Mitochondrial DNA and Kidney Function

Mitochondria play an essential role in cellular energy production and multiple hemostatic pathways. Not surprisingly, therefore, mitochondrial dysfunction is a hallmark of many diseases (1–3). Mitochondria are unique among cellular organelles in that they have their own genome, encoding 13 peptides involved in oxidative phosphorylation. In contrast to nuclear DNA that exists in two copies, up to several thousand copies of mitochondrial DNA exist per cell. Cellular copy numbers of mitochondrial DNA vary with the energy demand of single cells and tissues. In addition, they represent the balance between stress factors impairing mitochondrial function and the intrinsic capacity to maintain mitochondrial integrity. Reactive oxygen species (ROS) produced by oxidative phosphorylation represent a major challenge for mitochondrial function and integrity and have a strong influence on the regulation of cellular differentiation, apoptosis, autophagy, metabolic adaptation, immune cell activation, and inflammation (2). Mitochondrial dysfunction is generally characterized as a loss of efficiency in oxidative phosphorylation (2). Once established, it enhances oxidative stress and its adverse consequences. Mitochondrial DNA is particularly vulnerable to oxidative damage, and enzymes maintaining mitochondrial DNA integrity are of critical importance for cellular function. The number of copies of mitochondrial DNA is considered to be a surrogate for mitochondrial function/dysfunction, especially in energy-dependent tissues. Mitochondrial DNA copy number can be measured from peripheral blood, where it reflects mostly the content of white blood cells. Using this approach in large patient cohorts revealed associations of low mitochondrial DNA copy numbers with a variety of chronic diseases and reduced life expectancy (3).

In this issue of CJASN, He et al. (4) report studies on the association of mitochondrial DNA copy number with the risk for progression of kidney disease and mortality in 2943 individuals with CKD enrolled into the Chronic Renal Insufficiency Cohort (CRIC) study. They observed that patients in the lowest tertile compared with the highest tertile of mitochondrial DNA copy number had a 28% higher risk for CKD progression. This association was found in patients with diabetes as well as in patients with albuminuria but not in those without diabetes or albuminuria. This study is the first to establish an association between reduced mitochondrial DNA copy number and kidney disease progression in individuals with established CKD. A previous analysis in the Atherosclerosis Risk in Communities study found that 1490 of 9000 individuals who developed CKD during a median follow-up of 19.6 years had lower mitochondrial DNA copy number at baseline (5). Among almost 5000 participants of the German Chronic Kidney Disease (GCKD) study, which like CRIC, enrolled participants with established CKD, a lower mitochondrial DNA copy number was associated with prevalent cardiovascular disease after adjustment for other cardiovascular disease risk factors (6). In GCKD, mitochondrial DNA copy numbers were also lower in individuals with diabetes as compared with those without diabetes. Moreover, in GCKD, a low mitochondrial DNA copy number was a predictor of all-cause mortality (6). In contrast, He et al. (4) found no difference in the prevalence of cardiovascular disease or diabetes mellitus in the three tertiles of mitochondrial DNA copy number in the CRIC study, nor did they find an association of mitochondrial DNA copy number with mortality.

Thus, despite the growing evidence for an association between low mitochondrial DNA copy number and adverse outcomes, there appear to be differences in the risk relationships. Such differences could be related to patient characteristics, sample sizes, and event rates, but they might also reflect differences in methodology, as recently discussed by Chong et al. (7). The measurement by quantitative PCR, especially when a plasmid normalization is used, is currently considered as the gold standard feasible for application in large epidemiologic studies (8). Alternatively, mitochondrial DNA copy numbers can be estimated from raw probe signal intensities from single nucleotide polymorphism (SNP) array data used for genotypic inference in genome-wide association studies. The quality of these estimates very much depends on the SNP microarray used. The MitoPipeline algorithm applied by He et al. (4) was reported to have a correlation coefficient of about r=0.50 with measurements done by quantitative PCR (7), such as applied in GCKD, which is not a perfect correlation. However, a major advantage of using SNP array data is that the raw probe signal intensities are available from widely used microarray data so that data on mitochondrial DNA copy number are obtained as a by-product. Additionally, even if the “measurement error” is quite high, this can be partially compensated for by high sample numbers considering how many millions of microarrays have already been genotyped worldwide. It will be important in the future to bring studies together that have performed a similar phenotyping of diseases to increase statistical power and detect associations with high reliability. Moreover, methodologies probably need to be refined to explore the potential of mitochondrial DNA copy numbers as a useful biomarker in individuals.

