Renal function and mitochondrial cytopathy (MC): more questions than answers?

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.

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.

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.

Kidney epithelial targeted mitochondrial transcription factor A deficiency results in progressive mitochondrial depletion associated with severe cystic disease

Abnormal mitochondrial function is a well-recognized feature of acute and chronic kidney diseases. To gain insight into the role of mitochondria in kidney homeostasis and pathogenesis, we targeted mitochondrial transcription factor A (TFAM), a protein required for mitochondrial DNA replication and transcription that plays a critical part in the maintenance of mitochondrial mass and function. To examine the consequences of disrupted mitochondrial function in kidney epithelial cells, we inactivated TFAM in sine oculis-related homeobox 2-expressing kidney progenitor cells. TFAM deficiency resulted in significantly decreased mitochondrial gene expression, mitochondrial depletion, inhibition of nephron maturation and the development of severe postnatal cystic disease, which resulted in premature death. This was associated with abnormal mitochondrial morphology, a reduction in oxygen consumption and increased glycolytic flux. Furthermore, we found that TFAM expression was reduced in murine and human polycystic kidneys, which was accompanied by mitochondrial depletion. Thus, our data suggest that dysregulation of TFAM expression and mitochondrial depletion are molecular features of kidney cystic disease that may contribute to its pathogenesis.

Impact of Intravenous Iron on Oxidative Stress and Mitochondrial Function in Experimental Chronic Kidney Disease

Background: Mitochondrial dysfunction is observed in chronic kidney disease (CKD). Iron deficiency anaemia (IDA), a common complication in CKD, is associated with poor clinical outcomes affecting mitochondrial function and exacerbating oxidative stress. Intravenous (iv) iron, that is used to treat anaemia, may lead to acute systemic oxidative stress. This study evaluated the impact of iv iron on mitochondrial function and oxidative stress. Methods: Uraemia was induced surgically in male Sprague-Dawley rats and studies were carried out 12 weeks later in two groups sham operated and uraemic (5/6 nephrectomy) rats not exposed to i.v. iron versus sham operated and uraemic rats with iv iron. Results: Induction of uraemia resulted in reduced iron availability (serum iron: 31.1 ± 1.8 versus 46.4 ± 1.4 µM), low total iron binding capacity (26.4 ± 0.7 versus 29.5 ± 0.8 µM), anaemia (haematocrit: 42.5 ± 3.0 versus 55.0 ± 3.0%), cardiac hypertrophy, reduced systemic glutathione peroxidase activity (1.12 ± 0.11 versus 1.48 ± 0.12 U/mL), tissue oxidative stress (oxidised glutathione: 0.50 ± 0.03 versus 0.36 ± 0.04 nmol/mg of tissue), renal mitochondrial dysfunction (proton/electron leak: 61.8 ± 8.0 versus 22.7 ± 5.77) and complex I respiration (134.6 ± 31.4 versus 267.6 ± 26.4 pmol/min/µg). Iron therapy had no effect on renal function and cardiac hypertrophy but improved anaemia and systemic glutathione peroxidase (GPx) activity. There was increased renal iron content and complex II and complex IV dysfunction. Conclusion: Iron therapy improved iron deficiency anaemia in CKD without significant impact on renal function or oxidant status.

Five non-mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes phenotype adult patients with m.3243A>G mutation after kidney transplantation: follow-up and review of the literature

Background

Renal involvement in patients with the m.3243A>G mutation may result in end-stage renal disease (ESRD) requiring renal replacement therapy. Although kidney transplantations have been performed in a small number of patients, short- and long-term follow-up data are lacking.

Methods

We describe five patients with the m.3243A<G mutation who received a kidney transplant, including follow-up data up to 13 years. We also summarize all cases (n = 13) of kidney transplantation in m.3243A>G carriers described in the literature.

Results

Proteinuria with or without renal failure was the first clinical presentation of renal involvement in 13 of 18 (72%) patients. Focal segmental glomerulosclerosis (FSGS) was found in 9 of 13 (69%) biopsies. Sixteen of 18 (84%) patients developed hearing loss. All patients were diagnosed with diabetes mellitus, of whom eight (44%) developed the disease after transplantation. All patients with reported follow-up data (13/18) had stable kidney function from 6 months to 13 years of follow-up after transplantation.

