Mutations in the MPV17 gene are responsible for rapidly progressive liver failure in infancy

Two patients with hepatic mtDNA depletion syndromes and marked elevations of S-adenosylmethionine and methionine

This paper reports studies of two patients proven by a variety of studies to have mitochondrial depletion syndromes due to mutations in either their MPV17 or DGUOK genes. Each was initially investigated metabolically because of plasma methionine concentrations as high as 15-21-fold above the upper limit of the reference range, then found also to have plasma levels of S-adenosylmethionine (AdoMet) 4.4-8.6-fold above the upper limit of the reference range. Assays of S-adenosylhomocysteine, total homocysteine, cystathionine, sarcosine, and other relevant metabolites and studies of their gene encoding glycine N-methyltransferase produced evidence suggesting they had none of the known causes of elevated methionine with or without elevated AdoMet. Patient 1 grew slowly and intermittently, but was cognitively normal. At age 7 years he was found to have hepatocellular carcinoma, underwent a liver transplant and died of progressive liver and renal failure at age almost 9 years. Patient 2 had a clinical course typical of DGUOK deficiency and died at age 8 ½ months. Although each patient had liver abnormalities, evidence is presented that such abnormalities are very unlikely to explain their elevations of AdoMet or the extent of their hypermethioninemias. A working hypothesis is presented suggesting that with mitochondrial depletion the normal usage of AdoMet by mitochondria is impaired, AdoMet accumulates in the cytoplasm of affected cells poor in glycine N-methyltransferase activity, the accumulated AdoMet causes methionine to accumulate by inhibiting activity of methionine adenosyltransferase II, and that both AdoMet and methionine consequently leak abnormally into the plasma.

Hepatocerebral form of mitochondrial DNA depletion syndrome due to mutation in MPV17 gene

Mitochondrial DNA depletion syndromes (MDSs) are autosomal recessive diseases characterized by a severe decrease in mitochondrial DNA content leading to dysfunction of the affected organ. Autosomal recessive mutations in MPV17 have been identified in the hepatocerebral form of MDS. We describe the clinical features, biochemical and molecular results of a Saudi infant with a new mutation of MPV17 and compared the features to those of previously reported cases. We stress the importance of such rare cases particularly in countries with high consanguineous marriage rate.

Measurement of mitochondrial DNA copy number

Mitochondrial disorders are complex and heterogeneous diseases that may be caused by molecular defects in either the nuclear or mitochondrial genome. The biosynthesis and maintenance of the integrity of the mitochondrial genome is solely dependent on a number of nuclear proteins. Defects in these nuclear genes can lead to mitochondrial DNA (mtDNA) depletion (Spinazzola et al. Biosci Rep 27:39-51, 2007). The mitochondrial DNA (mtDNA) depletion syndromes (MDDSs) are autosomal recessive disorders characterized by a significant reduction in mtDNA content. These genes include POLG, DGUOK, TK2, TYMP, MPV17, SUCLA2, SUCLG1, RRM2B, and C10orf2, all nine genes have mutations reported to cause various forms of MDDSs. In this chapter, we outline the real-time quantitative polymerase chain reaction (qPCR) analysis of mtDNA content in muscle or liver tissues.

Diagnostic challenges of mitochondrial disorders: complexities of two genomes

Mitochondrial disorders causing respiratory chain dysfunction comprise a group of genetically and clinically heterogeneous diseases. This heterogeneity reflects both the biochemical complexity of oxidative phosphorylation and the genetic contribution of both the nuclear and mitochondrial genomes to the respiratory chain. Current approaches to diagnose and classify mitochondrial disorders incorporate clinical, biochemical, and histological criteria, as well as DNA-based molecular diagnostic testing. While the identification of pathogenic mutations is generally accepted as definitive, the large number of candidate nuclear genes, the involvement of two genomes, and potential heteroplasmy of pathogenic mitochondrial DNA (mtDNA) frequently complicate successful molecular diagnostic confirmation. The strategy for pursuing a diagnosis derives from the integration of family history, clinical findings, biochemical evaluations, histopathological analyses, neuroradiological results, and the availability of different tissues for analyses. Screening for common point mutations and large deletions in mtDNA is usually the first step. Specific subsets of known nuclear disease genes can be screened by direct sequencing for cases of recognizable patterns of respiratory chain deficiencies or clinically identifiable syndromic presentations. Measurement of mtDNA content in affected tissues such as muscle and liver allows screening for mtDNA depletion syndromes. The growing list of known disease-causing genes and the promise of next generation sequencing technologies will undoubtedly improve diagnostic accuracy and genetic counseling for this challenging group of disorders.

Nuclear gene defects in mitochondrial disorders

Most mitochondrial cytopathies in infants are caused by mutations in nuclear genes encoding proteins targeted to the mitochondria rather than by primary mutations in the mitochondrial DNA. Over the past few years, the awareness of the number of disease-causing mutations in different nuclear genes has grown exponentially. These genes encode the various subunits of each respiratory chain complex, the ancillary proteins involved in the assembly of these subunits, proteins involved in mitochondrial DNA replication and maintenance, proteins involved in mitochondrial protein synthesis, and proteins involved in mitochondrial dynamics. This increased awareness has added a challenging dimension to the current diagnostic workup of mitochondrial cytopathies. The advent of new technologies such as next-generation sequencing should facilitate the resolution of this dilemma.

