Mitochondrial Diseases

Diagnosis

Indications for Testing

• Individuals with multiple complex neurologic features or a single neurological symptom with other system involvement
• Children presenting with lactic acidosis
• Individuals with clinical symptoms characteristic of a specific mitochondrial disorder
• Individuals with any progressive multisystem disorder of unknown etiology
• Presymptomatic testing for at-risk family members
• Family history – assess inheritance pattern to help direct molecular testing

Laboratory Testing

• Metabolic evaluation generally precedes molecular genetic testing unless a specific disorder is suspected
  • Blood
    • Chemistry panel
    • Liver function studies
    • Blood lactate/pyruvate ratio
    • Ammonia
    • Creatine kinase (MM fraction) – rarely elevated
    • Plasma acylcarnitine profile
    • Ketone
    • Fasting glucose
    • Plasma amino acids
    • Coenzyme Q – deficient isolated myopathy, cerebellar ataxia, encephalomyopathy, Leigh syndrome
  • Urine
    • Urinalysis
    • Organic acids
    • Amino acids
  • Cerebrospinal fluid
    • Routine studies
    • Lactate/pyruvate ratio
    • Amino acids
• Molecular genetic testing
  • Molecular testing for mitochondrial DNA (mtDNA) mutations may require testing on DNA extracted from skeletal muscle; nuclear gene mutations and some mtDNA mutations can be detected in DNA from peripheral blood
  • Mitochondrial genome mutation scanning/sequencing
  • Mitochondrial genome duplication/deletion testing
  • DNA testing for nuclear genes associated with mitochondrial disorders
  • Targeted testing for a family-specific mutation in at-risk or symptomatic family members
  • For classic syndrome presentation, genetic testing may be initial testing of choice

Imaging Studies

• CT – often normal
  • May demonstrate punctate calcifications
  • May see edema or atrophy (cerebral or cerebellar)
• MRI
  • T2 signal that resembles stroke-like lesions
  • Abnormal myelination

Other Testing

• Biochemistry
  • Analysis of electron transport chain activity
  • ATP synthesis measures in fibroblasts
  • Biochemical results may suggest genetic testing
    • Complex I deficiency – analysis of mitochondrial DNA and nuclear encoded genes
    • Complex II deficiency – analysis of SDHA, SDHB, CDHC, SDHD
    • Complex III deficiency – analysis of MTCYB, 10 nuclear structural genes, BCS1L
    • Complex IV deficiency – analysis of mitochondrial DNA cytochrome coxidase assembly factors (SURF1, SCO1, SCO2, COX10, COX15)
    • Multiple complex deficiencies – analysis of mitochondrial DNA and nuclear DNA mitochondrial maintenance and translation genes
    • Coenzyme Q deficiency – analysis of CABC1, COQ2, COQ9, PDSS1, PDSS2, ETFDH, APTX
• Muscle biopsy
  • Light microscopy – histochemistry
    • Detection of ragged red fibers (most common in mitochondrial mutations) by Gomori trichrome stains
      • Subsarcolemmal accumulation of mitochondria on muscle pathology
    • Cytochrome coxidase-deficient fibers
  • Electron microscopy
    • Increase in mitochondrial number or size, increased lipid and glycogen droplets, increased mitochondrial matrix
  • Can also perform liver, cardiac, or skin biopsy
• Neurophysiologic studies
  • Electroencephalography for individuals with suspected encephalopathy or seizures
  • Electromyography/nerve conduction velocity for individuals with limb weakness, sensory issues, or areflexia
• Electrocardiography/echocardiography
  • Evaluate cardiomyopathy or atrioventricular conduction defects
• Auditory/ophthalmologic examinations to confirm defects

Differential Diagnosis

• Neurological
  • Children/infants
    • Inborn errors of metabolism
    • Hypothyroidism
    • Muscular dystrophy
    • Developmental delay
    • Lysosomal storage diseases
  • Adults
    • Wilson disease
    • Multiple sclerosis
    • Coenzyme Q disease
    • Dementia
    • Migraine disorder
    • Parkinson disease
    • Sarcoidosis
    • Vasculidities
    • Connective tissue diseases
    • Infectious – fungal, viral
    • Chronic demyelinating disease
    • Paraneoplastic syndrome
    • Stroke
    • Seizure disorder
• Endocrinological
  • Diabetes mellitus
  • Thyroid disease (hypo/hyper)
  • Autoimmune adrenal disease
• Hepatic disease
  • Reye syndrome
  • Wilson disease
  • Acute hepatitis B or C
• Myopathy
  • Dermatomyositis
  • Chronic demyelinating inflammatory polyneuropathy
  • Paraneoplastic syndrome
  • Guillain-Barré syndrome
• Lactic acidosis
  • Sepsis
  • Polymyositis
  • Inborn errors of metabolism

Clinical Background

Mitochondrial diseases are a group of disorders originating from mutations in nuclear DNA or mitochondrial DNA (mtDNA) and resulting in a wide spectrum of pathological conditions, often with significant neurologic and myelopathic symptoms. Many commonly seen conditions can be classified as discrete clinical syndromes; however, the presentation and severity of the conditions may vary, creating challenges in diagnosis and treatment.