Conclusions

Renal involvement in carriers of the m.3243A>G mutation most commonly leads to proteinuria and FSGS and may lead to ESRD. Proper recognition of the mitochondrial origin of the renal disease in these patients is important for adequate treatment selection and suitable supportive care. This case series and review of the available literature on long-term follow-up after kidney transplantation shows it is feasible for non-mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes phenotype carriers of the m.3243A>G mutation to be considered for kidney transplantation in case of ESRD. These patients should not be excluded from transplant solely for their mitochondrial diagnosis.

Case study of an inborn error manifested in the elderly: A woman with adult-onset mitochondrial disease mimicking systemic vasculitis

Mitochondrial diseases are a group of disorders presenting mainly during infancy due to pathological dysfunction of the mitochondrial respiratory chain. We report a case of mitochondrial disease in an elderly woman complaining of generalized myalgia. A 69-year-old woman was admitted due to fatigue, general weakness, and a drowsy mental status. A brain magnetic resonance imaging (MRI) demonstrated multifocal lesions of increased T2 signal intensity, and laboratory findings were consistent with Fanconi syndrome. During her hospital course, she developed seizures, stress-induced cardiomyopathy, and respiratory failure. A muscle biopsy demonstrated ragged-red fibers in the muscle tissues seen in mitochondrial myopathy. We confirmed an 8 kb deletion in her mitochondrial DNA. Following treatment with l-carnitine, coenzyme Q10, and supportive measures, brain lesions on MRI scans disappeared, and the general symptoms gradually improved.

SLC30A9 mutation affecting intracellular zinc homeostasis causes a novel cerebro-renal syndrome

A novel autosomal recessive cerebro-renal syndrome was identified in consanguineous Bedouin kindred: neurological deterioration was evident as of early age, progressing into severe intellectual disability, profound ataxia, camptocormia and oculomotor apraxia. Brain MRI was normal. Four of the six affected individuals also had early-onset nephropathy with features of tubulo-interstitial nephritis, hypertension and tendency for hyperkalemia, though none had rapid deterioration of renal function. Genome wide linkage analysis identified an ∼18 Mb disease-associated locus on chromosome 4 (maximal logarithm of odds score 4.4 at D4S2971; θ = 0). Whole exome sequencing identified a single mutation in SLC30A9 within this locus, segregating as expected within the kindred and not found in a homozygous state in 300 Bedouin controls. We showed that SLC30A9 (solute carrier family 30 member 9; also known as ZnT-9) is ubiquitously expressed with high levels in cerebellum, skeletal muscle, thymus and kidney. Confocal analysis of SH-SY5Y cells overexpressing SLC30A9 fused to enhanced green fluorescent protein demonstrated vesicular cytosolic localization associated with the endoplasmic reticulum, not co-localizing with endosomal or Golgi markers. SLC30A9 encodes a putative zinc transporter (by similarity) previously associated with Wnt signalling. However, using dual-luciferase reporter assay in SH-SY5Y cells we showed that Wnt signalling was not affected by the mutation. Based on protein modelling, the identified mutation is expected to affect SLC30A9's highly conserved cation efflux domain, putatively disrupting its transmembrane helix structure. Cytosolic Zn2+ measurements in HEK293 cells overexpressing wild-type and mutant SLC30A9 showed lower zinc concentration within mutant rather than wild-type SLC30A9 cells. This suggests that SLC30A9 has zinc transport properties affecting intracellular zinc homeostasis, and that the molecular mechanism of the disease is through defective function of this novel activity of SLC30A9 rather than by a defect in its previously described role in transcriptional activation of Wnt signalling.

Compound heterozygous RMND1 gene variants associated with chronic kidney disease, dilated cardiomyopathy and neurological involvement: a case report

Background

Nuclear gene mutations are being increasingly recognised as causes of mitochondrial disease. The nuclear gene RMND1 has recently been implicated in mitochondrial disease, but the spectrum of pathogenic variants and associated phenotype for this gene, has not been fully elucidated.

