Membranous nephropathy: a single disease or a pattern of injury resulting from different diseases.

Membranous nephropathy (MN) is defined as disease entity characterized by thickening of the glomerular basement membranes due to subepithelial (SE) deposition of immune complexes. It is typically classified into primary MN (70%) when there is no disease association, and secondary MN (30%) when there is an underlying disease association such as lupus, malignancy, infections or drugs. Phospholipase A2 receptor (PLA2R) and thrombospondin type-1 domain-containing 7A (THSD7A) are target antigens in 70% and 1-5% of primary MN, respectively. The antigens in the remaining MN were not known. Recently, multiple novel proteins/target antigens have been identified in MN. These include exostosin 1/2, neural epidermal growth-like 1 protein, semaphorin 3B, protocadherin 7 and neural cell adhesion molecule 1. Some of these antigens are present in the setting of primary MN, some in secondary MN and some in both, thus blurring the lines between primary and secondary MN. Preliminary studies show that each of the new antigen-associated MN has distinct clinical, kidney biopsy findings and outcome data. We propose that each new protein/antigen-associated MN is a specific disease that results in the common MN pattern of injury characterized by thickened glomerular basement membrane (GBM) with or without spikes or pinholes on light microscopy, granular immunoglobulin G with or without complement 3 on immunofluorescence microscopy and SE electron-dense deposits on electron microscopy. In other words, MN is truly only a pattern of injury resulting from specific diseases that cause deposition of SE immune deposits along the GBM. It is of paramount importance to ascertain the specific disease entity causing the MN pattern not only for precise diagnosis and management, but also for future studies on these newly described diseases.