br Introduction Advances in single nucleotide polymorphism S

Introduction Advances in single nucleotide polymorphism (SNP) array technologies and Next Generation Sequencing have made it practical to perform Genome-Wide Association Studies (GWAS), an unbiased genomic approach to identify genetic factors that account for cancer susceptibility. Following initial GWAS (Zheng et al., 2009; Turnbull et al., 2010), subsequent fine-mapping studies (>50) have implicated genetic variants located at the CCDC170/C6ORF97-ESR1 locus (6q25.1) as being associated with the risk of breast cancer. Estrogen receptor α (ERα), the protein encoded by the ESR1 gene, binds to estrogen, and the estrogen-ERα axis promotes the growth of breast epithelial GS-9620 and thereby contributes to breast cancer risk (Ali and Coombes, 2000). It is therefore logical to hypothesize that the breast cancer-associated SNPs at the CCDC170/C6ORF97-ESR1 locus impact function of the ESR1 gene (Hein et al., 2012; Koller et al., 2013; Paternoster et al., 2013; Yang et al., 2013). However, thus far, few studies have identified any strong causal variants regulating ESR1 function or expression (Cai et al., 2011; Stacey et al., 2010). Interestingly, the CCDC170-ESR1 intergenic rs2146210 SNP was found to have stronger risk-association in ER- breast tumors than those in ER+ breast tumors, which suggests that this risk variant is likely ESR1-independent (Hein et al., 2012; Mulligan et al., 2011; Stacey et al., 2010; Zheng et al., 2009). Nonsense mutations (e.g. p.E48* and p.Q405*) in the CCDC170 gene have been reported in sporadic breast cancer and other cancers by both the Cancer Genome Atlas (TCGA) and Cancer Genome Project. Importantly, several tumor-specific gene rearrangements from the second noncoding exon of ESR1 to the sixth and/or seventh coding exon(s) of CCDC170 were also reported by several studies using high-throughput RNA-seq (Robinson et al., 2011; Sakarya et al., 2012; Veeraraghavan et al., 2014). This ESR1-CCDC170 gene arrangement presents in ~14% of ER+ breast cancer and could be one of the most important recurrent gene fusions in breast cancer (Veeraraghavan et al., 2014). This recent study by Veeraghavan et al. demonstrated that N-terminally truncated CCDC170 proteins were produced as a result of this ESR1-CCDC170 rearrangement (Veeraraghavan et al., 2014). Ectopic expression of these truncated proteins increased breast cancer cell motility and enhanced the transformation of normal mammary epithelial cells (MECs) (Veeraraghavan et al., 2014), indicating the important role of CCDC170 gene abnormalities in breast cancer initiation and/or progression. Taken together, the findings from GWAS, TCGA, cell culture, and mouse xenograft studies strongly indicate that a variety of perturbations of the CCDC170 protein are capable of driving breast cancer. Despite the wealth of genetic information relating to the CCDC170 gene, nothing was known about the encoded protein. Here, we initially show that the CCDC170 locus is associated with significant Differential Allele Specific Expression (DASE), which supports specifically a link to breast cancer risk. As nothing was known about the molecular function of the CCDC170 protein, the present work focused largely on identifying a potential molecular mechanism for CCDC170-associated breast cancer risk and progression. We demonstrate that the CCDC170 protein, a predicted coiled-coil domain containing (CCDC) protein, associates with the Golgi apparatus, stabilizes perinuclear microtubules (MTs), and plays an essential role in the known process of MT-dependent Golgi organization. Distinct Golgi-derived MTs that extend into the cytoplasm are now understood to contribute to cell polarity and directional migration. We hypothesized that breast cancer-related perturbations of the CCDC170 Golgi-MT network could lead to altered cell polarity and migration, and thereby drive breast cancer initiation and progression. We provide evidence that dysregulation of CCDC170 indeed affects polarized cell migration.