Mitochondrial DNA mutations in renal disease: an overview

The relationship between kidney disease and mitochondria: a bibliometric study

BACKGROUND

Due to its highly reabsorptive function, the kidney is a mitochondria-dependent organ. Research on the association between mitochondria and kidney disease has always been a serious focus of researchers, with many publications. Bibliometrics is a secondary analysis of published literature that extracts relevant information to gain insights into hotspots and trends in the field. Through bibliometric analysis, we aimed to understand the development trends and hotspots in the field of research on the association between kidney disease and mitochondria.

METHOD

Three bibliometric mapping tools (Biblimetrix R Package, VOS Viewer, CiteSpace) were used to provide an overview of the literature and analyze the co-occurrence of keywords and reference citations.

RESULTS

A total of 2672 relevant research articles were included. The co-occurrence network identified three clusters related to the association between mitochondria and kidney disease, including experimental methods, research mechanisms, and disease phenotypes. We found that research in this field has shifted from disease-level studies to mechanism-based studies, with the most prominent disease being diabetic nephropathy and the most prominent pathogenic mechanism being related to mitochondrial ROS production.

CONCLUSION

The bibliometric analysis provided a comprehensive understanding of the progress of research on the role of mitochondria in kidney disease, enriching the review literature in this field.

Genetic determinants of renal scarring in children with febrile UTI

BACKGROUND

Febrile urinary tract infections (UTIs) are among the most severe bacterial infections in infants, in which a subset of patients develops complications. Identifying infants at risk of recurrent infections or kidney damage based on clinical signs is challenging. Previous observations suggest that genetic factors influence UTI outcomes and could serve as predictors of disease severity. In this study, we conducted a nationwide survey of infant genotypes to develop a strategy for infection management based on individual genetic risk. Our aims were to identify genetic susceptibility variants for renal scarring (RS) and genetic host factors predisposing to dilating vesicoureteral reflux (VUR) and recurrent UTIs.

METHODS

To assess genetic susceptibility, we collected and analyzed DNA from blood using exome genotyping. Disease-associated genetic variants were identified through bioinformatics analysis, including allelic frequency tests and odds ratio calculations. Kidney involvement was defined using dimercaptosuccinic acid (DMSA) scintigraphy.

RESULTS

In this investigation, a cohort comprising 1087 infants presenting with their first episode of febrile UTI was included. Among this cohort, a subset of 137 infants who underwent DMSA scanning was subjected to gene association analysis. Remarkable genetic distinctions were observed between patients with RS and those exhibiting resolved kidney involvement. Notably, the genetic signature indicative of renal scarring prominently featured mitochondrial genes.

CONCLUSIONS

In this nationwide study of genetic susceptibility to RS after febrile UTIs in infancy, we identified a profile dominated by mitochondrial polymorphisms. This profile can serve as a predictor of future complications, including RS and recurrent UTIs.

The Roles of CircRNAs in Mitochondria

Mitochondria participate in varieties of cellular events. It is widely accepted that human mitochondrial genome encodes 13 proteins, 2 rRNAs, and 22 tRNAs. Gene variation derived from human nuclear genome cannot completely explain mitochondrial diseases. The advent of high-throughput sequencing coupled with novel bioinformatic analyses decode the complexity of mitochondria-derived transcripts. Recently, circular RNAs (circRNAs) from both human mitochondrial genome and nuclear genome have been found to be located at mitochondria. Studies about the roles and molecular mechanisms underlying trafficking of the nucleus encoded circRNAs to mitochondria and mitochondria encoded circRNAs to the nucleus or cytoplasm in mammals are only beginning to emerge. These circRNAs have been associated with a variety of diseases, especially cancers. Here, we discuss the emerging field of mitochondria-located circRNAs by reviewing their identification, expression patterns, regulatory roles, and functional mechanisms. Mitochondria-located circRNAs have regulatory roles in cellular physiology and pathology. We also highlight future perspectives and challenges in studying mitochondria-located circRNAs, as well as their potential biomedical applications.

