Mitochondrial DNA Depletion Syndrome (MDDS) is another syndrome caused by problems with mitochondrial DNA. MDDS are associated with many clinical features, some of which can be fatal in childhood. The type and severity of symptoms will often depend on the particular gene that is affected.
In some cases, the main feature is progressive muscle weakness, which can make breathing difficult and lead to respiratory failure. Other causes affect the brain, with seizures, stroke-like episodes, movement disorders or developmental delay common.
A third syndrome, Leigh syndrome (also known as Leigh disease), is a mitochondrial disease that usually affects young children. As a severe condition it typically causes difficulties with movement, posture and mental capacity.
Importantly, Leigh syndrome is caused by a number of different genetic mistakes (mutations) found in either the nuclear or mitochondrial DNA and so can be inherited in many different ways.