Mitochondria and the Many Disorders That Compose “Mitochondrial Disease”
Mitochondria (singular mitochondrion) are tiny subunits present inside every cell of the human body except red blood cells, from the skin to the deep internal organs. Mitochondria’s main role is transforming food and oxygen that enter the cells into useful energy, creating more than 90 percent of the energy needed by the body to sustain life and support. The term “mitochondrion” derives from the Greek mitts (“thread”), and chondrion (“granule” or “grain-like”). More technically, mitochondria are double membrane-bound organelles — specialized subunits within cells with a specific function that are found in most eukaryotic cells (cells that have a nuclear envelope, cytoplasmic organelles, and a cytoskeleton).
Mitochondria are tiny, ranging from 0.5 to 1.0 micrometers in diameter, with considerable observable variations in the structure and size of the organelle, and are sometimes described as “the powerhouses of the cell” because they generate most of the cell’s supply of adenosine triphosphate (ATP), used as a source of chemical energy, and are involved in various other tasks, such as signaling, cellular differentiation, and cell death, as well as maintaining control of the cell cycle and cell growth.
However, the Pittsburgh, Pennsylvania-based United Mitochondrial Disease Foundation contends that regarding mitochondria primarily as energy-converters is an over-simplification that has slowed progress toward understanding the biology underlying mitochondrial disease. The UMDF notes that it takes about 3000 genes to make a mitochondrion, and mitochondrial DNA encodes just 37 of these genes, with the remaining preponderance of genes encoded in the cell nucleus and the resultant proteins transported to the mitochondria. Ergo: only about three percent of the genes necessary to make a mitochondrion (i.e.: 100 of 3000) are allocated for making ATP, while more than 95 percent (i.e.: 2900 of 3000) are involved with other functions tied to the specialized duties of the differentiated cell in which the mitochondrion resides, and these duties change as we develop from embryo to adult and as our tissues grow, mature, and adapt to the postnatal environment. It is these other non-ATP-related functions that are intimately involved with most of the major metabolic pathways used by a cell to build, break down, and recycle its molecular building blocks, and the cells themselves can’t even make the RNA and DNA they need to grow and function without the function of mitochondria. The building blocks of RNA and DNA are purines and pyrimidines, and it is the mitochondria that contain the rate-limiting enzymes for pyrimidine biosynthesis (dihydroorotate dehydrogenase) and heme synthesis (d-amino levulinic acid synthetase) required to make hemoglobin.
Similarly, liver cells’ mitochondria are specialized to detoxify ammonia in the urea cycle, and mitochondria are also required for cholesterol metabolism, estrogen and testosterone synthesis, neurotransmitter metabolism, and free radical production and detoxification, in addition to breaking down (oxidizing) the fat, protein, and carbohydrates we eat and drink.
Researchers at the National Institutes of Health report the first clear evidence that muscle cells distribute energy primarily by the rapid conduction of electrical charges through a vast, interconnected network of mitochondria in a way resembling the wire grid that distributes power throughout a city. The study, published the journal Nature, entitled: “Mitochondrial reticulum for cellular energy distribution in muscle“ (Nature 523, 617–620 (30 July 2015) dpi:10.1038/nature14614) provides an unprecedented, detailed window into showing how the system that rapidly distributes energy throughout the cell to power muscle contraction works. These observations solve the problem of how muscles rapidly distribute energy within the cells for movement.
Mitochondria have also been implicated in a number of human diseases, including a class of pathology known collectively as mitochondrial disorders as well as cardiac dysfunction and heart failure. Findings of a recent small University of California study suggest that autism may also be correlated with mitochondrial defects as well.
“Mitochondrial Disease” Includes Many Different Disorders
The term “Mitochondrial disease” refers to a broad range of disorders, each of which involves a mitochondrial dysfunction, with many more believed to have yet to be discovered. Because mitochondria perform so many different functions in different tissues, there are literally hundreds of different mitochondrial diseases that result from either inherited or spontaneous mutations in mtDNA or nDNA, which lead to altered functions of the proteins or RNA molecules that normally reside in mitochondria.
