Tubulointerstitial nephritis associated with a novel mitochondrial point mutation

Heteroplasmic and homoplasmic m.616T>C in mitochondria tRNAPhe promote isolated chronic kidney disease and hyperuricemia

Inherited kidney diseases are the fifth most common cause of end-stage renal disease (ESRD). Mitochondrial dysfunction plays a vital role in the progression of inherited kidney diseases, while mitochondrial-transfer RNA (mt-tRNA) variants and their pathogenic contributions to kidney disease remain largely unclear. In this study, we identified the pathogenic mt-tRNA^Phe 616T>C mutation in 3 families and documented that m.616T>C showed a high pathogenic threshold, with both heteroplasmy and homoplasmy leading to isolated chronic kidney disease and hyperuricemia without hematuria, proteinuria, or renal cyst formation. Moreover, 1 proband with homoplamic m.616T>C presented ESRD as a child. No symptoms of nervous system evolvement were observed in these families. Lymphoblast cells bearing m.616T>C exhibited swollen mitochondria, underwent active mitophagy, and showed respiratory deficiency, leading to reduced mitochondrial ATP production, diminished membrane potential, and overproduction of mitochondrial ROS. Pathogenic m.616T>C abolished a highly conserved base pair (A31-U39) in the anticodon stem-loop which altered the structure of mt-tRNA^Phe, as confirmed by a decreased melting temperature and slower electrophoretic mobility of the mutant tRNA. Furthermore, the unstable structure of mt-tRNA^Phe contributed to a shortage of steady-state mt-tRNA^Phe and enhanced aminoacylation efficiency, which resulted in impaired mitochondrial RNA translation and a significant decrease in mtDNA–encoded polypeptides. Collectively, these findings provide potentially new insights into the pathogenesis underlying inherited kidney disease caused by mitochondrial variants.

Gitelman-Like Syndrome Caused by Pathogenic Variants in mtDNA

BACKGROUND

Gitelman syndrome is the most frequent hereditary salt-losing tubulopathy characterized by hypokalemic alkalosis and hypomagnesemia. Gitelman syndrome is caused by biallelic pathogenic variants in SLC12A3, encoding the Na⁺-Cl^- cotransporter (NCC) expressed in the distal convoluted tubule. Pathogenic variants of CLCNKB, HNF1B, FXYD2, or KCNJ10 may result in the same renal phenotype of Gitelman syndrome, as they can lead to reduced NCC activity. For approximately 10 percent of patients with a Gitelman syndrome phenotype, the genotype is unknown.

METHODS

We identified mitochondrial DNA (mtDNA) variants in three families with Gitelman-like electrolyte abnormalities, then investigated 156 families for variants in MT-TI and MT-TF, which encode the transfer RNAs for phenylalanine and isoleucine. Mitochondrial respiratory chain function was assessed in patient fibroblasts. Mitochondrial dysfunction was induced in NCC-expressing HEK293 cells to assess the effect on thiazide-sensitive ²²Na⁺ transport.

RESULTS

Genetic investigations revealed four mtDNA variants in 13 families: m.591C>T (n=7), m.616T>C (n=1), m.643A>G (n=1) (all in MT-TF), and m.4291T>C (n=4, in MT-TI). Variants were near homoplasmic in affected individuals. All variants were classified as pathogenic, except for m.643A>G, which was classified as a variant of uncertain significance. Importantly, affected members of six families with an MT-TF variant additionally suffered from progressive chronic kidney disease. Dysfunction of oxidative phosphorylation complex IV and reduced maximal mitochondrial respiratory capacity were found in patient fibroblasts. In vitro pharmacological inhibition of complex IV, mimicking the effect of the mtDNA variants, inhibited NCC phosphorylation and NCC-mediated sodium uptake.

CONCLUSION

Pathogenic mtDNA variants in MT-TF and MT-TI can cause a Gitelman-like syndrome. Genetic investigation of mtDNA should be considered in patients with unexplained Gitelman syndrome-like tubulopathies.

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.

Brain dysfunction in tubular and tubulointerstitial kidney diseases

Kidney function has two important elements: glomerular filtration and tubular function (secretion and reabsorption). A persistent decrease in glomerular filtration rate (GFR), with or without proteinuria, is diagnostic of chronic kidney disease (CKD). While glomerular injury or disease is a major cause of CKD and usually associated with proteinuria, predominant tubular injury, with or without tubulointerstitial disease, is typically non-proteinuric. CKD has been linked with cognitive impairment, but it is unclear how much this depends on a reduced GFR, altered tubular function or the presence of proteinuria. Since CKD is often accompanied by tubular and interstitial dysfunction, we explore here for the first time the potential role of the tubular and tubulointerstitial compartments in cognitive dysfunction. To help address this issue, we have selected a group of primary tubular diseases with preserved GFR, in which to review the evidence for any association with brain dysfunction. Cognition, mood, neurosensory, and motor disturbances are not well characterized in tubular diseases, possibly because they are subclinical and less prominent than other clinical manifestations. The available literature suggests that brain dysfunction in tubular and tubulointerstitial diseases is usually mild and is more often seen in disorders of water handling. Brain dysfunction may occur when severe electrolyte and water disorders in young children persist over a long period of time before the diagnosis is made. We have chosen as examples to highlight this topic, Bartter and Gitelman syndromes and nephrogenic diabetes insipidus. We discuss current published findings, some unanswered questions, and propose topics for future research.

