Determination of enzyme activities for prenatal diagnosis of respiratory chain deficiency

Investigation of oxidative phosphorylation activity and complex composition in mitochondrial disease

A multidisciplinary approach to the laboratory diagnosis of mitochondrial disease has long been applied, with crucial information provided by deep clinical phenotyping, blood investigations, and biomarker screening as well as histopathological and biochemical testing of biopsy material to support molecular genetic screening. In an era of second and third generation sequencing technologies, traditional diagnostic algorithms for mitochondrial disease have been replaced by gene agnostic, genomic strategies including whole-exome sequencing (WES) and whole-genome sequencing (WGS), increasingly supported by other 'omics technologies (Alston et al., 2021). Whether a primary testing strategy, or one used to validate and interpret candidate genetic variants, the availability of a range of tests aimed at determining mitochondrial function (i.e., the assessment of individual respiratory chain enzyme activities in a tissue biopsy or cellular respiration in a patient cell line) remains an important part of the diagnostic armory. In this chapter, we summarize several disciplines used in the laboratory investigation of suspected mitochondrial disease, including the histopathological and biochemical assessment of mitochondrial function, as well as protein-based techniques to assess the steady-state levels of oxidative phosphorylation (OXPHOS) subunits and assembly of OXPHOS complexes via traditional (immunoblotting) and cutting-edge (quantitative proteomic) approaches.

Assessment of mitochondrial respiratory chain enzymes in cells and tissues

Measurement of the individual enzymes involved in mitochondrial oxidative phosphorylation (OXPHOS) forms a key part of diagnostic investigations in patients with suspected mitochondrial disease, and can provide crucial information on mitochondrial OXPHOS function in a variety of cells and tissues that are applicable to many research investigations. In this chapter, we present methods for analysis of mitochondrial respiratory chain enzymes in cells and tissues based on assays performed in two geographically separate diagnostic referral centers, as part of clinical diagnostic investigations. Techniques for sample preparation from cells and tissues, and spectrophotometric assays for measurement of the activities of OXPHOS complexes I-V, the combined activity of complexes II+III, and the mitochondrial matrix enzyme citrate synthase, are provided. The activities of mitochondrial respiratory chain enzymes are often expressed relative to citrate synthase activity, since these ratios may be more robust in accounting for variability that may arise due to tissue quality, handling and storage, cell growth conditions, or any mitochondrial proliferation that may be present in tissues from patients with mitochondrial disease. Considerations for adaption of these techniques to other cells, tissues, and organisms are presented, as well as comparisons to alternate methods for analysis of respiratory chain function. In this context, a quantitative immunofluorescence protocol is also provided that is suitable for measurement of the amount of multiple respiratory chain complexes in small diagnostic tissue samples. The analysis and interpretation of OXPHOS enzyme activities are then placed in the context of mitochondrial disease tissue pathology and diagnosis.

Recent perspectives of pediatric mitochondrial diseases

Mitochondrial disorders are amongst the most common groups of inborn errors of metabolism. They are caused by deficiencies in the final pathway of the cellular energy production, the mitochondrial respiratory chain. The disorders are clinically and genetically heterogeneous and the aetiology could be found in the mitochondrial, or in the nuclear genome. We searched important e-databases for the collection of latest literature on the mitochondrial disease especially in pediatric population. Most of the studies in the recent past have focused on the understanding of the clinical phenotypes and pathophysiological mechanisms. Leigh syndrome is a common severe, neurodegenerative disease of early childhood. A defect in the POLG gene is another common observation in most of the cases leading to Alpers syndrome. The review concludes that pediatric mitochondrial disorders are severe, progressive and usually multi-systemic. Further, whole genome sequencing is an excellent diagnostic option.

Phosphocreatine protects against LPS-induced human umbilical vein endothelial cell apoptosis by regulating mitochondrial oxidative phosphorylation

