Overlap of C3G and MPGN
There is an overlap between classification of disease using the terms MPGN and C3G (shown in figure). All patients with C3 dominant staining on IF (including those with an MPGN pattern of glomerular injury) should be classified as C3G. Electron microscopy findings then sub-classifies C3G into DDD and C3GN. Disorders of complement dysregulation are the hallmark of C3G and should be sought. CFHR5 nephropathy is a form of C3GN in which a specific genetic defect in CFHR5 has been described in some familial cases of C3GN.
Cases of MPGN that do not fulfil the criteria of C3G should be termed immune-complex MPGN. Disorders of complement dysregulation have been identified in cases of immune-complex MPGN and should be sought once secondary causes of MPGN have been excluded. This overlap of MPGN and C3G in which a disorder of complement dysregulation could be considered is summarised in the figure below.
Once secondary causes have been excluded, an abnormality of complement dysregulation should be considered in MPGN and C3G. In these overlapping conditions, C3 deposition is usually present. MPGN is shown in dashed circle and includes IC-MPGN (Ig and C3 deposition – Green) and C3G (Dominant C3 deposition). C3G is shown in large circle (pink) any can associate with an MPGN pattern of glomerular injury. Specific forms of C3G include C3GN, DDD and CFHR5 nephropathy.