Mitochondrial DNA mutations in renal disease: an overview

Kidneys have a high energy demand to facilitate the reabsorption of the glomerular filtrate. For this reason, renal cells have a high density of mitochondria. Mitochondrial cytopathies can be the result of a mutation in both mitochondrial and nuclear DNA. Mitochondrial dysfunction can lead to a variety of renal manifestations. Examples of tubular manifestations are renal Fanconi Syndrome, which is often found in patients diagnosed with Kearns-Sayre and Pearson's marrow-pancreas syndrome, and distal tubulopathies, which result in electrolyte disturbances such as hypomagnesemia. Nephrotic syndrome can be a glomerular manifestation of mitochondrial dysfunction and is typically associated with focal segmental glomerular sclerosis on histology. Tubulointerstitial nephritis can also be seen in mitochondrial cytopathies and may lead to end-stage renal disease. The underlying mechanisms of these cytopathies remain incompletely understood; therefore, current therapies focus mainly on symptom relief. A better understanding of the molecular disease mechanisms is critical in order to improve treatments.

Mitochondrial Disease and the Kidney With a Special Focus on CoQ10 Deficiency

Mitochondrial cytopathies include a heterogeneous group of diseases that are characterized by impaired oxidative phosphorylation, leading to multi-organ involvement and progressive clinical deterioration. Most mitochondrial cytopathies that cause kidney symptoms are characterized by tubular defects, but glomerular, tubulointerstitial, and cystic diseases have also been described. Mitochondrial cytopathies can result from mitochondrial or nuclear DNA mutations. Early recognition of defects in the coenzyme Q10 (CoQ10) biosynthesis is important, as patients with primary CoQ10 deficiency may be responsive to treatment with oral CoQ10 supplementation, in contrast to most mitochondrial diseases. A literature search was conducted to investigate kidney involvement in genetic mitochondrial cytopathies and to identify mitochondrial and nuclear DNA mutations involved in mitochondrial kidney disease. Furthermore, we identified all reported cases to date with a CoQ10 deficiency with glomerular involvement, including 3 patients with variable renal phenotypes in our clinic. To date, 144 patients from 95 families with a primary CoQ10 deficiency and glomerular involvement have been described based on mutations in PDSS1, PDSS2, COQ2, COQ6, and COQ8B/ADCK4. This review provides an overview of kidney involvement in genetic mitochondrial cytopathies with a special focus on CoQ10 deficiency.

Renal manifestations of primary mitochondrial disorders

The aim of the present review was to summarize and discuss previous findings concerning renal manifestations of primary mitochondrial disorders (MIDs). A literature review was performed using frequently used databases. The study identified that primary MIDs frequently present as mitochondrial multiorgan disorder syndrome (MIMODS) at onset or in the later course of the MID. Occasionally, the kidneys are affected in MIDs. Renal manifestations of MIDs include renal insufficiency, nephrolithiasis, nephrotic syndrome, renal cysts, renal tubular acidosis, Bartter-like syndrome, Fanconi syndrome, focal segmental glomerulosclerosis, tubulointerstitial nephritis, nephrocalcinosis, and benign or malign neoplasms. Among the syndromic MIDs, renal involvement has been most frequently reported in patients with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes syndrome, Kearns-Sayre syndrome, Leigh syndrome and mitochondrial depletion syndromes. Only in single cases was renal involvement also reported in chronic progressive external ophthalmoplegia, Pearson syndrome, Leber's hereditary optic neuropathy, coenzyme-Q deficiency, X-linked sideroblastic anemia and ataxia, myopathy, lactic acidosis, and sideroblastic anemia, pyruvate dehydrogenase deficiency, growth retardation, aminoaciduria, cholestasis, iron overload, lactacidosis, and early death, and hyperuricemia, pulmonary hypertension, renal failure in infancy and alkalosis syndrome. The present study proposes that the frequency of renal involvement in MIDs is probably underestimated. Diagnosis of renal involvement follows general guidelines and treatment is symptomatic. Thus, renal manifestations of primary MIDs require recognition and appropriate management, as they determine the outcome of MID patients.

Focal segmental glomerulosclerosis associated with mitochondrial disease

Primary mitochondrial diseases (MD) are complex, heterogeneous inherited diseases caused by mutations in either the mitochondrial or nuclear DNA. Glomerular diseases in MD have been reported with tRNA mutation m.3243A>G causing a syndrome of mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS). We describe here a case of focal segmental glomerulosclerosis (FSGS) associated with a new tRNA mutation site. A 34-year-old man with a history of living related kidney transplantation, diabetes, hearing loss, and developmental delay presented to the outpatient clinic with complaints of new behavioral difficulties, worsening symptoms, and brain involvement on imaging. Physical examination was remarkable for difficulty hearing, a pattern of dysarthric speech, and cerebellar gait. Laboratory investigations including lactate levels were unremarkable. Based on this set of clinical circumstances, concern for an underlying genetic abnormality was raised. Multiple metabolic tests were unremarkable. Whole exome sequencing revealed a mitochondrial MT-TW tRNA change at position m.5538G>A. Genotype-phenotype correlations are consistent with this tRNA mutation as a cause of his symptoms. To the best of our knowledge, this is the first case describing FSGS-associated MD caused by an m.5538 G>A mutation. Consideration of an underlying MD should be made in patients presenting with deafness, neurologic changes, diabetes, and renal failure.