Novel large-range mitochondrial DNA deletions and fatal multisystemic disorder with prominent hepatopathy

Hepatic involvement in mitochondrial cytopathies rarely manifests in adulthood, but is a common feature in children. Multiple OXPHOS enzyme defects in children with liver involvement are often associated with dramatically reduced amounts of mtDNA. We investigated two novel large scale deletions in two infants with a multisystem disorder and prominent hepatopathy. Amount of mtDNA deletions and protein content were measured in different post-mortem tissues. The highest levels of deleted mtDNA were in liver, kidney, pancreas of both patients. Moreover, mtDNA deletions were detected in cultured skin fibroblasts in both patients and in blood of one during life. Biochemical analysis showed impairment of mainly complex I enzyme activity. Patients manifesting multisystem disorders in childhood may harbour rare mtDNA deletions in multiple tissues. For these patients, less invasive blood specimens or cultured fibroblasts can be used for molecular diagnosis. Our data further expand the array of deletions in the mitochondrial genomes in association with liver failure. Thus analysis of mtDNA should be considered in the diagnosis of childhood-onset hepatopathies.

Mitochondrial DNA content varies with pathological characteristics of breast cancer

Changes in mitochondrial DNA (mtDNA) content in cancers have been reported with controversial results, probably due to small sample size and variable pathological conditions. In this study, mtDNA content in 302 breast tumor/surrounding normal tissue pairs were evaluated and correlated with the clinico-pathological characteristics of tumors. Overall, mtDNA content in tumor tissues is significantly lower than that in the surrounding normal tissues, P < 0.00001. MtDNA content in tumor tissues decreased with increasing tumor size. However, when the tumor is very large (>50 cm³), mtDNA content started to increase. Similarly, mtDNA content decreased from grades 0 and I to grade II tumors, but increased from grade II to grade III tumors. Tumors with somatic mtDNA alterations in coding region have significantly higher mtDNA content than tumors without somatic mtDNA alterations (P < 0.001). Tumors with somatic mtDNA alterations in the D-Loop region have significantly lower mtDNA content (P < 0.001). Patients with both low and high mtDNA content in tumor tissue have significantly higher hazard of death than patients with median levels of mtDNA content. mtDNA content in tumor tissues change with tumor size, grade, and ER/PR status; significant deviation from the median level of mtDNA content is associated with poor survival.

Mitochondrial hepatopathies in the newborn period

Mitochondrial disorders recognized in the neonatal period usually present as a metabolic crisis combined with one or several organ manifestations. Liver disorder in association with a respiratory chain deficiency may be overlooked since liver dysfunction is common in severely sick newborn infants. Lactacidosis, hypoglycemia, elevated serum transaminases and conjugated bilirubin are common signs of mitochondrial hepatopathy. Hepatosplenomegaly may occur in severe cases. A clinical picture with fetal growth restriction, postnatal lactacidosis, hypoglycemia, coagulopathy, and cholestasis, especially in combination with neurological symptoms or renal tubulopathy, should alert the neonatologist to direct investigations on mitochondrial disorder. A normal lactate level does not exclude respiratory chain defects. The most common liver manifestation caused by mutated mitochondrial DNA (deletion) is Pearson syndrome. Recently, mutations in several nuclear DNA genes have been identified that lead to mitochondrial hepatopathy, e.g. mitochondrial depletion syndrome caused by DGUOK, MPV17, SUCLG1, POLG1, or C10ORF2 mutations. A combination of lactacidosis, liver involvement, and Fanconi type renal tubulopathy is common when the complex III assembly factor BCS1L harbors mutations, the most severe disease with consistent genotype-phenotype correlation being the GRACILE syndrome. Mutations in nuclear translation factor genes (TRMU, EFG1, and EFTu) of the respiratory chain enzyme complexes have recently been identified. Diagnostic work-up of neonatal liver disorder should include assessment of function and structure of the complexes as well as mutation screening for known genes. So far, treatment is mainly symptomatic.

Early-onset severe neuromuscular phenotype associated with compound heterozygosity for OPA1 mutations

INTRODUCTION

Pathogenic mutations in the OPA1 gene are the most common identifiable cause of autosomal dominant optic atrophy (DOA), which is characterized by selective retinal ganglion cell loss, a distinctive pattern of temporal pallor of the optic nerve and a typical color vision deficit, with variable effects on visual acuity. Haploinsufficiency has been suggested as the major pathogenic mechanism for DOA. Here we present two siblings with severe ataxia, hypotonia, gastrointestinal dysmotility, dysphagia, and severe, early-onset optic atrophy who were found to be compound heterozygotes for two pathogenic OPA1 mutations. This example expands the clinical phenotype of OPA1-associated disorders and provides additional evidence for semi-dominant inheritance.

METHODS AND RESULTS

Molecular analysis of the OPA1 gene in this family by Sanger sequencing revealed compound heterozygosity for two mutations in trans configuration, a p.I382 M missense mutation and a p.V903GfsX3 frameshift deletion in both affected siblings. Electron microscopy of a skeletal muscle biopsy of the older sibling revealed dense osmiophilic bodies within the mitochondria. Mitochondrial DNA (mtDNA) content was within normal limits, and electron transport chain analysis showed no deficiencies of the mitochondrial respiratory chain enzymes. Multiple mtDNA deletions were not found.

