Rivenbark Ashley G, Livasy Chad A, Boyd Courtney E, Keppler Daniel, Coleman William B
Department of Pathology and Laboratory Medicine, University of North Carolina School of Medicine, Chapel Hill, NC 27599, USA.
Exp Mol Pathol. 2007 Oct;83(2):188-97. doi: 10.1016/j.yexmp.2007.03.008. Epub 2007 Apr 18.
CST6 is a breast tumor suppressor gene that is expressed in normal breast epithelium, but is epigenetically silenced as a consequence of promoter hypermethylation in metastatic breast cancer cell lines. In the current study, we investigated the expression and methylation status of CST6 in primary breast tumors and lymph node metastases. 25/45 (56%) primary tumors and 17/20 (85%) lymph node metastases expressed significantly lower levels of cystatin M compared to normal breast tissue. Bisulfite sequencing demonstrated CST6 promoter hypermethylation in 11/23 (48%) neoplastic lesions analyzed, including 3/11 (27%) primary tumors and 8/12 (67%) lymph node metastases. In most cases (12/23, 52%), the expression of cystatin M directly reflected CST6 promoter methylation status. In remaining lesions (8/23, 35%) loss of cystatin M was not associated with CST6 promoter hypermethylation, indicating that other mechanisms can account for loss of CST6 expression. These results show that methylation-dependent silencing of CST6 occurs in a subset of primary breast cancers, but more frequently in metastatic lesions, possibly reflecting progression-related genomic events. To examine this possibility, primary breast tumors and matched lymph node metastases were analyzed. In 2/3 (67%) patients, primary tumors were positive for cystatin M and negative for CST6 promoter methylation, and matched metastatic lesions lacked cystatin M expression and CST6 was hypermethylated. This observation suggests that progression-related epigenetic alterations in CST6 gene expression can accompany metastatic spread from a primary tumor site. Overall, the results of the current investigation suggest that methylation-dependent epigenetic silencing of CST6 represents an important mechanism for loss of CST6 during breast tumorigenesis and/or progression to metastasis.
CST6是一种乳腺肿瘤抑制基因，在正常乳腺上皮中表达，但在转移性乳腺癌细胞系中由于启动子高甲基化而发生表观遗传沉默。在本研究中，我们调查了CST6在原发性乳腺肿瘤和淋巴结转移中的表达及甲基化状态。与正常乳腺组织相比，45例原发性肿瘤中有25例（56%）和20例淋巴结转移中有17例（85%）的胱抑素M表达水平显著降低。亚硫酸氢盐测序显示，在分析的23个肿瘤病变中有11个（48%）存在CST6启动子高甲基化，其中包括11例原发性肿瘤中的3例（27%）和12例淋巴结转移中的8例（67%）。在大多数情况下（23例中的12例，52%），胱抑素M的表达直接反映了CST6启动子的甲基化状态。在其余病变中（23例中的8例，35%），胱抑素M的缺失与CST6启动子高甲基化无关，这表明其他机制也可导致CST6表达缺失。这些结果表明，CST6的甲基化依赖性沉默发生在一部分原发性乳腺癌中，但在转移性病变中更常见，这可能反映了与进展相关的基因组事件。为了验证这一可能性，我们分析了原发性乳腺肿瘤和配对的淋巴结转移灶。在2/3（67%）的患者中，原发性肿瘤胱抑素M呈阳性，CST6启动子甲基化呈阴性，而配对的转移灶缺乏胱抑素M表达且CST6发生了高甲基化。这一观察结果表明，CST6基因表达中与进展相关的表观遗传改变可能伴随原发性肿瘤部位的转移扩散。总体而言，本研究结果表明，CST6的甲基化依赖性表观遗传沉默是乳腺肿瘤发生和/或进展至转移过程中CST6缺失的重要机制。
