Genetic variation in cholinergic muscarinic-2 receptor gene modulates M2 receptor binding in vivo and accounts for reduced binding in bipolar disorder

Abstract

Genetic variation in the cholinergic muscarinic-2 (M(2)) receptor gene (CHRM2) has been associated with the risk for developing depression. We previously reported that M(2)-receptor distribution volume (V(T)) was reduced in depressed subjects with bipolar disorder (BD) relative to depressed subjects with major depressive disorder (MDD) and healthy controls (HCs). In this study, we investigated the effects of six single-nucleotide polymorphisms (SNPs) for CHRM2 on M(2)-receptor binding to test the hypotheses that genetic variation in CHRM2 influences M(2)-receptor binding and that a CHRM2 polymorphism underlies the deficits in M(2)-receptor V(T) observed in BD. The M(2)-receptor V(T) was measured using positron emission tomography and [(18)F]FP-TZTP in unmedicated, depressed subjects with BD (n=16) or MDD (n=24) and HCs (n=25), and the effect of genotype on V(T) was assessed. In the controls, one SNP (with identifier rs324650, in which the ancestral allele adenine (A) is replaced with one or two copies of thymine (T), showed a significant allelic effect on V(T) in the pregenual and subgenual anterior cingulate cortices in the direction AA<AT<TT. In contrast, in BD subjects with the TT genotype, V(T) was significantly lower than in BD subjects with the AT genotype in these regions. The BD subjects homozygous for the T -allele also showed markedly lower V(T) (by 27 to 37% across regions) than HCs of the same genotype. Post hoc analyses suggested that T homozygosity was associated with a more severe illness course, as manifested by lower socioeconomic function, poorer spatial recognition memory and a greater likelihood of having attempted suicide. These data represent novel preliminary evidence that reduced M(2)-receptor V(T) in BD is associated with genetic variation within CHRM2. The differential impact of the M(2)-receptor polymorphism at rs324650 in the BD and HC samples suggests interactive effects with an unidentified vulnerability factor for BD.

Figure 1. Association between regional distribution volume values (V_T) and genetic variation in the single nucleotide polymorphism at marker rs324650 on the muscarinic 2 receptor gene, CHRM2 for healthy controls

After applying the false discovery rate correction for multiple testing the results in the pregenual anterior cingulate corticex (pgACC) is significant (p_corrected<0.05). Abbreviations: pgACC pregenual anterior cingulate cortex, sgACC subgenual prefrontal cortex.

Figure 2. Reduced hippocampal [¹⁸F]FP-TZTP V_T in subjects with BD and the TT-genotype of rs324650 in the CHRM2 gene

A significant interaction was detected between rs324650 genotype and group in the hippocampus (F=5.42, p=0.01) that is accounted for by reduced [¹⁸F]FP-TZTP V_T in BD subjects homozygous for the non-ancestral T-allele of rs324650 relative to heterozygotic BD-subjects (T=2.50, p=0.03) and relative to controls of the same genotype (T=2.74, p=0.019).

Figure 3. Cognitive performance A) impairment in BD and MDD groups relative to healthy controls, B) impairment in BD subjects homozygous for the T-allele versus A-carriers for rs324650

A. BD and MDD groups showed impairment in attention and memory evidenced by a greater number of errors of omission (*, RVIP, F=2.48, p=0.009), and reduced percentage correct responses (**, DMS, F=3.447, p=0.039) relative to controls, respectively. The HC group has no error bars because they are performing at 100%. B. In BD, spatial recognition memory performance is impaired in subjects homozygous for the T-allele versus A-carriers (§, T=3.36, p=0.005).