APOL1 Nephropathy: Linking Genetics and Mechanisms

SUMMARY: Two coding variants (G1 and G2, also referred to as "risk variants" or RV) in the APOL1 gene account for much of the high rate of kidney disease in people of recent African ancestry. Evidence supports the idea that APOL1 risk variants (RV) promote kidney disease through toxic, gain-of-function activity despite a largely recessive pattern of risk inheritance. Investigators have speculated that recessive gain-of-function toxicity may be due to a dose threshold requiring two risk alleles or alternatively a “G0 rescue” model where G0 can protect against the toxic effect of the risk variants, potentially via direct interaction. The work proposed here will provide insight into this fundamental question of recessive, gain-of-function biology that is essential for understanding the mechanisms of APOL1 disease. We have generated a series of APOL1 BAC transgenic mice to study the mechanism of APOL1 kidney disease in vivo. RV mice respond to interferon, a powerful inducer of APOL1 expression, with robust proteinuria and foot process effacement while G0 mice develop neither. In Aim 1, we will use a suite of hemizygous, heterozygous, homozygous, and multicopy BAC transgenic mice including both G0 and risk genotypes to explore the relationship between gene dose and glomerular phenotype. We will perform an in-depth molecular characterization just below and above the critical interferon dose that is necessary for disease development. In Aim 2, we will explore the role of multimerization in APOL1-mediated cytotoxicity. Oligomerization may be an essential step for APOL1 ion channel formation and APOL1 RV appear more prone to multimerization than G0 APOL1 proteins. We will test the differential propensity of APOL1 genotypes to interact with one another and map the oligomerization domains. We will test the functional consequences of multimerization both in cells and in vivo. In Aim 3, we will examine how interferon changes APOL1 behavior and the cellular milieu to enhance toxicity, motivated by the observation that APOL1 kidney disease often occurs in the setting of high interferon states such as HIV, COVID-19, and lupus. While interferons trigger APOL1 kidney disease at least in part by upregulating APOL1 gene expression, our mouse models indicate that factors beyond the high risk APOL1 genotype and increased expression may be required for disease penetrance. We hypothesize that other interferon stimulated genes may enhance the toxicity of RV or help mitigate the toxicity of G0. Alternatively, APOL1 may itself be modified by the interferon program through expression of alternative transcripts, post translational modifications, or different trafficking patterns. We will identify elements of the interferon-stimulated cellular milieu that may regulate the quantitative threshold where APOL1 becomes toxic.