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

  • Specific biochemical testing
    • Analysis of electron transport chain activity
    • ATP synthesis measures in fibroblasts
    • Biochemical results may suggest further 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 BCS1L, MTCYB, 10 nuclear structural genes
      • Complex IV deficiency – analysis of mitochondrial DNA cytochrome coxidase assembly factors (COX10, COX15, SCO1, SCO2, SURF1)
      • Complex V deficiency – analysis of ATPAF2
      • Multiple complex deficiencies – analysis of mitochondrial DNA and nuclear DNA mitochondrial maintenance and translation genes
      • Coenzyme Q deficiency – analysis of APTX, CABC1, COQ2, COQ9, ETFDH, PDSS1, PDSS2
  • Muscle biopsy
    • Light microscopy – histochemistry
      • Detection of ragged red fibers (most common in mitochondrial mutations) by Gomori trichrome stain
        • 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
    • Other acceptable tissues – liver, cardiac, and 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 in U.S.
  • Age – all
  • 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
    • Tissues most affected are dependent upon aerobic metabolism or have a high energy requirement
  • 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; valproate sensitivity
    • Musculoskeletal – weakness, myopathy (proximal>distal, upper extremities>lower)
    • Neurologic – developmental delay, ataxia, spasticity, dystonia, hypotonia, bulbar signs, chorea, seizures, myoclonus, stroke-like episodes, encephalopathy
    • Ophthalmologic – optic atrophy, retinitis pigmentosa, ptosis, diplopia, cataract, vision loss
    • 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
Common mitochondrial syndromes

Common Mitochondrial Syndromes

Disorder

Major Clinical Features

Inheritance

Commonly Associated Genes/Mutations

Chronic progressive external ophthalmoplegia (CPEO)

External ophthalmoplegia

Bilateral ptosis

Mild proximal myopathy

Sporadic

mtDNA large deletions confined to skeletal muscle in ~50% of cases

Maternal

Various mtDNA point mutations

Autosomal dominant/

recessive

C10orf2, OPA1, POLG, POLG2, RRMB2, SLC25A4, SPG7, TYMP, etc.

Kearns-Sayre syndrome (KSS)

PEO onset <20 years

Pigmentary retinopathy

CSF protein >1g/L

Cerebellar ataxia

Heart block

Sporadic (infrequently maternally transmitted)

mtDNA large deletions detected in ~90% of cases

m.8470_1344del4977 is most frequent

Leber hereditary optic neuropathy (LHON)

Subacute painless bilateral visual failure

Median age of onset 24 years

Males:females 4:1

Maternal

90% of cases due to one of three common mtDNA mutations

MT-ND4 m.11778G>A

MT-ND6 m.14484T>C

MT-ND1 m.3460G>A

Mutations in other mtDNA genes are rare causes of LHON

Leigh syndrome (LS)

Progressive neurological disease with motor and intellectual developmental delay

Cerebellar and brain stem signs

Infantile onset

Maternal

mtDNA large deletions causative for <5% of LS

MT-ATP6 (10% of cases)

Other mtDNA genes (10-20% of cases)

Autosomal recessive or X-linked recessive

Majority of cases due to nuclear gene mutations (numerous genes implicated)

Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS)

Stroke-like episodes at age <40 years

Encephalopathy with seizures and/or dementia

Mitochondrial myopathy, evidenced by lactic acidosis and/or ragged-red fibers

Maternal

MT-TL1 (accounts for majority of cases)

m.3243A>G

m.3271T>C

m.3252A>G

MT-ND5

m.13513G>A

Mutations in other mtDNA genes are rare

Myoclonic epilepsy with ragged-red fibers (MERRF)

Myoclonus

Seizures

Cerebellar ataxia

Myopathy

Maternal

MT-TK (accounts for ~90% of cases)

m.8344A>G

m.8356T>C

m.8363G>A

m.8361G>A

Causative mutations in other mtDNA genes, including MT-TP and MT-TF, account for <5% of cases