Phosphocreatine (PCr) is an exogenous energy substance, which provides phosphate groups for adenosine triphosphate (ATP) cycle and promotes energy metabolism in cells. However, it is still unclear whether PCr has influenced on mitochondrial energy metabolism as well as oxidative phosphorylation (OXPHO) in previous studies. Therefore, the aim of the present study was to investigate the regulation of PCr on lipopolsaccharide (LPS)-induced human umbilical vein endothelial cells (HUVECs) and mitochondrial OXPHO pathway. PCr protected HUVECs against LPS-induced apoptosis by suppressing the mitochondrial permeability transition, cytosolic release of cytochrome c (Cyt C), Ca(2+), reactive oxygen species and subsequent activation of caspases, and increasing Bcl2 expression, while suppressing Bax expression. More importantly, PCr significantly improved mitochondrial swelling and membrane potential, enhanced the activities of ATP synthase and mitochondrial creatine kinase (CKmt) in creatine shuttle, influenced on respiratory chain enzymes, respiratory control ratio, phosphorus/oxygen ratio and ATP production of OXPHO. Above PCr-mediated mitochondrial events were effectively more favorable to reduced form of flavin adenine dinucleotide (FADH2) pathway than reduced form of nicotinamide-adenine dinucleotid pathway in the mitochondrial respiratory chain. Our results revealed that PCr protects against LPS-induced HUVECs apoptosis, which probably related to stabilization of intracellular energy metabolism, especially for FADH2 pathway in mitochondrial respiratory chain, ATP synthase and CKmt. Our findings suggest that PCr may play a certain role in the treatment of atherosclerosis via protecting endothelial cell function.

Biochemical analyses of the electron transport chain complexes by spectrophotometry

In the diagnostic work-up of patients with suspected mitochondrial disease, evaluating the activity of the individual oxidative phosphorylation (OXPHOS) complexes is crucial. Here, we describe spectrophotometric assays for OXPHOS enzymology that can be applied to both tissue samples and cultured cells. These assays are designed to assess the enzymatic activity of the individual OXPHOS complexes I-V, along with the Krebs cycle enzyme citrate synthase as a mitochondrial control. As well, we include an assay for the coupled energy transfer between complexes II and III. Determining the enzymatic activities can be valuable in defining isolated or multicomplex disorders and may be relevant to the design of future molecular investigations.

Energy metabolism in umbilical endothelial cells from preterm and term neonates

AIM

The aim of this study was to investigate the impact of gestational age on energy metabolism in human umbilical vein endothelial cells (HUVECs) of preterm and term neonates.

METHODS

Activities of respiratory chain (RC) complexes I-V, citrate synthase (CS), overall mitochondrial fatty acid oxidation (FAO), carnitine palmitoyltransferase 2 (CPT2), glycolytic enzymes as well as energy-rich phosphates in HUVECs from uncomplicated term and preterm pregnancies were measured. Neonatal acylcarnitine profiles were analyzed postpartum.

RESULTS

Activities of RC complexes II+III, IV, V, and CS were higher in HUVECs from immature pregnancies. Overall FAO did not change, whereas CPT2 activity was higher in term neonates. RC complexes II-V and CS correlated inversely to gestational age, as well as CPT2 activity within the term cohort. Phosphofructokinase activity increased with maturation; lactate dehydrogenase and hexokinase as well as energy-rich phosphates remained constant. In blood, long-chain acylcarnitines were higher in term neonates.

CONCLUSIONS

Gestational age-dependent differences of energy-providing pathways in HUVECs were shown. Alterations of RC complexes with gestational age may be an adaptive process to cope with metabolic stress during birth; reduced oxidative phosphorylation and high glycolytic activity make HUVECs less susceptible to peripartum hypoxic damage. We hypothesize that HUVECs of premature neonates are metabolically maladapted to birth, which may be responsible for perinatal complications.

Developmental aspects of respiratory chain from fetus to infancy

Reviewing the recent literature on the role of mitochondria during fetal development paradoxically reveals two features: the importance of mitochondria in these early developmental phases, and the scarcity of information available for humans. Indeed, most of the available information on the role of mitochondria during development comes from studies of animal models that do not necessarily strictly apply to humans. In this paper, we attempted to collect information existing on humans, together with data from animal studies essentially presented as corroboration. This makes clear that a complex interacting network of energetic, genetic and epigenetic factors governs the impact of mitochondrial function on early development in humans. This complexity presumably also accounts for our poor understanding of the consequences of impaired mitochondrial function on prenatal development, or conversely, of the impact of development on the expression of such deficiencies.

Impact of selected inborn errors of metabolism on prenatal and neonatal development

In general, data regarding maturational processes of different metabolic pathways in the very vulnerable fetal and neonatal period are rare. This review is to substantiate the impact of selected inborn errors of metabolism on this critical period of life and their clinical manifestation. Significant adaptation of mitochondrial/energy-, carbohydrate-, lysosomal-, and amino acid-metabolism occurs during early prenatal and neonatal development. In utero, metabolic environment has an impact on the development of the fetus as well as fetal organ maturation. Defects of distinct metabolic pathways could therefore already be of significant relevance in utero and for clinical manifestations in the early fetal and neonatal period. Disturbances of these pathways may influence intrauterine growth and health. Production of a toxic intrauterine milieu, energy-deficiency, modification of membrane function, or disturbance of the normal intrauterine expression of genes may be responsible for fetal compromise and developmental disorders. Three categories of metabolic disorders will be discussed: the "intoxication type" (classical galactosemia, ornithine transcarbamylase deficiency, and "maternal phenylketonuria"), the "storage type" (Morbus Niemann Pick type C), and the "energy deficient type" (including long-chain fatty acid oxidation disorders, pyruvate dehydrogenase deficiency, and respiratory chain defects). For these disorders, the pathophysiology of early manifestation, special aspects regarding the prenatal and neonatal period, and diagnostic as well as therapeutic options are presented.

