Re: Are bipolar disorder and obsessive compulsiveness linked?

Well Tineke, both bipolar disorder (BD) and obsessive compulsive disorder 
(OCD) have a genetic component associated with them, but their complex 
genetics have been difficult to understand and are constantly being 
refined.  The reason for this is probably due to the fact that these 
disorders, like almost all other psychiatric disorders, have a 
multifactorial genetic foundation.  That is to say that there is probably 
more than one gene involved in the disease and these genes have different 
modes of inheritance and penetration (penetration refers to whether a gene 
displays its phenotype in an animal).  In addition, the problem of 
studying the genetics of these diseases is confounded with the probability 
that there are environmental influences involved.  Still, we have managed 
to acquire a better understanding over the past several years.
   The incidence of BD in the general population is approximately 0.8%; 
however, the risk of having BD increases if one or both of your parents 
have BD.  If one parent has BD your relative risk of having BD is about 
25%, if both parents have BD your relative risk increases to about 75%.  
Studies of monozygotic (MZ, i.e. genetically identical) and dizygotic (DZ, 
i.e. fraternal) twins also demonstrate a genetic component for BD.  MZ 
twins have a concordance rate (i.e. both have the disease) of 62% whereas 
DZ twins have a concordance rate of 8%.  Unfortunately, there has been no 
unequivocal genetic locus found for BD.  The strongest evidence suggests 
some unknown gene (or genes) located on chromosomes 18 and/or 21 may be 
important, but genetic loci on at least 6 other chromosomes have also been 
implicated.  The best evidence to date suggests that there are 3 loci 
(think genes) involved in the disease and that each of these loci 
contribute equally.
   OCD has been less studied than BD even though its incidence in the 
general population is greater at 2.3%.  The relative risk of OCD for 
primary relatives has been found to be between 9% and 28% depending on the 
inclusion criteria used to define OCD.  Fewer twin studies have been 
performed for OCD than for BD and the data suggests an average concordance 
between MZ twins of about 50%.  While these data indicate a genetic 
component to OCD, there is no clear genetic model yet available, though 
one study I found suggested a dominant transmission with higher penetrance 
in females.  There has been some research performed examining the 
comorbidity of BD and OCD.  I found studies estimating the percentage of 
patients displaying both BD and OCD or between 15-35%, so it does appear 
that there may be a genetic link between these diseases.  I did not find 
any twin studies that looked specifically at this issue.  It is important 
to remember that for diseases like BD and OCD there is a large range of 
behavior displayed by the patients and that separate studies usually use 
different inclusion criteria when assessing patients.  I hoped this 
answered your question.