In the News: Dense Deposit Disease and ApoE- the new connection

C3 glomerulopathy arises from irregularities in the alternative pathway of complement. It manifests as two types: C3 glomerulonephritis (C3GN) and dense deposit disease (DDD), identifiable by bright C3 staining in the glomeruli under immunofluorescence. EM distinguishes DDD by dense deposits along the glomerular basement membranes, contrasting with non-dense deposits in C3GN. A fascinating new study investigating 12 cases each of DDD, C3GN, and pretransplant kidney controls, laser microdissection (LCM) followed by mass spectrometry (MS) revealed a significant accumulation of complement proteins and regulatory factors in both C3GN and DDD compared to controls. Notably, DDD exhibited a much higher concentration of C5-9 and apolipoprotein E (ApoE) compared to C3GN. 

Image courtesy: pathologyoutlines.com 

ApoE staining aligned with dense deposit patterns in DDD but not in C3GN or controls, validated in 31 C3G cases. This is fascinating as perhaps ApoE staining may serve as a diagnostic tool for DDD, particularly when EM is unavailable, as it reflects the enriched presence of ApoE in dense deposits, distinguishing DDD from C3GN.

ApoE is a 34 kDa lipoprotein, that facilitates lipid transportation by binding to lipids. As we are aware, in various diseases like atherosclerosis, Alzheimer’s, and amyloidosis, ApoE plays pivotal roles in plaque formation and fibril assembly. Additionally, I learnt that it is also detected in fibrillary glomerulonephritis, immunotactoid glomerulonephritis, and monoclonal Ig deposition disease, potentially acting as a scaffolding protein. While its accumulation is significant in DDD, it's also found in other diseases but in lesser amounts. Staining for ApoE aids in diagnosing DDD alongside proliferative glomerulonephritis and bright C3 staining. Kidney accumulation of ApoE occurs predominantly in lipoprotein glomerulopathy and ApoE-related glomerulopathies. ApoE interacts with complement factors, inhibiting inflammation and regulating complement pathways. Moreover, it's a component of age-related macular degeneration but hasn't been previously identified in DDD dense deposits. This study suggests ApoE binds to heparan sulfate in the glomerular basement membrane, potentially acting as a chaperone for C5-9 proteins, contributing to dense deposit formation. Further investigation is warranted to confirm ApoE's interaction with C5-9 proteins and potential treatment strategies as a result. Kudos to the authors for making this connection.