MMTV-Myc tumor development in E2F-null backgrounds

Advances in genomic signatures have begun to dissect breast cancer heterogeneity, and application of these signatures will allow the prediction of which pathways are important in tumor development. Here we used genomic signatures to predict involvement of specific E2F transcription factors in Myc-induced tumors. We genetically tested this prediction by interbreeding Myc transgenics with mice lacking various activator E2F alleles. Tumor latency decreased in the E2F1 mutant background and significantly increased in both the E2F2 and E2F3 mutants. Investigating the mechanism behind these changes revealed a reduction in apoptosis in the E2F1 knockout strain. E2F2 and E2F3 mutant backgrounds alleviated Myc effects on the mammary gland, reducing the susceptible tumor target population. Gene expression data from tumors revealed that the E2F2 knockout background resulted in fewer tumors with EMT, corresponding with a reduction in probability of Ras activation. In human breast cancer we found that a low probability of E2F2 pathway activation was associated with increased relapse-free survival time. Together these data illustrate the predictive utility of genomic signatures in deciphering the heterogeneity within breast cancer and illustrate the unique genetic requirements for individual E2Fs in mediating tumorigenesis in both mouse models and human breast cancer. MMTV-Myc tumors were generated in an E2F wild-type, E2F1 null, E2F2 null and E2F3 heterozygous background. When the primary tumor reached the endpoint, the tumors were flash frozen. 20 tumors from each genotype were selected for microarray analysis.

随着基因组特征（genomic signatures）研究的进展，研究者已得以解析乳腺癌的异质性；而这类特征的应用，则可预测肿瘤发生过程中的关键通路。本研究中，我们借助基因组特征预测了特定E2F转录因子在Myc诱导肿瘤中的参与作用。我们通过将Myc转基因小鼠与携带不同激活型E2F等位基因缺失的小鼠进行杂交，对上述预测开展了遗传学验证。结果显示，在E2F1突变背景的小鼠中，肿瘤潜伏期缩短；而在E2F2与E2F3突变的小鼠中，肿瘤潜伏期则显著延长。对上述表型变化背后机制的探究显示，E2F1敲除小鼠品系的细胞凋亡水平有所降低。E2F2与E2F3的突变背景可缓解Myc对乳腺的作用，减少易感肿瘤靶细胞群体。肿瘤的基因表达数据显示，E2F2敲除背景下出现上皮间质转化（EMT）的肿瘤更少，这与Ras激活概率的降低相一致。在人类乳腺癌样本中，我们发现E2F2通路激活概率较低与无复发生存期延长显著相关。综上，本研究数据证实了基因组特征在解析乳腺癌异质性方面的预测价值，同时也阐明了不同E2F家族成员在小鼠模型与人类乳腺癌的肿瘤发生过程中各自独特的遗传学需求。本研究在E2F野生型、E2F1敲除、E2F2敲除及E2F3杂合的遗传背景中构建了MMTV-Myc诱导的肿瘤模型。当原位肿瘤达到实验终点时，将肿瘤组织快速冷冻保存。每个基因型选取20例肿瘤样本用于微阵列（microarray）分析。