Role of Energy Metabolism and Mitochondrial Function in Inflammatory Bowel Disease.

Inflammatory bowel disease (IBD) is a chronic recurring inflammation of the intestine which can be debilitating for those with intractable disease. However, the etiopathogenesis of inflammatory bowel disorders remains to be solved. The hypothesis that mitochondrial dysfunction is a crucial factor in the disease process is being validated by an increasing number of recent studies. Thus mitochondrial alteration in conjunction with previously identified genetic predisposition, changes in the immune response, altered gut microbiota, and environmental factors (eg, diet, smoking, and lifestyle) are all posited to contribute to IBD. The implicated factors seem to affect mitochondrial function or are influenced by mitochondrial dysfunction, which explains many of the hallmarks of the disease. This review summarizes the results of studies reporting links between mitochondria and IBD that were available on PubMed through March 2021. The aim of this review is to give an overview of the current understanding of the role of mitochondria in the pathogenesis of IBD.

We address the effect of energy metabolism and mitochondrial function on the pathogenesis of inflammatory bowel disease. Because many studies on this topic have been published recently, it is important to give an overview of the results of that work.

Expression of Oxidative Phosphorylation Complexes and Mitochondrial Mass in Pediatric and Adult Inflammatory Bowel Disease.

INTRODUCTION: Inflammatory bowel disease (IBD), which includes Crohn's disease (CD) and ulcerative colitis (UC), is a multifactorial intestinal disorder but its precise etiology remains elusive. As the cells of the intestinal mucosa have high energy demands, mitochondria may play a role in IBD pathogenesis. The present study is aimed at evaluating the expression levels of mitochondrial oxidative phosphorylation (OXPHOS) complexes in IBD. Material and Methods. 286 intestinal biopsy samples from the terminal ileum, ascending colon, and rectum from 124 probands (34 CD, 33 UC, and 57 controls) were stained immunohistochemically for all five OXPHOS complexes and the voltage-dependent anion-selective channel 1 protein (VDAC1 or porin). Expression levels were compared in multivariate models including disease stage (CD and UC compared to controls) and age (pediatric/adult). RESULTS: Analysis of the terminal ileum of CD patients revealed a significant reduction of complex II compared to controls, and a trend to lower levels was evident for VDAC1 and the other OXPHOS complexes except complex III. A similar pattern was found in the rectum of UC patients: VDAC1, complex I, complex II, and complex IV were all significantly reduced, and complex III and V showed a trend to lower levels. Reductions were more prominent in older patients compared to pediatric patients and more marked in UC than CD. CONCLUSION: A reduced mitochondrial mass is present in UC and CD compared to controls. This is potentially a result of alterations of mitochondrial biogenesis or mitophagy. Reductions were more pronounced in older patients compared to pediatric patients, and more prominent in UC than CD. Complex I and II are more severely compromised than the other OXPHOS complexes. This has potential therapeutic implications, since treatments boosting biogenesis or influencing mitophagy could be beneficial for IBD treatment. Additionally, substances specifically stimulating complex I activity should be tested in IBD treatment.

Age-Related Deterioration of Mitochondrial Function in the Intestine.

Aging is an important and inevitable biological process in human life, associated with the onset of chronic disease and death. The mechanisms behind aging remain unclear. However, changes in mitochondrial function and structure, including reduced activity of the mitochondrial respiratory chain and increased production of reactive oxygen species-thus oxidative damage-are believed to play a major role. Mitochondria are the main source of cellular energy, producing adenosine triphosphate (ATP) via oxidative phosphorylation. Accumulation of damaged cellular components reduces a body's capacity to preserve tissue homeostasis and affects biological aging and all age-related chronic conditions. This includes the onset and progression of classic degenerative diseases such as cardiovascular disease, kidney failure, neurodegenerative diseases, and cancer. Clinical manifestations of intestinal disorders, such as mucosal barrier dysfunction, intestinal dysmotility, and chronic obstipation, are highly prevalent in the elderly population and have been shown to be associated with an age-dependent decline of mitochondrial function. This review summarizes our current understanding of the role of mitochondrial dysfunction in intestinal aging.