Another important question is whether the observed associations between low mitochondrial DNA copy numbers and adverse outcomes are causal. This cannot be answered with certainty by any of the observational studies performed so far. Mendelian randomization studies using genetic variants that have an influence on mitochondrial DNA copy number as a genetic instrument would be one approach. In the case that the variants that are associated with a lifelong lower mitochondrial DNA copy number (in blood) are also associated with progression of CKD, this would provide strong support for a causal association. A recent genome-wide association study identified 71 genetic loci associated with mitochondrial DNA copy number. These loci together, however, explained only 2% of the total variance of mitochondrial DNA copy numbers (7). Therefore, larger studies will be required to have sufficient statistical power to explore causality. Because the number of studies including patients with CKD is increasing, future collaborative initiatives may shed new light on causal associations between mitochondrial function and different diseases.

One of the inherent limitations of interpreting mitochondrial DNA copy numbers in circulating cells of peripheral blood is that the extent to which they reflect the mitochondrial genome in different tissues remains unclear. However, there is ample evidence supporting the association between mitochondrial function and disease progression in the kidney. The kidney is one of the most energy-demanding organs in the human body. In fact, it has the second-highest mitochondrial content and oxygen consumption following the heart. Within the kidney, the proximal tubule in the kidney accounts for the greatest proportion of oxygen consumption. Because of the special countercurrent arrangement of intrarenal microvessels and the high oxygen demand, large portions of the kidney operate at the verge of hypoxia, rendering many kidney cells particularly sensitive to mitochondrial dysfunction (9,10). Several mitochondrial cytopathies caused by inherited or sporadic mutations in genes within mitochondrial or nuclear DNA that affect mitochondrial functions manifest with kidney phenotypes, and some lead to CKD. Experimental evidence has also implicated mitochondrial damage in the kidneys in the pathogenesis of common etiologies of CKD, such as diabetes, hypertension, metabolic syndrome, and chronic renal ischemia. This evidence includes morphologic abnormalities of mitochondria and functional data implicating overproduction of ROS in epithelial damage and fibrosis. Urinary mitochondrial DNA copy numbers may serve as noninvasive biomarkers of mitochondrial dysfunction in the kidneys (10). Comparing mitochondrial DNA copy numbers determined in urine and blood may offer a future opportunity to differentiate systemic from kidney-specific mitochondrial dysfunction. It remains to be determined whether mitochondrial DNA copy numbers extracted and measured from cells or cellfree mitochondrial DNA copy numbers are more reliable biomarkers.

Given their important pathophysiologic role, mitochondrial damage and dysfunction are also recognized as interesting therapeutic targets. Diverse approaches to maintain mitochondrial function include the regulation of ROS metabolism, antagonism of mitochondrial oxidants, promotion of mitochondrial biogenesis and ATP synthesis, inhibition of mitochondrial fragmentation, and inhibition of the mitochondrial permeability transition pore. Studies like the one published by He et al. (4) encourage us to pursue such approaches and lend further support to the hypothesis that maintaining mitochondria homeostasis may prevent kidney pathogenesis and disease progression.

Disclosures

K.-U. Eckardt reports consultancy agreements with Akebia, Bayer, Boehringer Ingelheim, Otsuka, and Travere; reports research funding from Amgen, AstraZeneca, Bayer, Evotec, Fresenius, Genzyme, Shire, and Vifor; reports honoraria from Akebia, AstraZeneca, Bayer, Boehringer Ingelheim, Genzyme, Otsuka, Travere, and Vifor; serves on the editorial boards of BMJ and Kidney International; and is chair of the steering committee of the GCKD study. F. Kronenberg reports consultancy agreements with Amgen, CRISPR Therapeutics, Kaneka, and Novartis; reports honoraria from Amgen and Novartis; serves as Co-Editor of Atherosclerosis; and is a member of the steering committee of the GCKD study.

Funding

None.

Author Contributions

F. Kronenberg and K.-U. Eckardt wrote the editorial.