Germ cell neoplasia in situ (GCNIS) and the pathogenesis of testicular germ cell cancer
Camacho Moll, Maria Elena
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Testicular germ cell cancer (TGCC) has been increasing in incidence over recent decades, and is currently the most common malignancy amongst young men resulting in significant morbidity. These tumours are believed to arise from premalignant germ cell neoplasia in situ (GCNIS) cells, which originate from the aberrant germ cell differentiation from gonocyte to spermatogonia during fetal/early postnatal life. GCNIS cells remain dormant in the testis until puberty when they are activated to become tumours. Therefore, GCNIS cells remain in a pre-invasive stage during early childhood and early adulthood prior to the development of a seminoma or non-seminoma TGCC. GCNIS cells are phenotypically similar to gonocytes with expression of stem cell/early germ cell markers including OCT4, PLAP and LIN28. Furthermore, proteins which are expressed in more mature germ cells (spermatogonia) such as MAGE-A4 have also been shown to be expressed in GCNIS cells and these studies have indicated that GCNIS cells are a heterogeneous population in terms of protein expression profile. The relationship between the protein expression profile of individual GCNIS cells populations and their oncogenic potential has not been fully explored. GCNIS cells are located in the seminiferous tubules supported by somatic Sertoli cells. These cells have been previously reported to exhibit an immature protein expression profile in GCNIS tubules from patients with testis cancer, suggesting that the germ stem cell niche in GCNIS tubules resembles that of a fetal one. Associations between Sertoli cell maturation and GCNIS progression into tumour formation has not been fully investigated. Oncogenes are key players in the regulation of oncogenic potential of cancer cells. Gankyrin is an oncogene that has been shown to down-regulate OCT4, and interact with MAGE-A4 in hepatocellular carcinoma and colorectal cancer, where Gankyrin interaction with MAGE-A4 reduces the oncogenic potential of tumour cells. In this study I aimed to investigate the heterogeneity of GCNIS in relation to disease stage and Sertoli cell development. We also aimed to determine the role of Gankyrin in TGCC cell survival and invasion. The co-expression of early germ cells proteins such as OCT4, LIN28 and PLAP was characterized in GCNIS cells during childhood and adulthood pre-invasive TGCC and in invasive disease characterized by the presence of a testicular tumour. These results show that LIN28 was expressed in 95% of OCT4 GCNIS cells, whereas PLAP expression in GCNIS cells increased as the disease progressed from childhood pre-invasive disease to invasive seminoma (32.3% v 76%; p<0.05). In contrast there was a reduction in the proportion of MAGE-A4 expressing GCNIS cells with disease progression. The MAGE-A4 expressing population was also less proliferative than the MAGE-A4 negative GCNIS population. The methylation status of GCNIS cells was then investigated. EZH2 a methyltransferase previously reported to be important for TGCC development, was expressed in GCNIS cells at all stages of disease, however the histone 3 modification H3K27me3 (mediated by EZH2) was expressed in a significantly higher percentage of the proliferative OCT4+/MAGE-A4- GCNIS cells compared with the OCT4+/MAGEA4+ population (11.7% v 1.1%; p<0.01) which could indicate a repressive role for H3K27me3 over MAGE-A4 expression. Next, it was determined whether an association between Sertoli cell maturation status and progression of TGCC could be observed. The maturation status of Sertoli cells was studied using proteins indicative of immature (desmin, cytokeratin, fibronectin and AMH) and mature (vimentin and androgen receptor) Sertoli cells. These studies demonstrated heterogeneity of Sertoli cells maturation in GCNIS-containing tubules. Desmin, fibronectin, AMH and vimentin expression did not show any association with TGCC progression. Cytokeratin was expressed in Sertoli cells of human fetal testis up to second trimester of fetal life, absent in tubules with active spermatogenesis but heterogeneously present in GCNIS, demonstrating that cytokeratin expression is indicative of the presence of GCNIS. Androgen receptor was weakly present in Sertoli cells from human fetal testis and pre-pubertal pre-invasive TGCC testis whereas in GCNIS of adult pre-invasive testis and invasive samples, androgen receptor was abundantly expressed in Sertoli cells of GCNIS-containing tubules. These combined results for cytokeratin and androgen receptor suggest that Sertoli cells from GCNIS-containing tubules, in pre-invasive and invasive TGCC patients are partially differentiated. Gankyrin expression was characterised in fetal germ cells, GCNIS cells and TGCC tissue. In fetal testis nuclear Gankyrin was absent in OCT4+/MAGE-A4- (gonocyte) population whereas it was present in a subpopulation of OCT4-/MAGE-A4+ (spermatogonia) germ cells. In GCNIS cells from TGCC patients nuclear Gankyrin was expressed in 87%, 63.3%, 91.5% and 79% in childhood pre-invasive, adult pre-invasive, seminoma and non-seminoma GCNIS cells respectively. Finally, in seminoma cells, Gankyrin was expressed in the cytoplasm indicating a change in localisation as the GCNIS cells become invasive. We used siRNA to knockdown Gankyrin in NT2 (a TGCC cell line) cells in-vitro and demonstrated a decrease in cell number, suggesting that Gankyrin might play a role in TGCC progression and invasiveness. Gankyrin down-regulation also resulted in an increase in p53 and p21 mRNA level. Given the role of P53 and p21 in cisplatin cytotoxic effect in TGCC we went on to investigate the role of Gankyrin in cisplatin resistance using NT2 cells. We demonstrate that Gankyrin mediated cisplatin resistance through the p53/p21 pathway, upregulating apoptosis rates through BAX and FAS, whilst there was no effect on cell proliferation, cell cycle or cell migration. In conclusion, we have shown that GCNIS cells are heterogeneous and their phenotype can determine their oncogenic potential. We also show that Sertoli cells from GCNIS-containing tubules undergo partial differentiation displaying markers of immature and mature Sertoli cells, with a heterogeneous association of cytokeratin with GCNIS presence. We also demonstrate that the oncogene Gankyrin has a role in NT2 cells survival and cisplatin resistance indicating that manipulation of Gankyrin may have a role in the treatment of TGCC.