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.

M itochondrial cytopathies include a heterogeneous group of diseases with impaired oxidative phosphorylation. 1 Mitochondrial disorders are characterized by a progressive clinical deterioration of the disease and the presence of some degree of encephalopathy in most patients. However, other organs with high metabolic rates can also be severely affected. 2 The kidney is an energy-demanding organ in the human body and all nephron segments have different mitochondrial densities and distributions due to different energy demands along the nephron. 3À5 Kidney symptoms caused by mitochondrial dysfunction are often characterized by tubular defects, however, glomerular, tubulointerstitial, and cystic diseases have also been described. 6 Moreover, kidney manifestations are rarely isolated and often part of a multisystemic disorder with symptoms of encephalopathy, cardiomyopathy, sensorineural hearing loss, muscle weakness, retinopathy, and/or diabetes mellitus. 2 Mitochondrial cytopathies are caused by inherited or sporadic mitochondrial deoxyribonucleic acid (mtDNA) or nuclear DNA (nDNA) mutations in genes that affect mitochondrial functions. Disease-causing mutations that result in disorders of mitochondrial energy metabolism have been reported at present in >250 genes. 7 Although each individual defect is rare, the overall prevalence of mitochondrial disorders in the general population is approximately 1 in 4300. 8,9 For mitochondrial diseases with kidney involvement, 2 important genetic causes should be highlighted: defects in the mtDNA3243A>G and in the CoQ 10 biosynthesis. The m.3243A>G mutation in the transfer ribosomal ribonucleic acid (tRNA) Leu gene is one of the most common mtDNA point mutations. The phenotypic expression of m.3243A>G mutation can be highly variable and cause a wide range of clinical manifestations. Although kidney involvement is not very common, several patients with the m.3243A>G mutation have developed proteinuria and kidney failure. 10 In patients with primary CoQ 10 deficiency, proteinuria and/or kidney dysfunction might be the only manifestation at presentation. CoQ 10 deficiency is highly relevant for clinicians because, in contrast to most mitochondrial disorders, patients with primary CoQ 10 deficiency may respond to treatment with CoQ 10 supplementation. 11 Early diagnosis is crucial, as oral supplementation of CoQ 10 can limit disease progression, prevent neurological deterioration, and improve clinical symptoms. 12 As established neurologic and/or kidney damage is irreversible, this underlines the importance of early diagnosis and treatment. In this review, an overview of kidney involvement in genetic mitochondrial cytopathies is provided with a special focus on CoQ 10 deficiency.

MITOCHONDRIAL (DYS)FUNCTION
The mitochondrial genome is composed of a single, double-stranded, circular loop that lacks introns and contains 37 genes, encoding 2 ribosomal RNA, 22 transfer RNAs, and 13 structural protein subunits of the respiratory chain complexes I, III, and IV, and protein complex V. 13À15 Complex II contains only nDNA-encoded subunits, whereas the other respiratory chain complexes and complex V comprise both mtDNA-encoded and nDNA-encoded subunits. 8 The mitochondrial respiratory chain is composed of 4 protein complexes and 2 electron carriers: CoQ 10 and cytochrome c ( Figure 1). CoQ 10 , also known as ubiquinone, is present in all cell membranes as a lipid molecule with a variety of biological functions. 16 Besides its function as electron carrier, CoQ 10 is involved in the b-oxidation of fatty acids, pyrimidine synthesis, detoxification of hydrogen sulfide, and protection from reactive oxygen species (ROS). 2,[17][18][19] Mitochondrial cytopathies refer to inherited or sporadic mtDNA or nDNA mutations in genes that affect mitochondrial functions. Different from nDNA, various amounts of mtDNA copies are present in a cell depending on its energy demand. mtDNA is highly susceptible to damage and mutations, with a 10-to 1000-fold greater mutation rate than nDNA. 20 The threshold level for mitochondria to become dysfunctional can vary between tissues due to differences in energy dependence. The threshold for disease is lower in tissues that are highly dependent on oxidative metabolism, such as kidney tubules. 2 phosphorylation cause 1 major problems: a reduction in ATP production, and an increase in ROS production. In approximately one-third of patients, the first symptoms of mitochondrial defects develop within the first weeks of life, and more than 80% of patients are symptomatic by the age of 2 years. 22