Mutations in mitochondrial DNA causing tubulointerstitial kidney disease

Tubulointerstitial kidney disease is an important cause of progressive renal failure whose aetiology is incompletely understood. We analysed a large pedigree with maternally inherited tubulointerstitial kidney disease and identified a homoplasmic substitution in the control region of the mitochondrial genome (m.547A>T). While mutations in mtDNA coding sequence are a well recognised cause of disease affecting multiple organs, mutations in the control region have never been shown to cause disease. Strikingly, our patients did not have classical features of mitochondrial disease. Patient fibroblasts showed reduced levels of mitochondrial tRNAPhe, tRNALeu1 and reduced mitochondrial protein translation and respiration. Mitochondrial transfer demonstrated mitochondrial transmission of the defect and in vitro assays showed reduced activity of the heavy strand promoter. We also identified further kindreds with the same phenotype carrying a homoplasmic mutation in mitochondrial tRNAPhe (m.616T>C). Thus mutations in mitochondrial DNA can cause maternally inherited renal disease, likely mediated through reduced function of mitochondrial tRNAPhe.

Mitochondria Damage and Kidney Disease

The kidney is a vital organ that demands an extraordinary amount of energy to actively maintain the body's metabolism, plasma hemodynamics, electrolytes and water homeostasis, nutrients reabsorption, and hormone secretion. Kidney is only second to the heart in mitochondrial count and oxygen consumption. As such, the health and status of the energy power house, the mitochondria, is pivotal to the health and proper function of the kidney. Mitochondria are heterogeneous and highly dynamic organelles and their functions are subject to complex regulations through modulation of its biogenesis, bioenergetics, dynamics and clearance within cell. Kidney diseases, either acute kidney injury (AKI) or chronic kidney disease (CKD), are important clinical issues and global public health concerns with high mortality rate and socioeconomic burden due to lack of effective therapeutic strategies to cure or retard the progression of the diseases. Mitochondria-targeted therapeutics has become a major focus for modern research with the belief that maintaining mitochondria homeostasis can prevent kidney pathogenesis and disease progression. A better understanding of the cellular and molecular events that govern mitochondria functions in health and disease will potentially lead to improved therapeutics development.

Maleic Acid--but Not Structurally Related Methylmalonic Acid--Interrupts Energy Metabolism by Impaired Calcium Homeostasis

Maleic acid (MA) has been shown to induce Fanconi syndrome via disturbance of renal energy homeostasis, though the underlying pathomechanism is still under debate. Our study aimed to examine the pathomechanism underlying maleic acid-induced nephrotoxicity. Methylmalonic acid (MMA) is structurally similar to MA and accumulates in patients affected with methymalonic aciduria, a defect in the degradation of branched-chain amino acids, odd-chain fatty acids and cholesterol, which is associated with the development of tubulointerstitial nephritis resulting in chronic renal failure. We therefore used MMA application as a control experiment in our study and stressed hPTECs with MA and MMA to further validate the specificity of our findings. MMA did not show any toxic effects on proximal tubule cells, whereas maleic acid induced concentration-dependent and time-dependent cell death shown by increased lactate dehydrogenase release as well as ethidium homodimer and calcein acetoxymethyl ester staining. The toxic effect of MA was blocked by administration of single amino acids, in particular L-alanine and L-glutamate. MA application further resulted in severe impairment of cellular energy homeostasis on the level of glycolysis, respiratory chain, and citric acid cycle resulting in ATP depletion. As underlying mechanism we could identify disturbance of calcium homeostasis. MA toxicity was critically dependent on calcium levels in culture medium and blocked by the extra- and intracellular calcium chelators EGTA and BAPTA-AM respectively. Moreover, MA-induced cell death was associated with activation of calcium-dependent calpain proteases. In summary, our study shows a comprehensive pathomechanistic concept for MA-induced dysfunction and damage of human proximal tubule cells.

Renal manifestations of genetic mitochondrial disease

Mitochondrial diseases can be related to mutations in either the nuclear or mitochondrial genome. Childhood presentations are commonly associated with renal tubular dysfunction, but renal involvement is less commonly reported outside of this age-group. Mitochondrial diseases are notable for the significant variability in their clinical presentation and the broad spectrum of genes implicated in their etiology. These features contribute to the challenges of establishing a definitive diagnosis and understanding the pathogenetic mechanisms leading to kidney involvement in these diseases. Here, we review the deoxyribonucleic acid variants in the mitochondrial and nuclear genomes that have been associated with a kidney phenotype, and examine some of the possible pathogenic mechanisms that may contribute to the expression of a renal phenotype.

