Reimagining the kidney biopsy in the era of diagnostic biomarkers of glomerular disease

The management of membranous nephropathy - an update

In the last decades several important advances have taken place in the understanding of the pathogenesis underlying membranous nephropathy, which have sparked renewed interest in its management. Four landmark trials in membranous nephropathy, and a fifth clinical trial-which was a pilot study-, have been published in the last years. The results from some of these trials have had a significant impact in the recommendations included in the 2021 Kidney Disease: Improving Global Outcomes (KDIGO) Guideline for the Management of Glomerular Diseases, which represent a significant step forward compared to the previous guideline in several aspects such as diagnosis, disease monitoring and treatment strategies. However, considering the rapidly evolving advances in the knowledge of membranous nephropathy and the recent publication of the STARMEN and RI-CYCLO trials, several recommendations contained in the guideline warrant updates. This manuscript provides a perspective of the Immunonephrology Working Group of the European Renal Association (ERA) regarding the management of membranous nephropathy in native kidneys of adult patients.

Establishment and application of an immunoassay for the simultaneous detection of IgG and its subtype IgG4 autoantibodies against M-type phospholipase A2 receptor

BACKGROUND

The renal biopsy is an accurate and reliable gold standard for membranous nephropathy (MN) diagnosis. However, it is an invasive procedure involving the risk of hemorrhage or infection. Thus, an alternative approach that can facilitate the effective diagnosis and treatment monitoring of idiopathic membranous nephropathy (IMN) is urgently needed.

METHODS

We established a dual-labeled time-resolved fluoroimmunoassay (TRFIA) to simultaneously detect phospholipase A2 receptor (PLA2R)-IgG4 and PLA2R-IgG antibodies. Utilizing this assay, we determined the ratio of autoantibodies in the serum of patients with different kidney diseases and normal controls.

RESULTS

The sensitivity of TRFIA for detecting anti-PLA2R-IgG and anti-PLA2R-IgG4 was 0.12 µg/mL and 0.001 µg/mL, respectively. Human IgA did not interfere with the assay. Compared to anti-PLA2R-IgG alone, the positive rate of IMN could be increased from 86.5 % to 91.7 % through the combined use of anti-PLA2R-IgG4 and the PLA2R-IgG4/IgG ratio. In contrast, the false-positive rates for the detection of IgA nephropathy, lupus nephropathy, diabetic nephropathy, and minimal change nephropathy decreased from 25 to 50 % to 0 %.

CONCLUSIONS

The dual-labeled PLA2R-IgG4/IgG-TRFIA for simultaneous detection of anti-PLA2R-IgG4 and anti-PLA2R-IgG will contribute to improved accuracy of IMN diagnosis.

Urinary expression of long non-coding RNA TUG1 in non-diabetic patients with glomerulonephritides

Metabolic alterations serve a significant role in the pathogenesis of kidney disease. Long non-coding RNA (lncRNA) taurine upregulated gene 1 (TUG1) is a known regulator of podocyte health and mitochondrial biogenesis. Although TUG1 protects against podocyte loss in models of diabetic nephropathy, it is unknown if urinary TUG1 expression is associated with clinical and histopathological findings in non-diabetic patients diagnosed with glomerulonephritides. In the present study, the expression of TUG1, podocyte-specific markers (nephrin and podocin) and mitochondrial biogenesis-associated mRNAs (transcription factor A mitochondrial, cytochrome C oxidase subunit 5A and peroxisome proliferator-activated receptor γ coactivator 1α) were examined in urinary sediment of non-diabetic patients with biopsy-confirmed glomerulonephritides and healthy controls. Urinary expression of TUG1 was significantly lower in patients with glomerulonephritides, particularly those diagnosed with Focal Segmental Glomerulosclerosis (FSGS). Furthermore, TUG1 levels were associated with urinary expression of podocyte-specific markers and mRNAs associated with mitochondrial biogenesis. Loss of TUG1 expression in urinary sediment was strongly associated with FSGS, highlighting the potential of this lncRNA and its mitochondrial biogenesis-associated targets as non-invasive biomarkers of assessing podocytopathy.

Molecular classification of membranous nephropathy

PURPOSE OF REVIEW

Continuous expansion of our knowledge in the pathogenesis of membranous nephropathy possible by the identification of antibodies recognized specific podocytes antigens results in unprecedent patient management strategy.

RECENT FINDINGS

Circulating anti-phospholipase A2 receptor (PLA2R) and anti-thrombospondin domain 7A (THSD7A) antibodies strongly relate with the modifications of podocytes biology leading to the new molecular diagnosis of membranous nephropathy. Immunization against THSD7A involves extra-renal mechanism. However, the pathway of anti-PLA2R immunization still remains unresolved. Experimental data highlight the crucial role of THSD7A in the attachment of podocytes to the glomerular basement membrane, rewarding the THSD7A pathogenicity, whereas the third of Koch's postulates is still not fulfilled for anti-PLA2R antibodies. The anti-PLA2R antibodies epitope spreading will possibly be even more specific marker improving the molecular classification of membranous nephropathy. Two immune epitopes have been identified in the N-terminal tail of THSD7A but without evidence of epitope spreading as for anti-PLA2R.

SUMMARY

In 2019, the Kidney Diseases: Improving Global Outcomes guidelines recognized anti-PLA2R antibodies (but not anti-THSD7A antibodies) as a valuable molecular risk factor for the pejorative evolution of kidney function and recommended their monitoring for the diagnosis and the assessment of membranous nephropathy immune activity. Screening for malignancy is particularly advised in THSD7A-mediated membranous nephropathy.

New Ways of Understanding Membranous Nephropathy

BACKGROUND

In the last decade, great advances have been made in the field of membranous nephropathy (MN). The autoimmune nature of the disease has been confirmed with the description of diverse antigens, and few but very important prospective trials regarding treatment alternatives have been published, changing profoundly the way we understand this entity. Nowadays, an individualized therapeutic scheme based on clinical and serologic data appears to be the most appropriate method to manage patients with MN. Although there is still a long way to go, it is expected that future scientific progress will enable a patient-centered medicine based on concept-driven therapies.

SUMMARY

MN is the most common cause of nephrotic syndrome (NS) in white adults. Approximately one-third of patients achieve spontaneous remission, one-third remain stable, and one-third have an aggressive course with persistent NS and deterioration of renal function. About 80% of patients have circulating autoantibodies to phospholipase A2 receptor 1. Numerous therapies have been described including alkylating agents, rituximab, and calcineurin inhibitors, but new drugs are currently being explored. Here, we review the most important aspects regarding MN with an emphasis on results of the most recent clinical trials and pathophysiologic advances. Key Messages: 1. Evolving pathophysiologic concepts and recently published clinical trials have deeply changed our view of MN. 2. Most patients with MN present autoantibodies against diverse glomerular antigens. 3. Currently, an individual patient-centered management based on clinical and serologic markers is the most adequate approach to treat patients with MN.