CONCLUSION

Compound heterozygosity of pathogenic OPA1 mutations may cause severe neuromuscular phenotypes in addition to early-onset optic atrophy. While a role for OPA1 in mtDNA maintenance has been discussed, compound biallelic pathogenic OPA1 mutations in our patients did not result in altered mtDNA copy number, mtDNA deletions, or deficiencies of the electron transport chain, despite the severe clinical phenotype.

Infantile-onset disorders of mitochondrial replication and protein synthesis

Most inherited mitochondrial diseases in infants result from mutations in nuclear genes encoding proteins with specific functions targeted to the mitochondria rather than primary mutations in the mitochondrial DNA (mtDNA) itself. In the past decade, a growing number of syndromes associated with dysfunction resulting from tissue-specific depletion of mtDNA have been reported in infants. MtDNA depletion syndrome is transmitted as an autosomal recessive trait and causes respiratory chain dysfunction with prominent neurological, muscular, and hepatic involvement. Mendelian diseases characterized by defective mitochondrial protein synthesis and combined respiratory chain defects have also been described in infants and are associated with mutations in nuclear genes that encode components of the translational machinery. In the present work, we reviewed current knowledge of clinical phenotypes, their relative frequency, spectrum of mutations, and possible pathogenic mechanisms responsible for infantile disorders of oxidative metabolism involved in correct mtDNA maintenance and protein production.

Deoxyguanosine kinase deficiency presenting as neonatal hemochromatosis

Mutations in DGUOK result in mitochondrial DNA (mtDNA) depletion and may present as neonatal liver failure. Neonatal hemochromatosis (NH(1)) is a liver disorder of uncertain and varied etiology characterized by hepatic and non-reticuloendothelial siderosis. To date, deoxyguanosine kinase (dGK(2)) deficiency has not been formally recognized in cases of NH. We report an African American female neonate with clinical and autopsy findings consistent with NH, and mtDNA depletion due to a homozygous mutation in DGUOK. This report highlights hepatocerebral mtDNA depletion in the differential of neonatal tyrosinemia, advocates considering dGK deficiency in cases of NH, and posits mitochondrial oxidative processes in the pathogenesis of NH.

Atypical presentation of Leigh syndrome associated with a Leber hereditary optic neuropathy primary mitochondrial DNA mutation

Leber hereditary optic neuropathy (LHON) is caused by point mutations in mitochondrial DNA (mtDNA), and is characterized by bilateral, painless sub-acute visual loss that develops during the second decade of life. Here we report the case of a five year old girl who presented with clinical and neuroradiological findings reminiscent of Leigh syndrome but carried a mtDNA mutation m.11778G>A (p.R340H) in the MTND4 gene usually observed in patients with LHON. This case is unusual for age of onset, gender, associated neurological findings and evolution, further expanding the clinical spectrum associated with primary LHON mtDNA mutations.

Real-time quantitative PCR analysis of mitochondrial DNA content

Mitochondrial disorders are a group of complex and heterogeneous diseases that may be caused by molecular defects in the nuclear or mitochondrial genome. The biosynthesis and integrity of the small 16.6-kb mitochondrial genome require a group of nuclear encoded genes. The mitochondrial DNA (mtDNA) depletion syndromes (MDDSs) are autosomal recessive disorders caused by molecular defects in nuclear genes, and characterized by a reduction in mtDNA content. To date, mutations in at least nine genes (POLG, DGUOK, TK2, TYMP, MPV17, SUCLA2, SUCLG1, RRM2B, and C10orf2) have been reported to cause various forms of MDDSs. In the clinical setting, a simple method to determine mtDNA depletion would be useful prior to undertaking gene sequence analysis. This unit outlines the real-time quantitative polymerase chain reaction (qPCR) analysis of mtDNA content in tissues. MtDNA content varies among different tissues and at different ages in the same individual. Detailed protocols for the selection of nuclear genes for normalization, PCR set up, validation procedures, tissue and age matched controls, and sensitivity and specificity in various tissues, as well as interpretation of results are discussed.

Fatal infantile lactic acidosis and a novel homozygous mutation in the SUCLG1 gene: a mitochondrial DNA depletion disorder

Mitochondrial DNA (mtDNA) depletion syndromes are autosomal recessive conditions in which the mtDNA copy number is greatly decreased in affected tissues. The encephalomyopathic group of these syndromes comprise mutations in SUCLA2 and SUCLG1 subunits [1]. In this report, we describe a patient with fatal infantile lactic acidosis associated with mutations in the SUCLG1 gene and mtDNA depletion. Histological and enzymatic abnormalities in skeletal muscle support the diagnosis of this recently described mitochondrial disorder. This case is unique in that prenatal imaging suggested the diagnosis and that the confirmatory molecular diagnosis was established at 2 weeks of age. We describe prenatal MRI and neonatal laboratory disturbances that can point the clinician toward consideration of this diagnosis when treating infantile lactic acidosis.

