ENERGY PRODUCTION AND FREE RADICALS

This page describes conditions that impair oxidative phosphorylation (oxphos), the process by which mitochondria capture the energy in pyruvate and fatty acids and store it as ATP. This process takes place in the electron transport chain (ETC). The ETC consists of five complexes of transmembrane proteins, located on the inner mitochondrial membrane. Complexes I-IV pump protons out of the mitochondrial matrix, building a proton gradient. Then protons reverse flow and pass through complex V generating ATP. A small proportion of oxygen that enters the ETC is converted to toxic byproducts, oxygen radicals (free radicals). Most of these are detoxified by protective cellular enzymes and vitamin antioxidants. If they are not neutralized, free radicals can damage lipids, proteins, and nucleic acids. This damage is more severe at the site of free radical generation, i.e., the mitochondria. Mitochondria are deficient in protective cellular mechanisms, most of which reside in the cytosol. ETC dysfunction increases free radical production. This creates a vicious cycle, which increases mitochondrial DNA (mtDNA) damage and causes worse ETC dysfunction and more free radical generation. Cellular damage in mitochondrial disorders is due to free radicals and energy deficiency. Both these factors can also trigger necrosis and apoptosis.

GENETICS OF MITOCHONDRIAL DISORDERS

The ETC consists of 90 proteins. Seventy seven of these are encoded by nuclear DNA (nDNA), synthesized in the RER, and imported into the mitochondria. The other 13 are encoded by mtDNA and are synthesized in the mitochondria. Each mitochondrion contains two to ten copies of the mitochondrial genome, a 16.6 kb double-stranded circular DNA molecule, attached to the inner mitochondrial membrane.  In addition to the 13 ETC proteins genes, the mitochondrial genome contains 2 genes encoding rRNAs and 22 genes encoding tRNAs, 37 genes in all.

Respiratory chain disease can be caused by mtDNA and nDNA mutations. The mtDNA mutations are (1) large scale rearrangements (deletions, duplications) of mtDNA; (2) point mutations of rRNA and tRNA genes; and (3) point mutations of protein-coding genes. Large scale rearrangements of mt DNA and point mutations of tRNA and rRNA genes affect protein synthesis and have a profound effect on all ETC complexes (except II which is encoded entirely by nDNA.)

KSS: Kearns-Sayre Syndrome
CPEO: Chronic Progressive External Ophthalmoplegia 
LHON: Leber Hereditary Optic Neuropathy
MELAS: Mitochondrial Encephalomyopathy with Lactic Acidosis and Strokelike Episodes
MERRF: Myoclonic Epilepsy with Ragged Red Fibers
NARP: Neuropathy, Ataxia, And Retinitis Pigmentosa
PS: Pearson Syndrome

The mtDNA is not autonomous; it depends on nuclear-encoded factors for its replication, transcription, and translation. In addition, nDNA encodes most subunits of the ETC and other proteins that are important for their assembly. The nDNA mutations that affect the ETC can be divided into (1) mitochondrial depletion syndromes (MDS); (2) mutations causing multiple mtDNA deletions; and (3) mutations causing isolated ETC deficiencies.

ARCO: Autosomal Recessive Cardiomyopathy, Ophthalmoplegia
CPEO: Chronic Progressive External Ophthalmoplegia 
LS: Leigh Syndrome
MNGIE: Mitochondrial Neurogastrointestinal Encephalomyopathy
NARP: Neuropathy, Ataxia, And Retinitis Pigmentosa

Nuclear gene defects are transmitted in a mendelian fashion: most are autosomal recessive. Because the ovum has numerous mitochondria and the sperm almost none, mutations of mitochondrial genes are transmitted through the mother (maternal inheritance). Nuclear gene defects affect all cells equally. Defects of mtDNA affect cells unevenly. Because of the random way in which mitochondria segregate in dividing cells, wild type and mutant mtDNA coexist in variable proportions in any given cell, a phenomenon called heteroplasmy. In nondividing cells, such as myocytes and neurons, this proportion is relatively stable. In dividing cells, it may shift such that, after several cell cycles, a given cell may come to contain mostly mutant mtDNA (mitotic segregation). Cellular dysfunction develops when the proportion of mutant mtDNA exceeds a certain threshold, typically 80%-90%. Thus, in mtDNA mutations, the genetic defect is dynamic and cell and tissue dysfunction is in a state of flux.  Consequently, the severity of disease in any given cell line cannot be predicted and the clinical phenotype shows great variability. With some exceptions, most mtDNA mutations are heteroplasmic, presumably because homoplasmy for mutant mtDNA would be lethal.

