POLG encodes for the catalytic subunit pol γA, which is part of mitochondrial DNA polymerase.Other causes are mutations of thymidine phosphorylase (TyMP), succinate-CoA ligase, alpha sub unit (SUCLG1) and TWNK (also known as PEO1 and C10orf2). Diagnosis
MDDS is diagnosed based on systemic symptoms presenting in infants, followed by a clinical examination and laboratory tests (for example, high lactate levels are common) medical imaging, and usually is finally confirmed and formally identified by genetic testing. Classification
MDDS are a group of genetic disorders that share a common pathology — a lack of functioning DNA in mitochondria. There are generally four classes of MDDS:
a form that primarily affects muscle associated with mutations in the TK2 gene;
a form that primarily affects the brain and muscle associated with mutations in the genes SUCLA2, SUCLG1, or RRM2B;
a form that primarily affects the brain and the liver associated with mutations in DGUOK, MPV17, POLG, or TWNK (also called PEO1); and
a form that primarily affects the brain and the gastrointestinal tract associated with mutations in ECGF1 (also called TYMP). Treatment
There are no treatments for MDDS, but some of the symptoms can be managed. For survivors living with MDDS, there are drugs to control epilepsy, and physical therapy can help with muscle control. Liver transplants may benefit people with liver involvement. Prognosis
Myopathic form
The TK2 related myopathic form results in muscle weakness, rapidly progresses, leading to respiratory failure and death within a few years of onset. The most common cause of death is pulmonary infection. Only a few people have survived to late childhood and adolescence.
Additionally, although physical signs of chronic liver dysfunction may not be present, many people experience liver impairment leading to liver failure.In MDDS associated with mutations in PEO1/C10orf2 that primarily affect the brain and the liver, symptoms emerge shortly after birth or in early infancy, with hypotonia, symptoms of lactic acidosis, enlarged liver, feeding problems, lack of growth, and delay of psychomotor skills. Neurologically, development is slowed or stopped, and epilepsy emerges, as do sensory problems like loss of eye control and deafness, and neuromuscular problems like a lack of reflexes, muscular atrophy, and twitching, and epilepsy.In MDDS associated with mutations in the genes associated with mutations in ECGF1/TYMP  that primarily affects the brain and the gastrointestinal tract, symptoms can emerge any time in the first fifty years of life; most often they emerge before the person turns 20. Weight loss is common as is a lack of the ability of the stomach and intestines to automatically expand and contract and thus move through it (called gastrointestinal motility) – this leads to feeling full after eating only small amounts of food, nausea, acid reflux,  All affected individuals develop weight loss and progressive gastrointestinal dysmotility manifesting as early satiety, nausea, diarrhea, vomiting, and stomach pain and swelling. People also develop neuropathy, with weakness and tingling. There are often eye problems, and intellectual disability. Causes
MDDS is caused by mutations that may be inherited from the parents or may form spontaneously during development of the fetus.