Changes in the expression of oxidative phosphorylation complexes in the aging intestinal mucosa.

OBJECTIVE: Mitochondria produce cellular energy via oxidative phosphorylation (OXPHOS), mediated by respiratory chain complexes I to IV and ATP synthase (complex V). Mitochondrial respiratory complexes have been shown to decline with age in several tissues. As the intestinal epithelium is a tissue with a high energy demand, the aim of the present study was to establish whether the expression profile of OXPHOS subunits in the intestinal mucosa changes during the aging process. DESIGN: Biopsies of intestinal mucosa with no evidence of endoscopic or histomorphologic abnormalities, taken from 55 patients (mean age 42 years, age range 4-82 years; 62% female), were divided into four age groups (4-19, 20-39, 40-59, ≥60 years). Sections from different intestinal segments (terminal ileum, ascending colon, and sigmoid colon/rectum) were stained immunohistochemically (IHC) for subunits of OXPHOS complexes I-V and the voltage-dependent anion-selective channel 1 protein (VDAC1, porin), a marker of mitochondrial mass. Scores for IHC staining were determined by multiplication of the staining intensity and the percentage of positive cells. In addition, the numbers of intestinal crypts staining positive, partly positive, and negative were assessed. RESULTS: The average protein expression levels of OXPHOS subunits increased continuously from childhood onward, peaked in persons aged 20 to 59 years, and declined thereafter. This was seen for complexes II to V in the terminal ileum, complexes I to V in the ascending colon, and complexes I to IV in the sigmoid colon/rectum. Across all age groups, no effect of age on expression of the porin subunit VDAC1 was detected. The number of complex I- and IV-negative crypts in different intestinal segments increased with age. CONCLUSION: The protein expression levels of OXPHOS complexes increases from childhood onward and declines in elderly individuals, while the numbers of crypts with partial or complete loss of expression of complexes I and IV increase continuously with age. These data suggest that the continued reductions in the levels of mitochondrial OXPHOS complexes in crypts might be compensated in adulthood, but that, ultimately, reduced expression levels occur in persons aged 60 years and older. These findings raise two important questions: first, can the process of aging could be delayed through (pharmacological) intervention of mitochondrial pathways, and second, pathophysiologically, are these findings associated with disorders of the intestinal mucosa, e.g. inflammation?

Alterations of Oxidative Phosphorylation Complexes in Papillary Thyroid Carcinoma.

The papillary thyroid carcinoma (PTC) is the most common malignant tumor of the thyroid gland, with disruptive mutations in mitochondrial complex I subunits reported at very low frequency. Furthermore, metabolic diversity of PTC has been postulated owing to variable messenger RNA (mRNA) expression of genes encoding subunits of the oxidative phosphorylation (OXHPOS) complexes. The aim of the present study was to evaluate the metabolic diversity of the OXPHOS system at the protein level by using immunohistochemical staining. Analysis of 18 human PTCs revealed elevated mitochondrial biogenesis but significantly lower levels of OXPHOS complex I in the tumor tissue (p < 0.0001) compared to the adjacent normal tissue. In contrast, OXPHOS complexes II⁻V were increased in the majority of PTCs. In three PTCs, we found pathologic mutations within mitochondrially encoded complex I subunits. Our data indicate that PTCs are characterized by an oncocytic metabolic signature that is in low complex I is combined with elevated mitochondrial mass and high complex II⁻V levels, which might be an important factor for tumor formation.

Oxidative Phosphorylation System in Gastric Carcinomas and Gastritis.

