Genetic Testing for APOL1 Genetic Variants in Clinical Practice

• genetic testing
• genetic variants
• genetic variation
• kidney disease
• end stage kidney disease

The identification of genetic variants in APOL1 as a major driver of kidney disease in blacks has ushered in an exciting new era in glomerular disease, offering the potential for precision medicine in a range of kidney diseases in individuals with sub-Saharan African ancestry (1). The presence of APOL1 high-risk genotypes, comprising any combination of two APOL1 kidney risk alleles, increases the risk for several kidney diseases compared with APOL1 low-risk individuals (defined as those carrying zero or one APOL1 kidney risk allele). These include FSGS, HIV-associated nephropathy, focal global glomerulosclerosis with interstitial and vascular changes (overlapping with the pathologic pattern formerly termed arterionephrosclerosis), sickle cell nephropathy, lupus nephritis associated with collapsing glomerulopathy, and unexplained ESKD. Nevertheless, it seems that most APOL1 high-risk individuals will not develop kidney disease, and our inability to predict a particular individual’s lifetime risk is a major missing piece.

APOL1 high-risk status is also associated with more rapid decline in eGFR in the general population and patients with established CKD, including some patients with diabetic kidney disease. Paradoxically, APOL1 high-risk status is associated with longer patient survival while on long-term hemodialysis. APOL1 high-risk status in kidney allografts predicts shorter allograft survival (2). Of concern, living donors with high-risk APOL1 genotypes are at higher risk for developing CKD compared with low-risk APOL1 kidney donors (3).

APOL1 high-risk subjects with FSGS and nephrotic-range proteiniuria have comparable reduction in proteinuria to APOL1 low-risk individuals, and yet, APOL1 high-risk subjects have worse long-term outcomes, with faster progression to ESKD (4). The reasons for this discrepancy are unclear. It may be that, in APOL1 high-risk individuals, therapy suppresses a podocyte cellular pathway that is related to proteinuria but does not affect other podocyte cellular pathways that drive progressive cell injury. Alternatively, the favorable effect of these agents on particular pathways may be transitory, such as by reducing IFN expression and thus, reducing APOL1 gene expression, but only while therapy is administered. Finally, it is possible that socioeconomic and psychologic factors interact in some way with APOL1 high-risk status. For example, because race is strongly related to APOL1 risk allele carriage rates and because in the United States and certain other countries, race remains correlated with adverse socioeconomic and psychologic factors, these sociopsychologic factors could contribute to the worse outcomes associated with APOL1 high-risk status.

Next to be considered is by far the most common manifestation of APOL1 kidney disease, the clinical syndrome of hypertension and CKD with subnephrotic proteinuria in which causes, such as diabetes and GN, have been excluded. Pathologic manifestations characteristic of individuals with APOL1 high-risk genotype and a pathologic diagnosis of arterionephrosclerosis include solidified and disappearing glomerulosclerosis, thyroidization-type tubular atrophy, and microcystic tubular dilations while having less arteriosclerosis than others. Analysis of the APOL1 high-risk genotypes in the African-American Study of Kidney Disease in Hypertension demonstrated that APOL1 high-risk individuals with hypertension and reduced eGFR tended to have heavier proteinuria and faster GFR loss compared with blacks with APOL1 low-risk genotypes (5).

The relationship between albuminuria or proteinuria and the APOL1-associated CKD is not fully understood. Several patterns may be recognized. Many individuals may have stable low-level proteinuria and nonetheless, manifest progressive CKD. Others may proceed through sequential phases of increasing albuminuria and falling GFR. Still others may present with nephrotic syndrome and rapid progression to ESKD. Because it seems that the first two patterns are most common, genetic testing to identify APOL1 high-risk individuals followed by regular screening for proteinuria beginning during teenage years or early adulthood and the initiation of antiproteinuric therapy on appearance of microalbumimnuria might yield clinical benefit, although this would have to tested in a randomized, controlled trial. Certainly, screening for albuminuria and institution of an angiotensin-converting enzyme inhibitor or an angiotensin receptor blocker are standard of care in individuals with diabetes mellitus, both type 1 and type 2. However, APOL1 high-risk individuals might experience an event, perhaps a prolonged viral infection, that acutely increases systemic IFN expression and results in sudden increases in kidney APOL1 gene expression. Defining kinetics of incipient APOL1 nephropathy as accumulative or crescendo or more likely, complex and distinct patterns among individuals will require long-term follow-up of APOL1 high-risk individuals, starting when they have normal kidney function. The findings might inform approaches to screening.