KIDNEY MANIFESTATIONS OF GENETIC MITOCHONDRIAL CYTOPATHIES
Kidney symptoms caused by mitochondrial dysfunction are often characterized by tubular or glomerular defects. Tubulointerstitial and cystic kidney diseases have also been described; however, the molecular mechanisms remain to be elucidated. 20

Tubular Defects
Tubular disorders are frequently reported in patients with mitochondrial cytopathies. Mutations in both mitochondrial and nuclear genes have been described to cause tubular defects (Tables 1 and 2, Figure 2). 10,23-71 Excellent overviews of mitochondrial and nuclear gene mutations causing tubular defects are provided in various reviews. 8,20,72À74 The most commonly reported problem is a proximal tubular defect, as proximal tubular cells are relatively vulnerable to oxidative stress. Most patients present with partial defects, including renal tubular acidosis (RTA), aminoaciduria, glycosuria, hypermagnesuria, or a combination of the above. 8,73 Renal Fanconi syndrome has been reported in children with specific mitochondrial syndromes, including KearnsÀSayre syndrome, Pearson syndrome, Leigh syndrome, and CoQ 10 deficiency. 70,75À79 KearnsÀSayre syndrome typically includes chronic progressive external ophthalmoplegia, ptosis, and pigmentary retinopathy in individuals less than 20 years of age. 80 Pearson syndrome occurs in infancy, and characteristically includes sideroblastic anemia and exocrine pancreatic dysfunction. 81

Glomerular Diseases
Podocytes are highly differentiated cells with limited regenerative capacity. 84 The main metabolic pathway for podocytes is anaerobic glycolysis and the fermentation of glucose to lactate. 85 Therefore, the mechanism of glomerular dysfunction is probably different from that of tubular dysfunction in patients with mitochondrial cytopathies. Brinkkoetter et al. hypothesize that a change in podocyte metabolism rather than the loss of mitochondrial oxidative phosphorylation is essential for damage to the podocyte. 85 Two major mitochondrial cytopathies associated with glomerular dysfunction are due to a m.3243A>G mutation in tRNA Leu gene or to genetic defects in the CoQ 10 biosynthesis pathway 8 (Tables 1 and 2, Figure 2).

