Mitochondrial diseases are a group of disorders that arise as a result of poorly functioning mitochondria, which fail to produce sufficient energy for the body.
When the mitochondria do not work properly, cells do not have enough energy to function properly—this results in a mitochondrial disease. Depending on which organ symptom is affected, symptoms may include loss of motor control, muscle weakness and pain, gastrointestinal disorders and swallowing difficulties, poor growth, cardiac disease, liver disease, diabetes, respiratory complications, seizures, visual or hearing problems, lactic acidosis, developmental delays, and susceptibility to infection.
According to the Cleveland Clinic, one in 5,000 individuals has a genetic mitochondrial disease. These disorders are caused by errors in DNA. DNA is the blueprint that instructs the body’s growth, development, and maintenance. Most DNA is found within the nucleus of the cell; however, a smaller amount of DNA is found within the cell’s mitochondria.
In fact, the mitochondria are the only components of the cell, other than the nucleus, to contain functioning genes.
In addition to genetic mitochondrial disorders, other health conditions and lifestyle/environmental factors can lead to secondary mitochondrial dysfunction diseases. These disorders include autism, Parkinson’s disease, Alzheimer’s disease, muscular dystrophy, Lou Gehrig’s disease, diabetes, and cancer. With the number and type of symptoms and organ systems involved, mitochondrial diseases are often mistaken for other, more common, diseases.
How are Mitochondrial Diseases Diagnosed?
Because mitochondrial diseases affect so many different organs of the body, and patients have so many different symptoms, mitochondrial diseases are difficult to diagnose. There is no single laboratory or diagnostic test used to confirm the diagnosis of a mitochondrial disease.
The first steps towards obtaining a diagnosis typically include undergoing a series of thorough tests including neurological and metabolic examinations that include blood and urine tests. A cerebral spinal fluid test (spinal tap) may also be required for patients with neurological symptoms.
Additional tests may be ordered depending on a patient’s symptoms. These include: Magnetic resonance imagine (MRI) or spectroscopy (MRS) for neurological symptoms, retinal exam or electroretinogram (ERG) for vision symptoms, electrocardiogram (EKG) or echocardiogram for symptoms of heart disease, audiogram or auditory-brainstem evoked responses (ABER) for hearing symptoms, blood test to detect thyroid dysfunction, and blood tests to perform genetic DNA testing. Biopsies of skin and muscle tissue may also be performed.
Mitochondrial Diagnostic Testing
Diagnosing mitochondrial disease can be a lengthy process and some tests are more accurate than others. A referral to a physician who specialize in these diseases is critical to making the diagnosis.
Muscle biopsy and genetic testing are most commonly used in the clinical setting for diagnosing mitochondrial disease. P31NMR Scanning has been effective in cancer diagnosis and is now being used as a specialized test to evaluate lactic acid build-up in mitochondrial disease. On the forefront of cutting-edge research, the Seahorse Analyzer is expanding the scope of mitochondrial dysfunction assessment in the study of cellular metabolism.
Biochemical testing of muscle fibers obtained from muscle biopsies can determine if mitochondria are functioning properly. Muscle biopsies are performed to examine tissue that is considered highly energy dependent. During a muscle biopsy, a small sample of muscle tissue is removed and inspected; histological examination looks at the structure of the muscle and its characteristics, especially looking for "ragged red fibers". However, histology can be normal in some patients suffering from mitochondrial disease. The energy production pathway can also be examined through study of the enzymes. Drawbacks exist in this testing because it can produce false positive or negative readings. Muscle biopsies are invasive procedures and when done on children require general anesthesia.
Mitochondrial or Nuclear DNA Genetic Testing
DNA genetic tests can be performed on most tissues, for example, from a blood
sample or muscle/liver biopsy. Genetic tests may identify a known disease-causing mutation(s) in the nuclear DNA or in the mitochondrial DNA. If a mutation is identified and a diagnosis is confirmed, this information may provide knowledge about the progression of the disease, determine possible treatments for specific associated symptoms, and also provide information as to whether other family members should be tested.
The P31NMR scan is a multinuclear magnetic resonance spectroscopy (MRS) that measures the ATP-ADP ration in vivo. This test can been used to measure brain lactic acid, the antioxidant glutathione (GSH), several high-energy phosphates (e.g. ATP, PCr), and other metabolites in mitochondrial disorders. Simply put, P31NMR scanning is able to directly measure the level of ATP in tissues. Since ATP is the energy currency molecule of the cell, an ATP deficiency may provide evidence that indicates a defect in mitochondrial functioning.
The Seahorse Analyzer
The Seahorse XF Extracellular Flux Analyzer enables the direct measurement of mitochondrial functioning and reserve energy-generating capacity from intact cells such as white blood cells obtained from a peripheral blood draw. This powerful technology is currently only available in the research setting. It would be a great step forward if it could be integrated into clinical use in the future.
During the past decade, the number of mitochondrial disease research publications has increased exponentially from less than 100 in 2004 to over 1400 in 2014. The field of mitochondrial disease research is rapidly expanding, with new and significant links of mitochondrial function to human disease. The weight of evidence linking mitochondrial dysfunction to multiple diseases will hopefully lead to better testing methods, and eventually, to new and effective treatments.
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