Abstract A06: The murine ortholog of the human cancer-associated 8q24 gene desert affects mammary tumorigenesis and alters mammary gland development through Myc-mediated global gene expression regulation

The vast majority of breast cancer-associated variants identified by genome-wide association studies (GWAS) are located to non-protein coding genomic regions. Although some have been shown to regulate the expression of specific genes, novel approaches to elucidate the in vivo mechanisms underlying such breast cancer susceptibility loci are needed. The gene desert located on human chromosomal band 8q24, proximal to MYC and PVT1, and distal to FAM84B, harbors 2 common, low-penetrance breast cancer variants. We generated a megadeletion mouse model lacking 430 Kb of sequence orthologous to the breast cancer-associated locus of the 8q24 gene desert. Homozygous megadeletion mice are viable, fertile, lactate sufficiently to nourish their pups, but maintain a 10% lower body weight mainly due to decreased adiposity. We found that the mutation altered mammary gland development, resulting in less branch points, terminal end buds and altered luminal/basal ratio. Using a reciprocal mammary gland transplantation assay, we found a strong donor effect and weaker host effect of the mutation on mammary gland development, indicating that the non-protein coding locus affects mammary cell-autonomous as well as non-mammary cell-autonomous processes. When crossed to the PyVT mouse model for luminal breast cancer and the C3(1)-TAg mouse model for basal breast cancer, we found that the mutation in homozygous state increased latency in both models, but stronger in the PyVT model. This finding is consistent with human genetic data indicating that one of the 8q24 breast cancer variants is more strongly associated with estrogen receptor (ER)+ (luminal) than with ER- disease. Strikingly, the development of lung metastasis was severely reduced in homozygous mutant PyVT carriers. To identify potential candidate causal genes through which the locus exerts its effects, we quantified transcript levels of genes located within genomic vicinity of the gene desert in mammary gland, prostate, colon, bladder, spleen, thymus and mammary tumor samples from wild-type and homozygous mutant mice. We found organ-specific effects on transcript level regulation, with the proto-oncogene Myc being downregulated in all organs/tumors, except for the bladder. RNAseq-based network analysis of global gene expression on mammary epithelium samples from wild-type and knockout mice revealed clusters of coexpressed isoforms mostly marking specific cell types. An integrative computational approach intersecting flow cytometry-derived cell type abundance data with differential expression, coexpression as well as functional and motif enrichment data revealed that Myc-related binding motifs are enriching in regions of the network responsible for differential expression of mammary gland development and immune genes. Taken together, our comparative genetics approach of targeting a non-protein coding breast cancer-susceptibility locus provides the opportunity for a novel strategy to mitigate the breast cancer-promoting effects of Myc that has great potential for the development of preventative therapies without side effects. Citation Format: Bart MG Smits. The murine ortholog of the human cancer-associated 8q24 gene desert affects mammary tumorigenesis and alters mammary gland development through Myc-mediated global gene expression regulation. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr A06.