m.3243A>G Mutation in tRNA Leu Gene
The m.3243A>G mutation in the tRNA Leu gene is one of the most common mtDNA point mutations, but the phenotypic expression can be highly variable. This mutation is associated with a wide range of clinical extrarenal manifestations including deafness, diabetes mellitus, neuromuscular involvement, hypertrophic cardiomyopathy, and macular dystrophy. 86 The m.3243A>G mutation was initially described in mitochondrial myopathy encephalopathy with lactic acidosis and stroke like episodes (MELAS) syndrome, a mitochondrial syndrome manifested in patients who are typically less than 40 years of age. 20,87 Low m.3243A>G mutation heteroplasmy levels cause maternally inherited diabetes and deafness (MIDD) syndrome. 88 The m.3243A>G mutation is associated with focal segmental glomerulosclerosis (FSGS), tubulointerstitial nephritis, and cystic kidney disease (Table 1). 23 The kidney disease associated with the m.3243A>G mutation generally corresponds to a glomerulopathy with proteinuria, which is below the nephrotic range in two-thirds of patients. The majority of patients are diagnosed with kidney disease in their second or third decade of life, and CKD is present in one-half of these cases. 86 CoQ 10 Deficiency Primary CoQ 10 deficiency is a clinically and genetically heterogeneous disorder. Clinical phenotypes range from fatal infantile multisystem disorders to isolated glomerular involvement. In addition, variability exists regarding the age of onset, the different organs involved, and clinical response to CoQ 10 supplementation. CoQ 10 is present in the normal diet, but at insufficient levels to supply mitochondria. Therefore, de novo biosynthesis in mitochondria is needed, a complex pathway that involves several proteins encoded by COQ genes (Figure 3). 16 PDSS1 and PDSS2 encode a subunit of the enzyme required for synthesis of the decaprenyl tail of CoQ 10 ( Figure 3). Only 1 patient with a PDSS1 mutation and glomerular involvement has been reported in literature. 94 The patient presented before the age of 6 months with nephrotic syndrome, failure to thrive, and developmental delay. She rapidly developed kidney failure and died at the age of 16 months. To date, PDSS2 mutations have been identified in 7 patients with glomerular mitochondrial cytopathies associated showed that a PDSS2 haplotype was associated with a significantly increased risk of FSGS and collapsing glomerulopathy in European American patients. 95 Although the number of reported patients with a PDSS2 mutation is limited, the patients . Schematic overview of the CoQ 10 biosynthesis pathway. At least 15 genes are involved in the CoQ 10 biosynthesis pathway; however, the biochemical pathway is not yet completely understood. The function of COQ genes shown in italics (COQ8A/ADCK3, COQ8B/ADCK4, and COQ9) has not been fully elucidated. COQ8A/ADCK3 and COQ8B/ADCK4 have a regulatory role, 89 COQ9 is a lipid-binding protein interacting with COQ7. 91 The question mark indicates that the enzyme involved in the reaction has not been identified. CoA, coenzyme A; COQ, coenzyme Q gene; PDSS, prenyldiphosphate synthase subunit; PP, pyrophosphate; 4HB, 4-hydroxybenzoate generally presented at a young age and rapidly developed kidney failure. Furthermore, severe extrarenal involvement was reported in these patients, resulting in a poor prognosis. Saiki et al. showed that CoQ 10 supplementation rescued proteinuria and interstitial nephritis in PDSS2-deficient mice. 96 In clinical practice, CoQ 10 supplementation (5À20 mg/kg per day) was started in 4 patients, and 2 of them showed improvement in their clinical condition. 97 COQ2 encodes the enzyme para-hydroxybenzoatepolyprenyl-transferase required for the second step of the final pathway of CoQ 10 biosynthesis (Figure 3). 98 Steroid-resistant nephrotic syndrome (SRNS) usually develops within the first 2 years of life and often represents the first symptom of the disease, with or without neurologic symptoms. As shown in Table 3, 12 of 19 patients showed a rapid decline in kidney function. Furthermore, 17 of 25 patients showed some degree of extrarenal involvement. Patients who presented with decreased kidney function did not show improvement of kidney function after CoQ 10 supplementation. In patients presenting with nephrotic syndrome, a decrease in proteinuria may occur. Patients with severe neurological involvement showed neurological deterioration irrespectively of treatment with CoQ 10 , even when started immediately after birth (Supplementary Table S1). As expected, no recurrence of proteinuria occurred after kidney transplantation. In contrast to other genes involved in the CoQ 10 biosynthesis, there is a genotypeÀphenotype correlation for mutations in the COQ2 gene. 99 Mutations in the COQ2 gene have been associated with a wide spectrum of phenotypes, including a rapidly fatal, neonatal onset, multisystemic disease 100 ; SRNS, which can be associated with encephalopathy; and late-onset encephalopathy with retinopathy mimicking multiple system atrophy. 8,99,101,102 As previously described by Desbats et al., patients with severe phenotypes harbored 2 alleles that significantly impaired CoQ 10 production. 99 In contrast, patients harboring a compound heterozygous COQ2 mutation showed a milder clinical phenotype, including a higher age of onset, less severe clinical symptoms, and absence of extrarenal manifestations.
The enzyme CoQ 10 monooxygenase 6 (COQ6) is required for biosynthesis of CoQ 10 and catalyzes the C5 hydroxylation step of the quinine ring ( Figure 3). Ozeir et al. showed that Coq6 is not required for the C1hydroxylation. 103 As shown in Table 3, most of the affected children presented with SRNS and sensorineural deafness, generally at older ages than those reported for patients with COQ2 mutations. 104 The effect of CoQ 10 supplementation was not easy to evaluate, as most reported patients rapidly developed kidney failure. However, patients who did not rapidly develop kidney failure showed improvement of proteinuria (Table 3, Supplementary Table S1). In contrast, CoQ 10 supplementation did not improve sensorineural deafness in most patients. COQ8B/ADCK4 (AarF Domain Containing Kinase-4) interacts with components of the CoQ 10 biosynthesis pathway (Figure 3). 105À107 COQ8B/ADCK4 is expressed in podocytes and localized to mitochondria and foot processes. 106 A total of 82 patients with a COQ8B/ADCK4 mutation and kidney involvement have been reported to date (Table 3). Symptoms typically present in puberty, and CKD was diagnosed at presentation in one-fourth of the reported patients. Moreover, 54 of 74 patients (73%) developed kidney failure, with a median time to kidney failure of 1 year. In most patients, a kidney biopsy showed FSGS. Interestingly, Park et al. reported accompanying medullary nephrocalcinosis in 6 Korean patients. 108 Treatment with CoQ 10 was started in less than 50% of the reported patients. In 17 of 30 patients (57%), no improvement of kidney function was seen, especially in patients who already presented with impaired kidney function. However, if started early, patients did show a decrease in proteinuria (Table 3).
CoQ 10 IN DEPTH Primary CoQ 10 deficiency is a clinically and genetically highly variable disorder, as illustrated by the patients reported in Supplementary Table S1. Early diagnosis is crucial. as oral supplementation of CoQ 10 can limit disease progression and improve clinical symptoms.