Case presentation

An 11-month-old boy presented with renal impairment associated with a truncal ataxia, bilateral sensorineural hearing loss, hypotonia, delayed visual maturation and global developmental delay. Over a 9-year period, he progressed to chronic kidney disease stage V and developed a dilated cardiomyopathy. Abnormalities in renal and muscle biopsy as well as cytochrome c oxidase activity prompted genetic testing. After exclusion of mitochondrial DNA defects, nuclear genetic studies identified compound heterozygous RMND1 (c.713A>G, p. Asn238Ser and c.565C>T, p.Gln189*) variants.

Conclusion

We report RMND1 gene variants associated with end stage renal failure, dilated cardiomyopathy, deafness and neurological involvement due to mitochondrial disease. This case expands current knowledge of mitochondrial disease secondary to mutation of the RMND1 gene by further delineating renal manifestations including histopathology. To our knowledge dilated cardiomyopathy has not been reported with renal failure in mitochondrial disease due to mutations of RMND1. The presence of this complication was important in this case as it precluded renal transplantation.

The urinary proteome and metabonome differ from normal in adults with mitochondrial disease

We studied the extent and nature of renal involvement in a cohort of 117 adult patients with mitochondrial disease, by measuring urinary retinol-binding protein (RBP) and albumin; established markers of tubular and glomerular dysfunction, respectively. Seventy-five patients had the m.3243A>G mutation and the most frequent phenotypes within the entire cohort were 14 with MELAS, 33 with MIDD, and 17 with MERRF. Urinary RBP was increased in 29 of 75 of m.3243A>G patients, whereas albumin was increased in 23 of the 75. The corresponding numbers were 16 and 14, respectively, in the 42 non-m.3243A>G patients. RBP and albumin were higher in diabetic m.3243A>G patients than in nondiabetics, but there were no significant differences across the three major clinical phenotypes. The urine proteome (mass spectrometry) and metabonome (nuclear magnetic resonance) in a subset of the m.3243A>G patients were markedly different from controls, with the most significant alterations occurring in lysosomal proteins, calcium-binding proteins, and antioxidant defenses. Differences were also found between asymptomatic m.3243A>G carriers and controls. No patients had an elevated serum creatinine level, but 14% had hyponatremia, 10% had hypophosphatemia, and 14% had hypomagnesemia. Thus, abnormalities in kidney function are common in adults with mitochondrial disease, exist in the absence of elevated serum creatinine, and are not solely explained by diabetes.

Maternally-inherited diabetes with deafness (MIDD) and hyporeninemic hypoaldosteronism

Maternally-inherited diabetes with deafness (MIDD) is a rare form of monogenic diabetes that results, in most cases, from an A-to-G transition at position 3243 of mitochondrial DNA (m.3243A>G) in the mitochondrial-encoded tRNA leucine (UUA/G) gene. As the name suggests, this condition is characterized by maternally-inherited diabetes and bilateral neurosensory hearing impairment. A characteristic of mitochondrial cytopathies is the progressive multisystemic involvement with the development of more symptoms during the course of the disease. We report here the case of a patient with MIDD who developed hyporeninemic hypoaldosteronism.

Redox-sensitive glycogen synthase kinase 3β-directed control of mitochondrial permeability transition: rheostatic regulation of acute kidney injury

