Mitochondrial Diseases

Indications for Testing

Children with multiple complex neurologic symptoms 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
Asymptomatic, 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
- Serum
  - Chemistry panel
  - Liver function studies
  - Pyruvate concentration/lactate
  - Ammonia
  - Creatinine kinase (MM) – rarely elevated
  - Plasma acylcarnitine
  - Ketone
  - Fasting glucose
  - Amino acids
  - Coenzyme Q – deficient isolated myopathy, cerebellar ataxia, encephalomyopathy, Leigh syndrome
- Urine
  - Urinalysis
  - Organic acids
  - Amino acids
- Cerebrospinal fluid
  - Routine studies
  - Lactate acid/pyruvate concentration
  - Amino acids
Molecular genetic testing
- Molecular testing for mtDNA mutations may require testing be performed 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
- Coenzyme Q
- 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
  - Metabolic diseases
  - 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

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 1/8,500
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 a discrete clinical syndrome based on characteristic clinical features; however, clinical overlap occurs
Some mitochondrial disorders only affect a single organ, such as in 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 Dermatological – 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, vomiting, pseudoobstruction Hematological – anemia, pancytopenia Hepatic – hepatic failure (very sensitive to valproate) Musculoskeletal – weakness, myopathy Neurological – myopathy (proximal > distal, upper extremities > lower), developmental delay, ataxia, spasticity, dystonia, hypotonia, bulbar signs, chorea, seizures, myoclonus, stroke Ophthalmological – optic atrophy, retinitis pigmentosa, ptosis, diplopia, cataract Otological – 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 Neurological – migraine, stroke, seizures, dementia, myopathy, peripheral neuropathy, ataxia, speech disturbances, bulbar signs, myoclonus, tremor Ophthalmological – optic atrophy, cataracts, progressive external ophthalmoplegia, ptosis, pigmentary retinopathy, vision loss, diplopia Otological – sensorineural deafness Reproductive – pregnancy loss in mid to late gestation, hypogonadism Respiratory – respiratory failure, nocturnal hypoventilation, recurrent aspiration, pneumonia

Features of Mitochondrial DNA-Associated Diseases

Children

Cardiac – biventricular hypertrophic cardiomyopathy, rhythm abnormalities, cardiac murmur, sudden death
Dermatological – 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, vomiting, pseudoobstruction
Hematological – anemia, pancytopenia
Hepatic – hepatic failure (very sensitive to valproate)
Musculoskeletal – weakness, myopathy
Neurological – myopathy (proximal > distal, upper extremities > lower), developmental delay, ataxia, spasticity, dystonia, hypotonia, bulbar signs, chorea, seizures, myoclonus, stroke
Ophthalmological – optic atrophy, retinitis pigmentosa, ptosis, diplopia, cataract
Otological – 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
Neurological – migraine, stroke, seizures, dementia, myopathy, peripheral neuropathy, ataxia, speech disturbances, bulbar signs, myoclonus, tremor
Ophthalmological – optic atrophy, cataracts, progressive external ophthalmoplegia, ptosis, pigmentary retinopathy, vision loss, diplopia
Otological – 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
- Leber hereditary optic neuropathy (LHON)
- Chronic progressive external ophthalmoplegia
- Maternally inherited diabetes and deafness
- Non-syndromic maternally inherited deafness
Examples of typically sporadic disorders caused by mtDNA deletions (can be maternally transmitted)
- Kearns-Sayre syndrome (KSS)
- Pearson syndrome
Presence of substantial intellectual disability or significant dysmorphic features should steer testing toward other disorders

Tests generally appear in the order most useful for common clinical situations

Test name: Mitochondrial Disorders Panel (mtDNA and 108 Nuclear Genes) Sequencing and Deletion/Duplication

ARUP #: 2006054

Methodology: Massive Parallel Sequencing

Use:

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

Limitations:

Diagnosis of mitochondrial disorders may be tissue specific

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

Heteroplasmy levels of <10% will not be detected

Test name: Mitochondrial Disorders (mtDNA) Sequencing

ARUP #: 2006065

Methodology: Massive Parallel Sequencing

Use:

Diagnose mtDNA disorders caused by mutations within the mitochondrial genome

Limitations:

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

Heteroplasmy levels of <15% will not be detected

Mutation within the nuclear genes will not be detected

Test name: Mitochondrial Disorders (108 Nuclear Genes) Sequencing

ARUP #: 2006050

Methodology: Massive Parallel Sequencing

Use:

Diagnose mitochondrial disorders caused by mutations within nuclear genes

Limitations:

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

Mutation within the mitochondrial genome will not be detected

Test name: Mitochondrial Genome (mtDNA and 108 Nuclear Genes) Deletion/Duplication

ARUP #: 2006061

Methodology: Exonic Oligonucleotide-based CGH Microarray

Use:

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

Limitations:

Heteroplasmy levels of <15% will not be detected

Point mutation within the mitochondrial genome and nuclear genes will not be detected

Test name: Ketones, Serum or Plasma

ARUP #: 0020034

Methodology: Visual

Use:

Rule out other metabolic disorders

Test name: Lactic Acid, Plasma

ARUP #: 0020045

Methodology: Enzymatic

Use:

Rule out other metabolic disorders

Test name: Lactic Acid, CSF

ARUP #: 0020516

Methodology: Enzymatic

Use:

Rule out other metabolic disorders

Test name: Pyruvic Acid

ARUP #: 0080310

Methodology: Quantitative Enzymatic

Use:

Rule out other metabolic disorders

Test name: Carnitine Panel

ARUP #: 0081110

Methodology: Tandem Mass Spectrometry

Use:

Rule out other metabolic disorders

Test name: Organic Acids, Plasma

ARUP #: 0099289

Methodology: Gas Chromatography/Mass Spectrometry

Use:

Rule out other metabolic disorders

Limitations:

Urine assay is preferred test when screening for inherited disorders of metabolism; plasma assay is not as sensitive

Test name: Organic Acids, Urine

ARUP #: 0098389

Methodology: Gas Chromatography/Mass Spectrometry

Use:

Rule out other metabolic disorders

Test name: Amino Acids Quantitative, Plasma

ARUP #: 0080710

Methodology: Ion Exchange Chromatography

Use:

Rule out other metabolic disorders

Test name: Amino Acids Quantitative, Urine

ARUP #: 0080044

Methodology: Ion Exchange Chromatography

Use:

Rule out other metabolic disorders

Test name: Acylcarnitine Quantitative Profile, Plasma

ARUP #: 0040033

Methodology: Tandem Mass Spectrometry

Use:

Rule out other metabolic disorders

Test name: Glucose, Plasma or Serum

ARUP #: 0020024

Methodology: Quantitative Enzymatic

Use:

Rule out other metabolic disorders

Additional Tests Available

Test name: Urinalysis, Complete

ARUP #: 0020350

Methodology: Reflective Photometry/Microscopic by Yellow IRIS

Comments:

Guidelines

Finsterer J, Harbo HF, Baets J, Van Broeckhoven C, Di Donato S, Fontaine B, De Jonghe P, Lossos A, Lynch T, Mariotti C, Schols L, Spinazzola A, Szolnoki Z, Tabrizi SJ, Tallaksen CM, Zeviani M, Burgunder JM, Gasser T. EFNS guidelines on the molecular diagnosis of mitochondrial disorders. Eur J Neurol. 2009; 16 (12) :1255-1264.PubMed

General

ARUP Institute for Clinical and Experimental Pathology®

Comprehensive Review: August 2011
Last Update: February 2012

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Developmental Delay (DD) or Intellectual Disability (ID) Testing

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Seizure Disorders