Expression of OCT4 in the primary germ cell tumors and thymoma in the mediastinum

Primary germ cell tumors (GCTs) and thymoma are both located in the anterior mediastinum. A previous study has postulated that octamer binding transcription factor (OCT4) is a nuclear transcription factor that is expressed in pluripotent embryonic germ cells. This study examined OCT4 expression in GCTs and thymoma originating from the mediastinum. A retrospective study included 46 consecutive patients with GCTs conducted between 1983 and 2005, and 22 consecutive thymoma in the mediastinum whose tumors had been surgically excised. The 46 primary GCTs in mediastinum included teratoma (n=27; 58.7%), seminoma (n=10; 21.7%), yolk sac tumor (n=6; 13%), embryonal carcinoma (n=1; 2.1%), and mixed GCTs (n=2; 4%; one consisted of teratoma and yolk sac tumor, and the other teratoma, yolk sac tumor, and seminoma); and 22 thymoma including World Health Organization type A (n=3, 13.6%), type AB (n=4, 18.2%), type B1 (n=6, 27.3%), type B2 (n=4, 13.6%), and type B3 (n=5, 22.7%). Each tumor was examined with hematoxylin and eosin staining, and with antibodies to OCT4. All 10 seminoma cases, 1 embryonal carcinoma case, and 1 mixed GCT case containing seminoma were immunopositive for OCT4. On the other hand, the 22 thymoma, 6 yolk sac tumor, 27 teratomas, and 1 case with mixed GCT without component of seminoma were immunonegative for OCT4. We conclude that immunostaining with antibodies to OCT4 is a useful diagnostic tool in the identification of seminomas and primary embryonal carcinomas in GCTs originating from the mediastinum.