Mitochondrial DNA depletion syndrome

Mitochondrial DNA depletion syndrome (MDS or MDDS) is any of a group of autosomal recessive disorders that cause a significant drop in mitochondrial DNA in affected tissues. 
Symptoms can be any combination of myopathic, hepatopathic, or encephalomyopathic. These syndromes affect tissue in the muscle, liver, or both the muscle and brain, respectively. The condition is typically fatal in infancy and early childhood, though some have survived to their teenage years with the myopathic variant and some have survived into adulthood with the SUCLA2 encephalomyopathic variant. 
There is currently no cure for any form of MDDS, though some preliminary treatments have shown a reduction in symptoms.