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

Laboratory Testing

  • Metabolic evaluation generally precedes molecular genetic testing unless a specific disorder is suspected from clinical presentation
    • 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 in 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 and 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

Imaging Studies

  • CT of head
    • Often normal
    • May demonstrate punctate calcifications
    • May see edema or atrophy – cerebral or cerebellar
  • MRI of head
    • 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, SDHC, 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

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 deletions generally occur de novo
    • mtDNA defects, 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
Test Name and Number Recommended Use Limitations Follow Up
Mitochondrial Disorders Panel (mtDNA and 108 Nuclear Genes) Sequencing and Deletion/Duplication 2006054
Method: Massive Parallel Sequencing/Exonic Oligonucleotide-Based CGH Microarray

Diagnose mitochondrial disorders caused by mutations within the mitochondrial genome and nuclear genes

Diagnosis of mitochondrial disorders may be tissue specific

Nuclear DNA mutations and large deletions/duplications within the mitochondrial genome are not detected

Heteroplasmy levels of <10% are not detected

 
Mitochondrial Disorders (mtDNA) Sequencing 2006065
Method: Massive Parallel Sequencing

Diagnose mtDNA disorders caused by mutations within the mitochondrial genome

Large deletions/duplications within the mitochondrial genome and nuclear genes will not be detected

Heteroplasmy levels of <15% are not detected

Mutation within the nuclear genes are not detected

 
Mitochondrial Disorders (108 Nuclear Genes) Sequencing 2006050
Method: Massive Parallel Sequencing

Diagnose mitochondrial disorders caused by mutations within nuclear genes

Large deletions/duplications within the mitochondrial genome and nuclear genes are not detected

Mutation within the mitochondrial genome are not detected

 
Mitochondrial Genome (mtDNA and 108 Nuclear Genes) Deletion/Duplication 2006061
Method: Exonic Oligonucleotide-based CGH Microarray

Diagnose mitochondrial disorders caused by deletions and duplications within the mitochondrial genome and nuclear genes

Heteroplasmy levels of <15% are not detected

Point mutation within the mitochondrial genome and nuclear genes are not detected

 
Cytogenomic SNP Microarray 2003414
Method: Genomic Microarray (Oligo-SNP Array)

Aid in excluding conditions that present as mitochondrial disorders

   
Additional Tests Available
 
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
Carnitine Panel 0081110
Method: Tandem Mass Spectrometry

Rule out other metabolic disorders

Panel includes free, total, and esterified carnitine; esterified/free ratio; and acylcarnitine

Beta-Hydroxybutyric Acid 0080045
Method: Quantitative Enzymatic

Screening test for evaluation of suspected mitochondrial disorders

Lactic Acid, CSF 0020516
Method: Enzymatic

Screening test for evaluation of suspected mitochondrial disorders

Lactic Acid, Plasma 0020045
Method: Enzymatic

Screening test for evaluation of suspected mitochondrial disorders

Pyruvic Acid 0080310
Method: Quantitative Enzymatic

Aid in diagnosis of mitochondrial disorders individually or when combined with plasma lactic acid testing

Organic Acids, Urine 0098389
Method: Gas Chromatography/Mass Spectrometry

Screening test for evaluation of suspected mitochondrial disorders

Amino Acids Quantitative by LC-MS/MS, Plasma 2009389
Method: Quantitative Liquid Chromatography/Tandem Mass Spectrometry

Screening test for evaluation of suspected mitochondrial disorders

Amino Acids Quantitative by LC-MS/MS, Urine 2009419
Method: Quantitative Liquid Chromatography/Tandem Mass Spectrometry

Screening test for evaluation of suspected mitochondrial disorders

Acylcarnitine Quantitative Profile, Plasma 0040033
Method: Tandem Mass Spectrometry

Rule out other metabolic disorders

Hepatic Function Panel 0020416
Method: Quantitative Enzymatic/Quantitative Spectrophotometry

Identify hepatic dysfunction

Panel includes albumin; ALP; AST; ALT; bilirubin, direct; protein, total; and bilirubin, total

Ammonia, Plasma 0020043
Method: Colorimetry

Screening test for evaluation of suspected mitochondrial disorders

Orotic Acid and Orotidine, Urine 0092458
Method: Liquid Chromatography-Tandem Mass Spectrometry

Screening test for evaluation of suspected mitochondrial disorders

Creatine Disorders Panel, Plasma or Serum 2002328
Method: Liquid Chromatography/Tandem Mass Spectrometry

Screening test for evaluation of suspected mitochondrial disorders

Creatine Disorders Panel, Urine 2002333
Method: Liquid Chromatography/Tandem Mass Spectrometry

Screening test for evaluation of suspected mitochondrial disorders

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