Switching of cellular energy production from oxidative phosphorylation (OXPHOS) by mitochondria to aerobic glycolysis occurs in many types of tumors. However, the significance of this switching for the development of gastric carcinoma and what connection it may have to Helicobacter pylori infection of the gut, a primary cause of gastric cancer, are poorly understood. Therefore, we investigated the expression of OXPHOS complexes in two types of human gastric carcinomas ("intestinal" and "diffuse"), bacterial gastritis with and without metaplasia, and chemically induced gastritis by using immunohistochemistry. Furthermore, we analyzed the effect of HP infection on several key mitochondrial proteins. Complex I expression was significantly reduced in intestinal type (but not diffuse) gastric carcinomas compared to adjacent control tissue, and the reduction was independent of HP infection. Significantly, higher complex I and complex II expression was present in large tumors. Furthermore, higher complex II and complex III protein levels were also obvious in grade 3 versus grade 2. No differences of OXPHOS complexes and markers of mitochondrial biogenesis were found between bacterially caused and chemically induced gastritis. Thus, intestinal gastric carcinomas, but not precancerous stages, are frequently characterized by loss of complex I, and this pathophysiology occurs independently of HP infection.

Deficiency of respiratory chain complex I in Hashimoto thyroiditis.

Oncocytic cells (OCs) are characterized by an accumulation of mitochondria and their occurrence in the thyroid gland of patients with Hashimoto thyroiditis (HT) is well known. However, their properties and functional relevance are poorly understood. We investigated OC lesions (n=212) in the thyroid of 12 HT patients. Loss of complex I protein was observed in oncocytic lesions of each of the patients. In addition to isolated complex I deficiency, 25% of oncocytic lesions showed combined deficiency of complex I and IV. Thus, we demonstrate for the first time a defect of respiratory chain complex I in OCs of HT patients.

Alterations of oxidative phosphorylation in meningiomas and peripheral nerve sheath tumors.

BACKGROUND: Changes in the mode of aerobic energy production are observed in many solid tumors, though the kinds of changes differ among tumor types. We investigated mitochondrial energy metabolism in meningiomas and peripheral nerve sheath tumors, taking into consideration the histologic heterogeneity of these tumors. METHODS: Oxidative phosphorylation (OXPHOS) complexes and porin (a marker for mitochondrial mass) were analyzed by immunohistochemical staining of meningiomas (n = 76) and peripheral nerve sheath tumors (schwannomas: n = 10; neurofibromas: n = 4). The enzymatic activities of OXPHOS complexes and citrate synthase were determined by spectrophotometric measurement. Western blot analysis of OXPHOS complexes, porin, and mitochondrial transcription factor A was performed. Furthermore, mitochondrial DNA copy number was determined. RESULTS: The tumors differed with regard to mitochondrial energy metabolism. Low levels of a subset of OXPHOS complexes were frequently observed in World Health Organization grade I meningiomas (percent of cases with a reduction; complex I: 63%; complex II: 67%; complex IV: 56%) and schwannomas (complex III: 40%, complex IV: 100%), whereas in neurofibromas a general reduction of all complexes was observed. In contrast, expression of complexes III and V was similar to that in normal brain tissue in the majority of tumors. Mitochondrial mass was comparable or higher in all tumors compared with normal brain tissue, whereas mitochondrial DNA copy number was reduced. CONCLUSIONS: The reduction of OXPHOS complexes in meningiomas and peripheral nerve sheath tumors has potential therapeutic implications, since respiratory chain-deficient tumor cells might be selectively starved by inhibitors of glycolysis or by ketogenic diet.

Clinical, biochemical, and genetic spectrum of seven patients with NFU1 deficiency.

