Is the risk of bipolar disorder in twins equal to the risk in singletons? A nationwide register-based study.
BACKGROUND
A previous study demonstrated a higher rate of schizophrenia in dizygotic twins than in the general population, and a higher rate of schizophrenia in siblings of dizygotic twins than in siblings of monozygotic twins and singletons, pointing to a common genetic predisposition for dizygotic twinning and schizophrenia. The aim of the present study was to investigate whether these findings also apply to bipolar disorder.


METHODS
Through record linkage between The Danish Twin Register, The Danish Psychiatric Central Register and The Danish Civil Registration System, the rate of bipolar disorder (diagnosed for the first time during admission to hospital) in dizygotic and monozygotic twins was compared with the rate in singletons, and the rate in siblings and parents of twins was compared with the rate in siblings and parents of singletons.


RESULTS
The rate of bipolar disorder was the same in dizygotic twins, monozygotic twins and singletons as well as for parents and siblings of dizygotic twins, monozygotic twins and singletons.


CONCLUSIONS
The study is a register-based study, only including hospitalized patients.


CONCLUSIONS
This study shows that there is an equal rate of bipolar disorder in twins and in singletons. Assuming that DZ twinning is under some genetic influence, a differential relationship between schizophrenia and DZ twinning on one hand and bipolar disorder and DZ twinning on the other hand may suggest differences in the genetic basis of the two diseases. The finding that the rate of bipolar disorder in monozygotic twins is the same as the rate of bipolar disorder in singletons supports studies finding no association between bipolar disorder and obstetric complications.
Mood Disorder Research Unit of Clinical Psychiatric Research, Aarhus University Psychiatric Hospital, Skovagervej 2, 8240 Risskov, Denmark. uk@psychiatri.aaa.dk
Ulla
U
Kläning
Thomas Munk
TM
Laursen
Rasmus W
RW
Licht
Kirsten Ohm
KO
Kyvik
Axel
A
Skytthe
Preben Bo
PB
Mortensen
D000328:Adult; D000368:Aged; D001714:Bipolar Disorder / Q000209:etiology* / Q000235:genetics*; D015331:Cohort Studies; D003718:Denmark; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008297:Male; D008875:Middle Aged; D012042:Registries / Q000706:statistics & numerical data*; D012307:Risk Factors; D035781:Siblings; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic