Post-translational modifications in the insoluble fraction of platelet-poor plasma distinguish pre-COVID-19 Postural Orthostatic Tachycardia Syndrome, Long COVID, and Long COVID-POTS: a preliminary study.

Abstract

Background: Pre-COVID-19 Postural Orthostatic Tachycardia Syndrome (PC-POTS), Long COVID with initial orthostatic hypotension (LC-IOH), and Long COVID-POTS (LC-POTS) are primarily chronic post-viral conditions with debilitating symptoms, despite routine laboratory findings often being normal. We previously identified amyloidogenic fibrinaloid microclot complexes (FMCs) in the insoluble fraction of platelet-poor plasma (PPP-IF) in Long COVID, but it remains unclear whether similar abnormalities occur in PC-POTS and LC-POTS, or whether post-translational modifications (PTMs) contribute to disease mechanisms.

Methods: Platelet-poor plasma from healthy controls (n = 18), PC-POTS (n = 20), LC-IOH (n = 19), and LC-POTS (n = 18) participants underwent proteomic analysis of the PPP-IF using double trypsin digestion and data-independent LC-MS/MS. Differential protein abundance, PTMs, and amyloidogenicity were compared across groups.

Results: Although few proteins were significantly dysregulated, extensive disease-specific PTM alterations were identified across coagulation, immune, and metabolic pathways. LC-IOH demonstrated prominent AGE- and oxidation-related PTMs on fibrinogen subunits, particularly fibrinogen alpha chain (FIBA), resembling diabetic glycation profiles. PC-POTS showed fewer fibrinogen PTMs but marked metabolic and immune dysregulation, including oxidised apoA1 and apoB and complement-associated proteins (C3, C4A/B, IC1), IGG1, and A2MG. LC-POTS shared coagulation-related features with LC-IOH and immune-related features with PC-POTS. Many dysregulated peptides were highly amyloidogenic in silico, consistent with β-sheet-rich FMCs. PTM dysregulation was substantially greater than protein-level changes.

Conclusions: This study provides the first evidence that PTMs within the PPP-IF has the potential to distinctly differentiate PC-POTS, LC-IOH, and LC-POTS. Because PC-POTS samples predated SARS-CoV-2, their PTM signatures reflect intrinsic disease biology independent of COVID-19. PTM profiling revealed pro-coagulant fibrinogen changes in LC-IOH and LC-POTS, metabolic-oxidative dysregulation in LC-IOH and PC-POTS, and immune dysregulation in PC-POTS and LC-POTS. These abnormalities were not detectable by routine assays and may contribute to microvascular dysfunction. The findings support disease-specific biomarker development and targeted therapeutic strategies across autonomic and post-viral disorders.