Inherited non-FGF23-mediated phosphaturic disorders: A kidney-centric review

Phosphate is freely filtered by the glomerulus and reabsorbed exclusively in the proximal tubule by two key transporters, NaPiIIA and NaPiIIC, encoded by SLC34A1 and SLC34A3, respectively. Regulation of these transporters occurs primarily through the hormone FGF23 and, to a lesser degree, PTH. Consequently, inherited non-FGF23 mediated phosphaturic disorders are due to generalised proximal tubular dysfunction, loss-of-function variants in SLC34A1 or SLC34A3 or excess PTH signalling. The corresponding disorders are Renal Fanconi Syndrome, Infantile Hypercalcaemia type 2, Hereditary Hypophosphataemic Rickets with Hypercalciuria and Familial Hyperparathyroidism. Several inherited forms of Fanconi renotubular syndrome (FRTS) have also been described with the underlying genes encoding for GATM, EHHADH, HNF4A and NDUFAF6. Here, we will review their pathophysiology, clinical manifestations and the implications for treatment from a kidney-centric perspective, focussing on those disorders caused by dysfunction of renal phosphate transporters. Moreover, we will highlight specific genetic aspects, as the availability of large population genetic databases has raised doubts about some of the originally proposed gene-disease associations concerning phosphate transporters or their associated proteins.

Post-transplant recurrence of focal segmental glomerular sclerosis: consensus statements

Focal segmental glomerular sclerosis (FSGS) is 1 of the primary causes of nephrotic syndrome in both pediatric and adult patients, which can lead to end-stage kidney disease. Recurrence of FSGS after kidney transplantation significantly increases allograft loss, leading to morbidity and mortality. Currently, there are no consensus guidelines for identifying those patients who are at risk for recurrence or for the management of recurrent FSGS. Our work group performed a literature search on PubMed/Medline, Embase, and Cochrane, and recommendations were proposed and graded for strength of evidence. Of the 614 initially identified studies, 221 were found suitable to formulate consensus guidelines for recurrent FSGS. These guidelines focus on the definition, epidemiology, risk factors, pathogenesis, and management of recurrent FSGS. We conclude that additional studies are required to strengthen the recommendations proposed in this review.

Mitochondrial Dysfunction in Kidney Tubulopathies

Mitochondria play a key role in kidney physiology and pathology. They produce ATP to fuel energy-demanding water and solute reabsorption processes along the nephron. Moreover, mitochondria contribute to cellular health by the regulation of autophagy, (oxidative) stress responses, and apoptosis. Mitochondrial abundance is particularly high in cortical segments, including proximal and distal convoluted tubules. Dysfunction of the mitochondria has been described for tubulopathies such as Fanconi, Gitelman, and Bartter-like syndromes and renal tubular acidosis. In addition, mitochondrial cytopathies often affect renal (tubular) tissues, such as in Kearns-Sayre and Leigh syndromes. Nevertheless, the mechanisms by which mitochondrial dysfunction results in renal tubular diseases are only scarcely being explored. This review provides an overview of mitochondrial dysfunction in the development and progression of kidney tubulopathies. Furthermore, it emphasizes the need for further mechanistic investigations to identify links between mitochondrial function and renal electrolyte reabsorption.

Focus on Mitochondrial Respiratory Chain: Potential Therapeutic Target for Chronic Renal Failure

The function of the respiratory chain is closely associated with kidney function, and the dysfunction of the respiratory chain is a primary pathophysiological change in chronic kidney failure. The incidence of chronic kidney failure caused by defects in respiratory-chain-related genes has frequently been overlooked. Correcting abnormal metabolic reprogramming, rescuing the "toxic respiratory chain", and targeting the clearance of mitochondrial reactive oxygen species are potential therapies for treating chronic kidney failure. These treatments have shown promising results in slowing fibrosis and inflammation progression and improving kidney function in various animal models of chronic kidney failure and patients with chronic kidney disease (CKD). The mitochondrial respiratory chain is a key target worthy of attention in the treatment of chronic kidney failure. This review integrated research related to the mitochondrial respiratory chain and chronic kidney failure, primarily elucidating the pathological status of the mitochondrial respiratory chain in chronic kidney failure and potential therapeutic drugs. It provided new ideas for the treatment of kidney failure and promoted the development of drugs targeting the mitochondrial respiratory chain.

