Epigenetic Regulation of Driver Genes in Testicular Tumorigenesis

Abstract

In testicular germ cell tumor type II (TGCT), a seminoma subtype expresses an induced pluripotent stem cell (iPSC) panel with four upregulated genes, OCT4/POU5F1, SOX17, KLF4, and MYC, and embryonal carcinoma (EC) has four upregulated genes, OCT4/POU5F1, SOX2, LIN28, and NANOG. The EC panel can reprogram cells into iPSC, and both iPSC and EC can differentiate into teratoma. This review summarizes the literature on epigenetic regulation of the genes. Epigenetic mechanisms, such as methylations of cytosines on the DNA string and methylations and acetylations of histone 3 lysines, regulate expression of these driver genes between the TGCT subtypes. In TGCT, the driver genes contribute to well-known clinical characteristics and the driver genes are also important for aggressive subtypes of many other malignancies. In conclusion, epigenetic regulation of the driver genes are important for TGCT and for oncology in general.

Keywords: LIN28; NANOG; POU5F1; SOX2; differentiation; epigenetics; testis cancer.

Figure 1

TGCT subtypes in the tumorigenesis of TGCT. (a). The epigenetic landscape for germ cells. The germ cells progress from toti-differentiated cells to unipotent cells, and a gene panel of OCT4, SOX2, LIN28, and NANOG (OSLN) can reprogram germ cells for induced pluri- to totipotent stem cells (iPSC). The red boxes show gene panels for iPSC. The progression of the TGCT subtypes implies a loss of chromosomes from tetraploid malignant germ cells over hypertriploid seminoma to hypotriploid embryonic carcinoma. (b). Microinvasive germ cell tumor (MGCT). MGCT is the first invasive step in the tumorigenesis of TGCT. MGCT is an intermediary precursor of TGCT that has malignant germ cells inside and outside seminiferous tubules in the testis. The figure shows MGCT stained for OCT4, the protein of POU5F1. (c). Embryonal carcinoma (EC). EC has a positive homogeneous immunohistochemical staining for three transcription factors for induced pluripotent stem cells (OCT4, SOX2, and NANOG) and a homogeneous negative staining for the protein of the tumor suppressor gene, RB1. EC stained immunohistochemically positive for OCT4 (1c1), SOX2 (1c2), LIN28 (1c3), and immunohistochemically negative for RB1 (1c4).

Figure 1

TGCT subtypes in the tumorigenesis of TGCT. (a). The epigenetic landscape for germ cells. The germ cells progress from toti-differentiated cells to unipotent cells, and a gene panel of OCT4, SOX2, LIN28, and NANOG (OSLN) can reprogram germ cells for induced pluri- to totipotent stem cells (iPSC). The red boxes show gene panels for iPSC. The progression of the TGCT subtypes implies a loss of chromosomes from tetraploid malignant germ cells over hypertriploid seminoma to hypotriploid embryonic carcinoma. (b). Microinvasive germ cell tumor (MGCT). MGCT is the first invasive step in the tumorigenesis of TGCT. MGCT is an intermediary precursor of TGCT that has malignant germ cells inside and outside seminiferous tubules in the testis. The figure shows MGCT stained for OCT4, the protein of POU5F1. (c). Embryonal carcinoma (EC). EC has a positive homogeneous immunohistochemical staining for three transcription factors for induced pluripotent stem cells (OCT4, SOX2, and NANOG) and a homogeneous negative staining for the protein of the tumor suppressor gene, RB1. EC stained immunohistochemically positive for OCT4 (1c1), SOX2 (1c2), LIN28 (1c3), and immunohistochemically negative for RB1 (1c4).

