Mitochondrial disease: a practical approach for primary care physicians

Mitochondrial dysfunction underlying outer retinal diseases

Dysfunction of photoreceptors, retinal pigment epithelium (RPE) or both contribute to the initiation and progression of several outer retinal disorders. Disrupted Müller glia function might additionally subsidize to these diseases. Mitochondrial malfunctioning is importantly associated with outer retina pathologies, which can be classified as primary and secondary mitochondrial disorders. This review highlights the importance of oxidative stress and mitochondrial DNA damage, underlying outer retinal disorders. Indeed, the metabolically active photoreceptors/RPE are highly prone to these hallmarks of mitochondrial dysfunction, indicating that mitochondria represent a weak link in the antioxidant defenses of outer retinal cells.

Migraine Variants or Episodic Syndromes That May Be Associated With Migraine and Other Unusual Pediatric Headache Syndromes

PURPOSE

To provide an overview of the clinical course for children and adolescents with migraine variants (M.V.), childhood periodic syndromes or the episodic syndromes that may be associated with migraine using the International Classification of Headache Disorders, 3rd Edition Beta version [ICHD-3] International Headache Society criteria for the diagnosis of each disorder.

METHOD

Migraine is a complex set of neurological symptoms. This review encompasses the subtypes of M.V. or episodic syndromes that may be associated with migraine within the children and adolescent population.

CONCLUSION

The episodic syndromes that may be associated with migraine or migraine variant is multilayered neurological disorder in young children and adolescents. Within the these generally pediatric syndromes there are associated disorders described in this review, to provide a clinical overview and including the less common forms of migraine, such as acute confusional migraine, trauma-triggered migraine, and transient global amnesia.

Mitochondrial oxidative phosphorylation disorders in children: Phenotypic, genotypic and biochemical correlations in 85 patients from South India

Mitochondrial oxidative phosphorylation (OXPHOS) disorders account for a variety of neuromuscular disorders in children. In this study mitochondrial respiratory chain enzymes were assayed in muscle tissue in a large cohort of children with varied neuromuscular presentations from June 2011 to December 2013. The biochemical enzyme deficiencies were correlated with the phenotypes, magnetic resonance imaging, histopathology and genetic findings to reach a final diagnosis. There were 85 children (mean age: 6.9±4.7years, M:F:2:1) with respiratory chain enzyme deficiency which included: isolated complex I (n=50, 60%), multiple complexes (n=24, 27%), complex IV (n=8, 9%) and complex III deficiencies (n=3, 4%). The most common neurological findings were ataxia (59%), hypotonia (59%) and involuntary movements (49%). A known mitochondrial syndrome was diagnosed in 27 (29%) and non-syndromic presentations in 57 (71%). Genetic analysis included complete sequencing of mitochondrial genome, SURF1, POLG1&2. It revealed variations in mitochondrial DNA (n=8), SURF1 (n=5), and POLG1 (n=3). This study, the first of its kind from India, highlights the wide range of clinical and imaging phenotypes and genetic heterogeneity in children with mitochondrial oxidative phosphorylation disorders.

Cellular Metabolism and Induced Pluripotency

The discovery of induced pluripotent stem cells (iPSCs) a decade ago, which we are celebrating in this issue of Cell, represents a landmark discovery in biomedical research. Together with somatic cell nuclear transfer, iPSC generation reveals the remarkable plasticity associated with differentiated cells and provides an unprecedented means for modeling diseases using patient samples. In addition to transcriptional and epigenetic remodeling, cellular reprogramming to pluripotency is also accompanied by a rewiring of metabolic pathways, which ultimately leads to changes in cell identities.

Mitochondrial diseases

Mitochondrial diseases are a group of genetic disorders that are characterized by defects in oxidative phosphorylation and caused by mutations in genes in the nuclear DNA (nDNA) and mitochondrial DNA (mtDNA) that encode structural mitochondrial proteins or proteins involved in mitochondrial function. Mitochondrial diseases are the most common group of inherited metabolic disorders and are among the most common forms of inherited neurological disorders. One of the challenges of mitochondrial diseases is the marked clinical variation seen in patients, which can delay diagnosis. However, advances in next-generation sequencing techniques have substantially improved diagnosis, particularly in children. Establishing a genetic diagnosis allows patients with mitochondrial diseases to have reproductive options, but this is more challenging for women with pathogenetic mtDNA mutations that are strictly maternally inherited. Recent advances in in vitro fertilization techniques, including mitochondrial donation, will offer a better reproductive choice for these women in the future. The treatment of patients with mitochondrial diseases remains a challenge, but guidelines are available to manage the complications of disease. Moreover, an increasing number of therapeutic options are being considered, and with the development of large cohorts of patients and biomarkers, several clinical trials are in progress.

Mitochondrial Dysfunction in Autism Spectrum Disorders

Autism spectrum disorders (ASD) are classified as neurodevelopmental disorders characterised by diminished social communication and interaction. Recently, evidence has accrued that a significant proportion of individuals with autism have concomitant diseases such as mitochondrial disease and abnormalities of energy generation. This has therefore led to the hypothesis that autism may be linked to mitochondrial dysfunction. We review such studies reporting decreased activity of mitochondrial electron transport chain (ETC) complexes and reduced gene expression of mitochondrial genes, in particular genes of respiratory chain complexes, in individuals with autism. Overall, the findings support the hypothesis that there is an association of ASD with impaired mitochondrial function; however, many of the studies have small sample sizes and there is variability in the techniques utilised. There is therefore a vital need to utilise novel imaging techniques, such as near-infrared spectroscopy, that will allow non-invasive measurement of metabolic markers for neuronal activity such as cytochrome c oxidase, in order to better establish the link between autism and mitochondrial dysfunction.

Neuropsychiatric Features in Primary Mitochondrial Disease

Mitochondrial diseases are a clinically heterogeneous group of disorders that ultimately result from dysfunction of the mitochondrial respiratory chain. There is some evidence to suggest that mitochondrial dysfunction plays a role in neuropsychiatric illness; however, the data are inconclusive. This article summarizes the available literature published in the area of neuropsychiatric manifestations in both children and adults with primary mitochondrial disease, with a focus on autism spectrum disorder in children and mood disorders and schizophrenia in adults.

Solid organ transplantation in primary mitochondrial disease: Proceed with caution

Solid organ transplants are rarely performed in both adult and pediatric patients with primary mitochondrial disease. Poor outcomes have been described in case reports and small case series. It is unclear whether the underlying genetic disease has a significant impact on post-transplant morbidity and mortality. Data were obtained for 35 patients from 17 Mitochondrial Disease Centers across North America, the United Kingdom and Australia. Patient outcomes were noted after liver, kidney or heart transplantation. Excluding patients with POLG-related disease, post-transplant survival approached or met outcomes seen in non-mitochondrial disease transplant patients. The majority of mitochondrial disease patients did not have worsening of their mitochondrial disease within 90-days post-transplant. Post-transplant complications, including organ rejection, were not a common occurrence and were generally treatable. Many patients did not have a mitochondrial disease considered or diagnosed prior to transplantation. In conclusion, patients with mitochondrial disease in this cohort generally tolerated solid-organ transplantation. Such patients may not need to be excluded from transplant solely for their mitochondrial diagnosis; additional caution may be needed for patients with POLG-related disease. Transplant teams should be aware of mitochondrial disease as an etiology for organ-failure and consider appropriate consultation in patients without a known cause of their symptoms.