Development of mitochondrial gene-editing strategies and their potential applications in mitochondrial hereditary diseases: a review

Mitochondrial DNA (mtDNA) is a critical genome contained within the mitochondria of eukaryotic cells, with many copies present in each mitochondrion. Mutations in mtDNA often are inherited and can lead to severe health problems, including various inherited diseases and premature aging. The lack of efficient repair mechanisms and the susceptibility of mtDNA to damage exacerbate the threat to human health. Heteroplasmy, the presence of different mtDNA genotypes within a single cell, increases the complexity of these diseases and requires an effective editing method for correction. Recently, gene-editing techniques, including programmable nucleases such as restriction endonuclease, zinc finger nuclease, transcription activator-like effector nuclease, clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeats-associated 9 and base editors, have provided new tools for editing mtDNA in mammalian cells. Base editors are particularly promising because of their high efficiency and precision in correcting mtDNA mutations. In this review, we discuss the application of these techniques in mitochondrial gene editing and their limitations. We also explore the potential of base editors for mtDNA modification and discuss the opportunities and challenges associated with their application in mitochondrial gene editing. In conclusion, this review highlights the advancements, limitations and opportunities in current mitochondrial gene-editing technologies and approaches. Our insights aim to stimulate the development of new editing strategies that can ultimately alleviate the adverse effects of mitochondrial hereditary diseases.

Hyporeninemic hypoaldosteronism in RMND1-related mitochondrial disease

BACKGROUND

RMND1 is a nuclear gene needed for proper function of mitochondria. A pathogenic gene will cause multiple oxidative phosphorylation defects. A renal phenotype consisting of hyponatremia, hyperkalemia, and acidosis is frequently reported, previously considered to be due to aldosterone insensitivity.

METHODS

Clinical features and pathophysiology of three patients will be reported. DNA of these patients was subjected to exome screening.

RESULTS

In the first family, one pathogenic heterozygous and one highly probable heterozygous mutation were detected. In the second family, a homozygous pathogenic mutation was present. The electrolyte disbalance was not due to aldosterone insensitivity but to low plasma aldosterone concentration, a consequence of low plasma renin activity. This disbalance can be treated. In all three patients, the kidney function declined. In the first family, both children suffered from an unexplained arterial thrombosis with dire consequences.

CONCLUSIONS

Hyporeninemic hypoaldosteronism is the mechanism causing the electrolyte disbalance reported in patients with RMND1 mutations, and can be treated.

Ataxia Syndrome With Hearing Loss and Nephronophthisis Associated With a Novel Homozygous Variant in XPNPEP3

Objectives

Biallelic variants in XPNPEP3 are associated with a rare mitochondrial syndrome characterized by nephronophthisis leading to kidney failure, essential tremor, hearing loss, seizures, and intellectual disability. Only 2 publications on this condition are available. We report a man with a complex ataxia syndrome, hearing loss, and kidney failure associated with a new biallelic variant in XPNPEP3.

Methods

Clinical evaluation, neuroimaging studies, a kidney biopsy, and whole genome sequencing (WGS) were applied. Since the phenotype was compatible with a mitochondrial disease, a muscle biopsy with morphological and mitochondrial biochemical investigations was performed.

Results

Axial ataxia, cerebellar atrophy, hearing loss, myopathy, ptosis, supranuclear palsy, and kidney failure because of nephronophthisis were the prominent features in this case. WGS revealed the novel biallelic variant c.766C>T (p.Gln256*) in XPNPEP3. A muscle biopsy revealed COX negative fibers, a few ragged red fibers, and ultrastructural mitochondrial changes. Enzyme activity in respiratory chain complex IV was reduced in muscle and fibroblasts.

Discussion

This is the first report of a slowly progressive cerebellar ataxia associated with a novel biallelic variant in XPNPEP3. Abnormalities typical for mitochondrial disease and the slow progression of kidney disease are also striking. Our report expands the spectrum of XPNPEP3-related diseases.