Diagnostic Methods and Monitoring
The diagnosis of primary CoQ 10 deficiency is established with the identification of pathogenic variants in any of the genes encoding for the proteins directly involved in CoQ 10 biosynthesis ( Figure 3). Nowadays, genetic testing using next-generation sequencing is substantially less time intensive than a few years ago; however, because treatment should be started as early as possible, additional diagnostic tests are still indispensable to enable rapid identification of CoQ 10 deficiency. Classic biochemical analyses remain important in the diagnostics of mitochondrial cytopathies. Specifically for CoQ 10 deficiency, analysis of the combined activity of complex I to III and/or II and III is important. Furthermore, initial diagnostic testing should include measurement of blood lactate and alanine (as a measure of impaired energy production in skeletal muscle and long-standing pyruvate accumulation, respectively), even though normal levels do not exclude CoQ 10 deficiency. 109,110 In addition, assessment of CoQ 10 levels may be helpful in diagnosis and follow-up. Measurement of CoQ 10 in skeletal muscle is considered the gold standard test for diagnosing CoQ 10 deficiency; however, with the availability of relatively rapid genetic diagnostics, muscle biopsies are not routinely performed anymore. 109,111 Measurement of CoQ 10 levels in skin fibroblasts may be another option in patients with a primary CoQ 10 deficiency. 90 Nevertheless, a less invasive screening test is desirable. CoQ 10 levels can be measured in different biological specimens. 93,112À116 CoQ 10 levels in plasma appear to be highly dependent on dietary intake and are therefore not reflective of the levels in tissues. 117 Moreover, CoQ 10 levels in peripheral cells do not seem to accurately reflect the amount of CoQ 10 in tissues, and the exact value of other biological specimens in the diagnostics of CoQ 10 deficiency remains to be elucidated.

Pathophysiology
The pathogenesis of CoQ 10 deficiency involves different aspects. Interestingly, variable degrees of CoQ 10 deficiency appear to cause different defects of ATP synthesis and oxidative stress. Quinzii et al. show that severe CoQ 10 deficiency (<20% of normal) results in significant bioenergetic defects without oxidative stress. In contrast, intermediate CoQ 10 deficiency (30%À40% of normal) causes moderate bioenergetic defects but a significant increase in ROS production, indicating that residual activity of the respiratory chain is indispensable to produce ROS. 118 Podocytes are known to be susceptible to oxidative damage, 119 whereas impaired mitochondrial biogenesis did not result in a developmental or pathological change in podocytes. 85 This supports the concept that podocytes are independent of mitochondrial ATP production but susceptible to oxidation. In line with this, Desbats et al. show that patients with a severe presentation of CoQ 10 deficiency (presentation at birth with multi-organ failure) due to a COQ2 mutation harbored 2 alleles that markedly impaired CoQ 10 production. In contrast, patients with a milder phenotype including isolated nephrotic syndrome had at least 1 allele that allowed significant residual CoQ 10 biosynthesis for ROS production. 99 Zhu et al. used a Drosophila model to investigate COQ2 nephropathy. Silencing coq2 led to abnormal localization of slit diaphragms, collapse of lacunar channels, and dysmorphic mitochondria. Furthermore, increased levels of ROS were found in this model, and dietary supplementation with CoQ 10 partially rescued these defects. 120 Mutations in COQ8B/ADCK4 account for the highest number of patients with kidney disease secondary to CoQ 10 deficiency (Supplementary  Table S1). Ashraf et al. showed that knockdown of COQ8B/ADCK4 in zebrafish resulted in the characteristic triad of nephrotic syndrome. 106 Moreover, COQ8B/ ADCK4 is required for CoQ 10 biosynthesis and mitochondrial function in podocytes. 121 Compared to other CoQ 10 biosynthesis defects, mutations in COQ8B/ADCK4 seem to result in a less severe clinical entity, with a more prominent kidney phenotype, higher age at onset of SRNS, and good patient survival owing to the lack of extrarenal manifestations. The selective glomerular phenotype of patients with COQ8B/ADCK4 mutations may be the result of relative enrichment of COQ8B/ ADCK4 and lacking expression of the related protein COQ8A/ADCK3 in podocytes, whereas COQ8A/ADCK3 expression exceeds that of COQ8A/ADCK4 in most other body tissues. 106 Ashraf et al. showed that knockdown of COQ8B/ADCK4 in podocytes reduced their migration phenotype, which could be reversed by the addition of CoQ 10 . 106 The relatively mild phenotype observed in patients with COQ8B/ADCK4 defects is probably related to the fact that the encoded enzyme has a modulatory function without catalytic activity, enabling residual CoQ 10 synthesis even in the complete absence of this protein. 8 Moreover, other genes may partially compensate for the absence of COQ8A/ADCK4. 122