Focal segmental glomerulosclerosis is associated with a PDSS2 haplotype and, independently, with a decreased content of coenzyme Q10

Focal segmental glomerulosclerosis (FSGS) and collapsing glomerulopathy are common causes of nephrotic syndrome. Variants in >20 genes, including genes critical for mitochondrial function, have been associated with these podocyte diseases. One such gene, PDSS2, is required for synthesis of the decaprenyl tail of coenzyme Q10 (Q10) in humans. The mouse gene Pdss2 is mutated in the kd/kd mouse model of collapsing glomerulopathy. We examined the hypothesis that human PDSS2 polymorphisms are associated with podocyte diseases. We genotyped 377 patients with primary FSGS or collapsing glomerulopathy, together with 900 controls, for 9 single-nucleotide polymorphisms in the PDSS2 gene in a case-control study. Subjects included 247 African American (AA) and 130 European American (EA) patients and 641 AA and 259 EA controls. Among EAs, a pair of proxy SNPs was significantly associated with podocyte disease, and patients homozygous for one PDSS2 haplotype had a strongly increased risk for podocyte disease. By contrast, the distribution of PDSS2 genotypes and haplotypes was similar in AA patients and controls. Thus a PDSS2 haplotype, which has a frequency of 13% in the EA control population and a homozygote frequency of 1.2%, is associated with a significantly increased risk for FSGS and collapsing glomerulopathy in EAs. Lymphoblastoid cell lines from FSGS patients had significantly less Q10 than cell lines from controls; contrary to expectation, this finding was independent of PDSS2 haplotype. These results suggest that FSGS patients have Q10 deficiency and that this deficiency is manifested in patient-derived lymphoblastoid cell lines.

Effects of airborne pollutants on mitochondrial DNA methylation

Background

Mitochondria have small mitochondrial DNA (mtDNA) molecules independent from the nuclear DNA, a separate epigenetic machinery that generates mtDNA methylation, and are primary sources of oxidative-stress generation in response to exogenous environments. However, no study has yet investigated whether mitochondrial DNA methylation is sensitive to pro-oxidant environmental exposures.

Methods

We sampled 40 male participants (20 high-, 20 low-exposure) from each of three studies on airborne pollutants, including investigations of steel workers exposed to metal-rich particulate matter (measured as PM₁) in Brescia, Italy (Study 1); gas-station attendants exposed to air benzene in Milan, Italy (Study 2); and truck drivers exposed to traffic-derived Elemental Carbon (EC) in Beijing, China (Study 3). We have measured DNA methylation from buffy coats of the participants. We measured methylation by bisulfite-Pyrosequencing in three mtDNA regions, i.e., the transfer RNA phenylalanine (MT-TF), 12S ribosomal RNA (MT-RNR1) gene and “D-loop” control region. All analyses were adjusted for age and smoking.

Results

In Study 1, participants with high metal-rich PM₁ exposure showed higher MT-TF and MT-RNR1 methylation than low-exposed controls (difference = 1.41, P = 0.002); MT-TF and MT-RNR1 methylation was significantly associated with PM₁ exposure (beta = 1.35, P = 0.025); and MT-RNR1 methylation was positively correlated with mtDNA copy number (r = 0.36; P = 0.02). D-loop methylation was not associated with PM₁ exposure. We found no effects on mtDNA methylation from air benzene (Study 2) and traffic-derived EC exposure (Study 3).

Conclusions

Mitochondrial MT-TF and MT-RNR1 DNA methylation was associated with metal-rich PM₁ exposure and mtDNA copy number. Our results suggest that locus-specific mtDNA methylation is correlated to selected exposures and mtDNA damage. Larger studies are needed to validate our observations.

Identification of the novel mutation m.5658T>C in the mitochondrial tRNA(Asn) gene in a patient with myopathy, bilateral ptosis and ophthalmoparesis

We report a heteroplasmic novel mutation m.5658T>C in the mt-tRNA(Asn) gene in a patient who initially presented myopathy, bilateral ptosis and ophthalmoparesis and several years later developed a non-nephrotic proteinuria. The muscle biopsy showed cytochrome c oxidase (COX) negative and ragged red fibers and in the kidney biopsy that was taken in order to identify the causes of non-nephrotic proteinuria, a focal segmental glomerulosclerosis was observed. Using laser capture microdissection we isolated COX negative fibers and COX positive fibers from the muscle of the patient and determined that there was a clear increase in the mutation load in the COX negative muscle fibers. However, the low degree of mutation load found in the renal biopsy of the patient does not allow us to conclude that the m.5658T>C mutation is responsible for focal glomerulosclerosis. Additionally, we hypothesize that the mutated m.5658T nucleotide might be structurally relevant, as it is one of the fifteen nucleotides conserved in all the species analyzed and is situated contiguously to the discriminator base in the 3'end of the mt-tRNA, where the tRNase Z cleaves the 3' trailer sequence during mt-tRNA maturation.