Methylene Blue Induces Antioxidant Defense and Reparation of Mitochondrial DNA in a Nrf2-Dependent Manner during Cisplatin-Induced Renal Toxicity

Cisplatin is a platinum-based cytostatic drug that is widely used for cancer treatment. Mitochondria and mtDNA are important targets for platinum-based cytostatics, which mediates its nephrotoxicity. It is important to develop therapeutic approaches to protect the kidneys from cisplatin during chemotherapy. We showed that the exposure of mitochondria to cisplatin increased the level of lipid peroxidation products in the in vitro experiment. Cisplatin caused strong damage to renal mtDNA, both in the in vivo and in vitro experiments. Cisplatin injections induced oxidative stress by depleting renal antioxidants at the transcriptome level but did not increase the rate of H₂O₂ production in isolated mitochondria. Methylene blue, on the contrary, induced mitochondrial H₂O₂ production. We supposed that methylene blue-induced H₂O₂ production led to activation of the Nrf2/ARE signaling pathway. The consequences of activation of this signaling pathway were manifested in an increase in the expression of some antioxidant genes, which likely caused a decrease in the amount of mtDNA damage. Methylene blue treatment induced an increase in the expression of genes that were involved in the base excision repair (BER) pathway: the main pathway for mtDNA reparation. It is known that the expression of these genes can also be regulated by the Nrf2/ARE signaling pathway. We can assume that the protective effect of methylene blue is related to the activation of Nrf2/ARE signaling pathways, which can activate the expression of genes related to antioxidant defense and mtDNA reparation. Thus, the protection of kidney mitochondria from cisplatin-induced damage using methylene blue can significantly expand its application in medicine.

Analysis of mitochondrial DNA mutations in Pakistani population diagnosed with cardiovascular diseases

Heart and blood vessel disorders, such as coronary heart disease, brain vessel disease, rheumatic heart disease, and others, are together referred to as cardiovascular disease (CVD). In this study, we sought to determine how mitochondrial Leucine Transfer RNA genes and CVDs are related (MT-L1 and MT-L2). From CVD patients in Peshawar, a total of 27 saliva samples were taken. Leu-tRNA genes expressed by mitochondria were amplified using polymerase chain reaction after DNA was removed. Ten samples were sent for sequencing after PCR and gene cleaning. We obtained all of the sequenced results, which were subsequently aligned and evaluated against the mitochondrial revised Cambridge Reference Sequence (rCRS). However, in our sequenced samples, Leu-tRNA MT-L1 and MT-L2 genes were determined to be unaltered. Thus, it is suggested that a large population be taken into account while screening for mutations in the mitochondrial encoded Leu-tRNA MT-L1 and MT-L2 genes of cardiac patients in areas of Pakistan. Additionally, it is recommended that patients with cardiac problems should also have other mitochondrial encoded genes checked for potential mutations. This could result in the identification of genetic markers that could be used for early CVD screening in Pakistan.

A Multisystem Mitochondrial Disease Caused by a Novel MT-TL1 mtDNA Variant: A Case Report

BACKGROUND

Mitochondrial tRNA (MTT) genes are hotspot for mitochondrial DNA mutation and are responsible of half mitochondrial disease. MTT mutations are associated with a broad spectrum of phenotype often with complex multisystem involvement and complex genotype-phenotype correlations. MT-TL1 mutations, among which the m.3243A>G mutation is the most frequent, are associated with myopathy, maternal inherited diabetes and deafness, MELAS, cardiomyopathy, and focal segmental glomerulosclerosis.

CASE STUDY

Here we report the case of an Italian 49-years old female presenting with encephalomyopathy, chronic proteinuric kidney disease and a new heteroplasmic m.3274_3275delAC MT-TL1 gene mutation.

CONCLUSIONS

Our case demonstrates a systemic mitochondrial disease caused by the heteroplasmic m.3274_3275delAC MT-TL1 gene mutation, not yet described in the literature. A mitochondrial disease should be suspected in case of complex multisystem phenotypes, including steroid-resistant nephrotic syndrome with multisystemic involvement.

Environmental Chemical Exposures and Mitochondrial Dysfunction: a Review of Recent Literature

PURPOSE OF REVIEW

Mitochondria play various roles that are important for cell function and survival; therefore, significant mitochondrial dysfunction may have chronic consequences that extend beyond the cell. Mitochondria are already susceptible to damage, which may be exacerbated by environmental exposures. Therefore, the aim of this review is to summarize the recent literature (2012-2022) looking at the effects of six ubiquitous classes of compounds on mitochondrial dysfunction in human populations.

