Mutation links bipolar disorder to mitochondrial disease

Mutations in the gene ANT1 may confer a risk for bipolar disorder through a complex interplay between serotonin and mitochondrial signaling in the brain. 

Mitochondria are the vital organelles that deliver energy to all cells and mitochondrial damage has been found, for example, in brain imaging of bipolar patients and in post-mortem brains. 

Serotonergic neurons were found to deteriorate in a brain area called the dorsal raphe, which is a region also affected in Parkinson's disease--another condition that may have its roots in mitochondrial dysfunction. Source.