Each cell has 1000 to 2500 mitochondria in its cytoplasm. Mitochondria contain DNA in a single circular chromosome containing 37 genes that code for 13 proteins, various RNAs, and several regulating enzymes. However, > 90% of mitochondrial proteins are coded by nuclear genes. For practical purposes, all mitochondria are inherited from the cytoplasm of the egg; thus, mitochondrial DNA comes only from the mother.
Mitochondrial disorders (see also Mitochondrial Oxidative Phosphorylation Disorders Mitochondrial Oxidative Phosphorylation Disorders Impairment of oxidative phosphorylation often, but not always, causes lactic acidosis, particularly affecting the central nervous system, retina, and muscle. See also Approach to the Patient... read more ) can be due to mitochondrial or nuclear DNA abnormalities (eg, deletions, duplications, mutations Mutations and polymorphisms Certain situations represent aberrant inheritance, often because genes or chromosomes are altered. However, some of these alterations, such as mosaicism, are very common; others, such as polymorphisms... read more ). High-energy tissues (eg, muscle, heart, brain) are particularly at risk of malfunction due to mitochondrial abnormalities. Particular mitochondrial DNA abnormalities result in characteristic manifestations (see table ). Mitochondrial disorders are equally common among males and females.
(See also Overview of Genetics Overview of Genetics A gene, the basic unit of heredity, is a segment of DNA containing all the information necessary to synthesize a polypeptide (protein) or a functional RNA molecule. Protein synthesis, folding... read more .)
Pearls & Pitfalls
Mitochondrial abnormalities may occur in many common disorders such as some types of Parkinson disease Parkinson Disease Parkinson disease is a slowly progressive, degenerative disorder characterized by resting tremor, stiffness (rigidity), slow and decreased movement (bradykinesia), and eventually gait and/or... read more (which may involve large mitochondrial deletions in the cells of the basal ganglia) and many types of muscle disorders.
Some Mitochondrial Disorders
Maternal inheritance patterns characterize abnormalities of mitochondrial DNA. Thus, all offspring of an affected female are at risk of inheriting the abnormality, but no offspring of an affected male are at risk. Variability in clinical manifestations is the rule, and these abnormalities mimic a broad range of disorders, which often makes diagnosis extremely difficult. Variability may be due in part to variable mixtures of inherited mutant and normal mitochondrial genomes within cells and tissues.
Mitochondrial disorders have maternal inheritance patterns.
Because clinical manifestations can be subtle and variable, diagnosis can be very difficult.