Neurogenic weakness with ataxia and retinitis pigmentosa (NARP)

Late-childhood or adult-onset peripheral neuropathy

Ataxia

Pigmentary retinopathy

Maternal

MT-ATP6 (estimated to account for >50% of cases)

m.8993T>G

m.8993T>C

Pearson syndrome

Sideroblastic anemia of childhood

Pancytopenia

Exocrine pancreatic failure

Sporadic (infrequently maternally transmitted)

mtDNA large deletions

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 by Sequencing and Deletion/Duplication, 121 Nuclear Genes by Sequencing, 119 Nuclear Genes by Deletion/Duplication) 2006054
Method: Massive Parallel Sequencing/Exonic Oligonucleotide-Based CGH Microarray

Preferred test for identifying deletions, duplications, and point mutations in all 37 mitochondrial genes and in 108 nuclear genes known to cause mitochondrial disease

Refer to Additional Technical Information document for list of genes tested

Diagnosis of mitochondrial disorders may be tissue specific

Not detected

  • Mutations in genes not analyzed
  • Regulatory region and deep intronic mutations
  • Nuclear DNA mutations and large deletions/duplications within the mitochondrial genome

Not reported

  • Variants in the mitochondrial D-loop
  • Mosaic mutations in nuclear genes

mtDNA mutations present at <10% heteroplasmy may not be detected

Sequencing may detect variants of unknown clinical significance

Diagnostic errors can occur due to rare sequence variations

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

Assess for sequence variants in the mitochondrial genome (mtDNA) causing mitochondrial disorders

Diagnosis of mitochondrial disorders may be tissue specific

Not detected

  • Mutations in genes not analyzed
  • Regulatory region and deep intronic mutations
  • Mutations within the nuclear genes

Not reported

  • Variants in the mitochondrial D-loop

mtDNA mutations present at <10% heteroplasmy may not be detected

Sequencing may detect variants of unknown clinical significance

Diagnostic errors can occur due to rare sequence variations

 
Mitochondrial Disorders (121 Nuclear Genes) Sequencing 2006050
Method: Massive Parallel Sequencing

Assess for nuclear gene mutations causing mitochondrial disease

Refer to Additional Technical Information document for list of genes tested

Not detected

  • Mutations in genes not analyzed
  • Regulatory region and deep intronic mutations
  • Mutations within the mitochondrial genome

Not reported

  • Mosaic mutations in nuclear genes

Sequencing may detect variants of unknown clinical significance

Diagnostic errors can occur due to rare sequence variations

 
Mitochondrial Disorders (mtDNA and 119 Nuclear Genes) Deletion/Duplication 2006061
Method: Exonic Oligonucleotide-based CGH Microarray

Assess for large deletions/duplications in the mtDNA and nuclear genes associated with mitochondrial disease

Refer to Additional Technical Information document for list of genes tested

Not detected

  • Mutations in genes not analyzed
  • Regulatory region and deep intronic mutations
  • Point mutations within the mitochondrial genome and nuclear genes

Not reported

  • Variants in the mitochondrial D-loop
  • Mosaic mutations in nuclear genes

mtDNA mutations present at <10% heteroplasmy may not be detected

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

Aid in excluding conditions that may be clinically similar to 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, Serum or Plasma 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 (121 Nuclear Genes by Sequencing, 119 Nuclear Genes by Deletion/Duplication) 2006878
Method: Massive Parallel Sequencing/Exonic Oligonucleotide-based CGH Microarray

Diagnose mitochondrial disorders resulting from mutations within nuclear genes

Refer to Additional Technical Information document for list of genes tested

Mitochondrial Disorders (mtDNA) Sequencing and Deletion/Duplication 2006872
Method: Massive Parallel Sequencing/Exonic Oligonucleotide-based CGH Microarray

Diagnose mitochondrial disorders caused by mutations within the mitochondrial genome (mtDNA)  

Creatine Kinase, Total, Serum or Plasma 0020010
Method: Quantitative Enzymatic

Confirm muscle involvement