Bipolar is a highly inheritable disorder with an 80% chance of inheritance (Craddock, 1999). There has not been a single gene found that is the cause of this disorder. It is thought that several genes are involved and each raise the risk of onset. Some associated genes include: catechol-O-methyltransferase (COMT), GRK-3-BETA, monoamine oxidase (MAOA), G72/G30 gene, and the serotonin transporter gene. Most studies done need replication because they are usually family based studies. Certain families show certain genes affected, but as far as what genes are for sure involved still needs to be determined. Environmental factors also effect the onset of this disorder, which makes finding the cause much more difficult.
Cytokines are proteins that indicate internal responses to environmental stressors by promoting the inflammatory response. Cytokines have shown that they cause neuronal and glial apoptosis. Therefore, high levels of stress as a child produces high levels of cytokines and can be an important mediator in bipolar development.
During normal brain development, pruning of gray matter occurs as the brain matures. According to Gogtay's team, children with bipolar disorder have a more complicated pattern of gray matter in areas of the left hemisphere, losses in the right hemisphere, and losses in the mood regulation area in the mid-front part of the brain.
Gogtay N, Ordonez A, Herman DH, Hayashi KM, Greenstein D, Vaituzis C, Lenane M, Clasen L, Sharp W, Giedd JN, Jung D, Nugent Iii TF, Toga AW, Leibenluft E, Thompson PM, Rapoport JL. Dynamic mapping of cortical development before and after the onset of pediatric bipolar illness. J Child Psychol Psychiatry. 2007 Sep;48(9):852-62. PMID: 17714370
Craddock N, Jones I. Genetics of bipolar disorder. J Med Genet
Roybal, D.J., Singh, M.K., Cosgrove, V.E., Howe, M., Kelley, R., Barnea-Goraly, N., Chang, K.D. 2012. Biological evidence for a neurodevelopmental model of pediatric bipolar disorder. Isr J Psychiatry Relat Sci. (49(1): 28-43.