T‐cell factor 1 expression in germ cell tumors with trophoblastic differentiation

To the Editor: T-cell factor 1 (TCF-1) protein is a member of the LEF1/TCF family of transcription factors, which are constituents of the Wnt signaling pathway, that plays an important role in embryonal development, adult stem cell maintenance and tumor growth. TCF-1 binds to the DNA through a high mobility group and at least eight protein isoforms. The canonical Wnt operates in two modes, on and off. In the off mode, the Wnt ligand is absent and cytoplasmic complex, composed of axin, adenomatous polyposis coli (APC), Casein kinase 1 (Ck1) and Glycogen Synthase Kinase (Gsk3), stimulates the destruction of β-catenin. Consequently, as β-catenin fails to enter the nucleus and activates T-cell transcription factors, the TCFs instead bind to a group of co-repressor proteins and inhibit transcription of β-catenin dependent genes. In the on mode, Wnt ligand binding with membrane receptor inhibits cytoplasmic destruction complex, which allows translocation of β-catenin into nucleus and binding with amino-terminal end of TCF. As a consequence of this interaction, TCFs become transcriptional activators which bind to Wnt response elements of target genes, and increased activation of this signaling pathway may lead to development of cancer. Previous studies have indicated that the Wnt signaling pathway plays an important role in balancing cell proliferation, apoptosis and differentiation during the trophoblast differentiation. The β-catenin-induced TCF4 protein activates the GCM1/syncictin signaling pathway, which leads to fusion of the human choriocarcinoma cells. Expression of Wnt antagonist Wnt5a, found normally in human cytotrophoblast cells, is completely absent from choriocarcinoma cell line JAR, whereas exogenous recombinant Wnt5a applied to choriocarcinoma cells decreases their proliferation and induces apoptosis, suggesting it acts as a tumor suppressor for placental choriocarcinomas. Hypermethylation of the APC suppressor gene has been associated with all human choriocarcinoma cell lines. Dickkopf-related protein 1 (DKK1), another Wnt signaling pathway antagonist, is abundantly present in human cytotrophoblast cells, but it is absent from choriocarcinoma cells lines JAR and JEG3. When given exogenously to cells, it reduces proliferation of both choriocarcinoma cells lines and induces apoptosis of JAR cells. This tumor suppressor effect is believed to be accomplished through c-Jun N-terminal kinase, without directly involving Wnt signaling pathway. TCF-1 stimulates the growth of human malignant hematopoietic cells independently of β-catenin binding, i.e. its activity is accomplished through binding to ATF2 [activating transcription factor 2] family of proteins. It is thought that this alternative mechanism is responsible for development of some hematopoietic tumors. Increased expression of TCF-1 protein has been reported recently in various human malignancies including renal cell carcinoma and hepatocellular tumors (adenoma and carcinoma). The status of TCF-1 protein has not been analyzed in germ cell tumors of gonads including the tumors with trophoblastic differentiation. In the present report using immunohistochemistry (Clone: sc-8589, Santa Cruz Biotechnology, Santa Cruz, CA, USA, dilution 1:50) we explored TCF-1 expression in a subset of germ cell tumors with trophoblastic differentiation and compared it with adjacent normal/benign reproductive tissues (negative control) and tumor-infiltrating T-lymphocytes (positive control). TCF-1 protein was scored for the intensity and percentage in the choriocarcinoma cells. Tumor-infiltrating lymphocytes retained a strong, nuclear expression of TCF-1 protein (Fig. 1c), whereas normal and benign tissues (testis, ovary, endometrium and myometrium) were negative (Fig. 1a,c). Pure ovarian choriocarcinomas (n = 3) exhibited predominantly strong TCF-1 expression (score 3+) in the range 20–70% of the tumors cells (Fig. 1b). In mixed germ cell tumors of the testis (n = 5), choriocarcinoma cells also showed intense TCF-1 protein expression (Fig. 1d), whereas other germ cell compartments (seminoma, embryonal carcinoma, yolk sac tumor and teratoma) exhibited only weak (score 1+) and focal TCF-1 positivity (<10% of the cells) (Fig. 1c). Our preliminary data indicate that TCF-1 protein may be actively involved in pathogenesis of a subset of germ cell tumors with trophoblastic differentiation. Further functional and clinical studies should validate the relevance of TCF-1 protein in these tumors.