Preeclampsia and HELLP syndrome: impaired mitochondrial function in umbilical endothelial cells

Preeclampsia (PE) and hemolysis, elevated liver enzymes, and low platelet count (HELLP) syndrome have been linked to congenital fetal disorders of mitochondrial fatty acid oxidation (FAO). Different incidences may argue for the association of noncongenital alterations of mitochondrial energy metabolism with PE/HELLP syndrome. We studied human umbilical vein endothelial cells [HUVEC] as selected part of the feto-placental unit from uncomplicated (n = 46) and diseased (n = 27; 17 PE and 10 HELLP) pregnancies by measuring the overall FAO, carnitine palmitoyltransferase 2 (CPT2), respiratory chain (RC) complexes I-V, citratesynthase (CS), lactatedehydrogenase (LDH), hexokinase (HK), phosphofructokinase (PFK), and energy rich phosphates. Maternal and infantile acylcarnitines in blood were investigated post partum. Overall FAO, RC complexes II-V, and CS were significantly compromised in HUVEC from complicated pregnancies; impairment of complexes I + III was not significant. CPT2 and energy charges were unaffected. Lactatedehydrogenase and PFK from complicated pregnancies were upregulated, and HK remained constant. In blood, carnitine was elevated in diseased women and their children, acylcarnitines were higher in affected infants. Impaired mitochondrial function in HUVEC is associated with PE/HELLP syndrome and may be involved in the pathophysiology of these diseases.

Mitochondrial respiratory chain complex assembly and function during human fetal development

Oxidative phosphorylation (OXPHOS) deficiency may have early antenatal manifestations, probably related to the time course and/or tissue specificity of the disease gene expression during the embryo-fetal period. This feature hampers a fully reliable prenatal enzymological diagnosis of OXPHOS deficiency. We have studied OXPHOS in various human fetal tissues from 9 to 17 weeks of gestation. We found that the fetal respiratory chain complexes are fully assembled and functional at early stages of development in heart, liver, muscle, brain and kidney. We also observed a marked increase of respiratory chain activities and mitochondria content in postnatal compared to prenatal tissues. However, we were not able to detect obvious modification in the size, composition or activity of the various OXPHOS complexes during the second trimester of pregnancy that could account for the variations we observed in a pathological context. Therefore, we suggest that the time-dependent expression of respiratory chain deficiency either during fetal life or after birth could be related to the differential expression or regulation of the mutant proteins.

Grossesse chez une patiente atteinte de cytopathie mitochondriale

We report a case of a pregnant woman with a mitochondrial disorder affecting the energy-generating pathway of oxidative phosphorylation which was suggested when the patient presented the progressive clinical phenotype of a proximal tubular renal insufficiency, a muscular weakness of extremities, a bilateral optic neuropathy and a brain magnetic resonance imaging suggesting diffuse leucoencephalopathy. Her diagnosis was made on the basis of abnormal mitochondria on a muscle biopsy and of spectrophotometric deficiencies of the complexes I, II+III and IV of the respiratory chain. No specific molecular mutation could be detected. Her pregnancy was complicated by a severe preeclampsia, an insulin requiring gestational diabetes and a worrying renal failure which precipitated the premature delivery by cesarean section at 30 weeks gestation. The clinical course of the female neonate weighing 1030 grams was uneventful. At two Years of age she showed no sign of mitochondrial disease. But the postpartum course of the mother was complicated by seizures and a terminal renal failure leading presently to dialysis, but requiring a kidney transplantation in the near future.

Antenatal manifestations of mitochondrial respiratory chain deficiency

OBJECTIVE

To review the antenatal manifestations of disorders of oxidative phosphorylation.

STUDY DESIGN

A total of 300 cases of proven respiratory chain enzyme deficiency were retrospectively reviewed for fetal development, based on course and duration of pregnancy, antenatal ultrasonography and birth weight, length, and head circumference. Particular attention was given to fetal movements, oligo/hydramnios, fetal cardiac rhythm, fetal heart ultrasound, and ultrasonography/echo Doppler signs of brain, facial, trunk, limb, and organ anomalies.

