Membranoproliferative glomerulonephritis: no longer the same disease and may need very different treatment

Membranoproliferative glomerulonephritis (MPGN) is a pattern of glomerular injury that may be primary or secondary to infections, autoimmune diseases and haematological disorders. Primary C3G and IC-MPGN are rare and the prognosis is unfavourable. Based on immunofluorescence findings, MPGN has been classified into complement-mediated C3 glomerulopathy (C3G), and immune complex-mediated MPGN (IC-MPGN). However, this classification leaves a number of issues unresolved. The finding of genetic and acquired complement abnormalities in both C3G and IC-MPGN indicates that they represent a heterogeneous spectrum rather than distinct diseases. An unsupervised hierarchical clustering in a cohort of patients with primary C3G and IC-MPGN identified 4 distinct pathogenetic patterns, characterised by specific histologic and clinical features and genetic and acquired complement abnormalities. These results provide the groundwork for a more accurate diagnosis and the development of targeted therapies. The drugs that are currently used, such as corticosteroids and immunosuppressants, are frequently ineffective in primary C3G and IC-MPGN. Eculizumab, an anti-C5 monoclonal antibody, has been used occasionally in single cases or small series. However, only a few patients have achieved remission. This heterogeneous response could be related to the extent of terminal complement activation, which may vary substantially from patient to patient. Several drugs that target the complement system at different levels are under investigation for C3G and IC-MPGN. However, clinical trials to test new therapeutics will be challenging and heavily influenced by the heterogeneity of these diseases. This creates the need to characterise each patient to match the specific complement abnormality with the type of intervention.

Mycoplasma pneumoniae Infection Associated C3 Glomerulopathy Presenting as Severe Crescentic Glomerulonephritis

C3 glomerulopathy (C3GP) is a group of diseases caused by a deregulated complement system, which encompasses both dense deposit disease and C3 glomerulonephritis. Renal manifestations of C3GP are primarily of proliferative glomerulonephritis, and only a few case reports of crescentic glomerulonephritis (CGN) in association with C3GP are available. Here is a case of an adult South-Asian female, who was diagnosed as seropositive acute Mycoplasma pneumoniae infection, with associated systemic manifestations, including immune-type extravascular haemolysis and nephrotic range proteinuria. Subsequent renal biopsy revealed CGN with disrupted Bowman’s capsules and necrotizing lesions. Immunofluorescence showed coarse granular mesangial C3 deposits with negative IgM, IgG, IgA, and C1q. The immunomorphological phenotype raised two possibilities including C3GP and infection-related glomerulonephritis (IRGN). Persistent proteinuria with no evidence of resolution even after 6 months of follow-up favoured C3GP over IRGN. The patient proceeded to end-stage renal failure requiring renal replacement despite aggressive immunosuppression. This case illustrates the rare association of CGN with C3GP induced by Mycoplasma pneumoniae infection, highlighting the importance of correct diagnosis as well as timely identification of triggering factors in CGN on patient outcome.