Molecular genetics of mitochondrial disorders

Mitochondrial respiratory chain (RC) disorders (RCDs) are a group of genetically and clinically heterogeneous diseases because of the fact that protein components of the RC are encoded by both mitochondrial and nuclear genomes and are essential in all cells. In addition, the biogenesis, structure, and function of mitochondria, including DNA replication, transcription, and translation, all require nuclear-encoded genes. In this review, primary molecular defects in the mitochondrial genome and major classes of nuclear genes causing mitochondrial RCDs, including genes underlying mitochondrial DNA (mtDNA) depletion syndrome, as well as genes encoding RC subunits, complex assembly genes, and translation factors, are described. Diagnostic methodologies used to detect common point mutations, large deletions, and unknown point mutations in the mtDNA and to quantify mutation heteroplasmy are also discussed. Finally, the selection of nuclear genes for gold standard sequence analysis, application of novel technologies including oligonucleotide array comparative genomic hybridization, and massive parallel sequencing of target genes are reviewed.

Current molecular diagnostic algorithm for mitochondrial disorders

Mitochondrial respiratory chain disorders (RCD) are a group of genetically and clinically heterogeneous diseases, due in part to the biochemical complexity of mitochondrial respiration and the fact that two genomes, one mitochondrial and one nuclear, encode the components of the respiratory chain. Because of the large number of genes involved, attempts to classify mitochondrial RCD incorporate clinical, biochemical, and histological criteria, in addition to DNA-based molecular diagnostic testing. While molecular testing is widely viewed as definitive, confirmation of the diagnosis by molecular methods often remains a challenge because of the large number of genes, the two genome complexity and the varying proportions of pathogenic mitochondrial DNA (mtDNA) molecules in a patient, a concept termed heteroplasmy. The selection of genes to be analyzed depends on the family history and clinical, biochemical, histopathological, and imaging results, as well as the availability of different tissues for analysis. Screening of common point mutations and large deletions in mtDNA is typically the first step. In cases where tissue-specific, recognizable clinical syndromes or characteristic RC complex deficiencies and histochemical abnormalities are observed, direct sequencing of the specific causative nuclear gene(s) can be performed on white blood cell DNA. Measurement of mtDNA content in affected tissues such as muscle and liver allows screening for mtDNA depletion syndromes. The ever-expanding list of known disease-causing genes will undoubtedly improve diagnostic accuracy and genetic counseling.

Quantitative evaluation of the mitochondrial DNA depletion syndrome

BACKGROUND

The mitochondrial DNA (mtDNA) depletion syndromes (MDDSs) are autosomal recessive disorders characterized by a reduction in cellular mtDNA content. Mutations in at least 9 genes [POLG, polymerase (DNA directed), gamma; DGUOK, deoxyguanosine kinase; TK2, thymidine kinase, mitochondrial; TYMP, thymidine phosphorylase; MPV17, MpV17 mitochondrial inner membrane protein; SUCLA2, succinate-CoA ligase, ADP-forming, beta subunit; SUCLG1, succinate-CoA ligase, alpha subunit; RRM2B, RRM2B, ribonucleotide reductase M2 B (TP53 inducible); and C10orf2, chromosome 10 open reading frame 2 (also known as TWINKLE)] have been reported to cause mtDNA depletion. In the clinical setting, a simple method to quantify mtDNA depletion would be useful before undertaking gene sequence analysis.

METHODS

Real-time quantitative PCR (qPCR) was used to measure the mtDNA content in blood, muscle, and liver samples and in skin fibroblast cultures from individuals suspected of mitochondrial disorders, with or without deleterious mutations in genes responsible for MDDS.

RESULTS

The mtDNA content was quantified in 776 tissue samples (blood, n = 341; muscle, n = 325; liver, n = 63; skin fibroblasts, n = 47) from control individuals. mtDNA content increased with age in muscle tissue, decreased with age in blood samples, and appeared to be unaffected by age in liver samples. In 165 samples (blood, n = 122; muscle, n = 21; liver, n = 15; skin fibroblasts, n = 7) from patients with molecularly proven MDDSs, severe mtDNA depletion was detected in liver and muscle tissue with high specificity and sensitivity. Blood samples were specific but not sensitive for detecting mtDNA depletion, and skin fibroblasts were not valuable for evaluating mtDNA depletion. Mutations in the POLG, RRM2B, and MPV17 genes were prospectively identified in 1 blood, 1 liver, and 3 muscle samples.

CONCLUSIONS

Muscle and liver tissues, but not blood or skin fibroblasts, are potentially useful for rapid screening for mtDNA depletion with real-time qPCR.

Mitochondrial DNA depletion syndromes--many genes, common mechanisms

Mitochondrial DNA depletion syndrome has become an important cause of inherited metabolic disorders, especially in children, but also in adults. The manifestations vary from tissue-specific mtDNA depletion to wide-spread multisystemic disorders. Nine genes are known to underlie this group of disorders, and many disease genes are still unidentified. However, the disease mechanisms seem to be intimately associated with mtDNA replication and nucleotide pool regulation. We review here the current knowledge on the clinical and molecular genetic features of mitochondrial DNA depletion syndrome.