RECENT FINDINGS

The literature suggests that there are a number of biomarkers that are commonly used to identify mitochondrial dysfunction, each with certain advantages and limitations. Classes of environmental toxicants such as polycyclic aromatic hydrocarbons, air pollutants, heavy metals, endocrine-disrupting compounds, pesticides, and nanomaterials can damage the mitochondria in varied ways, with changes in mtDNA copy number and measures of oxidative damage the most commonly measured in human populations. Other significant biomarkers include changes in mitochondrial membrane potential, calcium levels, and ATP levels. This review identifies the biomarkers that are commonly used to characterize mitochondrial dysfunction but suggests that emerging mitochondrial biomarkers, such as cell-free mitochondria and blood cardiolipin levels, may provide greater insight into the impacts of exposures on mitochondrial function. This review identifies that the mtDNA copy number and measures of oxidative damage are commonly used to characterize mitochondrial dysfunction, but suggests using novel approaches in addition to well-characterized ones to create standardized protocols. We identified a dearth of studies on mitochondrial dysfunction in human populations exposed to metals, endocrine-disrupting chemicals, pesticides, and nanoparticles as a gap in knowledge that needs attention.

Renal involvement is frequent in adults with primary mitochondrial disorders: an observational study

Background

Mitochondrial functions are controlled by genes of both mitochondrial and nuclear DNA. Pathogenic variants affecting any of these are responsible for primary mitochondrial disorders (MIDs), which can be diagnosed during adulthood. Kidney functions are highly dependent on mitochondrial respiration. However, the prevalence of MID-associated nephropathies (MIDANs) is unknown in the adult population. We aimed to address this point and to provide a full characterization of MIDANs in this population.

Methods

We retrospectively included for observational study adults (≥16 years of age) with genetically diagnosed MID between 2000 and 2020 in our tertiary care academic centre when they had a chronic kidney disease (CKD) evaluation. MIDANs were ascertained by CKD occurring in MIDs. The phenotypic, biological, histopathological and genotypic characteristics were recorded from the medical charts.

Results

We included 80 MID-affected adults and ascertained MIDANs in 28/80 (35%). Kidney diseases under the care of a nephrologist occurred in only 14/28 (50%) of the adults with MIDAN. MIDANs were tubulointerstitial nephropathy in 14/28 patients (50%) and glomerular diseases in 9/28 (32.1%). In adults with MID, MIDAN was negatively associated with higher albumin levels {odds ratio [OR] 0.79 [95% confidence interval (CI) 0.67-0.95]} and vision abnormalities [OR 0.17 (95% CI 0.03-0.94)] and positively associated with hypertension [OR 4.23 (95% CI 1.04-17.17)].

Conclusion

MIDANs are frequent among adult MIDs. They are mostly represented by tubulointerstitial nephropathy or glomerular disease. Vision abnormalities, hypertension and albumin levels were independently associated with MIDANs. Our results pave the way for prospective studies investigating the prevalence of MIDANs among undetermined kidney disease populations.

Recent Advances in Mitochondrial Diseases: from Molecular Insights to Therapeutic Perspectives

Mitochondria are double-membraned cytoplasmic organelles that are responsible for the production of energy in eukaryotic cells. The process is completed through oxidative phosphorylation (OXPHOS) by the respiratory chain (RC) in mitochondria. Thousands of mitochondria may be present in each cell, depending on the function of that cell. Primary mitochondria disorder (PMD) is a clinically heterogeneous disease associated with germline mutations in mitochondrial DNA (mtDNA) and/or nuclear DNA (nDNA) genes, and impairs mitochondrial structure and function. Mitochondrial dysfunction can be detected in early childhood and may be severe, progressive and often multi-systemic, involving a wide range of organs. Understanding epigenetic factors and pathways mutations can help pave the way for developing an effective cure. However, the lack of information about the disease (including age of onset, symptoms, clinical phenotype, morbidity and mortality), the limits of current preclinical models and the wide range of phenotypic presentations hamper the development of effective medicines. Although new therapeutic approaches have been introduced with encouraging preclinical and clinical outcomes, there is no definitive cure for PMD. This review highlights recent advances, particularly in children, in terms of etiology, pathophysiology, clinical diagnosis, molecular pathways and epigenetic alterations. Current therapeutic approaches, future advances and proposed new therapeutic plans will also be discussed.