[Renal involvement in mitochondrial cytopathies]

Mitochondrial disorders are genetic defects of oxidative phosphorylation. Oxidative phosphorylation takes place in the mitochondrial inner membrane and consists of the oxidation of fuel molecules by oxygen and the concomitant energy transduction into ATP. The mitochondrial respiratory chain is a complex metabolic pathway. It is made of approximately 100 polypeptides, most of which are encoded in the nucleus whereas 13 are encoded in the mitochondria. Mitochondrial DNA is maternally inherited and its mutations are transmitted by the mother. During cell division, mitochondria are randomly partitioned in daughter cells. Therefore, in case normal and mutant DNA are present in the mother's cells, some lineage may have only mutant mitochondrial DNA or normal mitochondrial DNA while others may have both mutant and normal DNA, a condition named heteroplasmy. Renal involvement in mitochondrial cytopathies is rare. Patients most often present with a more or less complete de Toni-Debré-Fanconi syndrome. A few patients present with a nephrotic syndrome or with chronic tubulointerstitial nephritis. The investigation of patients with mitochondrial disorders include metabolic screening for abnormal oxidoreduction status in plasma, investigation of the mitochondrial respiratory chain, including polarographic and spectrophotometric studies, histopathologic studies and genetic studies.

A proposed consensus panel of organisms for determining evolutionary conservation of mt-tRNA point mutations

Assigning pathogenicity to mt-tRNA variants requires multiple strands of evidence. Evolutionary conservation is often considered mandatory, but lack of a standard panel of organisms to assess conservation complicates comparison between reports and undermines the value of conservation-based evidence. We demonstrate that intra-species MTT sequence variation is sufficiently low for sequence data from a single organism to adequately represent a species. On this basis, we propose a standardised panel of organisms for conservation assessment and describe integration of this conservation panel into a pathogenicity scoring system designed to assess mt-tRNA variation associated with mitochondrial disease.

Renal involvement in mitochondrial cytopathies

Mitochondrial cytopathies constitute a group of rare diseases that are characterized by their frequent multisystemic involvement, extreme variability of phenotype and complex genetics. In children, renal involvement is frequent and probably underestimated. The most frequent renal symptom is a tubular defect that, in most severe forms, corresponds to a complete De Toni-Debré-Fanconi syndrome. Incomplete proximal tubular defects and other tubular diseases have also been reported. In rare cases, patients present with chronic tubulo-interstitial nephritis or cystic renal diseases. Finally, a group of patients develop primarily a glomerular disease. These patients correspond to sporadic case reports or can be classified into two major defects, namely 3243 A>G tRNA(LEU) mutations and coenzyme Q10 biosynthesis defects. The latter group is particularly important because it represents the only treatable renal mitochondrial defect. In this Educational Review, the principal characteristics of these diseases and the main diagnostic approaches are summarized.

Renal mitochondrial cytopathies

Renal diseases in mitochondrial cytopathies are a group of rare diseases that are characterized by frequent multisystemic involvement and extreme variability of phenotype. Most frequently patients present a tubular defect that is consistent with complete De Toni-Debré-Fanconi syndrome in most severe forms. More rarely, patients present with chronic tubulointerstitial nephritis, cystic renal diseases, or primary glomerular involvement. In recent years, two clearly defined entities, namely 3243 A > G tRNA(LEU) mutations and coenzyme Q10 biosynthesis defects, have been described. The latter group is particularly important because it represents the only treatable renal mitochondrial defect. In this paper, the physiopathologic bases of mitochondrial cytopathies, the diagnostic approaches, and main characteristics of related renal diseases are summarized.

A new phenotype of mitochondrial disease characterized by familial late-onset predominant axial myopathy and encephalopathy

Axial myopathy is a rare neuromuscular disease that is characterized by paraspinal muscle atrophy and abnormal posture, most notably camptocormia (also known as bent spine). The genetic cause of familial axial myopathy is unknown. Described here are the clinical features and cause of late-onset predominant axial myopathy and encephalopathy. A 73-year-old woman presented with a 10-year history of severe paraspinal muscle atrophy and cerebellar ataxia. Her 84-year-old sister also developed late-onset paraspinal muscle atrophy and generalized seizures with encephalopathy. Computed tomography showed severe atrophy and fatty degeneration of their paraspinal muscles. Their mother and maternal aunt also developed bent spines. The existence of many ragged-red fibers and cytochrome c oxidase-negative fibers in the biceps brachii muscle of the proband indicated a mitochondrial abnormality. No significant abnormalities were observed in the respiratory chain enzyme activities; however, the activities of complexes I and IV were relatively low compared with the activities of other complexes. Sequence analysis of the mitochondrial DNA from the muscle revealed a novel heteroplasmic mutation (m.602C>T) in the mitochondrial tRNA^Phe gene. This familial case of late-onset predominant axial myopathy and encephalopathy may represent a new clinical phenotype of a mitochondrial disease.