Neonatal liver failure: a genetic and metabolic perspective

Liver failure in newborns can present formidable diagnostic challenges. The presentation of neonatal liver failure is variable and the initial assessment is crucial in the determination of potentially treatable causes. We present a case of neonatal hemochromatosis, review genetic and metabolic causes of neonatal liver failure, and outline an updated differential diagnosis of neonatal liver failure. In addition, we propose a comprehensive initial work-up of neonatal liver failure, and review current treatments for neonatal hemochromatosis.

MPV17-associated hepatocerebral mitochondrial DNA depletion syndrome: new patients and novel mutations

Mitochondrial DNA depletion syndromes are autosomal recessive diseases characterized by a severe decrease in mitochondrial DNA content leading to dysfunction of the affected organ. They are phenotypically heterogeneous and classified as myopathic, encephalomyopathic, or hepatocerebral. The latter group has been associated with mutations in TWINKLE,POLG1, DGUOK genes and recently with mutations in the MPV17 gene. MPV17 encodes a mitochondrial inner membrane protein and plays an as yet poorly understood role in mitochondrial DNA maintenance. Mutations in the MPV17 gene have been reported in patients who came to medical attention during infancy with liver failure, hypoglycemia, failure-to-thrive and neurological symptoms. In addition, a homozygous p.R50Q mutation has been identified in patients with Navajo neurohepatopathy. To date, 13 different mutations in 21 patients have been reported. We report eight new patients with seven novel mutations, including four missense mutations (c.262A>G (p.K88E), c.280G>C (p.G94R), c.293C>T (p.P98L), and c.485C>A (p.A162D)), one in-frame deletion (c.271_273del3 (p.L91del)), one splice site substitution (c.186+2T>C), and one insertion (c.22_23insC). The p.R50Q mutation, which occurs in a CpG dinucleotide, is the most common MPV17 mutation and, to date, has only been found in the homozygous state. Clinically, patients homozygous for p.R50Q or compound heterozygous for the p.G94R and p.P98L mutations have a better prognosis, with all the other mutations associated with early death if not treated by liver transplantation. Localizing the mutations within the predicted MPV17 protein structure reveals clustering of mutations in the region of the putative protein kinase C phosphorylation site.

A novel c.592-4_c.592-3delTT mutation in DGUOK gene causes exon skipping

Deoxyguanosine kinase (DGUOK) catalyzes the first step of the mitochondrial deoxypurine salvage pathway, the phosphorylation of purine deoxyribonucleosides. Mutations in the DGUOK gene have been linked to inherited mtDNA depletion syndromes, neonatal liver failure, nystagmus, and hypotonia. Previously, we reported the first case of a heterozygous unclassified c.592-4_c.592-3delTT alteration in a patient with DGUOK deficiency without the demonstration of its pathogenicity (Dimmock et al., 2008). This alteration was predicted to cause aberrant splicing based upon two computer algorithms. We now report a homozygous c.592-4_c.592-3delTT mutation found in two affected siblings of asymptomatic consanguineous parents. The proband presented with symptoms of idiopathic hepatitis, liver dysfunction, nystagmus, and retinal blindness. This individual died at 6months of age due to liver failure. This individual's affected sibling presented similarly and has remarkable elevations of tyrosine, methionine, and alanine. Many organic acids were elevated in urine, including lactic acid, Krebs cycle intermediates, and para-hydroxy compounds; ketone bodies were also present. RNA studies support aberrant splicing. Sequencing of cDNA detected exon 5 skipping in the two affected siblings, but not in the normal control. These results indicate that the homozygous c.592-4_c.592-3delTT is deleterious and responsible for the DGUOK deficiency. The parents were subsequently confirmed to be carriers of this mutation. In summary, we have demonstrated that c.592-4_c.592-3delTT is a pathogenic splice acceptor site mutation leading to DGUOK deficiency.

RHAMM (CD168) is overexpressed at the protein level and may constitute an immunogenic antigen in advanced prostate cancer disease

Localized prostate cancer (CaP) can be cured using several strategies. However, the need to identify active substances in advanced tumor stages is tremendous, as the outcome in such cases is still disappointing. One approach is to deliver human tumor antigen-targeted therapy, which is recognized by T cells or antibodies. We used data mining of the Cancer Immunome Database (CID), which comprises potential immunologic targets identified by serological screening of expression libraries. Candidate antigens were screened by DNA microarrays. Genes were then validated at the protein level by tissue microarrays, representing various stages of CaP disease. Of 43 targets identified by CID, 10 showed an overexpression on the complementary DNA array in CaP metastases. The RHAMM (CD168) gene, earlier identified by our group as an immunogenic antigen in acute and chronic leukemia, also showed highly significant overexpression in CaP metastases compared with localized disease and benign prostatic hyperplasia. At the protein level, RHAMM was highest in metastatic tissue samples and significantly higher in neoplastic localized disease compared with benign tissue. High RHAMM expression was associated with clinical parameters known to be linked to better clinical outcome. Patients with high RHAMM expression in the primaries had a significantly lower risk of biochemical failure. The number of viable cells in cell cultures was reduced in blocking experiments using hormone-sensitive and hormone-insensitive metastatic CaP cell lines. Acknowledging the proven immunogenic effects of RHAMM in leukemia, this antigen is intriguing as a therapeutic target in far-advanced CaP.

