Bipolar disorder, a mood swing of mania and depression, affects over 10 million people in the United States with an equal prevalence rate in both men and women, according to the National Alliance on Mental Illness, NAMI. Two types of bipolar disorders exist: bipolar 1 disorder, a psychological disorder where a person has experienced at least one episode of mania, and bipolar 2 disorder, where a person has never reached the full blown mania. The manifestations of BPD recur throughout an individual's lifetime and can persist to months. From a series of twin and genome-wide studies, researchers have concluded that bipolar disorder has a genetic component as well as an environmental influence, and that the complexity of this psychological disorder involves multiple genes.
Twin Studies
Studies of monozygotic, or identical twins, and dizygotic, or fraternal twins, have consistently shown that genetics play a significant role in bipolar disorder. Published in 2004 in "The American Journal of Psychiatry," Dr. Tuula Kieseppä and colleagues calculated the concordance and heritable rates of 25 pairs of twins with bipolar I disorder. The researchers found that the concordance rate for monozygotic twins was 43 percent compared to 6 percent for dizygotic twins. The concordance rate for monozygotic twins suggests that 43 percent of identical pairs have bipolar 1 disorder, whereas in dizygotic twins, only 6 percent. The significant difference in concordance rate suggests that genetics play a significant role in the contributions to the development of bipolar disorder in the study population.
Adoptive Family Studies
Despite the difficulty of carrying out an adoptive family study, and only a limited number has focused on the genetics contribution to bipolar disorder, the findings thus far, have suggested that a person can inherit bipolar disorder. In a literature, reviewed by Dr. Jordan Smoller and Dr. Christine Finn, in a 2003 issue of "American Journal of Medical Genetics," Dr. Julien Mendlewicz and Dr. John Rainer's 1977 study in "Naure" revealed that the frequency rate of bipolar disorder in biological parents of adoptees with the same psychological disorder is significantly higher at 31 percent than in adoptive parents at 12 percent. These findings suggest that children who have parents with bipolar disorder have an increased risk of developing bipolar disorder.
Genome-wide Association Studies
In the February 2008 issue of "Molecular Psychiatry," Dr. Francis McMahon and colleagues state that the genetic component of bipolar disorder, based on the study findings, encompasses the interactions of multiple risk genes. To identify the risk alleles, or characteristics of a trait, the researchers analyzed 10 single nucleotide polymorphisms, which contain the genome of individuals with and without bipolar disorder. A single nucleotide polymorphism, or a variation in DNA sequence due to one nucleotide difference: adenine, guanine, cytosine, or tyrosine, serves as an important marker in the identification of risk alleles that may contribute to the development of a particular disease. In the case of bipolar disorder, Dr. Francis McMahon and colleagues found that people with bipolar disorder tend to carry more risk alleles, which leads to the conclusion that as the number of risk alleles increases, the risk of developing bipolar disorder also increases.
References
- National Alliance on Mental Illness: Mental Illnesses--Bipolar Disorder
- "The American Journal of Psychiatry"; High Concordance of Bipolar I Disorder in a Nationwide Sample of Twins; Tuula Kieseppa, M.D. et al.; Oct. 2004
- "American Journal of Medical Genetics"; Family, Twin, and Adoption studies of Bipolar Disorder; Jordan Smoller, M.D. and Christine Finn, M.D; Nov. 2003
- "Molecular Psychiatry"; A Genome-wide Association Study Implicates Diacylglycerol Kinase eta (DGKH) and Several Other Genes in the Etiology of Bipolar Disorder; Francis J. McMahon, M.D. et al.; Feb. 2008