Reidy et al. (6) reported that fetal, and not maternal, genotype is associated with higher risk for preeclampsia with mothers carrying fetuses with two APOL1 risk alleles. Prospective studies are needed to determine whether early identification of increased preeclampsia risk associated with APOL1 risk status, by prompting more intensive prenatal care, improves maternal and fetal outcomes.

Now, 9 years after the identification of the APOL1 kidney risk variants, it remains unclear in what conditions APOL1 genetic testing is clinically indicated. There are several steps along the path to precision medicine. One step will be understanding the molecular and cellular pathways of cell injury induced by these variants, with the goal of identifying therapeutic targets that might be suitable for the design of specific therapies. Another step will be rigorous studies of both short-term (such as proteinuria) and long-term outcomes (progressive loss of kidney function, eventuating in ESKD) in APOL1 high-risk individuals. In the case of FSGS, APOL1 high-risk individuals respond just as well as others with regard to reduction of proteinuria, but they progress to ESKD faster than APOL1 low-risk blacks or whites (4). Approaches being tested now for APOL1 kidney disease include adrenal corticotropic hormone in humans (National Clinical Trial 02633046) and antisense oligonucleotides in APOL1 transgenic mice.

At present, there is no evidence that any particular therapeutic approach to the treatment of APOL1 high-risk individuals with glomerular disease or hypertension or those having had kidney transplant is superior to the standard treatments. This weakens the case for clinical APOL1 genetic testing at this time. Parsa et al. (7) concluded that the lack of an interaction between APOL1 status and treatment with an angiotensin-converting enzyme inhibitor medication with regard to progression to ESKD suggested that APOL1 high-risk individuals still benefit from this medication class. Future studies are likely to provide relevant information on these points. One argument is that genetic test results, when delivered to patient, might alter behavior in ways that would be conducive to healthy outcomes. These might include improved medication adherence, healthier diet, weight loss, smoking cessation, and other factors. Importantly, one must acknowledge that none of these interventions or lifestyle changes have been prospectively tested among individuals with this genetic risk. Recent studies of community attitudes have revealed a strong preference among at least some blacks to have more knowledge about APOL1 genetic risk and be offered the option of genetic testing (8). Knowledge of genetic test results might also alter the practice of the physician who, with knowledge of the increased risk of adverse outcome, might stress the same points. Clearly, these effects on physician and patient behavior should be tested in prospective studies. One can argue that return of genetic results requires a full understanding of the effect of return of these results. Yet, as is often the case, physicians and patients must often make decisions in absence of convincing data from well designed, well executed studies. In Table 1, we have presented our opinions regarding the evidence level that supports clinical APOL1 genetic testing at this time.

Perhaps the most compelling case for clinical APOL1 testing at present relates to kidney transplantation. Data from multiple groups have established that APOL1-associated risk operates at the level of the kidney. First, transplanted kidneys from living and deceased APOL1 high-risk donors fare slightly worse than non-APOL1 high-risk kidneys. Nevertheless, the allograft survival differences are modest, and the benefits of transplant outweigh the hazards of remaining on dialysis. Second and importantly, preliminary data suggested that living donors with an APOL1 high-risk genotype compared with those with a low-risk genotype have lower eGFR at follow-up and a faster decline in eGFR during the period after kidney donation (3). These observations are being evaluated on a broader scale in the National Institute of Diabetes and Digestive and Kidney Diseases–funded APOL1 Long Term Kidney Transplant Outcome Network (National Clinical Trial 03615235), which aims to determine the current kidney status of all United States kidney transplant recipients who received kidneys from blacks, a number estimated to be as high as 5000 potential study participants. At present, some physicians are testing potential living donors for APOL1 risk alleles before kidney donation and providing those with high-risk APOL1 genotypes information about increased risk for CKD, but at present, there is not a consensus recommendation to take this approach (9).

There are many clinical situations where the role of APOL1 genetic testing remains to be defined by focused clinical studies to establish whether genotype knowledge prompts behavioral change on the part of the individual or prompts interventions by clinicians that are known to lead to better health outcomes in many forms of CKD. These situations involve individuals with risk factors for kidney disease, including those with history of premature birth, hypertension, and obesity. This topic brings together issues of minority health and genetic testing (8) as well as the involvement of the affected communities (10). A key long-term goal is to develop effective therapy for APOL1 kidney disease by elucidating the pathways of kidney injury and targeting these pathways to enable preventive therapy in those at greatest risk and treat these conditions more effectively than we do at present.

• Copyright © 2020 by the American Society of Nephrology