Modifying the Mitochondrial Genome

Human mitochondria produce ATP and metabolites to support development and maintain cellular homeostasis. Mitochondria harbor multiple copies of a maternally inherited, non-nuclear genome (mtDNA) that encodes for 13 subunit proteins of the respiratory chain. Mutations in mtDNA occur mainly in the 24 non-coding genes, with specific mutations implicated in early death, neuromuscular and neurodegenerative diseases, cancer, and diabetes. A significant barrier to new insights in mitochondrial biology and clinical applications for mtDNA disorders is our general inability to manipulate the mtDNA sequence. Microinjection, cytoplasmic fusion, nucleic acid import strategies, targeted endonucleases, and newer approaches, which include the transfer of genomic DNA, somatic cell reprogramming, and a photothermal nanoblade, attempt to change the mtDNA sequence in target cells with varying efficiencies and limitations. Here, we discuss the current state of manipulating mammalian mtDNA and provide an outlook for mitochondrial reverse genetics, which could further enable mitochondrial research and therapies for mtDNA diseases.

Potentially diagnostic electron paramagnetic resonance spectra elucidate the underlying mechanism of mitochondrial dysfunction in the deoxyguanosine kinase deficient rat model of a genetic mitochondrial DNA depletion syndrome

A novel rat model for a well-characterized human mitochondrial disease, mitochondrial DNA depletion syndrome with associated deoxyguanosine kinase (DGUOK) deficiency, is described. The rat model recapitulates the pathologic and biochemical signatures of the human disease. The application of electron paramagnetic (spin) resonance (EPR) spectroscopy to the identification and characterization of respiratory chain abnormalities in the mitochondria from freshly frozen tissue of the mitochondrial disease model rat is introduced. EPR is shown to be a sensitive technique for detecting mitochondrial functional abnormalities in situ and, here, is particularly useful in characterizing the redox state changes and oxidative stress that can result from depressed expression and/or diminished specific activity of the distinct respiratory chain complexes. As EPR requires no sample preparation or non-physiological reagents, it provides information on the status of the mitochondrion as it was in the functioning state. On its own, this information is of use in identifying respiratory chain dysfunction; in conjunction with other techniques, the information from EPR shows how the respiratory chain is affected at the molecular level by the dysfunction. It is proposed that EPR has a role in mechanistic pathophysiological studies of mitochondrial disease and could be used to study the impact of new treatment modalities or as an additional diagnostic tool.

Hypoxia as a therapy for mitochondrial disease

Defects in the mitochondrial respiratory chain (RC) underlie a spectrum of human conditions, ranging from devastating inborn errors of metabolism to aging. We performed a genome-wide Cas9-mediated screen to identify factors that are protective during RC inhibition. Our results highlight the hypoxia response, an endogenous program evolved to adapt to limited oxygen availability. Genetic or small-molecule activation of the hypoxia response is protective against mitochondrial toxicity in cultured cells and zebrafish models. Chronic hypoxia leads to a marked improvement in survival, body weight, body temperature, behavior, neuropathology, and disease biomarkers in a genetic mouse model of Leigh syndrome, the most common pediatric manifestation of mitochondrial disease. Further preclinical studies are required to assess whether hypoxic exposure can be developed into a safe and effective treatment for human diseases associated with mitochondrial dysfunction.

Genetic aetiology of ophthalmological manifestations in children - a focus on mitochondrial disease-related symptoms

PURPOSE

To investigate the association of mutations in the mitochondrial DNA (mtDNA) or nuclear candidate genes with mitochondrial disease-related ophthalmic manifestations (nystagmus, ptosis, ophthalmoplegia, optic neuropathy and retinopathy) in children.

METHODS

A retrospective cohort of children (n = 98) was identified from the medical record files of a tertiary care hospital. The entire mtDNA and nuclear genes POLG1, OPA1 and PEO1 were analysed from the available DNA samples (n = 38). Furthermore, some nuclear candidate genes were investigated based on family history and phenotype. Rare mtDNA mutations were evaluated using in silico predictors and sequence alignment.

RESULTS

Three patients had previously identified mutations in mtDNA that are associated with optic neuropathy (in MT-ND6 and MT-ND1) and nystagmus (in tRNA Arg). Nine rare mutations in MT-ATP6 were identified in seven patients, of whom four manifested with retinopathy and three had clusters of MT-ATP6 mutations. Nuclear PEO1 and OPA1 were unchanged in all samples, but a patient with nystagmus had a heterozygous POLG1 mutation. The analysis of nuclear candidate genes revealed mutations in NDUF8 (patient with nystagmus), TULP1 (patient with optic neuropathy, nystagmus and retinopathy) and RP2 (patient with retinopathy) genes.

CONCLUSIONS

Children with retinopathy, nystagmus or optic neuropathy, especially together with developmental delay or positive family history, should be considered for mitochondrial disease. MT-ATP6 should be taken into account for children with retinopathy of unknown aetiology.

The Parent Experience of Caring for a Child with Mitochondrial Disease

Mitochondrial disease is a spectrum of progressive genetic disorders resulting from dysfunctions of cellular metabolism in the mitochondria that greatly compromise the lives of affected individuals, who are often children.

PURPOSE

This study described the parent experiences unique to caring for a child with mitochondrial disease.

METHODS

Internet surveys were made available to parents of children with a known mitochondrial disease. Surveys included demographic items and two questionnaires: Parent Experience of Child Illness (PECI) and Pediatric Inventory for Parents (PIP). Descriptive data were collected and correlations calculated to determine relationships between the parent experience and stress.

RESULTS

The majority of participants (n=231) were mothers (95%) of children with mitochondrial disease around the age of 10 years (M=9.85). Elevated scores were found in parent adjustment illness-related concerns regarding Guilt and Worry (M=2.30, SD=.650), Sorrow and Anger (M=2.09, SD=.730), Long-term Uncertainty (M=2.56, SD=.690), and Emotional Resources (M=2.36, SD=.615). Scores indicated elevated feelings of stress in terms of both difficulty and frequency. Significant correlations (p<0.01) were found between parent illness-related concerns and parenting stress.

CONCLUSIONS

The results of this study suggest that parents of a child with mitochondrial disease feel a burden of responsibility that exceeds the typical caregiver role, see their child as fragile, and have concerns about their child's future. Identification of these concerns can assist nurses to better meet the needs of these parents and families.

