Evidence of association between bipolar disorder and Citron on chromosome 12q24

SIR – Bipolar disorder (BP) has a predominantly genetic etiology that has been supported by twin, adoption, and family studies. Several genome-wide linkage analyses conducted for BP have identified a variety of susceptibility loci, including chromosome 4p16, 12q24, 18p11.2, 18q22, 21q21, 22q11–13, and Xq26.¹ Many of these loci overlap with those found in schizophrenia (SZ), suggesting common genetic susceptibility factors for both BP and SZ.

In a Scottish family, a hereditary balanced (1;11)(q42.1;q14.3) chromosomal translocation is segregated with major mental illnesses including BP, recurrent major depression, and SZ.² The translocation leads to an interference in the open reading frame of the Disrupted-in-Schizophrenia-1 (DISC1) gene that could produce a C-terminal truncated DISC1 protein. In a previous study, we conducted a yeast two-hybrid screening in order to identify possible protein interactors involved with cellular signaling of DISC1.³ Among more than 30 interacting proteins, we selected Citron for further analyses because the gene encoding Citron is located on chromosome 12q24, a frequently reported locus linked with BP.^{4, 5, 6}

Citron is a synaptic and cytoskeletal protein that can potentially modulate glutamatergic neurotransmission,⁷ a hypothesized mechanism for BP. In dividing cells, Citron is required for cytokinesis and is also implicated for neurogenesis.⁸ Considering its chromosomal location, interaction with DISC1, and possible roles in the synapse, we hypothesize that Citron may be one of several important candidate genes for BP. Therefore, we conducted a family-based association study of the Citron gene and BP.

In this study, we tested for association in the National Institutes of Mental Health (NIMH) Genetics Initiative wave 3 and wave 4 pedigrees, previously described.^{9, 10} The sample set available to us consisted of 307 nuclear families consisting of 1012 individuals, including probands diagnosed with BPI, BPII, schizoaffective disorder, or unipolar depression. We chose seven common single nucleotide polymorphisms (SNPs) roughly equidistant across the 195 kb Citron gene. These included rs278124, rs2285595, rs278109, rs2074052, rs203368, rs435136 (NCBI), and hCV3259834 (Celera). Taqman Assays-on-Demand (Applied Biosystems) were used to genotype all samples. Raw genotype data were checked for errors using MERLIN (http://www.sph.umich.edu/csg/abecasis/Merlin/index.html). Individuals with unlikely recombination events and families with Mendelian errors were excluded from our analysis. Marker-to-marker linkage disequilibrium (LD) and Hardy–Weinberg equilibrium were determined using Haploview (http://www.broad.mit.edu/mpg/haploview/index.php). Pedigrees were analyzed for the presence of association using the Family-Based Association Test (FBAT), a version of a transmission disequilibrium test (TDT) proposed by Rabinowitz and Laird (2000), to detect preferential transmission of single-markers and haplotypes to affected offspring. Monte-Carlo P-values were obtained in FBAT for single markers and haplotypes using 10 000 simulations. We analyzed haplotypes outlined by Haploview based on D′ measures of LD. We found evidence for transmission distortion for two individual SNPs, rs203368 (P=0.020) and rs435136 (P=0.012). Strong LD was observed between markers rs2285595, rs278109, rs203368, rs435136, and hCV3259834. We tested haplotypes composed of these markers and found that our single marker associations were reinforced with preferential transmissions to affected offspring in two- to five-marker haplotypes, which include rs203368 and rs435136 (0.011<P<0.065) (Table 1). No haplotypes without at least one of these markers were found to be significant.

Table 1 FBAT Results for Citron; 12q24.23

To our knowledge, this study provides the first evidence of a candidate gene for BP on chromosome 12q24, a linkage locus for BP common to many studies. Our evidence suggests a possible association of the Citron gene and BP in a family-based sample set. Of particular interest are two SNPs (rs203368 and rs435136) in the proximity of exons that code for the DISC1 binding domain on Citron. The rs203368 and rs435136 polymorphisms may have a key role in biology, but it is more likely that these are markers of causal genetic variation. If Citron plays a role in the pathophysiology of BP, we can predict at least two scenarios. First, the variation in the Citron gene may lead to abnormal expression, and result in altered Citron function. Second, we consider that the two polymorphisms are in LD with their surrounding region, which corresponds to the coding region for the DISC1 binding domain on Citron (between amino acids 499 and 680 (unpublished observation: AK and AS). These SNPs may either directly influence the binding of DISC1 and Citron proteins, or they may be markers of polymorphism(s) responsible for modulating binding. Alternately, we found the SNPs to have an association with BP in the study may reflect causal genetic variation in an adjacent gene. It is conceivable that Citron is one of a cluster of susceptibility genes for BP in the chromosomal region of 12q23–24. Together, our studies warrant further study of Citron and adjacent genes in larger samples and in other ethnic groups in the investigation of a possible etiology for major mood disorders.