Figure 1

TGCT subtypes in the tumorigenesis of TGCT. (a). The epigenetic landscape for germ cells. The germ cells progress from toti-differentiated cells to unipotent cells, and a gene panel of OCT4, SOX2, LIN28, and NANOG (OSLN) can reprogram germ cells for induced pluri- to totipotent stem cells (iPSC). The red boxes show gene panels for iPSC. The progression of the TGCT subtypes implies a loss of chromosomes from tetraploid malignant germ cells over hypertriploid seminoma to hypotriploid embryonic carcinoma. (b). Microinvasive germ cell tumor (MGCT). MGCT is the first invasive step in the tumorigenesis of TGCT. MGCT is an intermediary precursor of TGCT that has malignant germ cells inside and outside seminiferous tubules in the testis. The figure shows MGCT stained for OCT4, the protein of POU5F1. (c). Embryonal carcinoma (EC). EC has a positive homogeneous immunohistochemical staining for three transcription factors for induced pluripotent stem cells (OCT4, SOX2, and NANOG) and a homogeneous negative staining for the protein of the tumor suppressor gene, RB1. EC stained immunohistochemically positive for OCT4 (1c1), SOX2 (1c2), LIN28 (1c3), and immunohistochemically negative for RB1 (1c4).

Figure 2

Development of normal and malignant testicular germ cells. (a) Shows the development of normal testicular germ cells from fetal life to spermatozoa in adulthood, and the development of malignant testicular germ cells from tetraploid malignant germ cells to the testicular germ cell tumors type II subtypes. Seminomas are diagnosed at a higher age than non-seminomatous testicular germ cell tumors as reflection of a lower proliferation rate in seminoma. (b). Chromosome 12. The short arm of chromosome 12 (12p) has two candidate regions that are especially important for the tumorigenesis of testicular germ cell tumors type II, shown in red. The candidate regions have gene loci for three driver genes: CCND2, NANOG and LDHB. The candidate regions are shown in red. The figure also shows other genes that literature and OMIM relate to TGCT. (c). IPSC gene panel in TGCT subtypes. The combined OSKM/OSLN panels for seminoma, embryonal carcinoma, and teratoma have dramatic differences with up- and down-regulations of the genes. Down-regulated genes are indicated with red and up-regulated genes are indicated with green.

Figure 2

Development of normal and malignant testicular germ cells. (a) Shows the development of normal testicular germ cells from fetal life to spermatozoa in adulthood, and the development of malignant testicular germ cells from tetraploid malignant germ cells to the testicular germ cell tumors type II subtypes. Seminomas are diagnosed at a higher age than non-seminomatous testicular germ cell tumors as reflection of a lower proliferation rate in seminoma. (b). Chromosome 12. The short arm of chromosome 12 (12p) has two candidate regions that are especially important for the tumorigenesis of testicular germ cell tumors type II, shown in red. The candidate regions have gene loci for three driver genes: CCND2, NANOG and LDHB. The candidate regions are shown in red. The figure also shows other genes that literature and OMIM relate to TGCT. (c). IPSC gene panel in TGCT subtypes. The combined OSKM/OSLN panels for seminoma, embryonal carcinoma, and teratoma have dramatic differences with up- and down-regulations of the genes. Down-regulated genes are indicated with red and up-regulated genes are indicated with green.

Figure 3

Epigenetic regulation of gene expression. (a). Histone modifications and gene expression. Histone lysine methylations and acetylations can lead to repressed, poised or activated gene expressions. A combination of histone marks that act both to repress and activate a gene leads to a poised regulation of expression of the gene. (b). Sites for histone marks and gene regulation. Methylated histone 3 lysines differ in action sites on enhancers, promoters, and gene bodies of genes. Red indicates down-regulated gene expression and green indicates upregulated expression of the gene.

Figure 3

Epigenetic regulation of gene expression. (a). Histone modifications and gene expression. Histone lysine methylations and acetylations can lead to repressed, poised or activated gene expressions. A combination of histone marks that act both to repress and activate a gene leads to a poised regulation of expression of the gene. (b). Sites for histone marks and gene regulation. Methylated histone 3 lysines differ in action sites on enhancers, promoters, and gene bodies of genes. Red indicates down-regulated gene expression and green indicates upregulated expression of the gene.

Figure 4

A new trial of epidrugs in TGCT. Candidate patients are patients with high-risk NS metastatic TGCT with insufficient decay of serum tumor markers after the first course of BEP chemotherapy. The proposed trial investigates whether combination of epidrugs and BEP chemotherapy might give response equal to high-dose salvage chemotherapy. The new elements in the trial are shown in grey boxes. If a patient fails to present with sufficient decay of two serum tumor markers, the patient will be switched to a routine high-dose cisplatin salvage chemotherapy.