Mitochondria are the intracellular organelles that supply most of a cell's energy needs by producing adenosine triphosphate (ATP) through oxidative phosphorylation. More recently, mitochondria have also been found to play a central role in programmed cell death, or apoptosis.¹ In addition to these well-known tasks, mitochondria are also responsible for a variety of other metabolic functions specific to the almost 250 different cell types in the human body.²

Mitochondria are unique organelles in that they contain their own DNA (mtDNA), which is distinct from the DNA in the cell nucleus (nDNA). Thus, proper mitochondrial function depends on the coordinated expression of both the nuclear and mitochondrial genomes and therefore, mitochondrial dysfunction can arise from mutations in either genome (Figure 1).

This article is a brief review of mitochondrial genetics, disorders, and their diagnosis for neurologists, geneticists, genetic counselors, and other healthcare professionals involved in the care of people with complex neurological diseases that may be due to mitochondrial dysfunction.

Figure 1. The intricate function of the oxidative-phosphorylation complexes can be disrupted by defects in the subunits encoded by nDNA or by mtDNA or by defects in intergenomic communication between the two. The resulting deficits in the production of ATP have deleterious effects on a number of organ systems, causing the disorders shown. The red bar indicates the site of defects in intergenomic communication (depletion and multiple deletions of mitochondrial DNA). Adapted from Johns, DR: Mitochondrial DNA and Disease. The New England Journal of Medicine; 333: 638-644, 1995.

Mitochondrial DNA is a double-stranded, closed, circular molecule 16.5 kilo bases in length and containing 37 genes.³ These genes are arranged in two strands - a guanine-rich heavy (H) strand, and a cytosine-rich light (L) strand; and except for a short displacement (D) loop, the H strand is composed only of coding regions and therefore, contains no introns (Figure 2). These genes are so tightly packed that some of the open reading frames even overlap.

Figure 2. Mitochondrial Genome Mutations

The genetics of mitochondrial diseases is highly complex due to a series of factors that are discussed below:

Maternal inheritance During the fertilization process, the egg supplies essentially all of the mtDNA. Thus, mitochondrial DNA is almost exclusively maternally inherited in humans.^4,5 Mothers can transmit mutations in mtDNA to both sons and daughters, but only the daughters will pass on the mutations to their offspring.

Figure 3. Mitochondrial Inheritance. As mitochondria are inherited almost exclusively from the mother, defects in mtDNA will be passed on from the mother to her children, as illustrated in this pedigree.

Polyplasmy Every cell (except red blood cells) contains mitochondria, and every mitochondrion contains several mtDNAs. The exceptions to this rule are unfertilized eggs⁶ and platelets⁷, which contain only one mtDNA per mitochondrion. During cell division, mitochondria do not distribute evenly between daughter cells and as a result, the ratio of mutant to wild-type mtDNA can vary considerably among the daughter cells over time. Therefore, population genetics better describes mitochondrial inheritance than mendelian genetics. The mechanism of mitochondrial movement during cell division is a current research topic and many researchers believe that this is an elaborate process managed by complex cytoskeletal processes.⁸

Homoplasmy When all mtDNA in a cell are identical (wild-type or mutant), this condition is known as homoplasmy.⁵

Heteroplasmy Since there are many copies of mtDNA in every cell, when a mutation occurs, both normal and mutated mtDNA coexist within a patient's tissues, a condition known as heteroplasmy.⁴

Mutation accumulation Mitochondria do not have a well-developed repair mechanism, and they do not recombine.⁹ This results in accumulation of mutations as mtDNA is passed through maternal inheritance and also over an individual's life-time. Thus, a heteroplasmic cell can become homoplasmic with purely mutant mtDNA over many generations.¹⁰

Mitotic segregation As heteroplasmic cells divide, the proportions of normal and mutant mtDNA in daughter cells can shift. If and when the pathologic threshold for a particular tissue is exceeded, the phenotype can change. For example, some patients with severe sideroblastic anemia improve over time, which is likely due to selection of blood cells without the mutation during cell division.¹¹

Threshold effect A critical number of mutated mtDNA must be present before tissue dysfunction and clinical signs become apparent. The proportion of mutant mtDNA required for the presentation of a deleterious phenotype varies among individuals, organ systems, and within a given tissue. Tissues with high requirements for oxidative energy metabolism, such as muscle and brain, have relatively low thresholds and are particularly vulnerable to mtDNA mutation. It is therefore not surprising that most mtDNA disorders are encephalomyopathies, affecting primarily brain and muscle.⁴

The mitochondrial encephalomyopathies (discussed below) present with a variety of symptoms and specific inheritance patterns, and it is important to bear in mind the above-mentioned features of mtDNA in order to better understand these diseases.

Mitochondrial dysfunction can affect virtually every tissue and organ system in the human body. It can affect tissues individually, or in various combinations of tissue and organ systems. In addition, different mtDNA mutations can produce diverse and overlapping clinical presentations. Furthermore, due to accumulation of mutant mtDNA and mitotic segregation, an individual's phenotype can vary over time, ranging from dramatic worsening of symptoms to a shift in the disease itself. As an example, children with Pearson's syndrome (with deletions in mtDNA in blood) exhibiting sideroblastic anemia and exocrine pancreatic failure who survive into the second decade often develop Kearns-Sayre Syndrome (with the accumulation of mtDNA deletions in muscle and other tissues).¹⁰ This makes it very challenging to clinically diagnose a mitochondrial disorder.

