Clark Jarrod, Smith Steven S
City of Hope, 1500 E. Duarte Rd., Duarte, CA 91010, USA.
Cancer Genomics Proteomics. 2008 Sep-Oct;5(5):241-51.
The VNTR at c-Ha-ras resides in a hotspot for DNA methylation on chromosome 11 in human tumors, where it is flanked by two MspI restriction sites. We have investigated the nature of the MspI site polymorphism at the c-Ha-ras VNTR observed in variety of tumors including breast cancer.We find that the MspI site 5' to the VNTR is present in a Non-B DNA structure with single-strand character that renders it accessible to bisulfite modification under native conditions, while the MspI site 3' to the VNTR appears to reside in a normal B-form structure that is inaccessible to bisulfite. The non-B DNA structure accounts for the observed polymorphism since MspI cannot cleave single-stranded DNA and control experiments show that the MspI sites were neither mutated nor abnormally methylated. Southern blotting showed that structural polymorphism was present in tumor DNA and tumor adjacent normal tissue DNA but absent from lymphocyte DNA from the same patients. We conclude that the non-B DNA structural polymorphism detected in human tumors near the c-Ha-ras VNTR is a self-perpetuating epigenetic mark that manifests itself spontaneously during breast carcinogenesis in a methylation hot spot.
c-Ha-ras基因座上的可变数目串联重复序列（VNTR）位于人类肿瘤中11号染色体上DNA甲基化的热点区域，其两侧有两个MspI限制性酶切位点。我们研究了在包括乳腺癌在内的多种肿瘤中观察到的c-Ha-ras VNTR处MspI位点多态性的本质。我们发现，VNTR 5'端的MspI位点存在于具有单链特征的非B型DNA结构中，这使得它在天然条件下可被亚硫酸氢盐修饰，而VNTR 3'端的MspI位点似乎位于正常的B型结构中，亚硫酸氢盐无法作用于此。由于MspI不能切割单链DNA，且对照实验表明MspI位点既未发生突变也未出现异常甲基化，因此非B型DNA结构解释了所观察到的多态性。Southern印迹分析表明，肿瘤DNA和肿瘤旁正常组织DNA中存在结构多态性，但同一患者的淋巴细胞DNA中不存在。我们得出结论，在c-Ha-ras VNTR附近的人类肿瘤中检测到的非B型DNA结构多态性是一种自我延续的表观遗传标记，在乳腺癌发生过程中于甲基化热点区域自发显现。
