Kidney Disease-Associated APOL1 Variants Have Dose-Dependent, Dominant Toxic Gain-of-Function

Abstract

Significance Statement APOL1 alleles G1 and G2 are associated with high incidence and rapid progression of CKD in blacks of West African ancestry. The mechanism of APOL1 cytotoxicity is poorly understood, partly because cell-based models express variable amounts of APOL1 and yield contradictory results. Experiments using a stable human embryonic kidney cell line (HEK293) demonstrate that expression of G1 or G2 at relevant levels causes dose-dependent cytotoxicity, but the wild-type reference allele G0 does not. G0 does not reduce cytotoxicity of G1 or G2 APOL1 . The mutant alleles therefore introduce a dominant toxic gain-of-function. Dose-dependent G1 or G2 cytotoxicity could explain incomplete penetrance of APOL1 nephropathy. Reducing expression of G1 or G2 could represent a therapeutic strategy for APOL1 nephropathy. Background Two coding renal risk variants (RRVs) of the APOL1 gene (G1 and G2) are associated with large increases in CKD rates among populations of recent African descent, but the underlying molecular mechanisms are unknown. Mammalian cell culture models are widely used to study cytotoxicity of RRVs, but results have been contradictory. It remains unclear whether cytotoxicity is RRV-dependent or driven solely by variant-independent overexpression. It is also unknown whether expression of the reference APOL1 allele, the wild-type G0, could prevent cytotoxicity of RRVs. Methods We generated tetracycline-inducible APOL1 expression in human embryonic kidney HEK293 cells and examined the effects of increased expression of APOL1 (G0, G1, G2, G0G0, G0G1, or G0G2) on known cytotoxicity phenotypes, including reduced viability, increased swelling, potassium loss, aberrant protein phosphorylation, and dysregulated energy metabolism. Furthermore, whole-genome transcriptome analysis examined deregulated canonical pathways. Results At moderate expression, RRVs but not G0 caused cytotoxicity in a dose-dependent manner that coexpression of G0 did not reduce. RRVs also have dominant effects on canonical pathways relevant for the cellular stress response. Conclusions In HEK293 cells, RRVs exhibit a dominant toxic gain-of-function phenotype that worsens with increasing expression. These observations suggest that high steady-state levels of RRVs may underlie cellular injury in APOL1 nephropathy, and that interventions that reduce RRV expression in kidney compartments may mitigate APOL1 nephropathy.

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