This study provides initial evidence, from a family-based study, for association between RELN SNP rs362719 and BP in females from multiply affected pedigrees. Although candidate gene studies of RELN have been performed in schizophrenia and autism, this study represents, to our knowledge, the first focused association study of RELN in BP. Using a carefully characterized sample and a family-based design, we found overtransmission of the derived C allele in the overall sample to affected offspring (OR = 1.47), which was almost entirely accounted for by overtransmission to affected females (OR = 1.79). This finding remained significant after correction for multiple markers.
The Reelin protein is composed of a distinctive N-terminal domain followed by eight tandem repeats, each consisting of 350–390 amino acids. Each repeat comprises an extracellular growth factor (EGF)-like domain surrounded by two subrepeats. rs362719 is located 86 bp upstream of exon 42, which codes for the EGF-like domain of the fifth Reelin repeat [Royaux et al., 1997
]. Interestingly, recent in vitro studies have found that both the fifth and sixth Reelin repeats are essential for binding to the two Reelin receptors, ApoER2 and VLDLR [Chameau et al., 2009
]. Moreover, addition of the repeat 5–6 fragments was also sufficient to induce phosphorylation of Dab1, an “adapter” molecule responsible for triggering the Reelin intracellular signaling cascade. It is also possible that a potential risk allele is located in the intron itself, perhaps within a regulatory element with sex-specific effects.
Despite its important role in neurodevelopment and plasticity, genetic variation in RELN
has only been comprehensively studied by the recent GWASs of BP and schizophrenia, along with a Scandinavian case–control study of schizophrenia [Käahler et al., 2008
]. The latter study of 577 cases and 1,136 controls genotyped 71 markers in RELN
and found 6 nominal associations (P
values of 0.046–0.003) with schizophrenia, although the authors did not report a sex-specific analysis and it is unclear whether rs362719 was assayed. In the BP and schizophrenia GWAS results to date, there has been little support for association in RELN
. However, most of these studies have used the Affymetrix platform which does not assay rs362719. As shown in , rs362719 lies in a region of low LD that is poorly imputed; indeed, even with all genotypes of the HapMap phases I–III, the most closely linked SNP to rs362719 is insufficiently correlated (r2
= 0.44) to allow imputation with high confidence. To our knowledge, only one published GWAS of BP or schizophrenia has assayed this SNP [Need et al., 2009
]. This schizophrenia case–control study found no overall association with rs362719, but did not report sex-specific analyses for this marker.
Notably, our female-specific finding with RELN
parallels the results from a recent schizophrenia GWAS [Shifman et al., 2008
], albeit with a different marker. Although sex-specific associations may be vulnerable to higher rates of type I error [Patsopoulos et al., 2007
], our analyses were hypothesis driven and within the context of several converging lines of evidence suggesting a sex-specific effect for Reelin. For example, rat studies have provided evidence that the female hormone progesterone strongly upregulates the expression of Reelin in peripheral nerves [Roglio et al., 2008
]. In addition, studies of the heterozygous reeler mouse have found significant cell loss in the cerebellum of males, with no evident effect in females [Hadj-Sahraoui et al., 1996
]. The heterozygous reeler mouse was also found to have diminished expression of the oxytocin receptor [Liu et al., 2005
], which is key to one of the most well characterized sexually dimorphic signaling pathways in the brain [de Vries, 2008
]. While such mechanistic data are not available in humans, a meta-analysis of 12 post-mortem brain expression studies found RELN
levels to be moderately, but significantly, lower in females compared with males (–1.10 change, P
-value = 1.5 × 10–4
), as well as lower in BP cases compared with controls (–1.11 fold change; P
-value = 0.03), however, data on diagnosis-sex interaction is not available [Stanley Medical Research Institute Online Genomics Database, 2009
]. We further note that there have been several prior sex-specific genetic association findings in BP including those in GPR50
[Thomson et al., 2005
]. Such findings might reflect genetic correlates of well-known clinical variation in illness patterns between the sexes, such as those associated with the female reproductive cycle [Payne et al., 2007
The strengths of our study include the rigorous characterization of subjects using the DIGS, a well validated diagnostic instrument. Our study also benefits from a family-based design that is robust to spurious findings from population stratification. This is of particular concern for markers like rs362719, which show marked allele frequency differences between populations. However, one limitation of our family-based sample is that we had suboptimal power to detect alleles of small effect (OR < 1.4). A second limitation is that, because the tagSNP selection was based on Phase I of the HapMap project, we did not cover the common variation in RELN as comprehensively as a tagging approach based on the more recent Phase II version of HapMap would have. Third, our sex-specific analyses were exploratory and require confirmation in additional samples.