Mammographic density and genetics - A study of breast biopsies in relation to mammoraphic density. Homo sapiens

Introduction Mammographic density (MD), as assessed from film screen mammograms, is determined by the relative content of adipose, connective and epithelial tissue in the female breast. In epidemiological studies, a high percentage of MD confers a four to six fold risk elevation of developing breast cancer, even after adjustment for other known breast cancer risk factors. However, the biologic correlates of density are little known. Methods Gene expression analysis using whole genome arrays was performed on breast biopsies from 143 women; 79 women with no malignancy (healthy women) and 64 newly diagnosed breast cancer patients, both included from mammographic centres. Percent MD was determined using a previously validated, computerized method on scanned mammograms. Significance analysis of microarrays (SAM) was performed to identify genes influencing MD and generalized regression models were used to assess the independent contribution from different variables to MD. Results SAM-analysis identified 24 genes differentially expressed between samples from breasts with high and low MD. These genes included three uridine 5'-diphospho-glucuronosyltransferase (UGT) genes and the oestrogen receptor gene (ESR1). These genes were down-regulated in samples with high MD compared to those with low MD. The UGT gene products, which are known to inactivate oestrogen metabolites, were also down-regulated in tumour samples compared to samples from healthy individuals. Several single nucleotide polymorphisms (SNPs) in the UGT genes associated with the expression of UGT and other genes in their vicinity were identified. Conclusions Three UGT enzymes were lower expressed both in breast tissue biopsies from healthy women with high MD and in biopsies from newly diagnosed breast cancers. The association was strongest among young women and women using hormonal therapy. UGT2B10 predicts MD independently of age, hormone therapy and parity. Our results indicate that down-regulation of UGT genes in women exposed to female sex hormones is associated with high MD and might increase the risk of breast cancer. Overall design: Gene expression analysis of breast biopsies from 143 women, 79 non-cancer (healthy women with no cancer who had a mammogram taken) and 64 breast cancer.

引言
乳腺密度（Mammographic density, MD）是通过乳腺钼靶胶片评估得到的影像学指标，其量化依据为女性乳腺内脂肪组织、结缔组织与上皮组织的相对占比。在流行病学研究中，即便校正了其他已知的乳腺癌风险因素，高乳腺密度仍可使受试者的乳腺癌发病风险提升4至6倍。然而，目前学界对乳腺密度的生物学关联机制仍缺乏清晰认知。

方法
本研究对143名女性的乳腺活检组织样本开展全基因组芯片基因表达分析：其中79名为无恶性肿瘤的健康女性，64名为新确诊的原发性乳腺癌患者，所有受试者均招募自乳腺钼靶检查中心。采用此前经过临床验证的计算机辅助分析方法，对扫描存档的乳腺钼靶图像进行乳腺密度百分比测算。通过微阵列显著性分析（Significance Analysis of Microarrays, SAM）筛选与乳腺密度相关的差异表达基因，并运用广义回归模型评估各变量对乳腺密度的独立贡献程度。

结果
微阵列显著性分析共筛选得到24个在高、低乳腺密度样本中存在显著表达差异的基因，其中包含3个尿苷5'-二磷酸葡萄糖醛酸转移酶（uridine 5'-diphospho-glucuronosyltransferase, UGT）家族基因以及雌激素受体基因（ESR1）。与低乳腺密度样本相比，高乳腺密度样本中上述基因的表达水平均呈下调状态。已知可灭活雌激素代谢产物的UGT基因编码蛋白，在乳腺癌患者的肿瘤组织样本中，同样较健康个体的乳腺组织样本呈现表达下调。本研究还鉴定出UGT基因区域内多个与UGT自身及其邻近基因表达水平相关的单核苷酸多态性（single nucleotide polymorphism, SNP）位点。

结论
健康女性群体中高乳腺密度组的乳腺活检组织，以及新确诊乳腺癌患者的肿瘤活检组织中，均存在3种UGT酶的低表达现象。该关联在年轻女性以及接受激素治疗的女性亚群中最为显著。UGT2B10的基因表达水平可独立于年龄、激素治疗方案以及产次，对乳腺密度进行预测。本研究结果表明，暴露于女性性激素的女性体内UGT基因的表达下调，与高乳腺密度存在显著关联，或可提升个体的乳腺癌发病风险。

整体实验设计
对143名女性的乳腺活检组织开展基因表达分析，其中79名为无癌健康女性（曾接受乳腺钼靶检查），64名为原发性乳腺癌患者。