Application of dual-genome oligonucleotide array-based comparative genomic hybridization to the molecular diagnosis of mitochondrial DNA deletion and depletion syndromes

PURPOSE

Mitochondrial disorders constitute a group of clinically and genetically heterogeneous diseases for which molecular diagnosis has been a challenge. The current procedures for diagnosis of mitochondrial DNA deletion and depletion syndromes based on Southern analysis and quantitative polymerase chain reaction are particularly inefficient for determining important parameters of deletion endpoints and percent heteroplasmy. We have developed an improved approach for routine analyses of these disorders in a clinical laboratory.

METHODS

A custom-designed oligonucleotide array-based comparative genomic hybridization platform was developed to provide both tiled coverage of the entire 16.6-kb mitochondrial genome and high-density coverage of nuclear genes involved in mitochondrial biogenesis and function, for quick evaluation of mitochondrial DNA deletion and depletion.

RESULTS

For initial validation, the performance of this array was characterized in 20 samples with known mitochondrial DNA deletions and 12 with apparent depletions. All previously known deletions were clearly detected and the break points were correctly identified by the oligonucleotide array-based comparative genomic hybridization, within the limits of resolution of the array. The extent of mitochondrial DNA depletion and the percentage of deletion heteroplasmy were estimated using an automated computational approach that gave results comparable to previous methods. Conclusions from subsequent application of this approach with >300 new clinical samples have been in 100% concordance with those from standard methods. Finally, for one sample, we were able to identify an intragenic deletion in a nuclear gene that was responsible for the observed mitochondrial DNA depletion.

CONCLUSION

We conclude that this custom array is capable of reliably detecting mitochondrial DNA deletion with elucidation of the deletion break points and the percentage of heteroplasmy. In addition, simultaneous detection of the copy number changes in both nuclear and mitochondrial genomes makes this dual genome array of tremendous value in the diagnoses of mitochondrial DNA depletion syndromes.

Mitochondrial neurogastrointestinal encephalopathy due to mutations in RRM2B

BACKGROUND

Mitochondrial neurogastrointestinal encephalopathy (MNGIE) is a progressive neurodegenerative disorder associated with thymidine phosphorylase deficiency resulting in high levels of plasma thymidine and a characteristic clinical phenotype.

OBJECTIVE

To investigate the molecular basis of MNGIE in a patient with a normal plasma thymidine level.

DESIGN

Clinical, neurophysiological, and histopathological examinations as well as molecular and genetic analyses.

SETTING

Nerve and muscle center and genetic clinic. Patient A 42-year-old woman with clinical findings strongly suggestive for MNGIE.

MAIN OUTCOME MEASURES

Clinical description of the disease and its novel genetic cause.

RESULTS

Identification of mitochondrial DNA depletion in muscle samples (approximately 12% of the control mean content) prompted us to look for other causes of our patient's condition. Sequencing of genes associated with mitochondrial DNA depletion-POLG, PEO1, ANT1, SUCLG1, and SUCLA2-did not reveal deleterious mutations. Results of sequencing and array comparative genomic hybridization of the mitochondrial DNA for point mutations and deletions in blood and muscle were negative. Sequencing of RRM2B, a gene encoding cytosolic p53-inducible ribonucleoside reductase small subunit (RIR2B), revealed 2 pathogenic mutations, c.329G>A (p.R110H) and c.362G>A (p.R121H). These mutations are predicted to affect the docking interface of the RIR2B homodimer and likely result in impaired enzyme activity.

CONCLUSIONS

This study expands the clinical spectrum of impaired RIR2B function, challenges the notion of locus homogeneity of MNGIE, and sheds light on the pathogenesis of conditions involved in the homeostasis of the mitochondrial nucleotide pool. Our findings suggest that patients with MNGIE who have normal thymidine levels should be tested for RRM2B mutations.

Fluctuating liver functions in siblings with MPV17 mutations and possible improvement associated with dietary and pharmaceutical treatments targeting respiratory chain complex II

BACKGROUND/AIMS

To describe the clinical and biological findings of two Japanese siblings with novel MPV17 gene mutations (c.451insC/c.509C > T) manifesting hepatic mitochondrial DNA depletion syndrome.

METHODS

We observed these brothers and sought to determine the efficacy of treatment targeting respiratory chain complex II for the younger brother.

RESULTS

A 3-month-old boy had presented with profound liver dysfunction, failure to thrive, and watery diarrhea. Although he was then placed on a carbohydrate-rich diet, his liver function thereafter fluctuated greatly in association with viral infections, and rapidly deteriorated to liver failure. He underwent liver transplantation at 17 months of age but died at 22 months of age. The younger brother, aged 47 months at the time of this writing, presented with liver dysfunction from 8 months of age. His transaminase levels also fluctuated considerably fluctuations in association with viral infections. At 31 months of age, treatment with succinate and ubiquinone was initiated together with a lipid-rich diet using ketone milk. Thereafter, his transaminase levels normalized and never fluctuated, and the liver histology improved.

CONCLUSIONS

These cases suggested that the clinical courses of patients with MPV17 mutations are greatly influenced by viral infections and that dietary and pharmaceutical treatments targeting the mitochondrial respiratory chain complex II may be beneficial in the clinical management of MPV17 mutant patients.