C3 Glomerulopathy and Related Disorders in Children

Background and objectives: Membranoproliferative Glomerulonephritis (MPGN) and C3 Glomerulopathy are rare and overlapping disorders associated with dysregulation of the alternative complement pathway. Specific aetiological data for paediatric MPGN/C3 glomerulopathy are lacking, and outcome data are based upon retrospective studies without aetiological data. Design, setting, participants, and measurements: Eighty prevalent pediatric patients with MPGN/C3 glomerulopathy underwent detailed phenotyping and long-term follow-up within the National Registry of Rare Kidney Diseases (RaDaR). Risk factors for kidney survival were determined using COX proportional hazards model. Kidney and transplant graft survival was determined using Kaplan-Meier method. Results: Central histology review determined 39 C3 glomerulopathy, 31 immune-complex MPGN and 10 immune-complex glomerulonephritis (GN) cases. Patients were aged 2-15 (median 9 (IQR 7-11) years. Median complement C3 and C4 levels were 0.31g/L and 0.14g/L respectively; acquired (anti-complement autoantibodies) or genetic alternative pathway abnormalities were detected in 46% and 9% patients respectively, across all groups including immune-complex GN. Median follow-up was 5.18 (IQR 2.13-8.08) years. Eleven patients (14%) progressed to kidney failure with 9 transplants performed in 8 patients, 2 of which failed due to recurrent disease. Presence of >50% crescents on initial biopsy was the sole variable associated with kidney failure in multivariable analysis (Hazard Ratio 6.2, p = 0.045; 95% CI 1.05 to 36.6). Three distinct C3 glomerulopathy prognostic groups were identified according to presenting eGFR and >50% crescents on initial biopsy. Conclusions: Crescentic disease was a key risk factor associated with kidney failure in a national cohort of pediatric MPGN/C3 glomerulopathy and immune-complex GN. Presenting eGFR and crescentic disease help define prognostic groups in pediatric C3 glomerulopathy. Acquired abnormalities of the alternative pathway were commonly identified but not a risk factor for kidney failure.

FHR-5 Serum Levels and CFHR5 Genetic Variations in Patients With Immune Complex-Mediated Membranoproliferative Glomerulonephritis and C3-Glomerulopathy

BackgroundFactor H-related protein 5 (FHR-5) is a member of the complement Factor H protein family. Due to the homology to Factor H, the main complement regulator of the alternative pathway, it may also be implicated in the pathomechanism of kidney diseases where Factor H and alternative pathway dysregulation play a role. Here, we report the first observational study on CFHR5 variations along with serum FHR-5 levels in immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) and C3 glomerulopathy (C3G) patients together with the clinical, genetic, complement, and follow-up data.MethodsA total of 120 patients with a histologically proven diagnosis of IC-MPGN/C3G were enrolled in the study. FHR-5 serum levels were measured in ELISA, the CFHR5 gene was analyzed by Sanger sequencing, and selected variants were studied as recombinant proteins in ELISA and surface plasmon resonance (SPR).ResultsEight exonic CFHR5 variations in 14 patients (12.6%) were observed. Serum FHR-5 levels were lower in patients compared to controls. Low serum FHR-5 concentration at presentation associated with better renal survival during the follow-up period; furthermore, it showed clear association with signs of complement overactivation and clinically meaningful clusters.ConclusionsOur observations raise the possibility that the FHR-5 protein plays a fine-tuning role in the pathogenesis of IC-MPGN/C3G.

Long-term follow-up including extensive complement analysis of a pediatric C3 glomerulopathy cohort

Background C3 glomerulopathy (C3G) is a rare kidney disorder characterized by predominant glomerular depositions of complement C3. C3G can be subdivided into dense deposit disease (DDD) and C3 glomerulonephritis (C3GN). This study describes the long-term follow-up with extensive complement analysis of 29 Dutch children with C3G. Methods Twenty-nine C3G patients (19 DDD, 10 C3GN) diagnosed between 1992 and 2014 were included. Clinical and laboratory findings were collected at presentation and during follow-up. Specialized assays were used to detect rare variants in complement genes and measure complement-directed autoantibodies and biomarkers in blood. Results DDD patients presented with lower estimated glomerular filtration rate (eGFR). C3 nephritic factors (C3NeFs) were detected in 20 patients and remained detectable over time despite immunosuppressive treatment. At presentation, low serum C3 levels were detected in 84% of all patients. During follow-up, in about 50% of patients, all of them C3NeF-positive, C3 levels remained low. Linear mixed model analysis showed that C3GN patients had higher soluble C5b-9 (sC5b-9) and lower properdin levels compared to DDD patients. With a median follow-up of 52 months, an overall benign outcome was observed with only six patients with eGFR below 90 ml/min/1.73 m2 at last follow-up. Conclusions We extensively described clinical and laboratory findings including complement features of an exclusively pediatric C3G cohort. Outcome was relatively benign, persistent low C3 correlated with C3NeF presence, and C3GN was associated with higher sC5b-9 and lower properdin levels. Prospective studies are needed to further elucidate the pathogenic mechanisms underlying C3G and guide personalized medicine with complement therapeutics. Graphical abstract

