Hematology and diabetes: From hemoglobin A1c to CD59 glycation
Authors: Anne Nicholson-Weller
Journal: American Journal of Hematology (2013)

CD59 is a 19 kDa glycoprotein complement regulatory protein, which is attached to the plasma membrane of cells by a glycosylphosphatidyinositol (GPI) anchor. While there are three membrane complement regulatory proteins that act to regulate C3 activation (DAF, CD46, and CD35), there is only one membrane complement regulatory protein to regulate the C5b,6,7,8,and 9 complex, which forms a nongated pore in susceptible membranes, that is, CD59. Gliding on its GPI slipper to the site of inserted C5b-8, CD59 binds to C8 and thereby prevents pore formation by blocking the addition of C9 to the complex. Under normal circumstances, the terminal complement complex (C5b-9) allows a transient Ca++ influx and thus acts as a “hormone” without requirement for a specific receptor. C5b-9 signaling is involved in many types of tissue remodeling (reviewed in 1). While some C5b-9 activity is good, too much is pathologic and fully deserving of the C5b-9 complex being designated the “membrane attack complex”. One clinical condition associated with excess complement activation is diabetes where studies in different tissues demonstrate the co-localization of C5b-9 with abnormal microvasculature. Recently, a large prospective study of type 2 diabetic patients found that high levels of soluble C5b-9 at the time of hospitalization for an acute myocardial infarction was strongly predictive that a patient would have another cardiovascular event in the next 3.5 years 2. In this issue of the American Journal of Hematology, Ghosh and coworkers describe how they have developed an ELISA assay that recognizes soluble glycated CD59 (GCD59) found in the serum/plasma of diabetic patients 3. GCD59 is nonfunctional and its presence in the plasma membrane would put cells at greater risk of damage from C5b-9. For their assay, the authors developed a sandwich ELISA using a mouse monoclonal to capture all CD59 (native CD59 and GCD59); a novel rabbit monoclonal directed against the glycated epitope to specifically recognize GCD59; and labeled goat antirabbit Ig for detection. Importantly, they also made a synthesized glycated peptide of CD59 that functioned in this ELISA as a standard. Making the reagents for this assay was not trivial. In the first test of the assay, they compared the circulating levels of free GCD59 in the blood of 14 diabetics defined by a HbA1c value >6.5%, and 10 controls defined by a HbA1c value <6.0% and lack of a relevant ICD-9 code for diabetes. There was a clear difference in the means with diabetics having about three fold more GCD59 than nondiabetics (Fig. 4A, Ref. 3). The correlation between the levels of GCD59 and HbA1c for single diabetic patients exists (r = 0.77), but the graph looks more complicated and likely more interesting (Fig. 4A, Ref. 3). There may be a better fit for two lines suggesting different sample preparation and/or types of host pathology. What is important about this article is that it defines the means to ask many more questions relevant to diabetes and the role of complement in inflammation, for example, would the ratio of soluble CD59:GCD59 be informative? does GCD59 participate like CD59 in an endocytic pathway? what normal or pathologic conditions influence the phospholipase activity that cleaves CD59 and GCD59 from cell membranes? might some of the “soluble” CD59 and GCD59 be in vesicles? what are the cells of origin for GCD59, and might RBCs contribute to the soluble CD59 and GCD59 in the circulation? If the latter is true, perhaps some of the changes noted in RBCs from diabetic patients are attributable to dysregulated C5b-9 activity? We will stay tuned.