Metastasis accounts for the majority of cancer-related deaths. Although exact mechanisms are unknown, studies in our laboratory have demonstrated that germline polymorphisms could alter tumorigenesis and metastasis. Our lab developed a mouse genetics approach to investigate the effect of inherited single nucleotide polymorphisms (SNPs) on metastasis by crossing inbred mice of various backgrounds and subsequent mapping of quantitative trait loci (QTL) responsible for the phenotype. Using this approach, we crossed a highly metastatic (FVB) and low metastatic (MOLF) mouse strain and identified Arntl2, a circadian rhythm transcription factor, as a gene with differential expression that predicted distant metastasis-free survival in breast cancer patients. shRNA-mediated reduction in Arntl2 expression suppressed lung metastases, establishing Arntl2 as a metastasis modifier. Initial analysis revealed no polymorphisms in coding regions that could account for the differential expression in FVB and MOLF. Therefore, we performed whole genome sequencing of MOLF to identify SNP differences between MOLF and FVB. Overlapping this analysis with DNAse hypersensitivity sites in mouse mammary tumor cell lines revealed 13 polymorphisms in the predicted promoter region of Arntl2. FVB and MOLF promoters exhibited differential promoter activity in vitro, suggesting that these SNPs could explain the difference in Arntl2 expression between these two strains. In order to validate the causative role of the SNPs in vivo, metastatic cell lines were engineered using CRISPR technology to specifically replace the FVB with the MOLF promoter. Substitution of this promoter reduced Arntl2 transcript levels and subsequently decreased pulmonary metastases, as expected. Since Arntl2 is a transcription factor, current studies are focused on identifying Arntl2-regulated genes to investigate the pathways that are involved in modulating metastasis. This study will have important implications regarding the function of circadian rhythm in cancer progression and provide a potential molecular mechanism to explain the increased risk of breast cancers in nightshift workers. Furthermore, to the best of our knowledge this would be the first example of genome editing to validate polymorphisms in transcriptional control elements associated with inherited susceptibility for metastasis.
Citation Format: Ngoc-Han Ha, Kent W. Hunter. Polymorphisms in the Arntl2 promoter affect metastatic susceptibility in breast cancer. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Metastasis; 2015 Nov 30-Dec 3; Austin, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(7 Suppl):Abstract nr B12.