Renal Phenotype in Mitochondrial Diseases: A Multicenter Study

AIMS

This study aimed to investigate associations between renal and extrarenal manifestations of mitochondrial diseases and their natural history as well as predictors of renal disease severity and overall disease outcome. The secondary aim was to generate a protocol of presymptomatic assessment and monitoring of renal function in patients with a defined mitochondrial disease.

METHODS

A multicenter, retrospective cohort study was performed by the Mitochondrial Clinical and Research Network (MCRN). Patients of any age with renal manifestations associated with a genetically verified mitochondrial disease were included from 8 expert European centers specializing in mitochondrial diseases: Gothenburg, Oulu, Copenhagen, Bergen, Helsinki, Stockholm, Rotterdam, and Barcelona.

RESULTS

Of the 36 patients included, two-thirds had mitochondrial DNA-associated disease. Renal manifestations were the first sign of mitochondrial disease in 19%, and renal involvement was first identified by laboratory tests in 57% of patients. Acute kidney injury occurred in 19% of patients and was the first sign of renal disease in the majority of these. The most common renal manifestation was chronic kidney disease (75% with stage 2 or greater), followed by tubulopathy (44.4%), the latter seen mostly among patients with single large-scale mitochondrial DNA deletions. Acute kidney injury and tubulopathy correlated with worse survival outcome. The most common findings on renal imaging were increased echogenicity and renal dysplasia/hypoplasia. Renal histology revealed focal segmental glomerulosclerosis, nephrocalcinosis, and nephronophthisis.

CONCLUSION

Acute kidney injury is a distinct renal phenotype in patients with mitochondrial disease. Our results highlight the importance to recognize renal disease as a sign of an underlying mitochondrial disease. Acute kidney injury and tubulopathy are 2 distinct indicators of poor survival in patients with mitochondrial diseases.

Potential of Polyphenols to Restore SIRT1 and NAD+ Metabolism in Renal Disease

SIRT1 is an NAD⁺-dependent class III histone deacetylase that is abundantly expressed in the kidney, where it modulates gene expression, apoptosis, energy homeostasis, autophagy, acute stress responses, and mitochondrial biogenesis. Alterations in SIRT1 activity and NAD⁺ metabolism are frequently observed in acute and chronic kidney diseases of diverse origins, including obesity and diabetes. Nevertheless, in vitro and in vivo studies and clinical trials with humans show that the SIRT1-activating compounds derived from natural sources, such as polyphenols found in fruits, vegetables, and plants, including resveratrol, quercetin, and isoflavones, can prevent disease and be part of treatments for a wide variety of diseases. Here, we summarize the roles of SIRT1 and NAD⁺ metabolism in renal pathophysiology and provide an overview of polyphenols that have the potential to restore SIRT1 and NAD⁺ metabolism in renal diseases.

Clinicopathological Features of Mitochondrial Nephropathy

Introduction

The clinicopathologic characteristics of nephropathy associated with mitochondrial disease (MD) remain unknown. We retrospectively analyzed a cohort of patients with proteinuria, decreased glomerular filtration rate, or Fanconi syndrome who had a genetic mutation confirmed as the cause of MD, defined as mitochondrial nephropathy.

Methods

This nationwide survey included 757 nephrology sections throughout Japan, and consequently, data on 81 cases of mitochondrial nephropathy were collected.

Results

The most common renal manifestation observed during the disease course was proteinuria. Hearing loss was the most common comorbidity; a renal-limited phenotype was observed only in mitochondrial DNA (mtDNA) point mutation and COQ8B mutation cases. We found a median time delay of 6.0 years from onset of renal manifestations to diagnosis. Focal segmental glomerular sclerosis (FSGS) was the most common pathologic diagnosis. We then focused on 63 cases with the m.3243A>G mutation. The rate of cases with diabetes was significantly higher among adult-onset cases than among childhood-onset cases. Pathologic diagnoses were more variable in adult-onset cases, including diabetic nephropathy, nephrosclerosis, tubulointerstitial nephropathy, and minor glomerular abnormalities. During the median observation period of 11.0 years from the first onset of renal manifestations in patients with m.3243A>G, renal replacement therapy (RRT) was initiated in 50.8% of patients. Death occurred in 25.4% of the patients during the median observation period of 12.0 years. The median estimated glomerular filtration rate (eGFR) decline was 5.4 ml/min per 1.73 m²/yr in the cases, especially 8.3 ml/min per 1.73 m²/yr in FSGS cases, with m.3243A>G.

