Acquisition of Genomic Alterations During Mammary Gland

2-氨基-1-甲基-6-苯基咪唑并[4，5-B]吡啶（PhIP）是一种在人类饮食熟肉中发现的致突变化合物，是雌性Sprague-Dawley大鼠的乳腺致癌物。检测PhIP诱导的大鼠乳腺癌中已知调节细胞生长和凋亡的几个基因（外显子）的突变，包括p53（4-8）、p21 Waf 1（编码区）、Apc（14，15）、B-连环蛋白（3）、E-钙粘蛋白（9，13，15）、Bcl-x（编码区）、Bax（3）、IGFIIR（28）和TGF-β 1 R（3）。用单链构象多态性分析检测了30例肿瘤的DNA，但在这些基因/基因区域中未检测到突变。使用基于聚合酶链反应的方法，使用位于整个大鼠基因组中的已知微卫星区域的引物，进一步筛选来自癌和匹配正常组织的DNA的等位基因不平衡。在检测的53个标记中，有12个显示等位基因不平衡。在两个标记处检测到微卫星不稳定性（MSI），一个在4号染色体上，一个在6号染色体上。65%和96%的所有癌症检查（N=23）显示MSI在这些位点上的染色体4和6，分别支持的概念，MSI在PhIP诱导的乳腺癌发生中发挥作用。杂合性缺失（洛）是一种可能的肿瘤抑制基因的指示，在分布于3、10、11、14和X染色体上的10个标记处观察到。这些标记物的洛缺失频率范围为75%-94%，支持等位基因不平衡区域在本研究中检查的PhIP诱导的癌中很大程度上相似。当PhIP诱导的癌大鼠放置在高脂肪和低脂肪饮食进行了比较，没有独特的等位基因不平衡的区域，也没有等位基因不平衡的频率的统计学差异进行了观察。结果表明，高脂饮食，已知是一个促进PhIP诱导的大鼠乳腺癌发生，并没有影响癌中的等位基因失衡。有趣的是，DMBA诱导的乳腺癌在12个位点中的11个位点没有显示等位基因不平衡，而这些位点在PhIP诱导的乳腺癌中显示等位基因不平衡。这些研究结果表明，不同的化学致癌物诱导不同模式的等位基因失衡在大鼠乳腺癌的发生。洛缺失区域是否存在与乳腺癌相关的新的肿瘤抑制基因尚需进一步研究。乳腺癌发展的阶段和早期基因组变化目前正在这种大鼠模型中进行研究。

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a mutagenic compound found in the human diet in cooked meat, is a mammary gland carcinogen in female Sprague-Dawley rats. PhIP-induced rat mammary gland carcinomas were examined for mutations in several genes (exons) known to regulate cell growth and apoptosis including p53 (4-8), p21Waf1 (coding region), Apc (14, 15), B-catenin (3), E-cadherin (9,13,15), Bcl-x (coding region), Bax (3), IGFIIR (28), and TGFBIIR (3). DNA from 30 carcinomas were examined by single strand conformation polymorphism analysis but no mutations were detected in these genes/gene regions. DNA from carcinomas and matching normal tissue were further screened for allelic imbalance using a polymerase chain reaction-based approach with primers to known microsatellite regions located throughout the rat genome. Out of 53 markers examined, 12 revealed allelic imbalance. Microsatellite instability (MSI) was detected at two markers, one on chromosome 4 and one on chromosome 6. Sixty-five percent and 96% of all carcinomas examined (N=23) showed MSI at these loci on chromosomes 4 and 6, respectively, supporting the notion that MSI plays a role in PhIP-induced mammary carcinogenesis. Loss of heterozygosity (LOH), an indication of a possible tumor suppressor gene, was observed at ten markers distributed on chromosomes 3, 10, 11, 14, and X. The frequency of LOH at these markers ranged from 75%-94% supporting that the regions of allelic imbalance were largely similar for the PhIP-induced carcinomas examined in this study. When PhIP-induced carcinomas from rats placed on high fat and low fat diet were compared, no unique regions of allelic imbalance nor statistical differences in the frequency of allelic imbalance were observed. The results indicate that the high fat diet, known to be a promoter of PhIP-induced rat mammary carcinogenesis, did not influence allelic imbalance in the carcinomas. Interestingly, DMBA-induced mammary carcinomas did not show allelic imbalance at 11 of the 12 loci which showed allelic imbalance in PhIP-induced carcinomas. These findings suggest that distinct chemical carcinogens induce different patterns of allelic imbalance during rat mammary carcinogenesis. Further studies are needed to determine whether regions of LOH harbor potentially novel tumor suppressor genes involved in breast cancer. Stages of mammary gland cancer development and early genomic changes are currently being examined in this rat model.