Mutations in the mitochondrial seryl-tRNA synthetase cause hyperuricemia, pulmonary hypertension, renal failure in infancy and alkalosis, HUPRA syndrome

An uncharacterized multisystemic mitochondrial cytopathy was diagnosed in two infants from consanguineous Palestinian kindred living in a single village. The most significant clinical findings were tubulopathy (hyperuricemia, metabolic alkalosis), pulmonary hypertension, and progressive renal failure in infancy (HUPRA syndrome). Analysis of the consanguineous pedigree suggested that the causative mutation is in the nuclear DNA. By using genome-wide SNP homozygosity analysis, we identified a homozygous identity-by-descent region on chromosome 19 and detected the pathogenic mutation c.1169A>G (p.Asp390Gly) in SARS2, encoding the mitochondrial seryl-tRNA synthetase. The same homozygous mutation was later identified in a third infant with HUPRA syndrome. The carrier rate of this mutation among inhabitants of this Palestinian isolate was found to be 1:15. The mature enzyme catalyzes the ligation of serine to two mitochondrial tRNA isoacceptors: tRNA(Ser)(AGY) and tRNA(Ser)(UCN). Analysis of amino acylation of the two target tRNAs, extracted from immortalized peripheral lymphocytes derived from two patients, revealed that the p.Asp390Gly mutation significantly impacts on the acylation of tRNA(Ser)(AGY) but probably not that of tRNA(Ser)(UCN). Marked decrease in the expression of the nonacylated transcript and the complete absence of the acylated tRNA(Ser)(AGY) suggest that this mutation leads to significant loss of function and that the uncharged transcripts undergo degradation.

Unravelling the genetic basis of renal diseases; from single gene to multifactorial disorders

Chronic kidney disease is common with up to 5% of the adult population reported to have an estimated glomerular filtration rate of < 60 ml/min/1.73 m(2). A large number of pathogenic mutations have been identified that are responsible for 'single gene' renal disorders, such as autosomal dominant polycystic kidney disease and X-linked Alport syndrome. These single gene disorders account for < 15% of the burden of end-stage renal disease that requires dialysis or kidney transplantation. It has proved more difficult to identify the genetic susceptibility underlying common, complex, multifactorial kidney conditions, such as diabetic nephropathy and hypertensive nephrosclerosis. This review describes success to date and explores strategies currently employed in defining the genetic basis for a number of renal disorders. The complementary use of linkage studies, candidate gene and genome-wide association analyses are described and a collation of renal genetic resources highlighted.

Progressive carotid artery stenosis with a novel tRNA phenylalanine mitochondrial DNA mutation

Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) is a distinct clinical syndrome caused by mutations in the mitochondrial DNA. The pathogenesis of stroke-like episodes remains unknown but major vessels stenosis is not a cause of stroke-like episodes. We describe a novel heteroplasmic G617A transition in the mitochondrial transfer RNA phenylalanine gene in a patient with encephalomyopathy who presented with recurrent embolic ischemic strokes accompanied by transient occlusion of middle cerebral, anterior cerebral and internal carotid arteries. These ischemic strokes were presumed to be artery-to artery embolisms associated with carotid artery stenosis. Single muscle fiber analysis revealed the pathogenicity of the mutation although its causative role on carotid artery stenosis remains to be elucidated. This case expands phenotypic spectrum of mitochondrial disorders in terms of macroangiopathy, but macroangiopathy-related ischemic strokes should be distinguished from classic stroke-like episodes of MELAS that are speculated to be microangioapthy-related or non-ischemic neurovascular events.

Identification of novel mutations in five patients with mitochondrial encephalomyopathy

MELAS, MERRF, LHON and NARP, are well-established mitochondrial syndromes associated with specific point mutations of mitochondrial DNA (mtDNA). However, these recurrent mtDNA mutations account for only a minority of mitochondrial disease cases. To evaluate the impact of novel mtDNA mutations, we performed mtDNA sequence analysis in muscle and other tissues of 240 patients with different mitochondrial neuromuscular syndromes. We identified a total of 33 subjects with novel, private or uncommon mutations. Among these, five novel mutations were found in both paediatric and adult cases. We here report on the clinical description of these patients, as well as the biochemical and molecular genetic characterization of the corresponding mutations. Patients 1 and 2 showed changes in ND genes, patient 3 carried a heteroplasmic deletion in the COI gene, patients 4 and 5 carried heteroplasmic mutations in tRNA(Trp) and tRNA(Phe), respectively. Altogether, these data indicate that mtDNA analysis must become part of the routine screening for mitochondrial disorders.

Human mitochondrial transfer RNAs: role of pathogenic mutation in disease

The human mitochondrial genome encodes 13 proteins. All are subunits of the respiratory chain complexes involved in energy metabolism. These proteins are translated by a set of 22 mitochondrial transfer RNAs (tRNAs) that are required for codon reading. Human mitochondrial tRNA genes are hotspots for pathogenic mutations and have attracted interest over the last two decades with the rapid discovery of point mutations associated with a vast array of neuromuscular disorders and diverse clinical phenotypes. In this review, we use a scoring system to determine the pathogenicity of the mutations and summarize the current knowledge of structure-function relationships of these mutant tRNAs. We also provide readers with an overview of a large variety of mechanisms by which mutations may affect the mitochondrial translation machinery and cause disease.

