Undercarboxylated Matrix Gla Protein (ucMGP): a novel biomarker to predict coronary artery calcification
Tusar Giri, M.D., Ph.D.
Brian F. Gage, M.D., M.Sc.
Coronary heart disease and stroke are the leading causes of morbidity and mortality in the United States. Every year, more than 1.2 million Americans suffer from these cardiovascular diseases. Coronary artery calcification (CAC) is an independent risk factor for cardiovascular disease.
Because of its association with CAC, ucMGP is a promising biomarker. ucMGP is the undercarboxylated Matrix Gla Protein. Carboxylated Matrix Gla Protein (MGP) inhibits calcification, and tissue ucMGP is associated with cardiovascular disease. No plasma assay for ucMGP exists. Our one-year goal, therefore, is to develop a ucMGP plasma assay that will correlate with CAC.
Human and animal experiments suggest that vitamin K plays a crucial role in regulating MGP and therefore regulating calcification. The enzyme that carboxylates ucMGP requires vitamin K. Thus, higher levels of ucMGP may reflect inadequate availability of vitamin K. Vitamin K modifies the “Gla” in Matrix Gla Protein which stands for g-carboxyglutamic acid. This modification is identical to that affecting other proteins such as ucOC (uncarboxylated osteocalcin) and clotting factors. For example, higher ucOC levels in post-menopausal women also reflect inadequate availability of vitamin K.
Patients develop atherosclerosis, deficiency of clotting factors, and calcification of skin elastin when g-carboxylation is defective because of a defective enzyme (g-glutamyl carboxylase). Likewise, long-term use of the vitamin K antagonist Warfarin correlates with calcification on heart valves and may interfere with calcification on bones, predisposing to fractures. Furthermore, rats that take Warfarin plus vitamin D develop arterial calcification and local deposition of ucMGP, a phenotype that resembles MGP-deficient rodents.
The ucMGP biomarker may identify patients who would benefit from vitamin K supplementation. Indeed, in the rat arterial calcification model, Schurgers et al ascertained that high vitamin K intake may stop progression or lead to complete reversal of Warfarin-induced arterial calcification and arterial stiffness.
The only way to determine the circulating levels of ucMGP is to measure plasma levels of ucMGP. Unavailability of a serum-based assay for the measurement of ucMGP is thus a hindrance to research in this field, which we propose to amend. The EIA kit does not discriminate between ucMGP and MGP. Therefore, to develop the ucMGP assay we will remove the MGP from the serum using BaSO4 adsorption, and measure ucMGP directly. We will measure the ucMGP levels in CAC cases, and healthy controls involve case control study, to validate the assay.
Our long-term goals are to develop and validate this assay as a predictor of cardiovascular disease and to test the hypothesis that vitamin K supplementation will retard the progression of cardiovascular diseases in patients with an elevated level of ucMGP.