Pathogenesis and therapeutic options of APOL1-associated renal diseasese

• Applicant: Privatdozent Dr. Tobias Hermle
• Subject Area: Nephrology
• Term: from 2020 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 440399433

End-stage renal disease is approximately three times more common in African Americans compared to Americans of European ancestry. This disparity is not explained by socio-economic factors but most of the excess risk is attributable to two risk variants of the APOL1 gene. Homozygous carriers have a life-time risk exceeding 15% to develop debilitating renal disease such as focal-segmental glomerulosclerosis (FSGS) and hypertension-associated renal disease. Specific therapies are unavailable. Heterozygous carriers are protected from sleeping sickness making the risk variants exceedingly common in populations of African ancestry with millions of homozygous carriers at risk worldwide. Lacking orthologs in the major model organisms, the pathogenesis of APOL1-associated renal disease and even the intracellular function of APOL1 remain unclear. Several possible pathogenetic mechanisms have been proposed based on overexpression models but the reported findings are often in conflict. Studying subcellular localization of endogenous APOL1 in cultured podocytes and in a transgenic Drosophila model, we detected APOL1 within the endoplasmic reticulum (ER). APOL1-expression in Drosophila induced IRE1-dependent ER stress implying a possible pathogenetic role. APOL1 risk variant expression increased endocytic function of the podocyte-like nephrocytes in fly. APOL1-expressing cells are extruded apically from the epithelial layer of a wing precursor tissue, an effect that was more pronounced for the risk allele. To study the role of APOL1 risk variants for podocyte biology without overexpression, we propose to introduce the risk variant G2 via genome editing homozygously into a line of cultured podocytes (aim1). To attain an approximation of conditions in vivo, podocytes will be differentiated and kept in prolonged culture under exposure to interferon- which is known to enhance endogenous expression of APOL1. A functional analysis of this cell line compared to a control podocyte line without carrying the risk allele will help to distinguish between overexpression artefacts and the relevant mechanisms. To perform a complementary investigation in a rapid in vivo model, we will employ transgenic expression of human APOL1 variants in Drosophila (aim 2). We will study the effects of several APOL1 transgenes in the podocyte-like nephrocytes and the wing disc as an epithelial model. In particular, we will study ER stress signaling and explore the mechanisms underlying the phenotypes of increased nephrocyte function and apical cell extrusion that were specifically observed with the risk variant expression. Finally, using G2-APOL1-induced increase of nephrocyte function as a read-out, we will screen a library of FDA-approved drugs in a whole animal drug screen (aim 3) to identify protective compounds. This work program is likely to elucidate APOL1 function and discover potential therapeutic options.

• DFG Programme: Research Grants