Another subcategory is Mitochondrial myopathies — a group of neuromuscular diseases caused by damage to the mitochondria — with some examples including Kearns-Sayre syndrome (KSS), Leigh’s syndrome, Mitochondrial Depletion syndrome (MDS), Mitochondrial Encephalomyopathy, Lactic Acidosis and Stroke-like episodes (MELAS), Myoclonic epilepsy with Ragged Red Fibers (MERRG), Mitochondrial neurogastrointestinal encephalopathy syndrome (MNGIE), Neuropathy, Ataxia, and Retinitis Pigmentosa (NARP), Pearson syndrome, and Chronic Progressive External Opthalmoplegia (CPEO). Nerve cells in the brain and muscles require a great deal of energy and appear to be particularly vulnerable when mitochondrial dysfunction occurs.
Each particular condition is the result of a genetic mutation — a specific change in the genetic material of the mitochondria — that causes the mitochondria to fail, which in turn leads to less and less energy getting converted in the cells, which may stop working or die. Depending on the location of the affected cells, certain parts and/or functions of the body may no longer function properly, leading to health problems and symptoms that can range from mild to severe. Because of the complex interaction between the hundreds of genes and cells that must cooperate to keep the body’s metabolic machinery running smoothly, it is a hallmark of mitochondrial diseases that identical mtDNA mutations may not produce identical diseases. Genocopies are diseases that are caused by the same mutation but may not look the same clinically, which complicates diagnosis.
Mitochondrial disease primarily affects children, but adult onset is becoming more and more common, and there is a broad spectrum of metabolic, inherited and acquired disorders in adults in which abnormal mitochondrial function has been postulated or demonstrated. It is estimated that at least one in 6000 people and as many as one in 4000 have a mitochondrial disease, with approximately 20,000 people in the United States believed to have a form of mitochondrial myopathy, for which there is currently no cure, and approximately 1,000 to 4,000 American children born with the disorder each year. If mitochondrial failure is repeated throughout the body, whole systems begin to fail, and the life of the person in whom this is happening is severely compromised. Diseases of the mitochondria that appear to cause the most damage are ones affecting cells of the brain, heart, liver, skeletal muscles, kidney and the endocrine and respiratory systems. On the other hand, many people affected can have normal life spans with their disease well managed, and research is underway across the world that will enhance knowledge regarding these diseases and discovery of new treatments and therapies.
Symptoms of mitochondrial disease may include diminishment or loss of motor control, muscle weakness and pain, gastrointestinal disorders, swallowing difficulties, poor growth, fatigue, lack of endurance, poor balance, cardiac disease, liver disease, diabetes, respiratory complications, seizures, visual/hearing problems, lactic acidosis, developmental delays, skeletal muscle abnormalities, and susceptibility to infections, nervous system impairment.
Symptoms of mitochondrial myopathies include muscle weakness or exercise intolerance, heart failure or rhythm disturbances, dementia, movement disorders, stroke-like episodes, deafness, blindness, droopy eyelids, limited mobility of the eyes, vomiting, and seizures. Most mitochondrial myopathies occur before the age of 20, and often begin with exercise intolerance or muscle weakness. During physical activity, muscles may become easily fatigued or weak. Muscle cramping is rare, but may occur. Nausea, headache, and breathlessness are also associated with these disorders.
The National Institutes of Health (NIH) notes that while there is no specific treatment for any of the mitochondrial myopathies, physical therapy may extend the range of movement of muscles and improve dexterity, and vitamin therapies such as riboflavin, coenzyme Q, and carnitine (a specialized amino acid) may provide subjective improvement in fatigue and energy levels in some patients.
Article from "Mitochondrial Disease News" by Charles Moore published August 26, 2015