Disorders of the mitochondrial energy metabolism are clinically and genetically heterogeneous. An increasingly recognized subgroup is caused by defective mitochondrial iron-sulfur (Fe-S) cluster biosynthesis, with defects in 13 genes being linked to human disease to date. Mutations in three of them, NFU1, BOLA3, and IBA57, affect the assembly of mitochondrial [4Fe-4S] proteins leading to an impairment of diverse mitochondrial metabolic pathways and ATP production. Patients with defects in these three genes present with lactic acidosis, hyperglycinemia, and reduced activities of respiratory chain complexes I and II, the four lipoic acid-dependent 2-oxoacid dehydrogenases and the glycine cleavage system (GCS). To date, five different NFU1 pathogenic variants have been reported in 15 patients from 12 families. We report on seven new patients from five families carrying compound heterozygous or homozygous pathogenic NFU1 mutations identified by candidate gene screening and exome sequencing. Six out of eight different disease alleles were novel and functional studies were performed to support the pathogenicity of five of them. Characteristic clinical features included fatal infantile encephalopathy and pulmonary hypertension leading to death within the first 6 months of life in six out of seven patients. Laboratory investigations revealed combined defects of pyruvate dehydrogenase complex (five out of five) and respiratory chain complexes I and II+III (four out of five) in skeletal muscle and/or cultured skin fibroblasts as well as increased lactate (five out of six) and glycine concentration (seven out of seven). Our study contributes to a better definition of the phenotypic spectrum associated with NFU1 mutations and to the diagnostic workup of future patients.

Mutations in TTC19: expanding the molecular, clinical and biochemical phenotype.

BACKGROUND: TTC19 deficiency is a progressive neurodegenerative disease associated with isolated mitochondrial respiratory chain (MRC) complex III deficiency and loss-of-function mutations in the TT19 gene in the few patients reported so far. METHODS: We performed exome sequencing and selective mutational analysis of TTC19, respectively, in patients from three unrelated families presenting with initially unspecific clinical signs of muscular hypotonia and global developmental delay followed by regression, ataxia, loss of speech, and rapid neurological deterioration. One patient showed severe lactic acidosis at the neonatal age and during intercurrent illness. RESULTS: We identified homozygous mutations in all three index cases, in two families novel missense mutations (c.544 T > C/p.Leu185Pro; c.917 T > C/p.Leu324Pro). The younger sister of the severely affected patient 3 showed only mild delay of motor skills and muscular hypotonia so far but is also homozygous for the same mutation. Notably, one patient revealed normal activities of MRC complex III in two independent muscle biopsies. Neuroimaging of the severely affected patients demonstrated lesions in putamen and caudate nuclei, cerebellar atrophy, and the unusual finding of hypertrophic olivary nuclei degeneration. Reviewing the literature revealed striking similarities regarding neuroimaging and clinical course in pediatric patients with TTC19 deficiency: patterns consistent with Leigh or Leigh-like syndrome were found in almost all, hypertrophic olivary nucleus degeneration in all patients reported so far. The clinical course in pediatric patients is characterized by an initially unspecific developmental delay, followed by regression, progressive signs and symptoms of cerebellar, basal ganglia and brainstem affection, especially loss of speech and ataxia. Subsequently, neurological deterioration leading to a vegetative state occurs. CONCLUSIONS: Our findings add to the phenotypic, genetic, and biochemical spectrum of TTC19 deficiency. However, TTC19 deficient patients do show characteristic clinical and neuroimaging features, which may facilitate diagnosis of this yet rare disorder. Normal MRC complex III activity does not exclude the diagnosis.

Spectrum of combined respiratory chain defects.

