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  • alzheimer\'s disease Introduction Robust evidence sugges

    2019-09-11

    Introduction Robust evidence suggests that genetic factors influence an individual\'s susceptibility to BD (Craddock and Sklar, 2013). Yet, no single, high penetrance gene has been identified with direct causality (Craddock and Sklar, 2009). The largest GWAS to date for identifying genes conferring risk for the five major psychiatric disorders: schizophrenia, BD, major depression, attention deficit-hyperactivity disorder (ADHD) and autism spectrum disorder (ASD), identified the SYNE1 gene among the top ten risk loci (Cross-Disorder Group of the Psychiatric Genomics, 2013). Recent meta-analyses of the GWAS data identified SNPs in SYNE1 with genome-wide statistically significant association to BD (p<4×10) (Cross-Disorder Group of the Psychiatric Genomics, C, 2013, Psychiatric, G.C.B.D.W.G., 2011, Xu et al., 2014). This finding was validated in independent samples and the same risk allele was also found to be associated with recurrent unipolar depression (UD) (Green et al., 2013a, Green et al., 2013b). SYNE1 showed the strongest association with BD of any locus in the genome other than ANK3 (Cross-Disorder Group of the Psychiatric Genomics, 2013), which encodes ankyrin-G, a newly identified regulator of glutamatergic synapses implicated in psychiatric-related behaviors (Leussis et al., 2013, Smith et al., 2014). The BD associated SNPs in human SYNE1 are in a region of the gene homologous to alzheimer\'s disease specific transcripts of rat Syne1 encoding CPG2 (Cottrell et al., 2004). Cpg2 transcripts were identified and isolated from rat brain in a screen for neural activity-regulated genes (Loebrich and Nedivi, 2009, Nedivi et al., 1993), where their expression is responsive to seizure as well as to physiological levels of stimulation (Nedivi et al., 1993, Nedivi et al., 1996). Two Cpg2 transcripts have been reported and both are brain-specific splice variants of the Syne1 gene that are expressed predominantly in the hippocampus, neocortex, striatum, and cerebellum, brain regions strongly associated with electrophysiological paradigms of synaptic plasticity (Cottrell et al., 2004). The CPG2 protein localizes to excitatory synapses of excitatory neurons, where it facilitates glutamate receptor cycling, consistent with a role in synaptic plasticity (Cottrell et al., 2004). While the BD associated SNPs identified by GWAS are not necessarily themselves disease-causing mutations, their localization to the CPG2 locus is intriguing given CPG2\'s role in glutamate receptor internalization and prior implication of the glutamatergic synapse in BD (Cottrell et al., 2004, Sanacora et al., 2008). In addition to its association with BD and UD, the SYNE1 locus has been identified as a risk locus for multiple other genetic disorders. Variations in SYNE1 are known to cause autosomal recessive cerebellar ataxia type 1 (ARCA1) (Gros-Louis et al., 2007), Emery–Dreifuss type muscular dystrophy (Zhang et al., 2007a), and myogenic arthrogryposis, an autosomal recessive form of congenital muscular dystrophy (Attali et al., 2009). A severe de novo mutation identified by exome sequencing in sporadic cases of ASD has been mapped to SYNE1 (O\'Roak et al., 2011), in addition to an autosomal recessive autism mutation identified by homozygosity mapping in pedigrees with shared ancestry (Yu et al., 2013). As yet, the relationship between the different mutations and specific protein products from the SYNE1 locus is unclear. There is no comprehensive transcriptional map of this important gene and little is known regarding expression of the various transcripts in the human brain, or the cellular localization of their products in human neurons. Here, we use RNA sequencing (RNA-seq), chromatin immunoprecipitation sequencing (ChIP-seq) and rapid amplification of cDNA ends (RACE) to map the human SYNE1 transcriptome, providing a framework for localizing the multiple disease-associated mutations identified in this complex gene in relation to specific protein coding transcripts. Several CPG2 transcripts from the human SYNE1 gene were identified, including ones not previously annotated in public databases. The first full-length human CPG2 cDNA was cloned, and the corresponding transcript shown to be expressed in the human neocortex. Further, we raised specific polyclonal antibodies against human CPG2 and used them to confirm that the protein is expressed in several human brain regions. Using a lenti-viral gene knock down/replacement strategy and a surface receptor internalization assay, we demonstrate that human CPG2 localizes to dendritic spines in rat hippocampal neurons and is functionally equivalent to rat CPG2 in regulating glutamate receptor internalization. The conserved function of CPG2 between rat and human provides a platform for testing the effect of missense SNPs identified by BD patient exome sequencing on neuronal function.