Pathogenic mechanism of a human mitochondrial tRNAPhe mutation associated with myoclonic epilepsy with ragged red fibers syndrome

Human mitochondrial tRNA (hmt-tRNA) mutations are associated with a variety of diseases including mitochondrial myopathies, diabetes, encephalopathies, and deafness. Because the current understanding of the precise molecular mechanisms of these mutations is limited, there is no efficient method to treat their associated mitochondrial diseases. Here, we use a variety of known mutations in hmt-tRNA(Phe) to investigate the mechanisms that lead to malfunctions. We tested the impact of hmt-tRNA(Phe) mutations on aminoacylation, structure, and translation elongation-factor binding. The majority of the mutants were pleiotropic, exhibiting defects in aminoacylation, global structure, and elongation-factor binding. One notable exception was the G34A anticodon mutation of hmt-tRNA(Phe) (mitochondrial DNA mutation G611A), which is associated with MERRF (myoclonic epilepsy with ragged red fibers). In vitro, the G34A mutation decreases aminoacylation activity by 100-fold, but does not affect global folding or recognition by elongation factor. Furthermore, G34A hmt-tRNA(Phe) does not undergo adenosine-to-inosine (A-to-I) editing, ruling out miscoding as a possible mechanism for mitochondrial malfunction. To improve the aminoacylation state of the mutant tRNA, we modified the tRNA binding domain of the nucleus-encoded human mitochondrial phenylalanyl-tRNA synthetase, which aminoacylates hmt-tRNA(Phe) with cognate phenylalanine. This variant enzyme displayed significantly improved aminoacylation efficiency for the G34A mutant, suggesting a general strategy to treat certain classes of mitochondrial diseases by modification of the corresponding nuclear gene.

Mitochondrial DNA and its mutations: novel fields in a new era

Az utóbbi két évtizedet tartják a klinikai mitokondriális DNS-kutatás aranykorának. Folyamatosan bővül a patológiás variánsok száma, amelyek betegséggel társulnak, illetve bővül az ismeretanyag azokról az entitásokról, melyek hátterében a mitokondriális DNS kóros elváltozásai állnak. A cirkuláris mitokondriális DNS öröklődése eltér a Mendel-féle szabályoktól, anyai öröklésmenetet mutat; számos vonatkozásban eltérő sajátosságokkal rendelkezik a nukleáris DNS-hez viszonyítva. A molekuláris biológiai módszerek terjedésével egyre több kórkép ismerhető fel, noha a diagnosztika manapság is komoly kihívást jelent. Napjainkban a mitokondriális medicina számos orvosi szubspecialitáshoz kapcsolódóan jelentős előrelépéseket mutatott; így körvonalazódott a mitokondriális gasztroenterológia, endokrinológia, otológia, oftalmológia, nefrológia, hematológia, onkológia, reproduktív medicina és pszichiátria, mintegy az adott szubspecialitás mitokondriális DNS-sel kapcsolatos, többé-kevésbé részleges önállósodással megjelenő territóriuma. A jelen összefoglaló közlemény a mitokondriális medicina rövid, általános összefoglalása mellett e fejezetekre próbál rátekintést nyújtani.

Mitochondrial diseases: a nosological update

Mitochondrial diseases are disorders caused by impairment of the mitochondrial respiratory chain, characterized by clinical-genetic heterogeneity and frequent multisystemic involvement. It is difficult to establish a precise genotype/phenotype correlation and obtain a definitive nosology. Today's genetic classification distinguishes disorders caused by defects in the mitochondrial genome (sporadic or maternally-inherited) from disorders caused by defects in the nuclear genome (autosomally-inherited). We report an updated classification, briefly review the main clinical syndromes and describe the most recent genetic knowledge.

Mitochondrial DNA deletion in a girl with Fanconi's syndrome

We report a sporadic large-scale mitochondrial deletion in a paediatric patient with Fanconi's syndrome. Renal biopsy disclosed chronic interstitial nephritis. Ultrastructural examination of the renal tissue showed many giant atypical mitochondria. Histochemical stains revealed markedly reduced cytochrome c oxidase (COX). Genetic analysis disclosed a novel mitochondrial deletion of 7.3 kb in both peripheral blood and renal tissue. Mitochondrial diseases have heterogeneous clinical phenotypes; mutation analysis has proved to be an effective tool in confirming the diagnosis.

Mitochondrial myopathy with exercise intolerance and retinal dystrophy in a sporadic patient with a G583A mutation in the mt tRNA(phe) gene

We describe a second patient with the 583G>A mutation in the tRNA(phe) gene of mitochondrial DNA (mtDNA). This 17-year-old girl had a mitochondrial myopathy with exercise intolerance and an asymptomatic retinopathy. Muscle investigations showed occasional ragged red fibers, 30% cytochrome c oxidase (COX)-negative fibers, and reduced activities of complex I+IV in the respiratory chain. The mutation was heteroplasmic (79%) in muscle but undetectable in other tissues. Analysis of single muscle fibers revealed a significantly higher level of mutated mtDNA in COX-negative fibers. Our study indicates that the 583G>A mutation is pathogenic and expands the clinical spectrum of this mutation.