Mitochondrial dysfunction plays a pivotal role in necroapoptotic cell death and in the development of acute kidney injury (AKI). Evidence suggests that glycogen synthase kinase (GSK) 3β resides at the nexus of multiple signaling pathways implicated in the regulation of mitochondrial permeability transition (MPT). In cultured renal tubular epithelial cells, a discrete pool of GSK3β was detected in mitochondria. Coimmunoprecipitation assay confirmed that GSK3β physically interacts with cyclophilin F and voltage-dependent anion channel (VDAC), key MPT regulators that possess multiple GSK3β phosphorylation consensus motifs, suggesting that GSK3β has a direct control of MPT. Upon a strong burst of reactive oxygen species elicited by the pro-oxidant herbicide paraquat, the activity of the redox-sensitive GSK3β was drastically enhanced. This was accompanied by augmented phosphorylation of cyclophilin F and VDAC, associated with MPT and cell death. Inhibition of GSK3β by either the selective inhibitor 4-Benzyl-2-methyl-1,2,4-thiadiazolidine-3,5-dione (TDZD-8) or forced expression of a kinase-dead mutant obliterated paraquat-induced phosphorylation of cyclophilin F and VDAC, prevented MPT, and improved cellular viability. Conversely, ectopic expression of a constitutively active GSK3β amplified the effect of paraquat on cyclophilin F and VDAC phosphorylation and sensitized cells to paraquat-induced MPT and death. In vivo, paraquat injection elicited marked oxidant stress in the kidney and resulted in acute kidney dysfunction and massive tubular apoptosis and necrosis. Consistent with in vitro findings, the activity of GSK3β was augmented in the kidney after paraquat injury, associated with increased phosphorylation of cyclophilin F and VDAC and sensitized MPT. TDZD-8 blocked GSK3β activity in the kidney, intercepted cyclophilin F and VDAC phosphorylation, prevented MPT, attenuated tubular cell death, and ameliorated paraquat-induced AKI. Our data suggest that the redox-sensitive GSK3β regulates renal tubular injury in AKI by controlling the activity of MPT regulators.

Deficiency in mitochondrial complex I activity due to Ndufs6 gene trap insertion induces renal disease

AIMS

Defects in the activity of enzyme complexes of the mitochondrial respiratory chain are thought to be responsible for several disorders, including renal impairment. Gene mutations that result in complex I deficiency are the most common oxidative phosphorylation disorders in humans. To determine whether an abnormality in mitochondrial complex I per se is associated with development of renal disease, mice with a knockdown of the complex I gene, Ndufs6 were studied.

RESULTS

Ndufs6 mice had a partial renal cortical complex I deficiency; Ndufs6gt/gt, 32% activity and Ndufs6gt/+, 83% activity compared with wild-type mice. Both Ndufs6gt/+ and Ndufs6gt/gt mice exhibited hallmarks of renal disease, including albuminuria, urinary excretion of kidney injury molecule-1 (Kim-1), renal fibrosis, and changes in glomerular volume, with decreased capacity to generate mitochondrial ATP and superoxide from substrates oxidized via complex I. However, more advanced renal defects in Ndufs6gt/gt mice were observed in the context of a disruption in the inner mitochondrial electrochemical potential, 3-nitrotyrosine-modified mitochondrial proteins, increased urinary excretion of 15-isoprostane F2t, and up-regulation of antioxidant defence. Juvenile Ndufs6gt/gt mice also exhibited signs of early renal impairment with increased urinary Kim-1 excretion and elevated circulating cystatin C.

INNOVATION

We have identified renal impairment in a mouse model of partial complex I deficiency, suggesting that even modest deficits in mitochondrial respiratory chain function may act as risk factors for chronic kidney disease.

CONCLUSION

These studies identify for the first time that complex I deficiency as the result of interruption of Ndufs6 is an independent cause of renal impairment.

Mitochondrial tRNA(Phe) mutation as a cause of end-stage renal disease in childhood

BACKGROUND

We identified a mitochondrial tRNA mutation (m.586 G > A) in a patient with renal failure and symptoms consistent with a mitochondrial cytopathy. This mutation was of unclear significance due to the absence of consistent reports of linkage to specific disease phenotypes and any data pertaining to its effects on mitochondrial function.

CASE-DIAGNOSIS/TREATMENT

A 16-month-old girl with failure-to-thrive, developmental regression, persistent lactic acidosis, hypotonia, gastrointestinal dysmotility, adrenal insufficiency, and hematologic abnormalities developed hypertension and renal impairment with chronic tubulointerstitial fibrosis, progressing to renal failure with the need for peritoneal dialysis. Evaluation of her muscle and blood led to the identification of a mutation of the mitochondrial tRNA for phenylalanine, m.586 G > A.

CONCLUSIONS

The m.586 G > A mutation is pathogenic and a cause of end-stage renal disease in childhood. The mutation interferes with the stability of tRNA(Phe) and affects the translation of mitochondrial proteins and the stability of the electron transport chain.

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.

