Mitochondrial Diseases

Medical Experts

Contributor

Longo

Nicola Longo, MD, PhD

Professor, Pediatrics; Adjunct Professor of Pathology (Clinical), University of Utah

Chief, Medical Genetics Division; Medical Director, Biochemical Genetics and Newborn Screening, ARUP Laboratories

Contributor

Pasquali

Marzia Pasquali, PhD, FACMG

Professor of Pathology, Adjunct Professor of Pediatrics, University of Utah School of Medicine

Section Chief, Biochemical Genetics; Medical Director, Biochemical Genetics and Newborn Screening, ARUP Laboratories

Mitochondrial diseases are complex, clinically heterogeneous disorders that are caused by dysfunction of the mitochondrial respiratory chain. These diseases are often challenging to diagnose because they can be caused by pathogenic variants in nuclear DNA (nDNA) or mitochondrial DNA (mtDNA). Given that mitochondria are present in almost all cells of the body, clinical symptoms are diverse and can present in isolated organs or with multiple system involvement. There are no specific metabolic markers to identify a particular genetic defect. Initial laboratory tests such as lactate, pyruvate, urine organic acids, and plasma amino acids can guide the clinician toward possible mitochondrial disease and inform the genetic testing strategy. However, initial biochemical tests are not diagnostic, and genetic testing is required for definitive diagnosis.

Quick Answers for Clinicians

Which inheritance patterns are observed for mitochondrial disorders, and how do they affect laboratory testing?

Because mitochondrial disorders can arise from pathogenic variants in nuclear DNA (nDNA) or mitochondrial DNA (mtDNA), inheritance patterns vary widely. Mutations in nDNA can result in autosomal recessive, autosomal dominant, or X-linked recessive inheritance. mtDNA deletions are generally de novo, but can also be inherited from the mother. mtDNA defects, point variations, and duplications are typically maternally inherited. Additionally, individuals with mtDNA variants often display heteroplasmy (the coexistence of normal and mutant mtDNA in the same cell). Inheritance patterns are suggestive of specific mitochondrial disorders and as such can inform the testing strategy.

Indications for Testing

In general, testing for mitochondrial disease should be considered when a patient presents with a progressive multisystem disorder or a cluster of clinical features that indicate a discrete clinical syndrome. Some common clinical features of mitochondrial diseases include ptosis, external ophthalmoplegia, proximal myopathy, exercise intolerance, cardiomyopathy, sensorineural deafness, optic atrophy, pigmentary retinopathy, and diabetes mellitus. The table below lists some common mitochondrial disorders and their major clinical features.

Clinical Features of Common Mitochondrial Disorders
Disorder	Major Clinical Features
Chronic progressive external ophthalmoplegia	External ophthalmoplegia Bilateral ptosis Mild proximal myopathy
Kearns-Sayre syndrome	Progressive external ophthalmoplegia onset <20 years of age Pigmentary retinopathy CSF protein >1 g/L Cerebellar ataxia Heart block (cardiac conduction defect)
Leber hereditary optic neuropathy	Subacute painless bilateral visual failure Median age of onset 24 years Males:females, 4:1
Leigh syndrome	Progressive neurologic disease with motor and intellectual developmental delay Cerebellar and brain stem signs Infantile onset
Mitochondrial encephalomyopathy with lactic acidosis and strokelike episodes	Strokelike episodes <40 years of age Encephalopathy with seizures and/or dementia Mitochondrial myopathy, evidenced by lactic acidosis and/or ragged-red fibers
Myoclonic epilepsy with ragged-red fibers	Myoclonus Seizures Cerebellar ataxia Myopathy
Neurogenic weakness with ataxia and retinitis pigmentosa	Late childhood-onset or adult-onset peripheral neuropathy Ataxia Pigmentary retinopathy
Pearson syndrome	Sideroblastic anemia of childhood Pancytopenia Exocrine pancreatic failure
CSF, cerebrospinal fluid

Laboratory Testing

Diagnosis

Initial Testing

Biochemical laboratory results often raise suspicion of a mitochondrial disorder and can be useful to monitor disease progression in such disorders. The Mitochondrial Medicine Society recommends evaluation of selected mitochondrial biomarkers in blood, urine, and CSF to aid in disease investigation.

Lactate measurements in blood and CSF may be elevated in some mitochondrial disorders; however, normal lactate levels do not exclude the possibility of a mitochondrial disorder. Measurement of lactate both preprandially and postprandially is useful to distinguish a mitochondrial disorder from a glycogen metabolism condition. Additionally, the lactate/pyruvate ratio can help distinguish respiratory chain dysfunction from pyruvate metabolism disorders such as pyruvate dehydrogenase deficiency.

Measurement of urine organic acids can be used to identify lactic aciduria, excess ketones (which build up because they are not properly utilized by dysfunctional mitochondria), and the presence of unusual compounds, which reflects impairment of the metabolic reactions that occur within mitochondria.

Amino acid analysis in blood or CSF can be used to identify an elevation in alanine, glycine, proline, and threonine with respiratory chain dysfunction. Urine amino acid analysis can reveal generalized amino aciduria, a sign of mitochondrial renal tubular dysfunction.

Measurement of plasma acylcarnitines can be used to differentiate between mitochondrial disorders and other causes of lactic acidosis such as organic acidemia and fatty acid oxidation defects. This is particularly useful in patients with phenotypes that overlap with other inborn metabolic errors for which acylcarnitine analysis is diagnostic.

Other markers such as creatine phosphokinase, creatine, carnitine, and albumin may also be useful to evaluate mitochondrial disease and to monitor effects of supplementation. The Mitochondrial Medicine Society provides more information in the Diagnosis and management of mitochondrial disease: a consensus statement from the Mitochondrial Medicine Society.

Genetic Testing

Genetic testing is essential for the diagnosis of mitochondrial diseases. Next generation sequencing with gene dosage of nDNA and mtDNA in blood or affected tissues (muscle, buccal swab, urine sediment, liver biopsy) is recommended over testing for specific point mutations in cases of suspected mitochondrial disease. Tissue-based testing may identify mtDNA mutations, rearrangements, or deletions that are tissue specific. Genetic consultation is also advised.