CLINICAL ASPECTS OF MITOCHONDRIAL DISORDERS

Over 270 genetic entities of mitochondrial disorders have been recorded. They affect virtually all organ systems and cause hepatic, gastrointestinal, renal, hematopoietic, and endocrine abnormalities. However, the cells and organs that are most severely affected are those that have the highest energy consumption, namely the brain and skeletal and cardiac muscle (mitochondrial encephalomyopathies). The major mitochondrial disorders have distinct core phenotypes but show also markedly varied and overlapping clinical features. Some mitochondrial disorders, e.g., Leber Hereditary Optic Neuropathy, affect a single organ. Most cause multi-organ dysfunction with prominent neurological abnormalities and muscle disease.
The neurological abnormalities include loss of vision and hearing, migraine headaches, seizures and myoclonus, focal neurological deficits, encephalopathy, psychomotor retardation, dementia, ataxia, spasticity, motor neuron disease, system degenerations, and peripheral neuropathy. Muscle disease may present with weakness, exercise intolerance, rhabdomyolysis, myopathic face, chronic fatigue, a fibromyalgia-like picture, and anormal EMG. Extraocular muscles are especially susceptible because they have a high proportion of type 1 (oxidative) fibers. Thus, ptosis and ophthalmoplegia are very common in mitochondriopathies.

LABORATORY DIAGNOSIS OF MITOCHONDRIAL DISORDERS

The laboratory investigation of mitochondrial disorders includes determination of lactic acid and lactate/pyruvate ratio in blood and CSF, a muscle biopsy with Gomori trichrome, Succinic Dehydrogenase (SDH), and Cytochrome C Oxidase (COX) histochemistry, enzyme analysis of muscle tissue for respiratory chain defects, and mitochondrial DNA analysis.

If clinical and laboratory findings suggest a mitochondrial disorder, a muscle biopsy with metabolic studies of the respiratory chain can confirm the diagnosis. Muscle tissue, blood cells, cultured skin fibroblasts, and cells obtained form urine sediment can be used to detect the DNA abnormalities of mitochondrial disorders. Many children with unexplained encephalopathy and acidosis are suspected to have mitochondrial or organic acid disorders and are subjected to extensive laboratory tests, including muscle biopsies. Most of the time, this work-up is negative. It is worth pointing out that the clinical manifestations and MRI findings of metabolic disease are nonspecific and the most common causes of lactic acidosis are hypoxic-encephalopathy and sepsis.

PATHOLOGY OF MITOCHONDRIAL DISORDERS

Several mitochondrial disorders are accompanied by a massive proliferation and enlargement of mitochondria in myofibers. On electron microscopic examination, these mitochondria are large and structurally abnormal. They have a concentric or other unusual arrangement of their cristae and some of them contain crystal-like inclusions.

Ragged red fibers Ragged red fibers. Gomori trichrome

Abnormal mitochondria in RRF Mitochondrial proliferation in a ragged red fiber.

These mitochondrial clusters appear as red subsarcolemmal deposits in cryostat sections of muscle stained with Gomori trichrome. Myofibers with such deposits are called ragged red fibers (RRFs). RRFs are the hallmark of mitochondrial disorders and occur in no other metabolic disease but are only present in about one-third of them. They may be few or may be the majority of myofibers in a biopsy. Thus absence of RRFs does not rule out a mitochondrial disorder. RRFs occur in large deletions and point mutations of mtDNA and in mutations of nDNA that cause multiple mtDNA deletions or reduction of mtDNA copy number.  The common denominator of these conditions is impairment of intramitochondrial protein synthesis.  They are less abundant or absent in mutations involving protein-coding genes.  COX is synthesized in mitochondria. Therefore, RRFs in mutations affecting protein synthesis are deficient in COX activity. RRFs occurring in mutations of ETC encoding genes (except for those that encode COX components) are generally COX-positive. RRFs are succinic SDH hyperreactive. The SDH reaction product is deep blue, hence RRFs are also ragged blue fibers. Proliferation and hypertrophy of mitochondria probably represents compensatory hypertrophy in an effort to overcome biochemical defects of these mitochondria.   

Biochemical analysis of the muscle biopsy may reveal ETC abnormalities in cases without RRFs. RRFs arise also from drug-induced mtDNA damage such as in zidovudine treatment, and occur in inclusion body myositis and polymyalgia rheumatica. In these conditions, they are thought to be due to mtDNA damage induced by free radicals.

The CNS pathology of mitochondrial disorders (see table below) affects gray and white matter. Gray matter lesions consist of hypoxic-ischemic neuronal changes affecting individual or groups of neurons (MELAS), neuronal loss (MERFF), and a vacuolization and vascular proliferation of the neuropil with relative sparing of neurons (LS). The white matter pathology is spongy myelinopathy, seen mainly in the KSS.

The most common mitochondrial disorders are listed below.