Epidemiology

• Prevalence – approximately 1/5,000
• Age – all ages
• Sex – M:F, equal

Inheritance

• Mitochondrial disorders may be caused by mutations in nuclear DNA or mtDNA
  • Nuclear gene defects may be inherited in an autosomal recessive or autosomal dominant manner
  • mtDNA defects are maternally transmitted
  • mtDNA deletions generally occur de novo
  • mtDNA point mutations and duplications are maternally inherited
• Affected individuals with mtDNA mutations often have a mixture of mutated and normal mtDNA within each cell (heteroplasmy)
  • Disease severity and the age of onset are affected by the amount of heteroplasmy and the number and type of cells containing the mtDNA mutation
  • Females with heteroplasmy but no clinical symptoms may have affected offspring
• Poor genotype/phenotype correlation exists; the same mutation may cause different clinical syndromes

Pathophysiology

• Mitochondria are ubiquitous, complex, intracellular organelles containing non-nuclear DNA
  • Each cell may contain hundreds to thousands of copies of mtDNA
• Mitochondria are essential in many cell processes, including the generation of adenosine triphosphate during oxidative metabolism
• Mutations in the mitochondrial genome or in nuclear DNA involved in the respiratory chain principally affect tissues that are heavily dependent on oxidative metabolism (eg, central nervous system, cardiovascular, musculoskeletal)

Clinical Presentation

• Many mitochondrial diseases can be classified as discrete clinical syndromes based on characteristic clinical features; however, clinical overlap occurs
• Some mitochondrial disorders affect only a single organ (eg, Leber hereditary optic neuropathy (LHON) and nonsyndromic sensorineural deafness)
• Mitochondrial disorders may present at any age
  • Presentation of nuclear DNA mutations typically occurs in childhood; mtDNA abnormalities are more likely to present in late childhood or adulthood
• Clinical presentation is highly variable
• Features of mitochondrial DNA-associated diseases

  Features of Mitochondrial DNA-Associated Diseases
  Children Cardiac – biventricular hypertrophic cardiomyopathy, rhythm abnormalities, cardiac murmur, sudden death Dermatologic – erythema, lipomatosis, reticular pigmentation, hypertrichosis, vitiligo, alopecia Endocrine – diabetes mellitus, adrenal failure, growth failure, hypothyroidism, hypogonadism, hypoparathyroidism Gastrointestinal – vomiting, failure to thrive, dysphagia, GI motility problems, pseudoobstruction Hematologic – anemia, pancytopenia Hepatic – hepatic failure (very sensitive to valproate) Musculoskeletal – weakness, myopathy Neurologic – myopathy (proximal > distal, upper extremities > lower), developmental delay, ataxia, spasticity, dystonia, hypotonia, bulbar signs, chorea, seizures, myoclonus, stroke Ophthalmologic – optic atrophy, retinitis pigmentosa, ptosis, diplopia, cataract Otologic – sensorineural deafness Renal – renal tubular defects (proximal defect renal tubular acidosis most common), nephrotic syndrome, tubulointerstitial nephritis Respiratory – central hypoventilation, apnea
  Adults Cardiac – heart failure, conduction block, cardiomyopathy, sudden death Endocrine – diabetes, thyroid disease, parathyroid disease Gastrointestinal – constipation, irritable bowel syndrome, dysphagia, anorexia, abdominal pain, diarrhea Musculoskeletal – rhabdomyolysis, muscle weakness, exercise intolerance Neurologic – migraine, stroke, seizures, dementia, myopathy, peripheral neuropathy, ataxia, speech disturbances, bulbar signs, myoclonus, tremor Ophthalmologic – optic atrophy, cataracts, progressive external ophthalmoplegia, ptosis, pigmentary retinopathy, vision loss, diplopia Otologic – sensorineural deafness Reproductive – pregnancy loss in mid to late gestation, hypogonadism Respiratory – respiratory failure, nocturnal hypoventilation, recurrent aspiration, pneumonia

• Examples of inherited mitochondrial disorders caused by nuclear DNA mutations
  • Autosomal recessive external ophthalmoplegia
  • Hypertrophic cardiomyopathy
  • Myoneurogastrointestinal encephalomyopathy
  • Leigh syndrome
  • Mitochondrial depletion syndrome
  • Dominant optic atrophy
• Examples of inherited disorders caused by mtDNA mutations
  • Mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS)
  • Myoclonic epilepsy associated with ragged red fibers (MERRF)
  • Neuropathy, ataxia, retinitis pigmentosa/maternally inherited Leigh syndrome
  • LHON
  • Chronic progressive external ophthalmoplegia
  • Maternally inherited diabetes and deafness
  • Nonsyndromic maternally inherited deafness
• Examples of typically sporadic disorders caused by mtDNA deletions (can be maternally transmitted)
  • Kearns-Sayre syndrome 
  • Pearson syndrome
• Presence of substantial intellectual disability or significant dysmorphic features suggests testing for other disorders

Indications for Laboratory Testing

• Tests generally appear in the order most useful for common clinical situations
• Click on number for test-specific information in the ARUP Laboratory Test Directory

Click the plus sign to expand the table of additional tests.

Test Name and Number	Comments
Urinalysis, Complete 0020350 Method: Reflectance Spectrophotometry/Microscopy
Mitochondrial Disorders (108 Nuclear Genes) Sequencing and Deletion/Duplication 2006878 Method: Massive Parallel Sequencing/Exonic Oligonucleotide-based CGH Microarray
Mitochondrial Disorders (mtDNA) Sequencing and Deletion/Duplication 2006872 Method: Massive Parallel Sequencing/Exonic Oligonucleotide-based CGH Microarray
Creatine Kinase, Total, Serum or Plasma 0020010 Method: Quantitative Enzymatic	Confirm muscle involvement
Amino Acids Quantitative, Urine 0080044 Method: Ion Exchange Chromatography