Relationship of Mitochondrial Enzymes to Fatigue Intensity in Men With Prostate Cancer Receiving External Beam Radiation Therapy

PURPOSE

Mitochondrial dysfunction is a plausible biological mechanism for cancer-related fatigue. Specific aims of this study were to (1) describe the levels of mitochondrial oxidative phosphorylation complex (MOPC) enzymes, fatigue, and health-related quality of life (HRQOL) before and at completion of external beam radiation therapy (EBRT) in men with nonmetastatic prostate cancer (PC); (2) examine relationships over time among levels of MOPC enzymes, fatigue, and HRQOL; and (3) compare levels of MOPC enzymes in men with clinically significant and nonsignificant fatigue intensification during EBRT.

METHODS

Fatigue was measured by the revised Piper Fatigue Scale and the Functional Assessment of Cancer Therapy-Fatigue subscale (FACT-F). MOPC enzymes (Complexes I-V) and mitochondrial antioxidant superoxide dismutase 2 were measured in peripheral blood using enzyme-linked immunosorbent assay at baseline and completion of EBRT. Participants were categorized into high or low fatigue (HF vs. LF) intensification groups based on amount of change in FACT-F scores during EBRT.

RESULTS

Fatigue reported by the 22 participants with PC significantly worsened and HRQOL significantly declined from baseline to EBRT completion. The HF group comprised 12 men with clinically significant change in fatigue (HF) during EBRT. Although no significant changes were observed in MOPC enzymes from baseline to EBRT completion, there were important differences in the patterns in the levels of MOPC enzymes between HF and LF groups.

CONCLUSION

Distinct patterns of changes in the absorbance of MOPC enzymes delineated fatigue intensification among participants. Further investigation using a larger sample is warranted.

Mutation of the mitochondrial carrier SLC25A42 causes a novel form of mitochondrial myopathy in humans

Myopathies are heterogeneous disorders characterized clinically by weakness and hypotonia, usually in the absence of gross dystrophic changes. Mitochondrial dysfunction is a frequent cause of myopathy. We report a simplex case born to consanguineous parents who presented with muscle weakness, lactic acidosis, and muscle changes suggestive of mitochondrial dysfunction. Combined autozygome and exome analysis revealed a missense variant in the SLC25A42 gene, which encodes an inner mitochondrial membrane protein that imports coenzyme A into the mitochondrial matrix. Zebrafish slc25a42 knockdown morphants display severe muscle disorganization and weakness. Importantly, these features are rescued by normal human SLC25A42 RNA, but not by RNA harboring the patient's variant. Our data support a potentially causal link between SLC25A42 mutation and mitochondrial myopathy in humans.

Growth differentiation factor 15 as a useful biomarker for mitochondrial disorders

Objective

The diagnosis of mitochondrial disorders (MDs) is occasionally difficult because patients often present with solitary, or a combination of, symptoms caused by each organ insufficiency, which may be the result of respiratory chain enzyme deficiency. Growth differentiation factor 15 (GDF‐15) has been reported to be elevated in serum of patients with MDs. In this study, we investigated whether GDF‐15 is a more useful biomarker for MDs than several conventional biomarkers.

Methods

We measured the serum levels of GDF‐15 and fibroblast growth factor 21 (FGF‐21), as well as other biomarkers, in 48 MD patients and in 146 healthy controls in Japan. GDF‐15 and FGF‐21 concentrations were measured by enzyme‐linked immunosorbant assay and compared with lactate, pyruvate, creatine kinase, and the lactate‐to‐pyruvate ratio. We calculated sensitivity and specificity and also evaluated the correlation based on two rating scales, including the Newcastle Mitochondrial Disease Rating Scale (NMDAS).

Results

Mean GDF‐15 concentration was 6‐fold higher in MD patients compared to healthy controls (2,711 ± 2,459 pg/ml vs 462.5 ± 141.0 pg/mL; p < 0.001). Using a receiver operating characteristic curve, the area under the curve was significantly higher for GDF‐15 than FGF‐21 and other conventional biomarkers. Our date suggest that GDF‐15 is the most useful biomarker for MDs of the biomarkers examined, and it is associated with MD severity.

Interpretation

Our results suggest that measurement of GDF‐15 is the most useful first‐line test to indicate the patients who have the mitochondrial respiratory chain deficiency. Ann Neurol 2015;78:Ann Neurol 2015;78:679–696

DNA Damage, DNA Repair, Aging, and Neurodegeneration

Aging in mammals is accompanied by a progressive atrophy of tissues and organs, and stochastic damage accumulation to the macromolecules DNA, RNA, proteins, and lipids. The sequence of the human genome represents our genetic blueprint, and accumulating evidence suggests that loss of genomic maintenance may causally contribute to aging. Distinct evidence for a role of imperfect DNA repair in aging is that several premature aging syndromes have underlying genetic DNA repair defects. Accumulation of DNA damage may be particularly prevalent in the central nervous system owing to the low DNA repair capacity in postmitotic brain tissue. It is generally believed that the cumulative effects of the deleterious changes that occur in aging, mostly after the reproductive phase, contribute to species-specific rates of aging. In addition to nuclear DNA damage contributions to aging, there is also abundant evidence for a causative link between mitochondrial DNA damage and the major phenotypes associated with aging. Understanding the mechanistic basis for the association of DNA damage and DNA repair with aging and age-related diseases, such as neurodegeneration, would give insight into contravening age-related diseases and promoting a healthy life span.

Selective elimination of mitochondrial mutations in the germline by genome editing

Mitochondrial diseases include a group of maternally inherited genetic disorders caused by mutations in mtDNA. In most of these patients, mutated mtDNA coexists with wild-type mtDNA, a situation known as mtDNA heteroplasmy. Here, we report on a strategy toward preventing germline transmission of mitochondrial diseases by inducing mtDNA heteroplasmy shift through the selective elimination of mutated mtDNA. As a proof of concept, we took advantage of NZB/BALB heteroplasmic mice, which contain two mtDNA haplotypes, BALB and NZB, and selectively prevented their germline transmission using either mitochondria-targeted restriction endonucleases or TALENs. In addition, we successfully reduced human mutated mtDNA levels responsible for Leber's hereditary optic neuropathy (LHOND), and neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP), in mammalian oocytes using mitochondria-targeted TALEN (mito-TALENs). Our approaches represent a potential therapeutic avenue for preventing the transgenerational transmission of human mitochondrial diseases caused by mutations in mtDNA. PAPERCLIP.

The use of cerebrospinal fluid and neuropathologic studies in neuropsychiatry practice and research

The gold standard for diagnosis of neurodegenerative diseases (ie, Alzheimer disease, frontotemporal dementia, Parkinson disease, dementia with Lewy bodies, amyotrophic lateral sclerosis) is neuropathologic examination at autopsy. As such, laboratory studies play a central role in antemortem diagnosis of these conditions and their differentiation from the neuroinflammatory, infectious, toxic, and other nondegenerative etiologies (eg, rapidly progressive dementias) that are encountered in neuropsychiatric practice. This article summarizes the use of cerebrospinal fluid (CSF) laboratory studies in the diagnostic evaluation of dementia syndromes and emerging CSF biomarkers specific for underlying neuropathology in neurodegenerative disease research.

