 (DM2) as a disease comparison cohort. BMP levels were significantly increased in the serum of patients with GNE myopathy compared with both healthy controls and DM2 patients. Together, these findings define a previously unrecognized circulating lipid alteration in GNE myopathy and provide new insight into disease-associated lipid dysregulation.