Apolipoprotein L1 and apolipoprotein A-IV and their association with kidney function

PURPOSE OF REVIEW

Chronic kidney disease (CKD) is a common disease with an estimated prevalence of 10-12%. There are pronounced differences between ethnicities with a 3-fold to 4-fold higher lifetime risk for end-stage kidney disease in African Americans compared to European Americans. The purpose of this review was to discuss recent findings on two apolipoproteins (apolipoprotein L1 and A-IV) in the context of kidney disease and kidney function.

RECENT FINDINGS

The observation that certain apolipoprotein L1 risk genotypes that are only present in African Americans might explain a major fraction of the ethnic differences for nondiabetic CKD has set the stage for this otherwise under-researched apolipoprotein. These risk genotypes on the one hand protect African Americans against African sleeping sickness but cause on the other hand several types of nondiabetic CKD. We are currently beginning to understand the mechanisms how apolipoprotein L1 is involved in the modification of lysosomal and cytoplasmic membranes. The second protein, apolipoprotein A-IV (apoA-IV), turned out to be an early marker of kidney impairment not only in patients with primary CKD but also in individuals from the general population. Genetic studies provided strong support of a causal effect of kidney function on apoA-IV concentrations.

SUMMARY

These two apolipoproteins have very distinct properties. Apolipoprotein L1 is causally involved in the development of nondiabetic CKD in African Americans. In contrast, apoA-IV is an early marker for kidney impairment.

Generalization of associations of kidney-related genetic loci to American Indians

BACKGROUND AND OBJECTIVES

CKD disproportionally affects American Indians, who similar to other populations, show genetic susceptibility to kidney outcomes. Recent studies have identified several loci associated with kidney traits, but their relevance in American Indians is unknown.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

This study used data from a large, family-based genetic study of American Indians (the Strong Heart Family Study), which includes 94 multigenerational families enrolled from communities located in Oklahoma, the Dakotas, and Arizona. Individuals were recruited from the Strong Heart Study, a population-based study of cardiovascular disease in American Indians. This study selected 25 single nucleotide polymorphisms in 23 loci identified from recently published kidney-related genome-wide association studies in individuals of European ancestry to evaluate their associations with kidney function (estimated GFR; individuals 18 years or older, up to 3282 individuals) and albuminuria (urinary albumin to creatinine ratio; n=3552) in the Strong Heart Family Study. This study also examined the association of single nucleotide polymorphisms in the APOL1 region with estimated GFR in 1121 Strong Heart Family Study participants. GFR was estimated using the abbreviated Modification of Diet in Renal Disease Equation. Additive genetic models adjusted for age and sex were used.

RESULTS

This study identified significant associations of single nucleotide polymorphisms with estimated GFR in or nearby PRKAG2, SLC6A13, UBE2Q2, PIP5K1B, and WDR72 (P<2.1 × 10(-3) to account for multiple testing). Single nucleotide polymorphisms in these loci explained 2.2% of the estimated GFR total variance and 2.9% of its heritability. An intronic variant of BCAS3 was significantly associated with urinary albumin to creatinine ratio. APOL1 single nucleotide polymorphisms were not associated with estimated GFR in a single variant test or haplotype analyses, and the at-risk variants identified in individuals with African ancestry were not detected in DNA sequencing of American Indians.

CONCLUSION

This study extends the genetic associations of loci affecting kidney function to American Indians, a population at high risk of kidney disease, and provides additional support for a potential biologic relevance of these loci across ancestries.

Chronic kidney disease: novel insights from genome-wide association studies

Chronic kidney disease (CKD) is common, affecting about 10% of the general population, and causing significant morbidity and mortality. Apart from the risk conferred by traditional cardiovascular risk factors, there is a strong genetic component. The method of a genome-wide association study (GWAS) is a powerful hypothesis-free approach to unravel this component by association analyses of CKD with several million genetic variants distributed across the genome. Since the publication of the first GWAS in 2005, this method has led to the discovery of novel loci for numerous human common diseases and phenotypes. Here, we review the recent successes of meta-analyses of GWAS on renal phenotypes. UMOD, SHROOM3, STC1, LASS2, GCKR, ALMS1, TFDP2, DAB2, SLC34A1, VEGFA, PRKAG2, PIP5K1B, ATXN2/SH2B3, DACH1, UBE2Q2, and SLC7A9 were uncovered as loci associated with estimated glomerular filtration rate (eGFR) and CKD, and CUBN as a locus for albuminuria in cross-sectional data of general population studies. However, less than 1.5% of the total variance of eGFR and albuminuria is explained by the identified variants, and the relative risk for CKD is modified by at most 20% per locus. In African Americans, much of the risk for end-stage nondiabetic kidney disease is explained by common variants in the MYH9/APOL1 locus, and in individuals of European descent, variants in HLA-DQA1 and PLA(2)R1 implicate most of the risk for idiopathic membranous nephropathy. In contrast, genetic findings in the analysis of diabetic nephropathy are inconsistent. Uncovering variants explaining more of the genetically determined variability of kidney function is hampered by the multifactorial nature of CKD and different mechanisms involved in progressive CKD stages, and by the challenges in elucidating the role of low-frequency variants. Meta-analyses with larger sample sizes and analyses of longitudinal renal phenotypes using higher-resolution genotyping data are required to uncover novel loci associated with severe renal phenotypes.