Systemic diseases associated with retinal dystrophies

Inherited retinal degeneration (IRD) may occur in isolation or as part of a multi-systemic condition. Ocular manifestations may be the presenting symptom of a syndromic disease and can include retinitis pigmentosa, cone-rod dystrophy, or maculopathy. Alternatively, patients affected with syndromic disease may already have other systemic manifestations at the time retinal disease is diagnosed. Some of these systemic diseases can cause significant morbidity. Here, we review several of these syndromic IRDs and their underlying genetic causes. Early recognition and referral for systemic evaluation and surveillance may lead to early intervention and an improved outcome. Obtaining a molecular diagnosis can be beneficial in securing a definitive diagnosis, especially in cases with atypical presentations. A genetic diagnosis may also be informative with regard to prognosis and potential therapies. Effective management and rehabilitation for patients with syndromic retinal dystrophy requires a comprehensive genetic-based team approach involving patients, family members, ophthalmologists, primary care physicians, and geneticists.

Zellweger syndrome and secondary mitochondrial myopathy

Defects in peroxisomes such as those associated with Zellweger syndrome (ZS) can influence diverse intracellular metabolic pathways, including mitochondrial functioning. We report on an 8-month-old female infant and a 6-month-old female infant with typical clinical, radiological and laboratory features of Zellweger syndrome; light microscopic and ultrastructural evidence of mitochondrial pathology in their muscle biopsies; and homozygous pathogenic mutations of the PEX16 gene (c.460 + 5G > A) and the PEX 12 gene (c.888_889 del p.Leu297Thrfs*12), respectively. Additionally, mitochondrial respiratory chain enzymology analysis in the first girl showed a mildly low activity in complexes II-III and IV. We also review five children previously reported in the literature with a presumptive diagnosis of ZS and additional mitochondrial findings in their muscle biopsies. In conclusion, this is the first study of patients with a molecularly confirmed peroxisomal disorder with features of a concomitant mitochondrial myopathy and underscores the role of secondary mitochondrial dysfunction in Zellweger syndrome, potentially contributing to the clinical phenotype.

The cellular and molecular progression of mitochondrial dysfunction induced by 2,4-dinitrophenol in developing zebrafish embryos

The etiology of mitochondrial disease is poorly understood. Furthermore, treatment options are limited, and diagnostic methods often lack the sensitivity to detect disease in its early stages. Disrupted oxidative phosphorylation (OXPHOS) that inhibits ATP production is a common phenotype of mitochondrial disorders that can be induced in zebrafish by exposure to 2,4-dinitrophenol (DNP), a FDA-banned weight-loss agent and EPA-regulated environmental toxicant, traditionally used in research labs as an uncoupler of OXPHOS. Despite the DNP-induced OXPHOS inhibition we observed using in vivo respirometry, the development of the DNP-treated and control zebrafish were largely similar during the first half of embryogenesis. During this period, DNP-treated embryos induced gene expression of mitochondrial and nuclear genes that stimulated the production of new mitochondria and increased glycolysis to yield normal levels of ATP. DNP-treated embryos were incapable of sustaining this mitochondrial biogenic response past mid-embryogenesis, as shown by significantly lowered ATP production and ATP levels, decreased gene expression, and the onset of developmental defects. Examining neural tissues commonly affected by mitochondrial disease, we found that DNP exposure also inhibited motor neuron axon arbor outgrowth and the proper formation of the retina. We observed and quantified the molecular and physiological progression of mitochondrial dysfunction during development with this new model of OXPHOS dysfunction, which has great potential for use in diagnostics and therapies for mitochondrial disease.

Protecting the mitochondrial powerhouse

Mitochondria are the oxygen-consuming power plants of cells. They provide a critical milieu for the synthesis of many essential molecules and allow for highly efficient energy production through oxidative phosphorylation. The use of oxygen is, however, a double-edged sword that on the one hand supplies ATP for cellular survival, and on the other leads to the formation of damaging reactive oxygen species (ROS). Different quality control pathways maintain mitochondria function including mitochondrial DNA (mtDNA) replication and repair, fusion-fission dynamics, free radical scavenging, and mitophagy. Further, failure of these pathways may lead to human disease. We review these pathways and propose a strategy towards a treatment for these often untreatable disorders.

Autism and intellectual disability associated with mitochondrial disease and hyperlactacidemia

Autism spectrum disorder (ASD) with intellectual disability (ID) is a life-long debilitating condition, which is characterized by cognitive function impairment and other neurological signs. Children with ASD-ID typically attain motor skills with a significant delay. A sub-group of ASD-IDs has been linked to hyperlactacidemia and alterations in mitochondrial respiratory chain activity. The objective of this report is to describe the clinical features of patients with these comorbidities in order to shed light on difficult diagnostic and therapeutic approaches in such patients. We reported the different clinical features of children with ID associated with hyperlactacidemia and deficiencies in mitochondrial respiratory chain complex II–IV activity whose clinical presentations are commonly associated with the classic spectrum of mitochondrial diseases. We concluded that patients with ASD and ID presenting with persistent hyperlactacidemia should be evaluated for mitochondrial disorders. Administration of carnitine, coenzyme Q10, and folic acid is partially beneficial, although more studies are needed to assess the efficacy of this vitamin/cofactor treatment combination.

Diagnostic approach in infants and children with mitochondrial diseases

Mitochondrial diseases are a heterogeneous group of disorders affecting energy production in the human body. The diagnosis of mitochondrial diseases represents a challenge to clinicians, especially for pediatric cases, which show enormous variation in clinical presentations, as well as biochemical and genetic complexity. Different consensus diagnostic criteria for mitochondrial diseases in infants and children are available. The lack of standardized diagnostic criteria poses difficulties in evaluating diagnostic methodologies. Even though there are many diagnostic tools, none of them are sensitive enough to make a confirmative diagnosis without being used in combination with other tools. The current approach to diagnosing and classifying mitochondrial diseases incorporates clinical, biochemical, neuroradiological findings, and histological criteria, as well as DNA-based molecular diagnostic testing. The confirmation or exclusion of mitochondrial diseases remains a challenge in clinical practice, especially in cases with nonspecific clinical phenotypes. Therefore, follow-up evolution of clinical symptoms/signs and biochemical data is crucial. The purpose of this study is to review the molecular classification scheme and associated phenotypes in infants and children with mitochondrial diseases, in addition to providing an overview of the basic biochemical reactions and genetic characteristics in the mitochondrion, clinical manifestations, and diagnostic methods. A diagnostic algorithm for identifying mitochondrial disorders in pediatric neurology patients is proposed.