Treatment
Oral administration of CoQ 10 is the treatment strategy for affected individuals with a CoQ 10 deficiency. As was previously shown by Montini et al. and various other clinical reports, CoQ 10 can block the progression of the disease. 12 Although severe neurological and kidney damage cannot be reversed, treatment may also be initiated to prevent development of additional extrarenal symptoms. Different formulations of CoQ 10 are available, including an oxidized form (ubiquinone), a reduced form (ubiquinol), and analogs such as idebenone. Recently, Kleiner et al. proposed that CoQ 10 supplementation causes an increase in CoQ 10 levels, rescuing sulfide oxidation and thereby preventing kidney failure. 19 Unfortunately, clinical studies regarding efficacy are lacking, 92 and the optimal dose and form of oral CoQ 10 are still under debate. In the literature, the daily dose of CoQ 10 ranges from 5 to 100 mg/kg (Supplementary Table S1), with most clinicians describing a dose between 30 and 50 mg/ kg per day. Atmaca et al. recently published long-term follow-up results of CoQ 10 supplementation in patients with a COQ8B/ADCK4 mutation, and observed maximum reduction of proteinuria at 6 months of treatment. 123 After a median follow-up duration of 25.3 months following CoQ 10 administration (20À30 mg/kg per day), proteinuria was significantly decreased, whereas kidney function was preserved. 123,124 Of note, 4 of 8 reported patients received angiotensin-converting enzyme inhibitors or an angiotensin receptor blocker in addition to CoQ 10 , which may have contributed to the decrease in proteinuria. For patients with mutations in the COQ2 gene, however, results were less favorable. Eroglu et al. reported 4 patients of 2 different families, in which 2 patients were started on CoQ 10 supplementation immediately after birth or at first presentation. 125 Despite early treatment and an initial good response in terms of nephrotic syndrome and hyperglycemia, the patients still developed severe neurological problems and eventually died at 31 and 14 months of age, respectively. 125 Interestingly, 3 patients with a COQ6 mutation reported in the literature and 1 of the patients with a homozygous pathogenic COQ2 mutation from our clinic (Supplementary File) showed a response to cyclosporine therapy with regard to the nephrotic syndrome. 126 CoQ 10 deficiency can directly lead to the opening of the mitochondrial permeability transition pore (MPTP). Moreover, an increased amount of ROS can also induce mitochondrial permeability transition. 127 Cyclosporine has the capacity to inhibit MPTP through interaction with cyclophilin D, an essential component of the MPTP, and thereby reduce permeability. 128 In line with this, Heeringa et al. showed that incubation of COQ6 knockdown podocytes with cyclosporine had a mild rescue effect. 104 Our additional hypothesis is that the increase in ROS due to CoQ 10 deficiency may have an impact on the cytoskeleton of the podocytes. Cyclosporine is known for its podocytestabilizing capacities and might therefore cause a partial response in these patients. 129 A new potential therapeutic approach was recently reported for mitochondrial dysfunction due to CoQ 10 deficiency. The treatment of Coq6 knockout mice with 2,4-dihydroxybenzoic acid (4-diHB), an analog of a CoQ precursor molecule, prevented kidney dysfunction and reversed podocyte migration rate impairment. 130 A potential role for 2,4diHB was suggested for the treatment of CoQ10 deficiency caused by COQ8B/ADCK4 mutations as well. 121 Moreover, Ozeir et al. demonstrated that analogs of 4-HB can bypass a deficient CoQ biosynthetic enzyme. 103 In addition, hydroxylated analogs of 4-HB, 3,4-dihydroxybenzoic acid and/or vanillic acid were marked as a potential therapeutic intervention for CoQ 10 deficiency due to a COQ6 mutation. 131,132 Currently, no curative treatment options for mitochondrial cytopathies are available, besides CoQ 10 supplementation for patients with a CoQ 10 deficiency. Typically, patients receive supportive treatment, and other treatment options involve dietary supplements, vitamins, and antioxidants. 133 A 2012 Cochrane review of mitochondrial therapies has found little evidence supporting the use of any vitamin or cofactor intervention. 134 Furthermore, there is early evidence for a REVIEW AM Schijvens et al.: Mitochondrial Disease and the Kidney 2154 therapeutic role of L-arginine and citrulline therapy for MELAS-related strokes. The evidence supporting the use of CoQ 10 in mitochondrial diseases other than primary CoQ 10 is sparse. 133

