Pathogenesis of IgA nephropathy and diabetic kidney disease : linking molecular profile to morphological and clinical picture

Abstract

Background: More than 10% of the global population suffers from chronic kidney disease (CKD), which poses a large impact on society and, more importantly, the life of the affected patient. Two of the most common causes of CKD are IgA Nephropathy (IgAN) and Diabetic Kidney Disease (DKD). As specific, non-invasive biomarkers are lacking, the diagnoses are made based on clinical signs, often together with a kidney biopsy. Moreover, none of the two diseases have as of yet specific treatments. The available medications aim to reduce known risk factors for disease progression but are often not sufficiently efficient. In fact, almost 30 % of patients with IgAN loose half their kidney function or start kidney replacement therapy (KRT) within 10 years from diagnosis. Close to 50% of all patients requiring KRT suffer from DKD.

Aims: To gain deeper understanding of the molecular mechanisms underlying disease onset and progression in IgAN and DKD.

Material and methods: This thesis includes patients with clinically and histopathologically verified IgAN or DKD that have undergone a kidney biopsy for clinical reasons. All biopsies were scored according to established histopathological scores and a comprehensive clinical review was performed by collecting data from patients’ medical records at time of biopsy and at follow-up. Control material was mainly obtained from living kidney donors (LD). Transcriptomics (microarray or RNA sequencing, RNASeq) was performed on microdissected kidney tissue. Specific proteins were investigated using immunofluorescence (IFL) and immuno electron microscopy (iEM).

Results: Study I. The potential biomarker dendrin was investigated in IgAN and its systemic variant IgA Vasculitis with Nephropathy (IgAVN). Dendrin mRNA levels were higher (p = 0.01) in IgAN/IgAVN compared to membranous nephropathy (MN) and controls. This upregulation was more prominent in patients with mild disease. No differences between groups were detected with IFL but on an ultrastructural level, using iEM, higher relative dendrin concentrations were observed in the podocyte nuclei in patients with slower annual estimated glomerular filtration rate (eGFR) decline and milder histopathological changes in IgAN/IgAVN patients. These results indicate that dendrin is protective in IgAN/IgAVN. Study II. RNASeq was performed on microdissected kidney biopsies from 19 patients with DKD and 20 LD. Gene ontology (GO) analysis showed upregulation of pathways related to inflammation and extra cellular matrix organization in the glomeruli, and immune and apoptosis pathways in the tubulointerstitium. We also compared the RNA transcript data to clinical variables at time of biopsy using weighted gene co-expression analysis (WGCNA). This analysis identified several gene modules associated with kidney function. Study III. Our RNASeq data from Study II was further analysed in relation to clinical longitudinal data, including eGFR decline, progression to KF and albuminuria. We found 265 genes that were differently expressed between patients with rapid and non-rapid progression, thus suggesting that a prognostic gene expression profile is associated with outcome in this group of DKD patients. Study IV. RNASeq was performed on microdissected kidney biopsies from 71 adult patients and 13 children with IgAN/IgAVN. Eleven LD were included as controls. We found upregulation of genes and pathways involved in the immune system in both the glomeruli and the tubulointerstitium in IgAN/IgAVN patients compared to LD. In addition, extensive transcriptomic differences were found between adults and children with IgAN/IgAVN, mainly related to cell division and mitochondrial function in glomeruli and inflammatory response in tubulointerstitium.

Taken together, the result from this thesis adds important information on molecular events underlying onset and disease progression of IgAN/IgAVN and DKD. We identified compartmental and disease specific transcriptomal patterns that may be useful in the identification of new, specific biomarkers, thus conveying important information on the road towards precision medicine which ultimately may contribute to the development of better treatment options for patients suffering from CKD.