Delayed diagnosis of congenital myasthenia due to associated mitochondrial enzyme defect

Clinical phenotypes of congenital myasthenic syndromes and primary mitochondrial disorders share significant overlap in their clinical presentations, leading to challenges in making the correct diagnosis. Next generation sequencing is transforming molecular diagnosis of inherited neuromuscular disorders by identifying novel disease genes and by identifying previously known genes in undiagnosed patients. This is evident in two patients who were initially suspected to have a mitochondrial myopathy, but in whom a clear diagnosis of congenital myasthenic syndromes was made through whole exome sequencing. In patient 1, whole exome sequencing revealed compound heterozygous mutations c.1228C > T (p.Arg410Trp) and c.679C > T (p.Arg227*) in collagen-like tail subunit (single strand of homotrimer) of asymmetric acetylcholinesterase (COLQ). In patient 2, in whom a deletion of exon 52 in Dystrophin gene was previously detected by multiplex ligation-dependent probe amplification, Sanger sequencing revealed an additional homozygous mutation c.1511_1513delCTT (p.Pro504Argfs*183) in docking protein7 (DOK7). These case reports highlight the need for careful diagnosis of clinically heterogeneous syndromes like congenital myasthenic syndromes, which are treatable, and for which delayed diagnosis is likely to have implications for patient health. The report also demonstrates that whole exome sequencing is an effective diagnostic tool in providing molecular diagnosis in patients with complex phenotypes.

Role of plasma amino acids and urinary organic acids in diagnosis of mitochondrial diseases in children

BACKGROUND

Diagnostic difficulty in mitochondrial diseases (MD) results not only from the wide spectrum of symptoms and signs but also from the absence of a reliable screening or diagnostic biomarker.

AIM

To investigate the likelihood of MD in patients with symptoms and signs impressive of MD through quantitative measurement of plasma amino acids, and urinary organic acids.

METHODS

Twenty patients with symptoms and signs suggestive of MD were further evaluated by quantitative plasma amino acids and urinary organic acids assay and neuroimaging.

RESULTS

Plasma amino acid results revealed elevation of alanine in 11, glycine in five, and proline in two patients. Abnormal urinary organic acid analysis was present in six patients; increased urinary lactate (20%), dicarboxylicaciduria (15%), and urinary ketone bodies (10%). Upon enrollment our patients scored as possible MD according to the MD scoring system. At the end of the study, five patients still scored as possible MD, eight patients as probable MD, and seven patients as definite MD. All patients with definite MD had elevated serum lactate. In three patients, elevated urinary lactate was the only abnormality. Alanine was elevated in all patients with definite MD, whereas proline was elevated in only one. Magnetic resonance imaging of the brain showed atrophic changes in one patient and bilateral basal ganglia hyperintensity in another.

CONCLUSION

Urinary organic acids and quantitative plasma amino acids can help in the diagnosis of MD, especially when the economic burden and absence of specialized centers limits the diagnosis.

Association of circulating cytochrome c with clinical manifestations of antiretroviral-induced toxicity

Diagnosis of antiretroviral therapy (ART) toxicity is complicated. Apoptosis has been implicated in ART toxicity. Cytochrome c (Cyt-C) is a mitochondrial protein found in plasma during pro-apoptotic states. We conducted a study of HIV-infected individuals on ART with (cases, n=21) and without (controls, n=21) clinical evidence of toxicity to determine if elevated plasma Cyt-C is associated with ART toxicity. When corrected for CD4 count, viral load, and duration of HIV infection, cases are 7.86 times more likely than controls to have plasma Cyt-C>0.216 ng/mL. Cyt-C could be a useful clinical tool to guide treatment decisions in this population.

Sleep disorders associated with primary mitochondrial diseases

STUDY OBJECTIVES

Primary mitochondrial diseases are caused by heritable or spontaneous mutations in nuclear DNA or mitochondrial DNA. Such pathological mutations are relatively common in humans and may lead to neurological and neuromuscular complication that could compromise normal sleep behavior. To gain insight into the potential impact of primary mitochondrial disease and sleep pathology, we reviewed the relevant English language literature in which abnormal sleep was reported in association with a mitochondrial disease.

DESIGN

We examined publication reported in Web of Science and PubMed from February 1976 through January 2014, and identified 54 patients with a proven or suspected primary mitochondrial disorder who were evaluated for sleep disturbances.

MEASUREMENTS AND RESULTS

Both nuclear DNA and mitochondrial DNA mutations were associated with abnormal sleep patterns. Most subjects who underwent polysomnography had central sleep apnea, and only 5 patients had obstructive sleep apnea. Twenty-four patients showed decreased ventilatory drive in response to hypoxia and/ or hyperapnea that was not considered due to weakness of the intrinsic muscles of respiration.

CONCLUSIONS

Sleep pathology may be an underreported complication of primary mitochondrial diseases. The probable underlying mechanism is cellular energy failure causing both central neurological and peripheral neuromuscular degenerative changes that commonly present as central sleep apnea and poor ventilatory response to hyperapnea. Increased recognition of the genetics and clinical manifestations of mitochondrial diseases by sleep researchers and clinicians is important in the evaluation and treatment of all patients with sleep disturbances. Prospective population-based studies are required to determine the true prevalence of mitochondrial energy failure in subjects with sleep disorders, and conversely, of individuals with primary mitochondrial diseases and sleep pathology.

Inflammatory mediators of cognitive impairment in bipolar disorder

OBJECTIVES

Recent studies have pointed to neuroinflammation, oxidative stress and neurotrophic factors as key mediators in the pathophysiology of mood disorders. Little is however known about the cascade of biological episodes underlying the cognitive deficits observed during the acute and euthymic phases of bipolar disorder (BD). The aim of this review is to assess the potential association between cognitive impairment and biomarkers of inflammation, oxidative stress and neurotrophic activity in BD.

METHODS

Scopus (all databases), Pubmed and Ovid Medline were systematically searched with no language or year restrictions, up to November 2013, for human studies that collected both inflammatory markers and cognitive data in BD. Selected search terms were bipolar disorder, depression, mania, psychosis, inflammatory, cognitive and neurotrophic.

RESULTS

Ten human studies satisfied the criteria for consideration. The findings showed that high levels of peripheral inflammatory-cytokine, oxidative stress and reduced brain derived neurotrophic factor (BDNF) levels were associated with poor cognitive performance. The BDNF val66met polymorphism is a potential vulnerability factor for cognitive impairment in BD.

CONCLUSIONS

Current data provide preliminary evidence of a link between the cognitive decline observed in BD and mechanisms of neuroinflammation and neuroprotection. The identification of BD specific inflammatory markers and polymorphisms in inflammatory response genes may be of assistance for therapeutic intervention.