Renal pathology in children with mitochondrial diseases

We studied renal involvement in 42 children with mitochondrial diseases (MDs). The diagnosis of MD was established by morphological, biochemical, and molecular genetic criteria. Renal disease was considered when patients had renal failure, nephrotic syndrome, Fanconi's syndrome or any symptomatic renal alteration. Mild tubular disorder was established if they had abnormal laboratory findings with no apparent clinical symptom. Renal involvement was found in 21 children (50%), of whom 8 had an apparent clinical picture and 13 a mild tubular disorder. Five patients with renal disease showed Debré-Toni-Fanconi's syndrome, 2 of them with decreased glomerular filtration rate (GFR). One case had nephrotic syndrome, another one presented decreased GFR, and the last one had a neurogenic bladder and bilateral hydronephrosis. Patients with mild renal disease showed tubular dysfunction with normal GFR. Renal involvement is frequent and present in about half of the children with MD. Thus, studies for evaluating kidney function should be performed on children with MD. Conversely, patients with tubulopathy of unknown origin or progressive renal disease should be investigated for the existence of MD, especially if associated with involvement of other organs or tissues. Southern blot analysis to search for large-scale mitochondrial DNA (mtDNA) rearrangements should be performed for patients with MD and kidney involvement.

A novel mutation in the mitochondrial tRNAPhe gene associated with mitochondrial myopathy

We report a novel heteroplasmic T-->C mutation at nt position 582 within the mitochondrial tRNA(Phe) gene of a 70-year-old woman with mitochondrial myopathy. No other family members were affected, suggesting that our patient was a sporadic case. The muscle showed frequent ragged red fibers and 43% cytochrome c oxidase deficient fibers. The mutation alters a conserved base pairing in the aminoacyl acceptor stem. The mutation load was 70% in muscle homogenate and varied from 0 to 95% in individual muscle fiber segments. Cytochrome c oxidase-negative fibers showed significantly higher levels of mutated mtDNA (>75%) than Cytochrome c oxidase-positive fibers (<55%). This mutation adds to the previously described four pathogenic mutations in the tRNA(Phe) gene.

Melas with point mutations involving tRNALeu (A3243G) and tRNAGlu(A14693g)

The syndrome of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episode (MELAS) is typically associated with a single point mutation in the mitochondrial genome (mtDNA). Because mtDNA is known to have a higher mutation rate than nuclear DNA, we speculate that some patients with MELAS syndrome may harbor more than one mutation in mtDNA. For this purpose, mtDNA extracted from muscle containing dysmorphic mitochondria from a 32-year-old man with MELAS was sequenced in its entirety to identify all possible mutations. The result showed a homoplasmic A14693G and a heteroplasmic A3243G. The A14693G transition was not present in 205 unrelated control individuals, was not seen in 76 species randomly selected from GenBank, and appears to disrupt the base pairing within the T-loop of mtDNA tRNA(Glu). His asymptomatic siblings' blood showed wild-type at these positions, whereas the blood of the patient's oligosymptomatic diabetic mother had a heteroplasmic A14693G and an apparent homoplasmic wild-type A3243, suggesting an association of A14693G with diabetes mellitus. This case demonstrates the importance of sequencing the mtDNA in its entirety to evaluate the molecular basis of mitochondriopathy.

The spectrum of systemic involvement in adults presenting with renal lesion and mitochondrial tRNA(Leu) gene mutation

The A3243G mutation of the mitochondrial tRNA(Leu) gene has been recently reported in rare patients with focal and segmental glomerulosclerosis (FSGS). However, the full spectrum of systemic and kidney manifestations in adults presenting with this mutation remains poorly defined. Assessment of renal and nonrenal manifestations was performed in nine patients with A3243G mutation and prominent kidney disease diagnosed in adulthood. At first renal evaluation, median age was 35 years. Renal lesions consisted of FSGS (n = 2), tubulointerstitial nephropathy (n = 3), or bilateral enlarged cystic kidneys (n = 1). All but one patient exhibited extrarenal manifestations: deafness (8 of 9) requiring hearing aid in half the cases, diabetes mellitus (3 of 9), neuromuscular involvement (2 of 9), hypertrophic cardiomyopathy (1 of 9), and macular dystrophy (1 of 9). After a median follow-up of 5 yr, five patients progressed to end-stage renal disease between the ages of 15 and 51 years, four being successfully transplanted. Similarly, extrarenal manifestations progressed since all patients had deafness and diabetes (including three posttransplants), while half had neuromuscular, cardiac, or retinal involvement. In the adult patients with A3243G mutation and renal involvement, preexisting deafness is almost consistently found. While FSGS remains the most typical lesion, tubulointerstitial nephropathy or bilateral, enlarged cystic kidneys may also be encountered. In most cases, diabetes mellitus, macular dystrophy, hypertrophic cardiomyopathy, or neuromuscular features occur later in the course of the disease. The severity of the clinical course is heterogeneous, with end-stage renal failure being reached between the second and sixth decades. Renal transplantation may be offered to these patients, despite a high incidence of steroid-induced diabetes mellitus.