CONCLUSION
In this review, an overview of kidney involvement in genetic mitochondrial cytopathies is provided. Kidney manifestations reported in the setting of mitochondrial dysfunction include tubular dysfunction, interstitial nephritis, glomerular pathology, and, in rare cases, cystic disease. Special focus on CoQ 10 deficiency was presented to emphasize early diagnosis of primary CoQ 10 deficiency, as oral supplementation of CoQ 10 may limit disease progression and improve clinical symptoms. A combination of genetic and metabolic diagnostics, including targeted whole-exome sequencing and blood lactate and alanine levels at presentation, should be performed in children with nephrotic syndrome presenting in adolescence or at a young age (<3 years) and in patients with nephrotic syndrome at any age with extrarenal symptoms. In case none of these criteria is present, we recommend awaiting steroid responsiveness for 4 weeks. In case patients do not respond to steroids after 4 weeks of treatment, we would advise performing genetic diagnostics. Moreover, starting CoQ 10 treatment should be considered in expectation of the final genetic diagnosis. Lifelong CoQ 10 supplementation should be considered, as sustained remission of nephrotic syndrome has been described and extrarenal symptoms may be prevented. Finally, a multidisciplinary approach is required, given the number of organs that can be involved in patients suffering from a mitochondrial cytopathy.

DISCLOSURE
All the authors declared no competing interests.

ACKNOWLEDGMENTS
This study was funded by the Dutch Kidney Foundation (grant number 15OKG16) (MFS). The authors would like to thank E.J. Steenbergen for his contribution in the pathological data and images of the patients reported in this manuscript.

SUPPLEMENTARY MATERIAL
Supplementary File (PDF) Supplementary Case Description. Detailed description of 3 patients. Supplementary References. Table S1. Overview of patients reported in literature with a PDSS1, PDSS2, COQ2, COQ6, or COQ8B/ADCK4 mutation and glomerular involvement. Table S2. Clinical characteristics of our 3 patients with a primary CoQ 10 deficiency. Figure S1. (A) Light microscopy image, patient 2. Many glomeruli show segmental (or sometimes global) collapse of the capillaries with epithelial hyperplasia in the Bowman space, consistent with collapsing type focal and segmental glomerulosclerosis (indicated with the arrow). There are no basement membrane abnormalities. There is tubulopathy with flattening of the cells, irregular vacuolation, and activated appearance of the nuclei, probably secondary to protein overload. Bar ¼ 50 mm. (B) Electron microscopy patient 2. (C) Electron microscopy patient 3. Electron microscopy images for patients 2 and 3. There is extensive podocyte foot-process effacement (indicated with the arrow), and there are large areas of podocyte detachment from the glomerular basement membrane. Segmentally, accumulation of electron-lucent material in the subepithelial space was observed (indicated by the asterisks). This material sometimes appeared vaguely laminated but there was no evident organization. These findings were considered consistent with massive podocyte injury/collapsing focal and segmental glomerulosclerosis. Electron microscopy patient 2, bar ¼ 5 mm; electron microscopy patient 3, bar ¼ 2 mm. Figure S2. Disease course of patient 3. In this graph the disease course of patient 3 is depicted. The patient went into remission with CsA treatment, and, after adequate CoQ 10 intake was guaranteed, CsA was discontinued. CoQ 10 , coenzyme Q 10 ; CsA, cyclosporine A.