LEIGH SYNDROME (LS)
Leigh syndrome. Dark areas of degeneration of neural tissue around the aqueduct. Image provided by Dr. Maie Herrick.	Leigh syndrome. Lesions of the medulla. Image provided by Dr. Maie Herrick.

Definition and clinical findings: A childhood mitochondrial encephalopathy that damages primarily the brainstem and basal ganglia causing hypotonia, ophthalmoplegia, nystagmus, and psychomotor regression. Mean age of death is 5 years.

Genetics: Most LS cases are autosomal recessive and are caused by nDNA mutations affecting ETC proteins and pyruvate dehydrogenase. X-linked and maternal inheritance is seen in some cases.

Pathological findings: Congestion, softening, and atrophy of basal ganglia and brainstem tectum and tegmentum. Attenuation of the neuropil, spongiosis and vascular proliferation with relative preservation of neurons. The topography of the lesions is somewhat similar to the Wernicke-Korsakoff syndrome. No ragged red fibers are seen in most cases

KEARNS-SAYRE SYNDROME (KSS) AND CHRONIC PROGRESSIVE EXTERNAL OPHTHALMOPLEGIA (CPEO).

Definition and clinical findings: A mitochondrial disorder caused by large mtDNA deletions, characterized clinically by ophthalmoplegia, weakness, ataxia, pigmentary retinopathy, loss of hearing, dementia, and seizures. The numerous non-neurological manifestations of KSS include hypertrophic and dilated cardiomyopathy, cardiac conduction abnormalities, impaired GI motility, diabetes mellitus and other endocrine abnormalities, short stature, and renal dysfunction.

Genetics: Most KSS and CPEO cases are sporadic and are caused by new mtDNA deletions that wipe out a large amount (up to 50%) of the mitochondrial genome including tRNA genes, thus affecting mitochondrial protein synthesis. 

Pathological findings: RRFs and spongy myelinopathy

MITOCHONDRIAL ENCEPHALOMYOPATHY WITH LACTIC ACIDOSIS AND STROKELIKE EPISODES(MELAS)

MELAS MELAS. Cortical atrophy and abnormal FLAIR signal in the insula bilaterally. The abnormal signal corresponds to lesions like those illustrated on the right.

MELAS. Small infarct. MELAS. Small infarcts (arrows). Same patient as the MRI on the left.

Definition and clinical findings: A mitochondrial disorder characterized by encephalopathy (seizures, dementia), recurrent stroke-like episodes at a young age, myopathy, and lactic acidosis. Ataxia, deafness, pigmentary retinopathy and short stature is seen in some patients.

Genetics: Maternally inherited mtDNA mutations involving the tRNA leucine gene cause 80% of MELAS cases.

Pathological findings: COX-positive RRFs, cortical and subcortical infarct-like lesions, pseudolaminar necrosis, and basal ganglia mineralization

MYOCLONIC EPILEPSY WITH RAGGED RED FIBERS (MERRF).

Definition and clinical findings: A mitochondrial disorder characterized by myoclonus, epilepsy, ataxia, and dementia.

Genetics: Maternally inherited mtDNA mutations affecting the tRNA lysine gene cause 90% of MERRF cases.

Pathological findings: RRFs and system degenerations involving cerebellar, brainstem, and spinal cord tracts and nuclei

LEBER HEREDITARY OPTIC NEUROPATHY (LHON)

LHON LHON optic nerve, myelin stain. Loss of myelin and axons more severe in the central and temporal (loer right) portion of the nerve, consistent with degeneration of the papillomacular bundle.

Definition and clinical findings: A mitochondrial disorder that causes painless progressive loss of central vision. Some LHON patients also have dystonia, pseudobulbar palsy, intellectual deterioration, and muscle weakness. Many patients have the Wolff-Parkinson-White syndrome. Female LHON patients may have multiple sclerosis-like manifesations.

Inheritance: LHON is maternally inherited and shows a male prevalence. It is caused by mtDNA mutations that encode subunits of ETC complex I.

Pathological findings: Loss of retinal ganglion cells in the perifoveal region (macula densa) and degeneration of the papillomacular bundle. There are no RRFs

Mitochondrial disorders are progressive and show significant clinical and genetic variability and overlap. Some phenotypes are caused by defects of mtDNA, others by defects of nDNA, and some by both. Similar syndromes can be caused by a variety of mutations and any given mutation may have a variable phenotype.

The spectrum of genetic mitochondrial disease in neurology and medicine is expanding. Additionally, mitochondrial DNA damage from free radicals can develop in absence of a genetic disorder. Because mitochondria replicate even in postmitotic cells such as neurons, such damage is propagated intracellularly and may get worse with time. Mitochondrial DNA damage and free radicals have been implicated in the pathogenesis of HIE, neurodegenerative diseases, and the gradual deterioration of tissues that occurs with advancing age.