Smith-Sørensen B, Lind G E, Skotheim R I, Fosså S D, Fodstad Ø, Stenwig A-E, Jakobsen K S, Lothe R A
Department of Tumour Biology, Institute for Cancer Research, The Norwegian Radium Hospital, 0310 Oslo, Norway.
Oncogene. 2002 Dec 12;21(57):8878-84. doi: 10.1038/sj.onc.1205978.
Testicular germ cell tumours are classified into two major histological subgroups, seminomas and nonseminomas. All tumours display several recurrent chromosomal aberrations, but few target genes have been identified. Previous studies have shown that genome-wide hypermethylation of CpG islands is significantly more prevalent in nonseminomas than in seminomas. We have studied two potential target genes in testicular cancer. A series of 70 tumours were analysed for methylation of CpG sites in the O(6)-methylguanine-DNA methyltransferase (MGMT) gene promoter, and in exon 1alpha of the cyclin-dependent kinase inhibitor 2A gene (CDKN2A). In addition, eight microsatellite markers within and flanking these genes at chromosome arms 10q and 9p, respectively, were analysed for allelic imbalances. Allele alterations were frequently seen at 9p loci (47 out of 70, 67%), but none of the tumours (none out of 55) showed methylation of CDKN2A. On the other hand, a high frequency of MGMT promoter methylation (32 out of 69, 46%) was found, as well as allelic imbalances at 10q markers (50 out of 70, 71%). A significantly higher methylation frequency was found in nonseminomas (24 out of 35, 69%) compared to seminomas (eight out of 33, 24%) (P=0.0003, Fisher's exact test). Immunohistochemical analysis of the MGMT protein in a subgroup (n=20) of the testicular tumours supported the hypothesis of gene silencing being the functional consequence of the promoter methylation. In summary, our data suggest that inactivation of MGMT contributes to development of nonseminomatous testicular cancer.
睾丸生殖细胞肿瘤分为两个主要的组织学亚组，即精原细胞瘤和非精原细胞瘤。所有肿瘤均显示出几种反复出现的染色体畸变，但仅鉴定出少数靶基因。先前的研究表明，非精原细胞瘤中CpG岛的全基因组高甲基化比精原细胞瘤中更为普遍。我们研究了睾丸癌中的两个潜在靶基因。对一系列70个肿瘤进行了分析，检测O(6)-甲基鸟嘌呤-DNA甲基转移酶（MGMT）基因启动子以及细胞周期蛋白依赖性激酶抑制剂2A基因（CDKN2A）外显子1α中CpG位点的甲基化情况。此外，分别对位于染色体臂10q和9p上这些基因内部及侧翼的八个微卫星标记进行了等位基因失衡分析。9p位点经常出现等位基因改变（70个中有47个，占67%），但所有肿瘤（55个中无一例）均未显示CDKN2A甲基化。另一方面，发现MGMT启动子甲基化频率较高（69个中有32个，占46%），以及10q标记处的等位基因失衡（70个中有50个，占71%）。与精原细胞瘤（33个中有8个，占24%）相比，非精原细胞瘤中的甲基化频率明显更高（35个中有24个，占69%）（P = 0.0003，Fisher精确检验）。对一部分睾丸肿瘤（n = 20）进行MGMT蛋白的免疫组织化学分析，支持了基因沉默是启动子甲基化的功能后果这一假说。总之，我们的数据表明MGMT的失活有助于非精原性睾丸癌的发生。