Inherited disorders of mitochondrial energy metabolism form a large and heterogeneous group of metabolic diseases. More than 250 gene defects have been reported to date and this number continues to grow. Mitochondrial diseases can be grouped into (1) disorders of oxidative phosphorylation (OXPHOS) subunits and their assembly factors, (2) defects of mitochondrial DNA, RNA and protein synthesis, (3) defects in the substrate-generating upstream reactions of OXPHOS, (4) defects in relevant cofactors and (5) defects in mitochondrial homeostasis. Deficiency of more than one respiratory chain enzyme is a common finding. Combined defects are found in 49 % of the known disease-causing genes of mitochondrial energy metabolism and in 57 % of patients with OXPHOS defects identified in our diagnostic centre. Combined defects of complexes I, III, IV and V are typically due to deficiency of mitochondrial DNA replication, RNA metabolism or translation. Defects in cofactors can result in combined defects of various combinations, and defects of mitochondrial homeostasis can result in a generalised decrease of all OXPHOS enzymes. Noteworthy, identification of combined defects can be complicated by different degrees of severity of each affected enzyme. Furthermore, even defects of single respiratory chain enzymes can result in combined defects due to aberrant formation of respiratory chain supercomplexes. Combined OXPHOS defects have a great variety of clinical manifestations in terms of onset, course severity and tissue involvement. They can present as classical encephalomyopathy but also with hepatopathy, nephropathy, haematologic findings and Perrault syndrome in a subset of disorders.

From ventriculomegaly to severe muscular atrophy: expansion of the clinical spectrum related to mutations in AIFM1.

The apoptosis-inducing factor (AIF) functions as a FAD-dependent NADH oxidase in mitochondria. Upon apoptotic stimulation it is released from mitochondria and migrates to the nucleus where it induces chromatin condensation and DNA fragmentation. So far mutations in AIFM1, a X-chromosomal gene coding for AIF, have been described in three families with 11 affected males. We report here on a further patient thereby expanding the clinical and mutation spectrum. In addition, we review the known phenotypes related to AIFM1 mutations. The clinical course in the male patient described here was characterized by phases with rapid deterioration and long phases without obvious progression of disease. At age 2.5 years he developed hearing loss and severe ataxia and at age 10 years muscle wasting, swallowing difficulties, respiratory insufficiency and external opthamoplegia. By next generation sequencing of whole exome we identified a hemizygous missense mutation in the AIFM1 gene, c.727G>T (p.Val243Leu) affecting a highly conserved residue in the FAD-binding domain. Summarizing what is known today, mutations in AIFM1 are associated with a progressive disorder with myopathy, ataxia and neuropathy. Severity varies greatly even within one family with onset of symptoms between birth and adolescence. 3 of 12 patients died before age 5 years while others were still able to walk during young adulthood. Less frequent symptoms were hearing loss, seizures and psychomotor regression. Results from clinical chemistry, brain imaging and muscle biopsy were unspecific and inconsistent.

The spectrum of pyruvate oxidation defects in the diagnosis of mitochondrial disorders.

Pyruvate oxidation defects (PODs) are among the most frequent causes of deficiencies in the mitochondrial energy metabolism and represent a substantial subset of classical mitochondrial diseases. PODs are not only caused by deficiency of subunits of the pyruvate dehydrogenase complex (PDHC) but also by various disorders recently described in the whole pyruvate oxidation route including cofactors, regulation of PDHC and the mitochondrial pyruvate carrier. Our own patients from 2000 to July 2014 and patients identified by a systematic survey of the literature from 1970 to July 2014 with a pyruvate oxidation disorder and a genetically proven defect were included in the study (n=628). Of these defects 74.2% (n=466) belong to PDHC subunits, 24.5% (n=154) to cofactors, 0.5% (n=3) to PDHC regulation and 0.8% (n=5) to mitochondrial pyruvate import. PODs are underestimated in the field of mitochondrial diseases because not all diagnostic centres include biochemical investigations of PDHC in their routine analysis. Cofactor and transport defects can be missed, if pyruvate oxidation is not measured in intact mitochondria routinely. Furthermore deficiency of the X-chromosomal PDHA1 can be biochemically missed depending on the X-inactivation pattern. This is reflected by an increasing number of patients diagnosed recently by genetic high throughput screening approaches. PDHC deficiency including regulation and import affect mainly the glucose dependent central and peripheral nervous system and skeletal muscle. PODs with combined enzyme defects affect also other organs like heart, lung and liver. The spectrum of clinical presentation of PODs is still expanding. PODs are a therapeutically interesting group of mitochondrial diseases since some can be bypassed by ketogenic diet or treated by cofactor supplementation. PDHC kinase inhibition, chaperone therapy and PGC1α stimulation is still a matter of further investigations.