Finding twinkle in the eyes of a 71-year-old lady: a case report and review of the genotypic and phenotypic spectrum of TWINKLE-related dominant disease

Progressive external ophthalmoplegia (PEO) can be caused by a disorder characterized by multiple mitochondrial DNA (mtDNA) deletions due to mutations in the TWINKLE gene, encoding a mtDNA helicase. We describe a 71-year-old woman who had developed PEO at age 55 years. She had cataracts, diabetes, paresthesias, cognitive defects, memory problems, hearing loss, and sensory ataxia. She had muscle weakness with ragged red fibers on biopsy. MRI showed static white matter changes. A c.908G>A substitution (p.R303Q) in the TWINKLE gene was identified. Multiple mtDNA deletions were detected in muscle but not blood by a PCR-based method, but not by Southern blot analysis. MtDNA copy number was maintained in blood and muscle. A systematic literature search was used to identify the genotypic and phenotypic spectrum of dominant TWINKLE-related disease. Patients were adults with PEO and symptoms including myopathy, neuropathy, dysarthria or dysphagia, sensory ataxia, and parkinsonism. Diabetes, cataract, memory loss, hearing loss, and cardiac problems were infrequent. All reported mutations clustered between amino acids 303 and 508 with no mutations at the N-terminal half of the gene. The TWINKLE gene should be analyzed in adults with PEO even in the absence of mtDNA deletions in muscle on Southern blot analysis, and of a family history for PEO. The pathogenic mutations identified 5' beyond the linker region suggest a functional role for this part of the protein despite the absence of a primase function in humans. In our patient, the pathogenesis involved multiple mtDNA deletions without reduction in mtDNA copy number.

Disorders from perturbations of nuclear-mitochondrial intergenomic cross-talk

In the course of evolution, mitochondria lost their independence, and mitochondrial DNA (mtDNA) became the 'slave' of nuclear DNA, depending on numerous nucleus-encoded factors for its integrity, replication and expression. Mutations in any of these factors may alter the cross-talk between the two genomes and cause Mendelian disorders characterized by qualitative (multiple deletions) or quantitative (depletion) alterations of mtDNA, or by defective translation of mtDNA-encoded respiratory chain components.

Glucose metabolism and diet-based prevention of liver dysfunction in MPV17 mutant patients

BACKGROUND/AIMS

To describe in detail the specific clinical and biological characteristics of three patients with MPV17 gene mutations, a rare hepatocerebral mitochondrial DNA depletion syndrome (MDS) and the positive effects of a novel dietetic treatment based on avoidance of fasting.

METHODS

We describe the case histories of three members of the same family with MPV17 mutations.

RESULTS

Two patients had a very severe and progressive liver disease: 1 died in the first year of life and the other underwent liver transplantation. The third patient, now 13 years of age, had a milder form of liver disease and developed progressive ataxia. Psychomotor involvement at onset of disease was mild or absent. No patient had severe hyperlactataemia. In vivo functional studies on two patients showed no hyperlactataemia even after intravenous and oral glucose loading, regular fasting hypoglycemia 3-4h after meals and no response to glucagon. Liver function tests improved when patients received continuous iv glucose infusion or were regularly fed every 3h.

CONCLUSIONS

These clinical and biochemical features allow us to differentiate patients with MPV17 mutations from other liver MDS and suggest that regular glucose intake at short intervals may be beneficial in slowing the progression of the disease.

Mitochondrial diseases: a cross-talk between mitochondrial and nuclear genomes

More than one billion years ago, mitochondria were free-living prokaryotic organisms with their own DNA. However, during the evolution, ancestral genes have been transferred from the mitochondrial to the nuclear genome so that mtDNA became dependent on numerous nucleus-encoded factors for its integrity, replication and expression. Mutations in any of these factors may alter the cross-talk between the two genomes and cause Mendelian diseases that affect mtDNA integrity or expression.

Early-onset liver mtDNA depletion and late-onset proteinuric nephropathy in Mpv17 knockout mice

In humans, MPV17 mutations are responsible for severe mitochondrial depletion syndrome, mainly affecting the liver and the nervous system. To gain insight into physiopathology of MPV17-related disease, we investigated an available Mpv17 knockout animal model. We found severe mtDNA depletion in liver and, albeit to a lesser extent, in skeletal muscle, whereas hardly any depletion was detected in brain and kidney, up to 1 year after birth. Mouse embryonic fibroblasts did show mtDNA depletion, but only after several culturing passages, or in a serumless culturing medium. In spite of severe mtDNA depletion, only moderate decrease in respiratory chain enzymatic activities, and mild cytoarchitectural alterations, were observed in the Mpv17^−/− livers, but neither cirrhosis nor failure ever occurred in this organ at any age. The mtDNA transcription rate was markedly increased in liver, which could contribute to compensate the severe mtDNA depletion. This phenomenon was associated with specific downregulation of Mterf1, a negative modulator of mtDNA transcription. The most relevant clinical features involved skin, inner ear and kidney. The coat of the Mpv17^−/− mice turned gray early in adulthood, and 18-month or older mice developed focal segmental glomerulosclerosis (FSGS) with massive proteinuria. Concomitant degeneration of cochlear sensory epithelia was reported as well. These symptoms were associated with significantly shorter lifespan. Coincidental with the onset of FSGS, there was hardly any mtDNA left in the glomerular tufts. These results demonstrate that Mpv17 controls mtDNA copy number by a highly tissue- and possibly cytotype-specific mechanism.

