Common variants underlying cognitive ability: further evidence for association between the SNAP-25 gene and cognition using a family-based study in two independent Dutch cohorts

The synaptosomal associated protein of 25 kDa (SNAP-25) gene, located on chromosome 20 p12-12p11.2 encodes a presynaptic terminal protein. SNAP-25 is differentially expressed in the brain, and primarily present in the neocortex, hippocampus, anterior thalamic nuclei, substantia nigra and cerebellar granular cells. Recently, a family-based genetic association was reported between variation in intelligence quotient (IQ) phenotypes and two intronic variants on the SNAP-25 gene. The present study is a follow-up association study in two Dutch cohorts of 371 children (mean age 12.4 years) and 391 adults (mean age 36.2 years). It examines the complete genomic region of the SNAP-25 gene to narrow down the location of causative genetic variant underlying the association. Two new variants in intron 1 (rs363043 and rs353016), close to the two previous reported variants (rs363039 and rs363050) showed association with variation in IQ phenotypes across both cohorts. All four single nucleotide polymorphisms were located in intron 1, within a region of about 13.8 kbp, and are known to affect transcription factor-binding sites. Contrary to what is expected in monogenic traits, subtle changes are postulated to influence the phenotypic outcome of complex (common) traits. As a result, functional polymorphisms in (non)coding regulatory sequences may affect spatial and temporal regulation of gene expression underlying normal cognitive variation.