Expanding the clinical and molecular spectrum of thiamine pyrophosphokinase deficiency: a treatable neurological disorder caused by TPK1 mutations.

Thiamine pyrophosphokinase (TPK) produces thiamine pyrophosphate, a cofactor for a number of enzymes, including pyruvate dehydrogenase and 2-ketoglutarate dehydrogenase. Episodic encephalopathy type thiamine metabolism dysfunction (OMIM 614458) due to TPK1 mutations is a recently described rare disorder. The mechanism of the disease, its phenotype and treatment are not entirely clear. We present two patients with novel homozygous TPK1 mutations (Patient 1 with p.Ser160Leu and Patient 2 with p.Asp222His). Unlike the previously described phenotype, Patient 2 presented with a Leigh syndrome like non-episodic early-onset global developmental delay, thus extending the phenotypic spectrum of the disorder. We, therefore, propose that TPK deficiency may be a better name for the condition. The two cases help to further refine the neuroradiological features of TPK deficiency and show that MRI changes can be either fleeting or progressive and can affect either white or gray matter. We also show that in some cases lactic acidosis can be absent and 2-ketoglutaric aciduria may be the only biochemical marker. Furthermore, we have established the assays for TPK enzyme activity measurement and thiamine pyrophosphate quantification in frozen muscle and blood. These tests will help to diagnose or confirm the diagnosis of TPK deficiency in a clinical setting. Early thiamine supplementation prevented encephalopathic episodes and improved developmental progression of Patient 1, emphasizing the importance of early diagnosis and treatment of TPK deficiency. We present evidence suggesting that thiamine supplementation may rescue TPK enzyme activity. Lastly, in silico protein structural analysis shows that the p.Ser160Leu mutation is predicted to interfere with TPK dimerization, which may be a novel mechanism for the disease.

ELAC2 mutations cause a mitochondrial RNA processing defect associated with hypertrophic cardiomyopathy.

The human mitochondrial genome encodes RNA components of its own translational machinery to produce the 13 mitochondrial-encoded subunits of the respiratory chain. Nuclear-encoded gene products are essential for all processes within the organelle, including RNA processing. Transcription of the mitochondrial genome generates large polycistronic transcripts punctuated by the 22 mitochondrial (mt) tRNAs that are conventionally cleaved by the RNase P-complex and the RNase Z activity of ELAC2 at 5' and 3' ends, respectively. We report the identification of mutations in ELAC2 in five individuals with infantile hypertrophic cardiomyopathy and complex I deficiency. We observed accumulated mtRNA precursors in affected individuals muscle and fibroblasts. Although mature mt-tRNA, mt-mRNA, and mt-rRNA levels were not decreased in fibroblasts, the processing defect was associated with impaired mitochondrial translation. Complementation experiments in mutant cell lines restored RNA processing and a yeast model provided additional evidence for the disease-causal role of defective ELAC2, thereby linking mtRNA processing to human disease.

Mitochondrial myopathy associated with a novel 5522G>A mutation in the mitochondrial tRNA(Trp) gene.

We report a novel pathogenic mutation of the mitochondrial transfer RNA (tRNA) gene for tryptophan in a patient with isolated myopathy and persistently elevated creatine kinase. Muscle studies revealed ragged red fibres and decreased activity of respiratory chain complex I and cytochrome c oxidase (COX). Sequencing of the 22 mitochondrial tRNA genes revealed a mutation m.5522G>A, which alters a conserved base pairing in the D-stem of the tRNA for tryptophan. The mutation was heteroplasmic with a mutational load between 88 and 99% in COX-negative fibres. This case contributes to the genetic heterogeneity of mitochondrial diseases caused by mutations in mitochondrial tRNA genes.

