Heritability of bipolar spectrum disorders. Unity or heterogeneity?
BACKGROUND
The purpose of this study was to investigate whether the three disorders in the bipolar spectrum, Bipolar I disorder, Bipolar II disorder and Cyclothymia, are various expressions of an underlying genetic commonality.


METHODS
A sample consisting of same-sexed mono (MZ)- and dizygotic (DZ) twins were identified using hospital and outpatient registers (N=303). DSM-III-R criteria were assessed by personal interviews. Cross tabulations were used to compare concordance rates for different definitions of the bipolar spectrum. Correlations in liability and estimation of the heritability (h) with biometrical model fitting were performed.


RESULTS
Concordance rates were higher among MZ- than DZ pairs for all the single diagnoses and main combinations of diagnoses. Cross-concordance between different diagnoses was observed. The heritability of Bipolar I was .73, of Bipolar I+II .77 and of Bipolar I+II+Cyclothymia .71.


CONCLUSIONS
Probands were not sampled from the general population. Most often the same person interviewed both twins in a pair. The statistical power was restricted in some sub-analyses.


CONCLUSIONS
The 'Bipolar Spectrum' category consisting of Bipolar I disorder, Bipolar II disorder and Cyclothymia constitute one entity with high heritability without detectable shared family environmental effects. Future genetic and clinical work might consider that all variants of the bipolar spectrum are an expression of one underlying genetic liability.
Edvardsen
Jack
J
Nordland Hospital Trust, Vesterålen District Psychiatric Centre, Norway. Jack.Edvardsen@nlsh.no
Torgersen
Svenn
S
Røysamb
Espen
E
Lygren
Sissel
S
Skre
Ingunn
I
Onstad
Sidsel
S
Oien
Per Anders
PA
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001714:Bipolar Disorder / Q000145:classification / Q000175:diagnosis / Q000235:genetics*; D015331:Cohort Studies; D003527:Cyclothymic Disorder / Q000175:diagnosis / Q000235:genetics; D039721:Diagnostic and Statistical Manual of Mental Disorders; D004200:Diseases in Twins / Q000235:genetics*; D005190:Family; D005260:Female; D005787:Gene Frequency; D018740:Genetic Heterogeneity; D020022:Genetic Predisposition to Disease / Q000235:genetics; D006801:Humans; D008297:Male; D008875:Middle Aged; D008957:Models, Genetic; D010375:Pedigree; D012042:Registries / Q000706:statistics & numerical data; D012307:Risk Factors; D014429:Twins, Dizygotic / Q000235:genetics; D014430:Twins, Monozygotic / Q000235:genetics