Haplotypes of catechol-O-methyltransferase modulate intelligence-related brain white matter integrity

Twin studies have indicated a common genetic origin for intelligence and for variations in brain morphology. Our previous diffusion tensor imaging studies found an association between intelligence and white matter integrity of specific brain regions or tracts. However, specific genetic determinants of the white matter integrity of these brain regions and tracts are still unclear. In this study, we assess whether and how catechol-O-methyltransferase (COMT) gene polymorphisms affect brain white matter integrity. We genotyped twelve single nucleotide polymorphisms (SNPs) within the COMT gene and performed haplotype analyses on data from 79 healthy subjects. Our subjects had the same three major COMT haplotypes (termed the HPS, APS and LPS haplotypes) as previous studies have reported as regulating significantly different levels of enzymatic activity and dopamine. We used the mean fractional anisotropy (FA) values from four regions and five tracts of interest to assess the effect of COMT polymorphisms, including the well-studied val158met SNP and the three main haplotypes that we had identified, on intelligence-related white matter integrity. We identified an association between the mean FA values of two regions in the bilateral prefrontal lobes and the COMT haplotypes, rather than between them and val158met. The haplotype-FA value associations modulated nonlinearly and fit an inverted U-model. Our findings suggest that COMT haplotypes can nonlinearly modulate the intelligence-related white matter integrity of the prefrontal lobes by more significantly influencing prefrontal dopamine variations than does val158met.