RNA-binding protein LIN28 is a marker for testicular germ cell tumors

LIN28 is an RNA-binding protein involved in maintaining the pluripotency of embryonic stem cells. Using formalin-fixed, paraffin-embedded tissue blocks, we performed immunohistochemical staining of LIN28 in 103 primary and 81 metastatic testicular germ cell tumors (54 intratubular germ cell neoplasias, unclassified type; 49 primary and 20 metastatic classic seminomas; 35 primary and 24 metastatic embryonal carcinomas; 35 primary and 15 metastatic yolk sac tumors; 23 primary and 12 metastatic teratomas; 6 primary and 10 metastatic choriocarcinomas; and 5 spermatocytic seminomas). The percentage of tumor cell stained was scored as 0 (0%), 1+ (≤30%), 2+ (31%-60%), 3+ (61%-90%), and 4+ (>90%). We stained LIN28 in 327 non-germ cell tumors to determine its specificity. We also compared LIN28 with SALL4 (Sal-like 4) and OCT4 (octamer-binding transcription factor 4) in all germ cell tumors. The staining was cytoplasmic for LIN28 and nuclear for SALL4 and OCT4. Strong 4+ LIN28 staining was seen in all 54 intratubular germ cell neoplasias, 59 embryonal carcinomas, and 50 yolk sac tumors. Positive LIN28 staining was seen in all 69 classic seminomas (1+ in 3, 3+ in 3, and 4+ in 63) (63, strong). Variable staining of LIN28 was seen in 10 of 35 teratomas (1+ to 3+, weak to strong intensity), 12 of 16 choriocarcinomas (1+ to 4+, weak to strong intensity), and 1 of 5 spermatocytic seminomas (2+, weak). Only 10 of 327 non-germ cell tumors showed 1+ weak LIN28 staining. Therefore, LIN28 is a highly sensitive marker for testicular intratubular germ cell neoplasias, classic seminomas, embryonal carcinomas, and yolk sac tumors with relatively high specificity. LIN28 can be used as a diagnostic marker for these tumors and has demonstrated a similar level of diagnostic utility as SALL4 (except for a few classic seminomas), although it does not show an advantage over SALL4. The major advantage of LIN28 over OCT4 is in diagnosing yolk sac tumors (yolk sac tumors negative for OCT4).