Approach to the diagnosis and treatment of cyclic vomiting syndrome: a large single-center experience with 106 patients

BACKGROUND

Cyclic vomiting syndrome is characterized by repeated, stereotypical vomiting episodes. The diagnosis is made by exclusion of other organic diseases, which can lead to extensive testing. It has been suggested that these patients can have mitochondrial dysfunction. The aim of the study was to examine the evaluation of our cyclic vomiting patients and to determine whether they had associated, undiagnosed metabolic abnormalities.

METHODS

This retrospective study included 106 patients aged <21 years at diagnosis. Information regarding medical history, laboratory, and imaging studies were collected. Metabolic studies in plasma and urine were obtained when patients were well and when patients were in a vomiting cycle, including plasma amino acids, acylcarnitines, and urine organic acids.

RESULTS

The mean age at diagnosis was 8.9 ± 5.0 years. Neuroimaging revealed previously unknown intracranial abnormalities in <10% of patients, none of whom explained the vomiting signs. Abdominal ultrasounds revealed abnormalities in 15% of patients during an acute episode and 7% of patients when well. Sixty-one patients had an upper gastrointestinal series, all of which were normal. A total of 92% of patients had laboratory testing with 38% indicating abnormalities possibly suggesting mitochondrial dysfunction.

CONCLUSIONS

This large, single-center study further evaluated the need for more focused evaluation in patients with suspected cyclic vomiting syndrome. Thirty-eight percent of our patients had abnormalities in blood and/or urine suggesting mitochondrial dysfunction, which requires more detailed investigation in the future.

Myocardial atrophy in children with mitochondrial disease and Duchenne muscular dystrophy

Purpose

Mitochondrial disease (MD) and Duchenne muscular dystrophy (DMD) are often associated with cardiomyopathy, but the myocardial variability has not been isolated to a specific characteristic. We evaluated the left ventricular (LV) mass by echocardiography to identify the general distribution and functional changes of the myocardium in patients with MD or DMD.

Methods

We retrospectively evaluated the echocardiographic data of 90 children with MD and 42 with DMD. Using two-dimensional echocardiography, including time-motion (M) mode and Doppler measurements, we estimated the LV mass, ratio of early to late mitral filling velocities (E/A), ratio of early mitral filling velocity to early diastolic mitral annular velocity (E/Ea), stroke volume, and cardiac output. A "z score" was generated using the lambda-mu-sigma method to standardize the LV mass with respect to body size.

Results

The LV mass-for-height z scores were significantly below normal in children with MD (-1.02±1.52, P<0.001) or DMD (-0.82±1.61, P=0.002), as were the LV mass-for-lean body-mass z scores. The body mass index (BMI)-for-age z scores were far below normal and were directly proportional to the LV mass-for-height z scores in both patients with MD (R=0.377, P<0.001) and those with DMD (R=0.330, P=0.033). The LV mass-for-height z score correlated positively with the stroke volume index (R=0.462, P<0.001) and cardiac index (R=0.358, P<0.001).

Conclusion

LV myocardial atrophy is present in patients with MD and those with DMD and may be closely associated with low BMI. The insufficient LV mass for body size might indicate deterioration of systolic function in these patients.

In vivo metabolic flux profiling with stable isotopes discriminates sites and quantifies effects of mitochondrial dysfunction in C. elegans

Mitochondrial respiratory chain (RC) disease diagnosis is complicated both by an absence of biomarkers that sufficiently divulge all cases and limited capacity to quantify adverse effects across intermediary metabolism. We applied high performance liquid chromatography (HPLC) and mass spectrometry (MS) studies of stable-isotope based precursor-product relationships in the nematode, C. elegans, to interrogate in vivo differences in metabolic flux among distinct genetic models of primary RC defects and closely related metabolic disorders.

METHODS

C. elegans strains studied harbor single nuclear gene defects in complex I, II, or III RC subunits (gas-1, mev-1, isp-1); enzymes involved in coenzyme Q biosynthesis (clk-1), the tricarboxylic acid cycle (TCA, idh-1), or pyruvate metabolism (pdha-1); and central nodes of the nutrient-sensing signaling network that involve insulin response (daf-2) or the sirtuin homologue (sir-2.1). Synchronous populations of 2000 early larval stage worms were fed standard Escherichia coli on nematode growth media plates containing 1,6-(13)C2-glucose throughout their developmental period, with samples extracted on the first day of adult life in 4% perchloric acid with an internal standard. Quantitation of whole animal free amino acid concentrations and isotopic incorporation into amino and organic acids throughout development was performed in all strains by HPLC and isotope ratio MS, respectively. GC/MS analysis was also performed to quantify absolute isotopic incorporation in all molecular species of key TCA cycle intermediates in gas-1 and N2 adult worms.

RESULTS

Genetic mutations within different metabolic pathways displayed distinct metabolic profiles. RC complex I (gas-1) and III (isp-1) subunit mutants, together with the coenzyme Q biosynthetic mutant (clk-1), shared a similar amino acid profile of elevated alanine and decreased glutamate. The metabolic signature of the complex II mutant (mev-1) was distinct from that of the other RC mutants but resembled that of the TCA cycle mutant (idh-1) and both signaling mutants (daf-2 and sir-2.1). All branched chain amino acid levels were significantly increased in the complex I and III mutants but decreased in the PDH mutant (pdha-1). The RC complex I, coenzyme Q, TCA cycle, and PDH mutants shared significantly increased relative enrichment of lactate+1 and absolute concentration of alanine+1, while glutamate+1 enrichment was significantly decreased uniquely in the RC mutants. Relative intermediary flux analyses were suggestive of proximal TCA cycle disruption in idh-1, completely reduced TCA cycle flux in sir-2.1, and apparent distal TCA cycle alteration in daf-2. GC/MS analysis with universally-labeled (13)C-glucose in adult worms further showed significantly increased isotopic enrichment in lactate, citrate, and malate species in the complex I (gas-1) mutant.

CONCLUSIONS

Stable isotopic/mass spectrometric analysis can sensitively discriminate primary RC dysfunction from genetic deficiencies affecting either the TCA cycle or pyruvate metabolism. These data are further suggestive that metabolic flux analysis using stable isotopes may offer a robust means to discriminate and quantify the secondary effects of primary RC dysfunction across intermediary metabolism.

AGC1 Deficiency Causes Infantile Epilepsy, Abnormal Myelination, and Reduced N-Acetylaspartate

BACKGROUND

Whole exome sequencing (WES) offers a powerful diagnostic tool to rapidly and efficiently sequence all coding genes in individuals presenting for consideration of phenotypically and genetically heterogeneous disorders such as suspected mitochondrial disease. Here, we report results of WES and functional validation in a consanguineous Indian kindred where two siblings presented with profound developmental delay, congenital hypotonia, refractory epilepsy, abnormal myelination, fluctuating basal ganglia changes, cerebral atrophy, and reduced N-acetylaspartate (NAA).

METHODS

Whole blood DNA from one affected and one unaffected sibling was captured by Agilent SureSelect Human All Exon kit and sequenced on the Illumina HiSeq2000. Mutations were validated by Sanger sequencing in all family members. Protein from wild-type and mutant fibroblasts was isolated to assess mutation effects on protein expression and enzyme activity.