The mitochondrial 13513G>A mutation is associated with Leigh disease phenotypes independent of complex I deficiency in muscle

The mitochondrial 13513G>A (D393N) mutation in the ND5 subunit of the respiratory chain complex I was initially described in association with MELAS syndrome. Recent observations have linked this mutation to Leigh disease. We screened for the 13513G>A mutation in a cohort of 265 patients with Leigh and Leigh-like disease. The mutation was found in a total of 5 patients. An additional patient who had clinical presentation consistent with a Leigh-like phenotype but with a normal brain MRI was added to the cohort. None of an additional 88 patients meeting MELAS disease criteria, nor 56 patients with respiratory chain deficiency screened for the 13513G>A were found positive for the mutation. The most frequent clinical manifestations in our patients were hypotonia, ocular and cerebellar involvement. Low mutation heteroplasmy in the range of 20-40% was observed in all 6 patients. This observation is consistent with the previously reported low heteroplasmy of this mutation in some patients with the 13513G>A mutation and complex I deficiency. However, normal complex I activity was observed in two patients in our cohort. As most patients with Leigh-like disease and the 13513G>A mutation have been described with complex I deficiency, this report adds to the previously reported subset of patients with normal respiratory complex function. We conclude that in any patient with Leigh or Leigh-like disease, testing for the 13513G>A mutation is clinically relevant and low mutant loads in blood or muscle may be considered pathogenic, in the presence of normal respiratory chain enzyme activities.

Sensory ataxic neuropathy with ophthalmoparesis caused by POLG mutations

Mutations in POLG gene are responsible for a wide spectrum of clinical disorders with altered mitochondrial DNA (mtDNA) integrity, including mtDNA multiple deletions and depletion. Sensory ataxic neuropathy with ophthalmoparesis (SANDO) caused by mutations in POLG gene, fulfilling the clinical triad of sensory ataxic neuropathy, dysarthria and/or dysphagia and ophthalmoparesis, has described in a few reports. Here we described five cases of adult onset autosomal recessive sensory ataxic neuropathy with ophthalmoplegia. All patients had ataxia, neuropathy, myopathy, and progressive external ophthalmoplegia (PEO). The muscle pathology revealed ragged-red and cytochrome c oxidase (COX) negative fibers in three patients. However, deficiencies in the activities of mitochondrial respiratory chain enzyme complexes were not detected in any of the patients' muscle samples. Multiple deletions of mtDNA were detected in blood and muscle specimens but mtDNA depletion was not found. Due to these diagnostic difficulties, POLG-related syndromes are definitively diagnosed based on the presence of deleterious mutations in the POLG gene.

Lethal hepatopathy and leukodystrophy caused by a novel mutation in MPV17 gene: description of an alternative MPV17 spliced form

It has recently been reported that mutations in MPV17 gene may be causative of mtDNA depletion syndrome (MDS). Patients with this alteration presented with severe liver failure, hypoglycemia, growth retardation and neurological symptoms during the first year of life. We report on the clinical, biochemical and molecular findings of a patient presenting with lethal hepatopathy, polyneuropathy, neurological regression and leukodystrophy associated with mutations in MPV17. Mitochondrial respiratory chain activities were low in liver and within reference values in muscle. However, levels of mtDNA were markedly reduced both in muscle and liver. A novel homozygous mutation in MPV17, c.70+5G>A (IVS1+5G>A), was identified. This intronic change causes the full-length cDNA loss, probably due to loss of strength of the splice donor site of exon 1. Western blot analysis, performed in liver homogenates, further corroborates these results as the amount of patient's protein was highly reduced, or almost absent, compared with that of controls. We also identified an additional alternative spliced form in controls and in the patient, due to exon 2 skipping, that has not previously been reported.

Alpers syndrome with prominent white matter changes

Alpers syndrome is a fatal neurogenetic disorder caused by the mutations in POLG1 gene encoding the mitochondrial DNA polymerase gamma (polgamma). Two missense variants, c.248T > C (p.L83P), c.2662G > A (p.G888S) in POLG1 were detected in a 10-year-old Chinese girl with refractory seizures, acute liver failure after exposure to valproic acid, cortical blindness, and psychomotor regression. The pathology of left occipital lobe showed neuronal loss, spongiform degeneration, astrocytosis, and demyelination. In addition, there were prominent white matter changes in a series of brain magnetic resonance imaging (MRI) and increased immunological factors in CSF.

Lack of founder effect for an identical mtDNA depletion syndrome (MDS)-associated MPV17 mutation shared by Navajos and Italians

Navajo neurohepatopathy is a hepato-cerebral variant of mitochondrial DNA depletion syndrome due to a specific mutation in MPV17, a gene located on human chromosome 2p. The same mutation was reported in an Italian family. To understand whether the MPV17 mutation was transmitted by descent from a common ancestor to Navajos and Italians we constructed a dense haplotype of the MPV17 locus using suitable single nucleotide polymorphisms. Complete discordance between Italian and Navajo haplotypes rules out the former hypothesis, suggesting that the mutation occurred independently in the two populations.