CHAPTER TEN
INHERITED METABOLIC DISORDERS

MITOCHONDRIAL DISORDERS

ENERGY PRODUCTION AND FREE RADICALS

This page describes conditions caused by abnormalities of mitochondrial DNA. These disorders 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 enzymes (Complexes I-V), 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 oxphos 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. Oxphos dysfunction increases free radical production. This creates a vicious cycle which increases mitochondrial DNA (mtDNA) damage and causes more oxphos 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 86 proteins. Seventy three of these are encoded by nuclear genes, synthesized in the RER, and imported into the mitochondria. Thirteen ETC proteins are encoded by mtDNA and are synthesized in the mitochondria. In addition, the mitochondrial genome contains 2 genes encoding rRNAs and 22 genes encoding tRNAs.

Mutations of rRNA and tRNA and large mt DNA deletions affect mitochondrial protein synthesis as a whole and cause deficiencies of multiple ETC complexes. Mutations of genes encoding ETC proteins affect the ETC complex in which these proteins occur.

Nuclear gene defects are transmitted in a mendelian fashion: most are autosomal recessive. Abnormalities of mitochondrial genes are transmited from mother to offspring (maternal inheritance) because only the ovum has mitochondria. 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 rapidly such that, after several cell cycles, a given cell may come to contain mostly mutant mtDNA (replicative segregation). Cellular dysfunction develops when the proportion of mutant mtDNA exceeds a certain threshold. 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 200 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 mycolonus, focal neurological deficits, encephalopathy, psychomotor retardation, ataxia, spasticity, motor neuron disease, system degenerations, and peripheral neuropathy. Muscle disease may also present with weakness, exercize intolerance, rhabdomyolysis, myopathic face, chronic fatigue, a fibromuscular dysplasia-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.

Almost any unexplained neurological disorder in a child or young adult, especially if it has a component of muscle disease, could be a mitochondrial disorder. However, many other genetic neurometabolic disorders may have a similar clinical picture. The differential diagnosis can be narrowed down with the use of laboratory and imaging studies.

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, and Cytochrome C Oxidase histochemistry, enzyme analysis of muscle tissue for respiratory chain defects, and mitochondrial DNA analysis.

NEUROIMAGING FINDINGS Abnormal signal in the basal ganglia, basal ganglia calcification, cerebral and cerebellar atrophy, bilateral striatal necrosis, cerebellar hypoplasia, infarcts, leukoencephalopathy

LABORATORORY FINDINGS Lactic acidosis, elevated lactate/pyruvate ratio in blood and CSF, elevated alanine in blood and CSF, elevated CK, myoglobinuria

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.

PATHOLOGY OF MITOCHONDRIAL DISORDERS

Several mitochondrial disorders are accompanied by a massive reactive 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	Abnormal mitochondria in RRF

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 regarded as the hallmark of mitochondrial disorders but are only present in about one-third of them. They occur in point mutations and large mtDNA deletions that impair mitochondrial protein synthesis. Less frequently RRFs appear in mutations of genes encoding ETC complexes. RRFs in mutations affecting protein synthesis are deficient in Cytochrome C Oxidase (COX) activity. COX is an electron transport carrier and is synthesized in mitochondria. RRFs occurring in mutations of ETC encoding genes (except for those that encode COX components) are generally COX-positive.

Biochemical analysis of the muscle biopsy may reveal respiratory chain 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 which they are thought to be caused by 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 Leigh syndrome Definition and clinical findings: A childhood mitochondrial encephalopathy that damages primarily the brainstem and basal ganglia causing hypotonia, ophthalmoplegia, nystagmus, and psychomotor regression. Genetics: Most LS cases are autosomal recessive and are caused by nuclear gene mutations affecting ETC proteins and pyruvate dehydrogenase. Pathological findings : Congestion, softening, and atrophy of basal ganglia and brainstem tectum and tegmentum. Attenuation of the neuropil 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 sporadic mitochondrial disorder caused by large mtDNA deletions, characterized clinically by ophthalmoplegia, weakness, ataxia, loss of vision and 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. Pathological findings: RRFs and spongy myelinopathy. MITOCHONDRIAL ENCEPHALOMYOPATHY WITH LACTIC ACIDOSIS AND STROKELIKE EPISODES(MELAS) MELAS MELAS-small infarct Definition and clinical findings: A mitochondrial disorder characterized by lactic acidosis, migraine headaches, seizures, recurrent unexplained strokes, psychomotor retardation, ataxia, deafness, and pigmentary retinopathy. Short stature is seen in some patients. Inheritance: Maternally inherited mtDNA mutations involving the tRNA leucine gene. Pathological findings: 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 myoclonic epilepsy, ataxia, and dementia. Genetics: Maternally inherited mtDNA mutations affecting the tRNA lysine gene. Pathological findings: RRFs and neuronal and tract degenerations involving multiple systems. LEBER HEREDITARY OPTIC NEUROPATHY (LHON) Definition and clinical findings: A mitochondrial disorder that causes painless progressive loss of central vision. Some LHON patients also have multiple sclerosis-like manifesations, dystonia, pseudobulbar palsy, intellectual deterioration, and muscle weakness. Many patients have the Wolff-Parkinson-White syndrome. 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. Many mtDNA mutations have well defined phenotypes but show also significant clinical and genetic variability and overlap. Some of the above entities are caused by defects of mitochondrial DNA and others by defects of nuclear DNA. Some have different versions that are caused by either. Similar syndromes can be caused by defects of different enzyme complexes and the same defect of nuclear DNA may be inherited in an autosomal dominant or recessive fashion.

The spectrum of genetic mitochondrial disease in neurology and medicine is expanding rapidly. Additionally, mitochondrial DNA damage from free radicals can developin 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 oftissues that occurs with advancing age.

Further reading:

DiMauro S, Davidzon G. Mitochondrial DNA and disease. Ann Med 2005; 37:222-32. PubMed

Morava E, van den Heuvel l, Hol f et al. Mitochondrial disease criteria. Diagnostic applications in children. Neurology 2006;67:1823-6. PubMed

Updated: January, 2007