RESULTS

A novel SLC25A12 homozygous missense mutation, c.1058G>A; p.Arg353Gln, segregated with disease in this kindred. SLC25A12 encodes the neuronal aspartate-glutamate carrier 1 (AGC1) protein, an essential component of the neuronal malate/aspartate shuttle that transfers NADH and H(+) reducing equivalents from the cytosol to mitochondria. AGC1 activity enables neuronal export of aspartate, the glial substrate necessary for proper neuronal myelination. Recombinant mutant p.Arg353Gln AGC1 activity was reduced to 15% of wild type. One prior reported SLC25A12 mutation caused complete loss of AGC1 activity in a child with epilepsy, hypotonia, hypomyelination, and reduced brain NAA.

CONCLUSIONS

These data strongly suggest that SLC25A12 disease impairs neuronal AGC1 activity. SLC25A12 sequencing should be considered in children with infantile epilepsy, congenital hypotonia, global delay, abnormal myelination, and reduced brain NAA.

Coenzyme Q10 as a therapy for mitochondrial disease

Treatment of mitochondrial respiratory chain (MRC) disorders is extremely difficult, however, coenzyme Q10 (CoQ10) and its synthetic analogues are the only agents which have shown some therapeutic benefit to patients. CoQ10 serves as an electron carrier in the MRC as well as functioning as a potent lipid soluble antioxidant. CoQ10 supplementation is fundamental to the treatment of patients with primary defects in the CoQ10 biosynthetic pathway. The efficacy of CoQ10 and its analogues in the treatment of patients with MRC disorders not associated with a CoQ10 deficiency indicates their ability to restore electron flow in the MRC and/or increase mitochondrial antioxidant capacity may also be important contributory factors to their therapeutic potential.

Biomarkers in autism

Autism spectrum disorders (ASDs) are complex, heterogeneous disorders caused by an interaction between genetic vulnerability and environmental factors. In an effort to better target the underlying roots of ASD for diagnosis and treatment, efforts to identify reliable biomarkers in genetics, neuroimaging, gene expression, and measures of the body's metabolism are growing. For this article, we review the published studies of potential biomarkers in autism and conclude that while there is increasing promise of finding biomarkers that can help us target treatment, there are none with enough evidence to support routine clinical use unless medical illness is suspected. Promising biomarkers include those for mitochondrial function, oxidative stress, and immune function. Genetic clusters are also suggesting the potential for useful biomarkers.

Multidisciplinary approach to the management of myopathies

PURPOSE OF REVIEW

Aside from some inflammatory myopathies and very few genetic disorders, there are no therapies that make most patients with myopathies stronger. Consequently, the management of these patients can be frustrating for patients and their families as well as the clinicians taking care of them. Treatment of these patients must involve a comprehensive approach focused on limiting the secondary effects of skeletal muscle weakness, managing comorbidities associated with specific diseases, and, most importantly, optimizing patients' functional abilities and quality of life in terms of their ability to accomplish activities of daily living. While the approach to each patient differs depending on their disease, certain common themes can be addressed in each patient. This review highlights an approach centered on four conceptual themes ("the Four S's"): Strength therapies, Supportive care, Symptomatic therapies, and pSychological support.

RECENT FINDINGS

Although relatively few well-designed studies have been done that highlight conservative management of patients with various myopathies, an emerging literature helps guide the clinician in certain key areas, especially in relation to cardiac and pulmonary management of these patients.

SUMMARY

While disease-altering therapies have proven elusive for many muscle diseases, a multimodal approach to the conservative and supportive care of these patients can markedly improve their quality of life. Pharmacologic treatment options for specific myopathies will not be addressed in this article but are covered elsewhere in this issue of CONTINUUM.

A novel diagnostic tool reveals mitochondrial pathology in human diseases and aging

The inherent complex and pleiotropic phenotype of mitochondrial diseases poses a significant diagnostic challenge for clinicians as well as an analytical barrier for scientists. To overcome these obstacles we compiled a novel database, www.mitodb.com, containing the clinical features of primary mitochondrial diseases. Based on this we developed a number of qualitative and quantitative measures, enabling us to determine whether a disorder can be characterized as mitochondrial. These included a clustering algorithm, a disease network, a mitochondrial barcode and two scoring algorithms. Using these tools we detected mitochondrial involvement in a number of diseases not previously recorded as mitochondrial. As a proof of principle Cockayne syndrome, ataxia with oculomotor apraxia 1 (AOA1), spinocerebellar ataxia with axonal neuropathy 1 (SCAN1) and ataxia-telangiectasia have recently been shown to have mitochondrial dysfunction and those diseases showed strong association with mitochondrial disorders. We next evaluated mitochondrial involvement in aging and detected two distinct categories of accelerated aging disorders, one of them being associated with mitochondrial dysfunction. Normal aging seemed to associate stronger with the mitochondrial diseases than the non-mitochondrial partially supporting a mitochondrial theory of aging.

Mitochondrial respiratory chain disease discrimination by retrospective cohort analysis of blood metabolites

Diagnosing primary mitochondrial respiratory chain (RC) dysfunction has long relied on invasive tissue biopsies, since no blood-based biomarker has been shown to have sufficiently high sensitivity and specificity across the myriad of individual clinical presentations. We sought to determine whether cohort-level evaluation of commonly obtained blood analytes might reveal consistent patterns to discriminate a heterogenous group of primary mitochondrial RC disease subjects both from control individuals and from subjects with pyruvate dehydrogenase deficiency.

METHODS

Following IRB approval, 62 biochemical analyte concentrations or ratios were retrospectively analyzed in three well-defined and intentionally heterogeneous subject cohorts reflective of clinical practice: [1] Primary mitochondrial disease (n=19); [2] pyruvate dehydrogenase deficiency (n=4); and [3] controls (n=27). Blood analyte categories included comprehensive chemistry profile, creatine kinase, lipoprotein profile, lactate, pyruvate, and plasma amino acid profile. Non-parametric analyses were used to compare the median of each analyte level between cohorts.

RESULTS

Disease cohorts differed significantly in their median levels of triglycerides, lactate, pyruvate, and multiple individual plasma amino acids. Primary mitochondrial disease was significantly discriminated at the cohort level from pyruvate dehydrogenase deficiency by greater pyruvate and alanine elevation in pyruvate dehydrogenase deficiency, as well as significantly increased branched chain amino acid (BCAA) levels and increased ratios of individual BCAAs to glutamate in mitochondrial disease. In addition, significant elevation of median blood triglyceride level was seen in the primary mitochondrial disease cohort.

CONCLUSIONS

Blood metabolite profile analysis can discriminate a heterogeneous cohort of primary mitochondrial disease both from controls and from pyruvate dehydrogenase deficiency. Elevated BCAA levels, either absolutely or when considered relative to the level of glutamate, are common metabolic sequelae of primary mitochondrial RC disease. Prospective study is needed to validate observed plasma metabolite alterations as a potential biomarker of disease both in larger cohorts and at the individual subject level.