RESULTS

Retrospective analyses detected low birth weight (<3rd percentile for gestational age) in 22.7% of cases (68/300, P<.000001). Intrauterine growth retardation was either isolated (48/300, 16%) or associated with otherwise unexplained anomalies (20/300, 6.7%, P<.0001). Antenatal anomalies were usually multiple and involved several organs sharing no common function or embryologic origin. They included polyhydramnios (6/20), oligoamnios (2/20), arthrogryposis (1/20), decreased fetal movements (1/20), ventricular septal defects (2/20), hypertrophic cardiomyopathy (4/20), cardiac rhythm anomalies (4/20), hydronephrosis (3/20), vertebral abnormalities, anal atresia, cardiac abnormalities, tracheoesophageal fistula/atresia, renal agenesis and dysplasia, and limb defects (VACTERL) association (2/20), and a complex gastrointestinal malformation (1/20).

CONCLUSIONS

Although a number of metabolic diseases undergo a symptom-free period, respiratory chain deficiency may have an early antenatal expression, presumably related to the time course of the disease gene expression in the embryofetal period. The mechanism triggering malformations is unknown and may include decreased ATP formation and/or an alteration of apoptotic events controlled by the mitochondria.

Prerequisites and strategies for prenatal diagnosis of respiratory chain deficiency in chorionic villi

Prenatal diagnosis for respiratory chain deficiencies is a complex procedure that requires a thorough diagnostic work-up of the index patient. This includes confirmation of the clinical and metabolic evaluations through histological and enzymatic examinations of tissue biopsies. Prenatal diagnosis currently relies on biochemical assays of respiratory chain complexes in chorionic villi or amniocytes and is possible by mutation analysis of nuclear genes in a limited but increasing proportion of cases. Based on a recent survey of prenatal diagnosis in families with complex I and complex IV deficiencies, performed at Nijmegen Centre for Mitochondrial Disorders (NCMD), prerequisites and strategies for performing prenatal diagnosis have been developed to increase reliability. Biochemical investigations in chorionic villi can be done reliably if the respiratory chain enzyme deficiency is expressed in both skeletal muscle and skin fibroblasts to rule out tissue specificity. No mitochondrial DNA defects must be suspected or established. The NCMD does not offer prenatal diagnosis until all the prerequisites have been confirmed. We expect prenatal diagnosis at the molecular level to become more feasible in time as the mutational spectrum broadens with advances in medical research.

Mitochondrial diseases

Mitochondrial disorders are caused by deficient respiratory chain function, resulting in a complex series of pathophysiological events. Genetic counselling is complicated because the respiratory chain subunits are encoded by both nuclear and mitochondrial DNA genes. Only a minority of the nuclear genes involved in mitochondrial function have been identified, and even fewer are associated with human mitochondrial disease. Mutations in mitochondrial DNA are particularly challenging because of the complexities of mitochondrial genetics: the mitochondrial DNA is strictly maternally inherited; there are 10(3)-10(4) copies of mitochondrial DNA in somatic cells; affected individuals often have a mixture of normal and mutated mitochondrial DNA (mitochondrial DNA heteroplasmy), the level of mutated mitochondrial DNA (the mitochondrial DNA mutation load) may vary widely between different maternally related individuals, between tissues and with time; a particular minimal threshold of mutated mitochondrial DNA is required to impair respiratory chain function; and there is not always a good correlation between mutant load and phenotype.

Mitochondrial disorders: genetics, counseling, prenatal diagnosis and reproductive options

Most patients with mitochondrial disorders are diagnosed by finding a respiratory chain enzyme defect or a mutation in the mitochondrial DNA (mtDNA). The provision of accurate genetic counseling and reproductive options to these families is complicated by the unique genetic features of mtDNA that distinguish it from Mendelian genetics. These include maternal inheritance, heteroplasmy, the threshold effect, the mitochondrial bottleneck, tissue variation, and selection. Although we still have much to learn about mtDNA genetics, it is now possible to provide useful guidance to families with an mtDNA mutation or a respiratory chain enzyme defect. We describe a range of current reproductive options that may be considered for prevention of transmission of mtDNA mutations, including the use of donor oocytes, prenatal diagnosis (by chorionic villus sampling or amniocentesis), and preimplantation genetic diagnosis, plus possible future options such as nuclear transfer and cytoplasmic transfer. For common mtDNA mutations associated with mitochondrial cytopathies (such as NARP, Leigh Disease, MELAS, MERRF, Leber's Hereditary Optic Neuropathy, CPEO, Kearns-Sayre syndrome, and Pearson syndrome), we summarize the available data on recurrence risk and discuss the relative advantages and disadvantages of reproductive options.

