We performed a whole-genome linkage scan in 15 extended pedigrees segregating severe BP from the population isolate of Antioquia, Colombia. While we identified no signals at a genome-wide level of significance, we found suggestive evidence for a range of candidate regions for BP, mood disorders in general, and psychosis. A lack of significant results at a genome-wide level has characterised linkage studies of BP for the past decades, reflecting the challenges implicated in the mapping of susceptibility variants for complex psychiatric disease, and making the replication of results all the more important. Interestingly, and encouragingly, many of our results replicate findings from previous studies, as discussed in detail below.
BP is a complex and heterogeneous condition, whose mode of inheritance does not easily conform to standard linkage analysis. The use of nonparametric methods provides a way of overcoming these difficulties, while the use of different diagnostic models can help explore the possible genetic overlap between BP, mood disorders and schizophrenia. Consequently, we performed NPL analysis under 3 different diagnostic models: BPI only; BPI, BPII and major unipolar depression; and psychosis (i.e. ‘narrow’, ‘broad’, and ‘psychosis models’, respectively). Additionally, parametric linkage analysis was used for the narrow diagnosis, using both a near-dominant and a recessive inheritance model. A similar combination of diagnostic definitions and genetic models has been previously used in a number of linkage studies of BP (e.g. [20
There was a substantial overlap in the results obtained with the narrow and broad diagnostic models, as both analyses highlighted loci on chromosomes 1p13–31, 1q25–31 and 12p11–q14. An overlap of linkage signals under these 2 diagnostic models has been previously reported [23
]. These observations suggest the existence of loci predisposing to mood disorders in general rather than to BP in particular, consistent with the observation of an increased rate of major depression in relatives of BP patients [29
The narrow diagnostic model (BPI) also produced some specific linkage signals on chromosomes 7p21–22, 5q12, 13q33 and 21q21–22, although results differed somewhat between parametric and nonparametric analyses (see table ). The loci on chromosomes 7p21–22, 13q33 and 21q21–22 were highlighted by the parametric analysis, possibly implicating that segregation patterns at these loci were broadly consistent with the model used for analysis. Segregation on chromosome 7p21–22 may hence best fit the recessive model, while segregation of loci on chromosomes 13q33 and 21q21–22 may best fit the near-dominant mode of inheritance. It is possible that these regions were not consistently picked up by the nonparametric analysis because of its reduced power (although NPL analysis did produce a p value of 0.0212 for the chromosome 7 region under the broad phenotypic model).
Results under the psychosis phenotype implicated different regions from those suggested by the 2 other diagnostic models (chromosomes 2q24–31, 10q21 and 16p12). There are very few published analyses of the psychosis phenotype in bipolar pedigrees; available results are consistent with our findings in that they implicate regions different from those linked to BP as such [30
], suggesting that there are genetic loci conferring susceptibility to psychosis independent of affective disorders per se
. The concept of a genetic predisposition to psychosis is in line with evidence for a shared genetic susceptibility between BP and schizophrenia [32
Several of the loci identified here have been reported in previous studies of BP. Both the 13q33 and 21q21–22 regions have repeatedly been implicated in susceptibility to BP [23
]. The region on 21q21 was one of the best-supported loci in our previous linkage scan [10
], and the present study provides further support for this region. Interestingly, most of the studies showing evidence for a locus on chromosome 21q also used a narrow diagnostic model similar to the one used here [23
]. Lin and colleagues [38
] also reported linkage of chromosome 21q22 for early-onset BP, a subtype of the disorder associated with increased severity of clinical symptoms [39
]. Interestingly, the 13q33 region harbours the DAOA
gene which has been found to be associated with both BP and schizophrenia [40
The loci on 1p13–31 and 1q25–31 implicated here have also both been reported to be involved in BP susceptibility [23
]. The chromosome 1p13–31 region had been highlighted by our initial genome scan [10
], and the present study has further strengthened this signal. The locus on the long arm of chromosome 1 has frequently been linked to schizophrenia [33
]. This region includes the genes encoding the regulator of G signalling protein, RGS4
, and the nitric oxide synthase 1 (neuronal) adaptor protein, NOS1AP
,which have both been associated with BP or schizophrenia [55
]. Interestingly, NOS1AP expression has been found to be increased in the prefrontal cortex of bipolar subjects [60
The strongest support for linkage in our study was observed for regions on chromosome 7p21.1–p22.2 (under the narrow affection model and assuming recessive inheritance), and 12q13 (under both narrow and broad affection models).
Evidence of linkage of BP to the chromosome 7p21 region has been reported in a set of 16 extended Portuguese Island pedigrees [61
]. The replication of this finding in the Colombian families is of interest because both populations share ancestral contributions from the Iberian Peninsula.
The chromosome 12q13 region has been implicated in a study of French Canadian BP pedigrees. Like the present study, the signal from the Canadian pedigrees was obtained using a broad diagnostic model including BPI, BPII, schizoaffective disorder and recurrent unipolar depression [34
]. However, the strongest support for linkage in the Canadian families appears to be in a region telomeric to the locus described here. An interesting candidate gene in the region identified here is Timeless
. This gene plays a crucial role in the regulation of circadian rhythms [62
], the dysfunction of which is a widespread phenomenon in BP [63
Amongst the regions implicated under the psychosis model, both chromosomes 2q24 and 16p12 have previously been linked to psychiatric disease. The 2q24 region has been implicated in both BP and schizophrenia [53
], and the chromosome 16p12 region has been implicated in severe psychotic BP [31
]. Interestingly, the most significant signal in the WTCCC BP genome-wide association study was for a SNP in the 16p12 region [6
]. This region harbours several interesting candidate genes, including DCTN5
, whose product appears to interact with that of DISC1
(Disrupted in Schizophrenia 1), a gene repeatedly linked to schizophrenia [66
In this expanded pedigree sample we failed to find further linkage support for the 5q31–34 region implicated by a subset of the families analysed previously [10
]. A possible explanation is interfamily heterogeneity. In particular the two largest families examined here (CO1 and CO28) stem from an isolated village, which could represent a subisolate within Antioquia. Neither of these two families provides support for a locus on chromosome 5q31–34, and due to their size, these two pedigrees have a strong impact on the total LOD-scores calculated. For a complex disorder like BP, genetic heterogeneity is not unlikely even within a population isolate.
Such difficulties highlight the need to readjust how results from linkage studies should be interpreted. Most individual studies might not have sufficient power to detect susceptibility loci for psychiatric disease on their own, and replication studies and meta-analyses are essential to evaluate the true significance of linkage findings. In this respect, it is interesting that regions highlighted by meta-analyses of genome-wide linkage studies of BP often do not coincide with the most significant signals of the individual studies included [47
]. Results from an individual study might therefore represent relatively larger genetic effects of importance only for a specific family or population. On the other hand, individually weaker signals could show greater consistency across studies, hence only reaching statistical significance through meta-analyses. Family studies have contributed, and continue to contribute, important information to the emerging picture of genetic predisposition to BP. The present study, which has provided further supporting evidence for a range of previously reported loci, but in which we have also identified a possible novel locus on chromosome 12q13, adds to this picture.