Practice patterns of mitochondrial disease physicians in North America. Part 1: diagnostic and clinical challenges

Mitochondrial medicine is a young subspecialty. Clinicians have a limited evidence base on which to formulate clinical decisions regarding diagnosis, treatment and patient management. Mitochondrial medicine specialists have cobbled together an informal set of rules and paradigms for preventive care and management based in part on anecdotal experience. The Mitochondrial Medicine Society (MMS) assessed the current state of clinical practice from diagnosis, to preventive care and treatment, as provided by various mitochondrial disease specialists in North America. We hope that by obtaining this information we can begin moving towards formulating a set of consensus criteria and establishing standards of care.

Mitochondrial deficiency in Cockayne syndrome

Cockayne syndrome is a rare inherited disorder characterized by accelerated aging, cachectic dwarfism and many other features. Recent work has implicated mitochondrial dysfunction in the pathogenesis of this disease. This is particularly interesting since mitochondrial deficiencies are believed to be important in the aging process. In this review, we discuss recent findings of mitochondrial pathology in Cockayne syndrome and suggest possible mechanisms for the mitochondrial dysfunction.

Clinical and molecular features of POLG-related mitochondrial disease

The inability to replicate mitochondrial genomes (mtDNA) by the mitochondrial DNA polymerase (pol γ) leads to a subset of mitochondrial diseases. Many mutations in POLG, the gene that encodes pol γ, have been associated with mitochondrial diseases such as myocerebrohepatopathy spectrum (MCHS) disorders, Alpers-Huttenlocher syndrome, myoclonic epilepsy myopathy sensory ataxia (MEMSA), ataxia neuropathy spectrum (ANS), and progressive external ophthalmoplegia (PEO). This chapter explores five important topics in POLG-related disease: (1) clinical symptoms that identify and distinguish POLG-related diseases, (2) molecular characterization of defects in polymerase activity by POLG disease variants, (3) the importance of holoenzyme formation in disease presentation, (4) the role of pol γ exonuclease activity and mutagenesis in disease and aging, and (5) novel approaches to therapy and avoidance of toxicity based on primary research in pol γ replication.

Mitochondrial disease in childhood: mtDNA encoded

Since the first description of a mitochondrial DNA (mtDNA)-associated disease in the late 1980s, there have been more than 275 mutations within the mtDNA genome described causing human disease. The phenotypic expression of these disorders is vast, as disturbances of the unique physiology of mitochondria can create a wide range of clinical heterogeneity. Features of heteroplasmy, threshold effect, genetic bottleneck, mtDNA depletion, mitotic segregation, and maternal inheritance have been identified and described as a result of novel biochemical and genetic controls of mitochondrial function. We hope that as we unfold this fascinating part of clinical medicine, the reader will see how alterations in the tapestry of mitochondrial biochemistry and genetics can give rise to human illness.

Molecular genetic testing for mitochondrial disease: from one generation to the next

Molecular genetic diagnostic testing for mitochondrial disease has evolved continually since the first genetic basis for a clinical mitochondrial disease syndrome was identified in the late 1980s. Owing to global limitations in both knowledge and technology, few individuals, even among those with strong clinical or biochemical evidence of mitochondrial respiratory chain dysfunction, ever received a definitive molecular diagnosis prior to 2005. Clinically available genetic diagnostic testing options improved by 2006 to include sequencing and deletion analysis of an increasing number of individual nuclear genes linked to mitochondrial disease, genome-wide microarray analysis for chromosomal copy number abnormalities, and mitochondrial DNA whole genome sequence analysis. To assess the collective effect of these tests on the genetic diagnosis of suspected mitochondrial disease, we report here results from a retrospective review of the diagnostic yield in patients evaluated from 2008 to 2011 in the Mitochondrial-Genetics Diagnostic Clinic at The Children's Hospital of Philadelphia. Among 152 patients aged 6 weeks to 81 years referred for clinical evaluation of multisystem presentations concerning for suspected mitochondrial disease, a genetic etiology was established that confirmed definite mitochondrial disease in 16.4% and excluded primary mitochondrial disease in 9.2%. Substantial diagnostic challenges remain owing to the clinical difficulty and frank low yield of a priori selecting individual nuclear genes to sequence based on particular symptomatic or biochemical manifestations of suspected mitochondrial disease. These findings highlight the particular utility of massively parallel nuclear exome sequencing technologies, whose benefits and limitations are explored relative to the clinical genetic diagnostic evaluation of mitochondrial disease.

Programming of respiratory health in childhood: influence of outdoor air pollution

PURPOSE OF REVIEW

This overview highlights recent experimental and epidemiological evidence for the programming effects of outdoor air pollution exposures during early development on lung function and chronic respiratory disorders, such as asthma and related allergic disorders.

RECENT FINDINGS

Air pollutants may impact anatomy and/or physiological functioning of the lung and interrelated systems. Programming effects may result from pollutant-induced shifts in a number of molecular, cellular, and physiological states and their interacting systems. Specific key regulatory systems susceptible to programming may influence lung development and vulnerability to respiratory diseases, including both central and peripheral components of neuroendocrine pathways and autonomic nervous system (ANS) functioning which, in turn, influence the immune system. Starting in utero, environmental factors, including air pollutants, may permanently organize these systems toward trajectories of enhanced pediatric (e.g., asthma, allergy) as well as adult disease risk (e.g., chronic obstructive pulmonary disease). Evidence supports a central role of oxidative stress in the toxic effects of air pollution. Additional research suggests xenobiotic metabolism and subcellular components, such as mitochondria are targets of ambient air pollution and play a role in asthma and allergy programming. Mechanisms operating at the level of the placenta are being elucidated. Epigenetic mechanisms may be at the roots of adaptive developmental programming.

SUMMARY

Optimal coordinated functioning of many complex processes and their networks of interaction are necessary for normal lung development and the maintenance of respiratory health. Outdoor air pollution may play an important role in early programming of respiratory health and is potentially amenable to intervention.

Challenges of bringing next generation sequencing technologies to clinical molecular diagnostic laboratories

Molecular diagnosis of complex dual genome mitochondrial disorders is a challenge. It requires the identification of deleterious mutations in one of the ~1,500 nuclear genes and the mitochondrial genome. If the molecular defect is in the mitochondrial genome, quantification of degree of mutation load (heteroplasmy) in affected tissues is important. Due to the extreme clinical and genetic heterogeneity, conventional sequence analysis of the candidate genes one-by-one is impractical, if not impossible. The newly developed massively parallel next generation sequencing (NGS) technique, that allows simultaneous sequence analysis of multiple target genes, when appropriately validated with deep coverage and proper quality controls, can be used as an effective comprehensive diagnostic approach in CLIA certified clinical laboratories.