Differences in global DNA methylation of testicular seminoma are not associated with changes in histone modifications, clinical prognosis, BRAF mutations or gene expression

Association Study between Polymorphisms in DNA Methylation-Related Genes and Testicular Germ Cell Tumor Risk

BACKGROUND

Testicular germ cell tumors (TGCT), histologically classified as seminomas and nonseminomas, are believed to arise from primordial gonocytes, with the maturation process blocked when they are subjected to DNA methylation reprogramming. SNPs in DNA methylation machinery and folate-dependent one-carbon metabolism genes have been postulated to influence the proper establishment of DNA methylation.

METHODS

In this pathway-focused investigation, we evaluated the association between 273 selected tag SNPs from 28 DNA methylation-related genes and TGCT risk. We carried out association analysis at individual SNP and gene-based level using summary statistics from the Genome Wide Association Study meta-analysis recently conducted by the international Testicular Cancer Consortium on 10,156 TGCT cases and 179,683 controls.

RESULTS

In individual SNP analyses, seven SNPs, four mapping within MTHFR, were associated with TGCT risk after correction for multiple testing (q ≤ 0.05). Queries of public databases showed that three of these SNPs were associated with MTHFR changes in enzymatic activity (rs1801133) or expression level in testis tissue (rs12121543, rs1476413). Gene-based analyses revealed MTHFR (q = 8.4 × 10-4), methyl-CpG-binding protein 2 (MECP2; q = 2 × 10-3), and ZBTB4 (q = 0.03) as the top TGCT-associated genes. Stratifying by tumor histology, four MTHFR SNPs were associated with seminoma. In gene-based analysis MTHFR was associated with risk of seminoma (q = 2.8 × 10-4), but not with nonseminomatous tumors (q = 0.22).

CONCLUSIONS

Genetic variants within MTHFR, potentially having an impact on the DNA methylation pattern, are associated with TGCT risk.

IMPACT

This finding suggests that TGCT pathogenesis could be associated with the folate cycle status, and this relation could be partly due to hereditary factors.

WNT signalling in the normal human adult testis and in male germ cell neoplasms

STUDY QUESTION

Is WNT signalling functional in normal and/or neoplastic human male germ cells?

SUMMARY ANSWER

Regulated WNT signalling component synthesis in human testes indicates that WNT pathway function changes during normal spermatogenesis and is active in testicular germ cell tumours (TGCTs), and that WNT pathway blockade may restrict seminoma growth and migration.

WHAT IS KNOWN ALREADY

Regulated WNT signalling governs many developmental processes, including those affecting male fertility during early germ cell development at embryonic and adult (spermatogonial) ages in mice. In addition, although many cancers arise from WNT signalling alterations, the functional relevance and WNT pathway components in TGCT, including germ cell neoplasia in situ (GCNIS), are unknown.

STUDY DESIGN, SIZE, DURATION

The cellular distribution of transcripts and proteins in WNT signalling pathways was assessed in fixed human testis sections with normal spermatogenesis, GCNIS and seminoma (2-16 individuals per condition). Short-term (1-7 h) ligand activation and long-term (1-5 days) functional outcomes were examined using the well-characterised seminoma cell line, TCam-2. Pathway inhibition used siRNA or chemical exposures over 5 days to assess survival and migration.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The cellular localisation of WNT signalling components was determined using in situ hybridisation and immunohistochemistry on Bouin's- and formalin-fixed human testis sections with complete spermatogenesis or germ cell neoplasia, and was also assessed in TCam-2 cells. Pathway function tests included exposure of TCam-2 cells to ligands, small molecules and siRNAs. Outcomes were measured by monitoring beta-catenin (CTNNB1) intracellular localisation, cell counting and gap closure measurements.

MAIN RESULTS AND THE ROLE OF CHANCE

Detection of nuclear-localised beta-catenin (CTNNB1), and key WNT signalling components (including WNT3A, AXIN2, TCF7L1 and TCF7L2) indicate dynamic and cell-specific pathway activity in the adult human testis. Their presence in germ cell neoplasia and functional analyses in TCam-2 cells indicate roles for active canonical WNT signalling in TGCT relating to viability and migration. All data were analysed to determine statistical significance.

LARGE SCALE DATA

No large-scale datasets were generated in this study.

LIMITATIONS, REASONS FOR CAUTION

As TGCTs are rare and morphologically heterogeneous, functional studies in primary cancer cells were not performed. Functional analysis was performed with the only well-characterised, widely accepted seminoma-derived cell line.

WIDER IMPLICATIONS OF THE FINDINGS

This study demonstrated the potential sites and involvement of the WNT pathway in human spermatogenesis, revealing similarities with murine testis that suggest the potential for functional conservation during normal spermatogenesis. Evidence that inhibition of canonical WNT signalling leads to loss of viability and migratory activity in seminoma cells suggests that potential treatments using small molecule or siRNA inhibitors may be suitable for patients with metastatic TGCTs.