Conclusion

Here, we described the clinicopathologic features and prognosis of mitochondrial nephropathy using large-scale data.

Hereditary kidney diseases associated with hypomagnesemia

In the kidney, a set of proteins expressed in the epithelial cells of the thick ascending loop of Henle and the distal convoluted tubule directly or indirectly play important roles in the regulation of serum magnesium levels. Magnesium reabsorption in the thick ascending loop of Henle occurs through a passive paracellular pathway, while in the distal convoluted tubule, the final magnesium concentration is established through an active transcellular pathway. The players involved in magnesium reabsorption include proteins with diverse functions including tight junction proteins, cation and anion channels, sodium chloride cotransporter, calcium-sensing receptor, epidermal growth factor, cyclin M2, sodium potassium adenosine triphosphatase subunits, transcription factors, a serine protease, and proteins involved in mitochondrial function. Mutations in the genes that encode these proteins impair their function and cause different rare diseases associated with hypomagnesemia, which may lead to muscle cramps, fatigue, epileptic seizures, intellectual disability, cardiac arrhythmias, and chronic kidney disease. The purpose of this review is to describe the clinical and genetic characteristics of these hereditary kidney diseases and the current research findings on the pathophysiological basis of these diseases.

BCS1L mutations produce Fanconi syndrome with developmental disability

Fanconi syndrome is a functional disorder of the proximal tubule, characterized by pan-aminoaciduria, glucosuria, hypophosphatemia, and metabolic acidosis. With the advancements in gene analysis technologies, several causative genes are identified for Fanconi syndrome. Several mitochondrial diseases cause Fanconi syndrome and various systemic symptoms; however, it is rare that the main clinical symptoms in such disorders are Fanconi syndrome without systematic active diseases like encephalomyopathy or cardiomyopathy. In this study, we analyzed two families exhibiting Fanconi syndrome, developmental disability and mildly elevated liver enzyme levels. Whole-exome sequencing (WES) detected compound heterozygous known and novel BCS1L mutations, which affect the assembly of mitochondrial respiratory chain complex III, in both cases. The pathogenicity of these mutations has been established in several mitochondria-related functional analyses in this study. Mitochondrial diseases with isolated renal symptoms are uncommon; however, this study indicates that mitochondrial respiratory chain complex III deficiency due to BCS1L mutations cause Fanconi syndrome with developmental disability as the primary indications.

Growth Retardation in the Course of Fanconi Syndrome Caused by the 4977-bp Mitochondrial DNA Deletion: A Case Report

Fanconi syndrome is one of the primary renal manifestations of mitochondrial cytopathies caused by mitochondrial DNA (mtDNA) mutation. The common 4977-bp mtDNA deletion has been reported to be associated with aging and diseases involving multiple extrarenal organs. Cases of Fanconi syndrome caused by the 4977-bp deletion were rarely reported previously. Here, we report a 6-year-old girl with growth retardation in the course of Fanconi syndrome. She had mild ptosis and pigmented retinopathy. Abnormal biochemical findings included low-molecular-weight proteinuria, normoglycemic glycosuria, increased urine phosphorus excretion, metabolic acidosis, and hypophosphatemia. Growth records showed that her body weight and height were normal in the first year and failed to thrive after the age of three. Using a highly sensitive mtDNA analysis methodology, she was identified to possess the common 4977-bp mtDNA deletion. The mutation rate was 84.7% in the urine exfoliated cells, 78.67% in the oral mucosal cells, and 23.99% in the blood sample. After three months of oral coenzyme Q10 and levocarnitine treatment in combination with standard electrolyte supplement, her condition was improved. This is a report of growth retardation as the initial major clinical presentation of Fanconi syndrome caused by the deletion of the 4977-bp fragment. Renal tubular abnormality without any other extrarenal dysfunction may be an initial clinical sign of mitochondrial disorders. Moreover, considering the heterogeneity of the phenotypes associated with mtDNA mutations, the risk of developing Kearns–Sayre syndrome (KSS) with age in this patient should be noted because she had ptosis, retinal involvement, and changes in the brain and skeletal muscle.