Possible role of mtDNA depletion and respiratory chain defects in aristolochic acid I-induced acute nephrotoxicity

This report describes an investigation of the pathological mechanism of acute renal failure caused by toxic tubular necrosis after treatment with aristolochic acid I (AAI) in Sprague-Dawley (SD) rats. The rats were gavaged with AAI at 0, 5, 20, or 80 mg/kg/day for 7 days. The pathologic examination of the kidneys showed severe acute tubular degenerative changes primarily affecting the proximal tubules. Supporting these results, we detected significantly increased concentrations of blood urea nitrogen (BUN) and creatinine (Cr) in the rats treated with AAI, indicating damage to the kidneys. Ultrastructural examination showed that proximal tubular mitochondria were extremely enlarged and dysmorphic with loss and disorientation of their cristae. Mitochondrial function analysis revealed that the two indicators for mitochondrial energy metabolism, the respiratory control ratio (RCR) and ATP content, were reduced in a dose-dependent manner after AAI treatment. The RCR in the presence of substrates for complex I was reduced more significantly than in the presence of substrates for complex II. In additional experiments, the activity of respiratory complex I, which is partly encoded by mitochondrial DNA (mtDNA), was more significantly impaired than that of respiratory complex II, which is completely encoded by nuclear DNA (nDNA). A real-time PCR assay revealed a marked reduction of mtDNA in the kidneys treated with AAI. Taken together, these results suggested that mtDNA depletion and respiratory chain defects play critical roles in the pathogenesis of kidney injury induced by AAI, and that the same processes might contribute to aristolochic acid-induced nephrotoxicity in humans.

Role of mitofusin 2 in the renal stress response

The role of mitofusin 2 (MFN2), a key regulator of mitochondrial morphology and function in the renal stress response is unknown. To assess its role, the MFN2 floxed gene was conditionally deleted in the kidney of mice (MFN2 cKO) by Pax2 promoter driven Cre expression (Pax2Cre). MFN2 cKO caused severe mitochondrial fragmentation in renal epithelial cells that are critical for normal kidney tubular function. However, despite a small (20%) decrease in nephron number, newborn cKO pups had organ or tubular function that did not differ from littermate Cre-negative pups. MFN2 deficiency in proximal tubule epithelial cells in primary culture induced mitochondrial fragmentation but did not significantly alter ATP turnover, maximal mitochondrial oxidative reserve capacity, or the low level of oxygen consumption during cyanide exposure. MFN2 deficiency also did not increase apoptosis of tubule epithelial cells under non-stress conditions. In contrast, metabolic stress caused by ATP depletion exacerbated mitochondrial outer membrane injury and increased apoptosis by 80% in MFN2 deficient vs. control cells. Despite similar stress-induced Bax 6A7 epitope exposure in MFN2 deficient and control cells, MFN2 deficiency significantly increased mitochondrial Bax accumulation and was associated with greater release of both apoptosis inducing factor and cytochrome c. In conclusion, MFN2 deficiency in the kidney causes mitochondrial fragmentation but does not affect kidney or tubular function during development or under non-stress conditions. However, MFN2 deficiency exacerbates renal epithelial cell injury by promoting Bax-mediated mitochondrial outer membrane injury and apoptosis.

Respiratory-chain deficiency presenting as diffuse mesangial sclerosis with NPHS3 mutation

Renal manifestations of mitochondrial cytopathies have been described, but nephrotic syndrome with respiratory-chain disorders have been described extremely rarely. We report a 9-month-old boy with a mitochondrial cytopathy preceded by a 2-month history of steroid-resistant nephrotic syndrome. Percutaneous renal biopsy revealed diffuse mesangial sclerosis, and mutational analysis was compatible with PLCE1 mutation. However, electron microscopic findings of renal tissue, sensorineural hearing loss, and other ocular and neurologic findings led us to suspect mitochondrial cytopathy. Muscle tissue analysis showed a deficiency of the respiratory chain complex IV. The clinical presentation of our patient is not typical for primary cytochrome oxidase (COX) deficiency but showed similarities with patients carrying AR mutations in COX10. This was the first case in the literature with both PLCE1 mutation and COX deficiency. We could not identify pathogenic mutations in the COX10 gene, suggesting that PLCE1 deficiency could be the cause of the secondary deficiency of COX. Another, more likely, possibility is that the mitochondriopathy phenotype is caused by another mutation homozygous by descent in a yet unidentified recessive gene.

