Claudin 1 is a tight junction protein which contributes to the transepithelial barrier that controls the transport of ions and small molecules in epithelial cells. Claudin 1 is also important for the organization of epithelial cell polarity and thus is crucial in maintaining the differentiated state of epithelial cells. An increasing number of studies have shown that the deregulated expression of the junctional proteins is directly or indirectly involved in cancer progression, including breast cancer. However, although claudin 1 forms the backbone of the tight junction, the role of claudin 1 in breast cancer is not known. Studies from our laboratory and others suggest that it may be that of a tumor suppressor, as it is often down regulated or absent in human invasive breast cancer. In the present study we explored mechanisms that may be involved in the down regulation of claudin 1 in human breast cancer.
Using claudin 1 primers, RT-PCR analysis of several human breast cancer tissue samples resulted in the identification of a number of cDNAs, ranging in size from 217bp-615bp. Sequencing of these cDNAs revealed that they were products of variant claudin 1 transcripts, exhibiting various deletions primarily involving exon 1. Further inspection of these sequences showed that the RNA from which they were derived could not be translated into full length claudin 1 protein because of existing frame shifts and premature stop codons which may partially explain the loss of the protein in these tumors. Using genomic DNA derived from the tumors, we sequenced the claudin 1 exon coding regions in order to ascertain whether deletions or mutations could explain the aberrant transcripts. We identified several single nucleotide polymorphisms (SNPs), however, the SNPs did not appear to be directly linked to these alternate splice variants. Additionally, methylation specific PCR assay of exon 1 of the claudin 1 gene suggests distinct methylation patterns within the breast tumors. This data, in conjunction with supporting evidence obtained from the Cancer Genome Atlas study, suggests that methylation is involved in the regulation of claudin 1 expression in human breast cancer.
In this study, we have identified for the first time variant claudin 1 transcripts in human breast cancer. These studies provide novel insights and reveal two possible mechanisms that may explain the down regulation of claudin 1during breast cancer progression. Since the breakdown of cell-cell interactions and the deregulated expression of the junctional proteins, are believed to be key steps in invasion and metastasis, understanding the molecular and genetic changes within tight junction molecules such as claudin 1 during breast tumorigenesis will be critical for facilitating more effective patient management.
Citation Format: Anne AA Blanchard, Xiuli Ma, Teresa Zelinski, Jiuyong Xie, Steven Cooper, Yvonne Myal. Identification of variant claudin 1 transcripts in human breast tumors. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 751. doi:10.1158/1538-7445.AM2013-751