ING2 Controls Mitochondrial Respiration via Modulating MRPL12 Ubiquitination in Renal Tubular Epithelial Cells

Mitochondrial injury of tubular epithelial cells (TECs) is the key pathogenic event underlying various kidney diseases and a potential intervening target as well. Our previous study demonstrated that ING2 is ubiquitously expressed at tubulointerstitial area within kidneys, while its role in regulating TEC mitochondrial respiration is not fully elucidated. To clarify the roles of ING2 in mitochondrial homeostasis of TECs and pathogenesis of acute ischemic kidney injury, Western blot, PCR, immunofluorescence, immunoprecipitation, and oxygen consumption rate assay were applied to address the roles of ING2 in modulating mitochondrial respiration. We further complemented these studies with acute ischemic kidney injury both in vitro and in vivo. In vitro study demonstrated ING2 could positively control TEC mitochondrial respiration. Concurrently, both mRNA and protein levels of mtDNA encoded respiratory chain components were altered by ING2, suggesting ING2 could regulate mtDNA transcription. In mechanism, ING2 could regulate the ubiquitination of a newly identified mitochondrial transcription factor MRPL12, thereby modulating its cellular stability and abundance. We also demonstrated ING2-mediated modulation on mtDNA transcription and mitochondrial respiration are involved in serum deprivation induced TEC injuries. Finally, immunohistochemistry study revealed that ING2 expression was significantly altered in kidney biopsies with acute ischemic kidney injury. In vivo study suggested that kidney specific ING2 overexpression could effectively ameliorate acute ischemic kidney injury. Our study demonstrated that ING2 is a crucial modulator of TEC mitochondrial respiration. These findings suggested a unrecognized role of ING2 in TEC mitochondrial energetic homeostasis and a potential intervening target for TEC mitochondrial injury associated pathologies.

Possible role of the mitochondrial genome in the pathogenesis of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common monogenic renal disease in adults and is due to heterozygous germ line variants in either PKD1, PKD2 or rarely other genes. It is characterized by marked intra-familial disease variability suggesting that other genetic and/or environmental factors are involved in determining the lifetime course ADPKD. Recently, research indicates that polycystin-mediated mitochondrial dysfunction and metabolic re-programming contributes to the progression of ADPKD. Although biochemical abnormalities have gained the most interest, variants in the mitochondrial genome could be one of the mechanisms underlying the phenotypic variability in ADPKD. This narrative review aims to evaluate the role of the mitochondrial genome in the pathogenesis of APDKD.

Distinct Mitochondrial Pathologies Caused by Mutations of the Proximal Tubular Enzymes EHHADH and GATM

The mitochondria of the proximal tubule are essential for providing energy in this nephron segment, whose ATP generation is almost exclusively oxygen dependent. In addition, mitochondria are involved in a variety of metabolic processes and complex signaling networks. Proximal tubular mitochondrial dysfunction can therefore affect renal function in very different ways. Two autosomal dominantly inherited forms of renal Fanconi syndrome illustrate how multifaceted mitochondrial pathology can be: Mutation of EHHADH, an enzyme in fatty acid metabolism, results in decreased ATP synthesis and a consecutive transport defect. In contrast, mutations of GATM, an enzyme in the creatine biosynthetic pathway, leave ATP synthesis unaffected but do lead to mitochondrial protein aggregates, inflammasome activation, and renal fibrosis with progressive renal failure. In this review article, the distinct pathophysiological mechanisms of these two diseases are presented, which are examples of the spectrum of proximal tubular mitochondrial diseases.

Excessively Enlarged Mitochondria in the Kidneys of Diabetic Nephropathy

Diabetic nephropathy (DN) is the most serious complication of diabetes and a leading cause of kidney failure and mortality in patients with diabetes. However, the exact pathogenic mechanisms involved are poorly understood. Impaired mitochondrial function and accumulation of damaged mitochondria due to increased imbalance in mitochondrial dynamics are known to be involved in the development and progression of DN. Accumulating evidence suggests that aberrant mitochondrial fission is involved in the progression of DN. Conversely, studies linking excessively enlarged mitochondria to DN pathogenesis are emerging. In this review, we summarize the current concepts of imbalanced mitochondrial dynamics and their molecular aspects in various experimental models of DN. We discuss the recent evidence of enlarged mitochondria in the kidneys of DN and examine the possibility of a therapeutic application targeting mitochondrial dynamics in DN.