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.

A novel mitochondrial MTND5 frameshift mutation causing isolated complex I deficiency, renal failure and myopathy

Isolated complex I deficiency is the most commonly reported enzyme defect in paediatric mitochondrial disorders, and may arise due to mutations in nuclear-encoded structural or assembly genes, or the mitochondrial genome. We present the clinical, biochemical and molecular genetic data in a young girl whose clinical picture is dominated by chronic renal failure, myopathy and persistent lactic acidosis. An isolated complex I deficiency in muscle was identified due to a novel mutation (m.12425delA) in the MTND5 gene. This single nucleotide deletion is heteroplasmic and detectable in several tissues from the proband but not her mother, suggesting a de novo mutation event. The description of the first frameshift mutation in a mitochondrial complex I gene affirms mitochondrial DNA mutations as an important cause of isolated complex I deficiency in children and the importance of whole mitochondrial genome sequencing in the diagnostic work-up to elucidate the underlying molecular genetic abnormality and provide important genetic advice.

Evaluation of renal function in children with mitochondrial respiratory chain complex defect: usefulness of cystatin C

AIM

Mitochondrial disorders with respiratory chain complex defect are known to show a broad spectrum of clinical phenotypes, though their contribution to renal disease has not received much attention. The aim of this study was to evaluate renal function using different markers of the glomerular filtration rate (GFR) in children with mitochondrial respiratory chain complex (MRC) defects.

METHODS

Twenty-two patients with MRC I defect confirmed by biochemical enzyme assays using muscle tissue, were evaluated for renal function with diethylenetriaminepentaacetic acid (DTPA) renal scintigraphy, urine creatinine clearance, serum creatinine (Scr) and cystatin C (Cys C) levels.

RESULTS

All patients showed lower levels within normal range of Scr (0.4-0.7 mg/dL, mean 0.47 mg/dL), and with no significant correlation with DTPA-GFR. Cys C showed significant correlation with DTPA-GFR (p < 0.01) providing higher diagnostic accuracy than creatinine in patients with MRC defect.

CONCLUSION

In mitochondrial disorder with MRC defect, Scr could not fully reflect renal function due to the relatively small body mass of patients. Considering the limited clinical availability of DTPA renal scintigraphy, Cys C should be taken as the first step to evaluate GFR in mitochondrial disorders.

An autopsy case of chronic progressive external ophthalmoplegia with renal insufficiency

An autopsy of a 44-year-old Japanese woman with mitochondrial cytopathy confirmed the presence of chronic progressive external ophthalmoplegia (CPEO). Immunohistochemistry using antimitochondrial antibody was performed to observe the ultrastructure of the skeletal muscle and renal tissues. The patient was born of consanguineous parents, developed normally, and was of average intelligence. At 22 years of age, the patient noticed hearing loss, and subsequently, over time, developed a progressive generalized muscle weakness, which included limitation of eye movement and ptosis. At age 41, a muscle biopsy was performed using the modified Gomori trichrome method and demonstrated the presence of ragged red fibers. After the evaluation of her results in conjunction with her clinical course, she was diagnosed with CPEO. Renal insufficiency was discovered at age 30, and the patient died at the age of 44 of respiratory failure caused by respiratory muscle weakness and pneumonia. The autopsy revealed fiber size variation within the skeletal muscle, and an antimitochondrial antibody analysis demonstrated the accumulation of mitochondria between the bundles of myofibrils, as well as in subsarcolemmal locations. Ultrastructurally, abnormal mitochondria with disoriented cristae and paracrystalline inclusions were seen. Although no remarkable histological changes were noted in the kidneys, tubular epithelial cells exhibited accumulated abnormal mitochondria, similar to those seen in the skeletal muscle. Because mitochondrial diseases can affect other energy-dependent organs in addition to the skeletal muscle, immunohistochemical examinations employing an antimitochondrial antibody are useful for obtaining further ultrastructural observations that can assist in making a distinct diagnosis of this systemic disorder.