Prenatal diagnosis of NADH:ubiquinone oxidoreductase deficiency

NADH:ubiquinone oxidoreductase (complex I of the mitochondrial respiratory chain) deficiency is a severe disorder with an often early fatal outcome. Prenatal diagnosis for complex I defects currently relies mainly on biochemical assays of complex I in fetal tissues such as chorionic villi (CV), and is only in a minority of cases possible by means of mutational analysis of nuclear-encoded genes of complex I. We report on our experience to date with prenatal diagnosis in pregnancies at risk for complex I deficiency. We measured complex I activity in native CV and/or cultured CV in 23 pregnancies in 15 families. In accordance with the results of the investigations in CV, 15 children were born clinically unaffected. Two prenatally diagnosed unaffected fetuses and two prenatally diagnosed affected fetuses were lost prematurely with spontaneous or provoked abortions, respectively. Two affected children were born (prenatally found to be affected). In two pregnancies a discrepancy between native and cultured cells was found. We conclude that prenatal diagnosis for complex I deficiency can be reliably performed. Pitfalls were encountered in using cultured CV as a result of maternal cell contamination (MCC). Future research on pathogenic nuclear mutations underlying complex I deficiency will extend the possibilities for prenatal diagnosis at the molecular level.

Mitochondrial disorders

The most relevant contribution to the elucidation of the molecular basis of mitochondrial disorders has come from the discovery of an impressive and ever expanding number of mutations of mitochondrial DNA. However, known mutations of mtDNA only account for a fraction of all the mitochondrial disorders in both infants and adults. A number of recent clinical and molecular observations indicate that many syndromes are caused by abnormalities in nuclear genes related to oxidative phosphorylation. Nuclear genes encode hundreds of proteins involved in mitochondrial biogenesis and oxidative phosphorylation. Nevertheless, the identification of the nuclear genes responsible for oxidative phosphorylation-related disorders has proceeded at a much slower pace, compared with the discovery and characterization of mtDNA mutations. The reasons for such a gap are numerous, including the rarity of the syndromes, their genetic heterogeneity, and our ignorance of this nuclear gene repertoire in humans. This scenario is rapidly changing, thanks to the discovery of several oxidative phosphorylation-related human genes, and to the identification in some of them of mutations responsible for different clinical syndromes. In addition, animal models have recently been generated, which will offer the opportunity to understand better the pathogenesis of specific oxidative phosphorylation defects, and to test in a rational and controlled fashion therapeutic strategies for the treatment of these disorders.

Prenatal diagnosis of respiratory chain deficiency by direct mutation screening

Respiratory chain deficiency (RCD) is responsible for a clinically heterogeneous group of early-onset untreatable disorders. Enzymological prenatal diagnosis (PD) can only be offered to a fraction of families. Moreover, due to the two-fold genetic origin of the respiratory chain (nuclear and mitochondrial DNA) and owing to the large number of nuclear genes involved in the respiratory chain assembly, maintenance and functioning, the identification of the disease causing gene in a given family remains challenging. Here, we report on PD of RCD by direct screening of NDUFV1, SDH-Fp, SCO1 and SURF1 mutations in five unrelated families with complex I, II and IV deficiency, respectively. The identification of the disease-causing gene in a given family with RCD is a major issue to provide both adequate genetic counselling and early, reliable PD.

Human NADH:ubiquinone oxidoreductase

NADH:ubiquinone oxidoreductase consists of at least 43 proteins; seven are encoded by the mitochondrial genome, while the remainder are encoded by the nuclear genome. A deficient activity of this enzyme complex is frequently observed in the clinical heterogeneous group of mitochondrial disorders, with Leigh (-like) disease as the main contributor. Enzyme complex activity measurement in skeletal muscle is the mainstay of the diagnostic process. Fibroblast studies are a prerequisite whenever prenatal enzyme diagnosis is considered. Mitochondrial DNA mutations are found in approximately 5-10% of all complex I deficiencies. Recently, all structural nuclear complex I genes have been determined at the cDNA level and several at the gDNA level. A comprehensive mutational analysis study of all complex I nuclear genes in a group of 20 patients exhibiting this deficiency revealed mutations in about 40%. Here, we describe the enzymic methods we use and the recent progress made in genomics and cell biology of human complex I.