Different Aspects of Classical Pathway Overactivation in Patients With C3 Glomerulopathy and Immune Complex-Mediated Membranoproliferative Glomerulonephritis

The rare and heterogeneous kidney disorder C3 glomerulopathy (C3G) is characterized by dysregulation of the alternative pathway (AP) of the complement system. C3G is often associated with autoantibodies stabilizing the AP C3 convertase named C3 nephritic factors (C3NeF). The role of classical pathway (CP) convertase stabilization in C3G and related diseases such as immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) remains largely unknown. Here, we investigated the CP convertase activity in patients with C3G and IC-MPGN. Using a refined two-step hemolytic assay, we measured the stability of CP convertases directly in the serum of 52 patients and 17 healthy controls. In four patients, CP convertase activity was prolonged compared to healthy controls, i.e. the enzymatic complex was stabilized. In three patients (2 C3G, 1 IC-MPGN) the convertase stabilization was caused by immunoglobulins, indicating the presence of autoantibodies named C4 nephritic factors (C4NeFs). Importantly, the assay also enabled detection of non-immunoglobulin-mediated stabilization of the CP convertase in one patient with C3G. Prolonged CP convertase activity coincided with C3NeF activity in all patients and for up to 70 months of observation. Crucially, experiments with C3-depleted serum showed that C4NeFs stabilized the CP C3 convertase (C4bC2a), that does not contain C3NeF epitopes. All patients with prolonged CP convertase activity showed clear signs of complement activation, i.e. lowered C3 and C5 levels and elevated levels of C3d, C3bc, C3bBbP, and C5b-9. In conclusion, this work provides new insights into the diverse aspects and (non-)immunoglobulin nature of factors causing CP convertase overactivity in C3G/IC-MPGN.

CFH and CFHR Copy Number Variations in C3 Glomerulopathy and Immune Complex-Mediated Membranoproliferative Glomerulonephritis

C3 Glomerulopathy (C3G) and Immune Complex-Mediated Membranoproliferative glomerulonephritis (IC-MPGN) are rare diseases characterized by glomerular deposition of C3 caused by dysregulation of the alternative pathway (AP) of complement. In approximately 20% of affected patients, dysregulation is driven by pathogenic variants in the two components of the AP C3 convertase, complement C3 (C3) and Factor B (CFB), or in complement Factor H (CFH) and Factor I (CFI), two genes that encode complement regulators. Copy number variations (CNVs) involving the CFH-related genes (CFHRs) that give rise to hybrid FHR proteins also have been described in a few C3G patients but not in IC-MPGN patients. In this study, we used multiplex ligation-dependent probe amplification (MLPA) to study the genomic architecture of the CFH-CFHR region and characterize CNVs in a large cohort of patients with C3G (n = 103) and IC-MPGN (n = 96) compared to healthy controls (n = 100). We identified new/rare CNVs resulting in structural variants (SVs) in 5 C3G and 2 IC-MPGN patients. Using long-read single molecule real-time sequencing (SMRT), we detected the breakpoints of three SVs. The identified SVs included: 1) a deletion of the entire CFH in one patient with IC-MPGN; 2) an increased number of CFHR4 copies in one IC-MPGN and three C3G patients; 3) a deletion from CFHR3-intron 3 to CFHR3-3′UTR (CFHR34–6Δ) that results in a FHR3-FHR1 hybrid protein in a C3G patient; and 4) a CFHR31–5-CFHR410 hybrid gene in a C3G patient. This work highlights the contribution of CFH-CFHR CNVs to the pathogenesis of both C3G and IC-MPGN.