Genomic mapping and cellular expression of human CPG2 transcripts in the SYNE1 gene

Bipolar disorder (BD) is a prevalent and severe mood disorder characterized by recurrent episodes of mania and depression. Both genetic and environmental factors have been implicated in BD etiology, but the biological underpinnings remain elusive. Recent genome-wide association studies (GWAS) for identifying genes conferring risk for schizophrenia, BD, and major depression, identified an association between single-nucleotide polymorphisms (SNPs) in the SYNE1 gene and increased risk of BD. SYNE1 has also been identified as a risk locus for multiple other neurological or neuromuscular genetic disorders. The BD associated SNPs map within the gene region homologous to part of rat Syne1 encompassing the brain specific transcripts encoding CPG2, a postsynaptic neuronal protein localized to excitatory synapses and an important regulator of glutamate receptor internalization. Here, we use RNA-seq, ChIP-seq and RACE to map the human SYNE1 transcriptome, focusing on the CPG2 locus. We validate several CPG2 transcripts, including ones not previously annotated in public databases, and identify and clone a full-length CPG2 cDNA expressed in human neocortex, hippocampus and striatum. Using lenti-viral gene knock down/replacement and surface receptor internalization assays, we demonstrate that human CPG2 protein localizes to dendritic spines in rat hippocampal neurons and is functionally equivalent to rat CPG2 in regulating glutamate receptor internalization. This study provides a valuable gene-mapping framework for relating multiple genetic disease loci in SYNE1 with their transcripts, and for evaluating the effects of missense SNPs identified by patient genome sequencing on neuronal function.