Genome-wide association studies suggest that APOL1-environment interactions more likely trigger kidney disease in African Americans with nondiabetic nephropathy than strong APOL1-second gene interactions

APOL1 polymorphisms and kidney disease: loss-of-function or gain-of-function?

The mechanism that explains the association of APOL1 variants with nondiabetic kidney diseases in African Americans remains unclear. Kidney disease risk is inherited as a recessive trait, and many studies investigating the intracellular function of APOL1 have indicated the APOL1 variants G1 and G2 are associated with cytotoxicity. Whether cytotoxicity results from the absence of a protective effect conferred by the G0 allele or is induced by a deleterious effect of variant allele expression has not be conclusively established. A central issue hampering basic biology studies is the lack of model systems that authentically replicate APOL1 expression patterns. APOL1 is present in humans and a few other primates and appears to have important functions in the kidney, as the kidney is the primary target for disease associated with the genetic variance. There have been no studies to date assessing the function of untagged APOL1 protein under native expression in human or primate kidney cells, and no studies have examined the heterozygous state, a disease-free condition in humans. A second major issue is the chronic kidney disease (CKD)-associated APOL1 variants are conditional mutations, where the disease-inducing function is only evident under the appropriate environmental stimulus. In addition, it is possible there may be more than one mechanism of pathogenesis that is dependent on the nature of the stressor or other genetic variabilities. Studies addressing the function of APOL1 and how the CKD-associated APOL1 variants cause kidney disease are challenging and remain to be fully investigated under conditions that faithfully model known human genetics and physiology.

Meso-American nephropathy: what we have learned about the potential genetic influence on chronic kidney disease development

Chronic kidney disease of unknown aetiology (CKDu) refers to the epidemic level of incidence of CKD in several low- and middle-income countries, usually near the equator, for which the aetiology has not been identified. CKDu represents a form of CKD hotspot, defined as a country, region, community or ethnicity with a higher than average incidence of CKD. In terms of the number of persons affected, the so-called hypertensive nephropathy of African Americans probably represents the largest CKD hotspot, which is largely driven by variants of the APOL1 gene, questioning the very existence of hypertensive nephropathy and illustrating how kidney disease driven by genetic predisposition may underlie some forms of hypertension. For CKDu, hard physical work leading to dehydration (the first global warming-related disease?) and local toxins are leading aetiological candidates. Meso-American nephropathy is probably the best-characterized CKDu. In this issue of CKJ, a systematic review and meta-analysis by Gonzalez et al. identified positive associations between Meso-American nephropathy and male gender, family history of CKD, high water intake and lowland altitude. We now discuss the potential relationship of family history to genetic predisposition and how a better understanding of CKDu may help advance the aetiological characterization of the nearly 50% of end-stage renal disease patients worldwide that have no known cause for CKD or have been assigned non-specific diagnoses.