Mitochondrial disorders can present with a wide variety of clinical signs and symptoms affecting multiple organ systems. The classic features of mitochondrial disorders include the indication of maternal inheritance, along with relatively common symptoms such as stroke, epilepsy, seizures, hearing loss, or peripheral neuropathy (Table 1). A simple rule of thumb is: "When a relatively well-known disease has features that set it apart from the pack, or involves three or more organ systems, think mitochondria."²

Table 1: The Spectrum of Mitochondrial Disease: When is a Disease Not What It Appears?

Adapted from Naviaux, RK and Longenecker, A The Spectrum of Mitochondrial Disease. Exceptional Parent, 43; 1997.

Five major categories of molecularly defined disorders result from mutations in mtDNA:

• Chronic Progressive External Ophthalmoplegia (CPEO) and Kearns-Sayre Syndrome (KSS) CPEO is characterized by ptosis, ophthalmoplegia, and limb myopathy. Patients with KSS also have atypical pigmentary retinopathy, ataxia, and heart block that usually begin before age 20.¹⁰ Skeletal muscle biopsy typically demonstrates ragged red fibers and more than 90% of patients with KSS and approximately 50% of patients with CPEO have a single large deletion in mtDNA.
• Mitochondrial myopathy, Encephalopathy, Lactic Acidosis, and Stroke-like episodes (MELAS) is a multisystem disorder characterized by short stature, recurrent strokes, lactic acidosis, and progressive mental deterioration.¹² Muscle biopsy typically demonstrates ragged red fibers.¹² Distinct mtDNA point mutations in the tRNA^Leu(UUR) gene at nucleotide positions 3243 or 3271 are present in more than 80% of patients with the MELAS syndrome.¹²
• Myoclonic Epilepsy with Ragged Red Fibers (MERRF) is characterized by myoclonus, ataxia, lactic acidosis, weakness, seizures, progressive dementia, and hearing loss.¹² Muscle biopsy typically demonstrates ragged red fibers.¹² Distinct mtDNA point mutations in the tRNA^lys gene at nucleotide positions 8344 or 8356 are present in more than 80% of patients with the MERRF syndrome.¹²
• Neuropathy, Ataxia, and Retinitis Pigmentosa (NARP) is characterized by ataxia, sensory neuropathy, retinal degeneration, mental deterioration, proximal weakness, and developmental delay.⁹ A point mutation at nucleotide position 8993 in the ATPase 6 gene of mtDNA is present in patients with NARP.⁹ When the percent of mutated mtDNA is very high, patients present with Leigh's disease.⁹
• Leber's Hereditary Optic Neuropathy (LHON) is characterized by painless, acute and subacute visual loss, and subsequent optic atrophy.⁹ The mean age of onset is 23 years, and three to four times more males are affected than females.⁹ The three primary pathogenic mtDNA point mutations associated with LHON are at positions 3460, 11778, and 14484, in the ND1, ND4, and ND6 genes respectively.⁹

Clinical features of the above mitochondrial diseases often overlap. This overlap is shown in the shaded boxes in Table 2.

Table 2: Clinical features associated with the mtDNA mutation syndromes.

Adapted from DiMauro, S et al, Mitochondrial Encephalomyopathies. Neurological Clinics, 8. 3: 483-506, 1990.

Molecular diagnostic testing allows for accurate identification of mitochondrial disorders in affected or suspected individuals. Polymerase chain reaction (PCR) is typically the methodology used to detect mutations in mitochondrial DNA, except in the case of KSS/CPEO in which Southern blot is used to detect deletions in mtDNA. These DNA tests can help confirm a clinical diagnosis and lead to appropriate genetic counseling and available therapies, such as Coenzyme Q or diet adjustments. A diagnostic evaluation flowchart of how to approach these disorders is illustrated in Figure 4, which depicts the evaluation of an individual with a suspected mitochondrial disorder.

Figure 4. Mitochondrial Disease Evaluation Flowchart illustrates general guidelines for determining when to order enzyme analysis and/or DNA analysis. It is not intended to be inclusive of all clinical and laboratory combinations.

Mitochondria have challenged physicians and cell physiologists for the last 50 years. We now know that they are one of the most complex organelles within the human body and are responsible for regulating a variety of functions from metabolism to cell death. Current research focuses on the interaction of nuclear and mitochondrial DNA, and the relationship between mtDNA mutations and the wide array of presentations that are present in individuals with these mutations.

Advances in molecular and diagnostic technology will help us better understand these complex diseases and help medical practitioners in better managing their patients.

Athena Diagnostics would like to acknowledge the following organizations for their contributions to this article:
New England Journal of Medicine
Molecular Genetics Department, Emory University
Neurological Clinics Journal
Exceptional Parent Magazine

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