A 1.1 million base pair X-chromosomal deletion covering the PDHA1 and CDKL5 genes in a female patient with West syndrome and pyruvate oxidation deficiency.

Mutations in the X-linked E1α subunit of the pyruvate dehydrogenase complex (PHDC) are the most frequent causes of PDHC deficiency. The clinical picture is heterogeneous depending on residual enzyme activity and X-inactivation. We report on a girl who presented at an age of 3 weeks with muscular hypotonia, vomiting, hyperlactatemia, microcephaly, enlarged ventricles, partial agenesis of the corpus callosum, and seizures. PDHA1 sequencing was normal in DNA from blood. In muscle, normal PDHC activity was measured while substrate oxidation rates revealed moderately diminished pyruvate oxidation. Quantitative PCR analysis revealed hemizygosity of the whole PDHA1 gene. Homozygosity mapping and determination of the breakpoint showed a 1.1 million base pair deletion on the X-chromosome including the CDKL5 and PDHA1 genes. The difficulty in the diagnosis of PDHC deficiency is evident: (1) enzyme activity can be normal depending on the X-inactivation; (2) large deletions can be missed by routine genetic analysis; and (3) only quantification of the PDHA1 gene content revealed the mutation in our patient. We recommend to revisit patients who are clinically suspicious for a mitochondrial disorder especially for hidden PDHA1 mutations, such as large deletions.

Lack of the mitochondrial protein acylglycerol kinase causes Sengers syndrome.

Exome sequencing of an individual with congenital cataracts, hypertrophic cardiomyopathy, skeletal myopathy, and lactic acidosis, all typical symptoms of Sengers syndrome, discovered two nonsense mutations in the gene encoding mitochondrial acylglycerol kinase (AGK). Mutation screening of AGK in further individuals with congenital cataracts and cardiomyopathy identified numerous loss-of-function mutations in an additional eight families, confirming the causal nature of AGK deficiency in Sengers syndrome. The loss of AGK led to a decrease of the adenine nucleotide translocator in the inner mitochondrial membrane in muscle, consistent with a role of AGK in driving the assembly of the translocator as a result of its effects on phospholipid metabolism in mitochondria.

Reduction of nuclear encoded enzymes of mitochondrial energy metabolism in cells devoid of mitochondrial DNA.

Mitochondrial DNA (mtDNA) depletion syndromes are generally associated with reduced activities of oxidative phosphorylation (OXPHOS) enzymes that contain subunits encoded by mtDNA. Conversely, entirely nuclear encoded mitochondrial enzymes in these syndromes, such as the tricarboxylic acid cycle enzyme citrate synthase (CS) and OXPHOS complex II, usually exhibit normal or compensatory enhanced activities. Here we report that a human cell line devoid of mtDNA (HEK293 ρ(0) cells) has diminished activities of both complex II and CS. This finding indicates the existence of a feedback mechanism in ρ(0) cells that downregulates the expression of entirely nuclear encoded components of mitochondrial energy metabolism.

Lipoic acid synthetase deficiency causes neonatal-onset epilepsy, defective mitochondrial energy metabolism, and glycine elevation.

Lipoic acid is an essential prosthetic group of four mitochondrial enzymes involved in the oxidative decarboxylation of pyruvate, α-ketoglutarate, and branched chain amino acids and in the glycine cleavage. Lipoic acid is synthesized stepwise within mitochondria through a process that includes lipoic acid synthetase. We identified the homozygous mutation c.746G>A (p.Arg249His) in LIAS in an individual with neonatal-onset epilepsy, muscular hypotonia, lactic acidosis, and elevated glycine concentration in plasma and urine. Investigation of the mitochondrial energy metabolism showed reduced oxidation of pyruvate and decreased pyruvate dehydrogenase complex activity. A pronounced reduction of the prosthetic group lipoamide was found in lipoylated proteins.