STUDY FUNDING AND COMPETING INTEREST(S)

This study was funded by National Health and Medical Research Council of Australia (Project ID 1011340 to K.L.L. and H.E.A., and Fellowship ID 1079646 to K.L.L.) and supported by the Victorian Government's Operational Infrastructure Support Program. None of the authors have any competing interests.

Pathogenesis of Testicular Germ Cell Neoplasia: A Conceptual Approach

Testicular germ cell tumors are a diverse group of neoplasms, consisting of the prepubertal type 1 tumors, pure teratoma, and pure yolk sac tumor, the type 2 tumors, which are biologically malignant, preceded by germ cell neoplasia in situ, and harbor chromosome 12p abnormalities, and the type 3 tumor, spermatocytic tumor, which features chromosome 9p amplification. These arise in distinct clinical settings, and harbor distinct genetic abnormalities, immunohistochemical properties, and morphologic features. Here we have attempted to unify embryology, morphology, immunohistochemistry, and genetics in order to distill this fascinating group of neoplasms into what we hope is a useful framework for understanding their classification.

New insights into germ cell tumor genomics

BACKGROUND

Testicular germ cell tumors (GCTs) represent the most common malignancy in young men. While GCTs represent a model for curable solid tumors due to exquisite chemosensitivity, mortality for patients with GCT comprises the most life years lost for non-pediatric malignancies. Given limited options for patients with platinum-resistant disease, improved insight into GCT biology could identify novel therapeutic options for patients with platinum-resistant disease. Recent studies into molecular characteristics of both early stage and advanced germ cell tumors suggest a role for rationally targeted agents and potentially immunotherapy.

RECENT DEVELOPMENTS

Recent GWAS meta-analyses have uncovered additional susceptibility loci for GCT and provide further evidence that GCT risk is polygenic. Chromosome arm level amplifications and reciprocal loss of heterozygosity have been described as significantly enriched in GCT compared to other cancer types. Contemporary analyses confirm ubiquitous gain of isochromosome 12 and mutations in addition to previously described GCT-associated genes such as KIT and KRAS. Alterations within the TP53-MDM2 signal transduction pathway appear to be enriched among patients with platinum-resistant disease. Potentially actionable targets, including alterations in TP53-MDM2, Wnt/β-catenin, PI3K, and MAPK signaling, are present in significant proportions of patients with platinum-resistant disease and may be exploited as therapeutic options. Pre-clinical and early clinical data also suggest a potential role for immunotherapy among patients with GCTs.

CONCLUSION

Molecular characterization of GCT patients may provide biologic rationale for novel treatment options in patients with platinum-resistant disease.

Cell-to-cell variation of chromosomal number in the adult testicular germ cell tumors: a comparison of chromosomal instability among histological components and its putative role in tumor progression

By allelotyping analysis, we previously reported a putative progression pathway from germ cell neoplasia in situ (GCNIS) to seminoma, then to embryonal carcinoma in mixed-type testicular germ cell tumors (TGCTs), and detected that loss of heterozygosity events in seminoma components in mixed tumors were more frequent than those in pure seminomas. To elucidate a role of chromosomal instability in the progression of non-seminomatous germ cell tumor (NSGCT), we performed fluorescence in situ hybridization with centromeric probes for chromosomes 1, 7, 8, 12, 17, and X on a cohort of 52 TGCT cases with 103 histologically distinct components: 39 GCNIS lesions (16 and 23 in tumors with and without NSGCT components, respectively), 39 seminomas (27 as pure seminomas and 12 in mixed tumors), and 25 embryonal carcinomas. On a total component basis, both the mean copy number per tumor cell nucleus and the deviations from the modal number of all chromosomes examined significantly increased from GCNIS to seminoma, then to embryonal carcinoma with few exceptions. Seminoma components in mixed tumors showed a significantly greater extent of chromosomal instability in chromosomes 8 and 12 than pure seminomas, whereas no statistically significant difference was observed between GCNIS lesions with and without NSGCT components. These results suggest that not only aneuploidy, but also the cell-to-cell variation of chromosomal number is a sensitive indicator of chromosomal instability and would be implicated in the progression of NSGCT.

Seminoma component of mixed testicular germ cell tumor shows a higher incidence of loss of heterozygosity than pure-type seminoma

Using analysis of allelic loss (loss of heterozygosity [LOH]), we previously reported a putative progression pathway from germ cell neoplasia in situ (GCNIS) to seminoma and then to embryonal carcinoma in mixed-type testicular germ cell tumors. To identify the genetic backgrounds related to the progression of nonseminomatous germ cell tumor, patterns of LOH were studied in seminoma components in mixed tumors (18 cases), pure seminomas (20 cases), and coexisting GCNIS lesions. Each tumor was assessed for LOH at 22 polymorphic loci located on 12 chromosomal arms: 3q, 5q, 6p, 9p, 10q, 11p, 12p, 12q, 13q, 17p, 17q, and 18q. For all informative loci, the frequency of LOH in seminoma components in mixed tumors was significantly higher than that in pure seminomas (32% [96/302 loci] versus 19% [60/323 loci], P < .0001). The frequency of LOH in GCNIS lesions was not significantly different between the 2 tumor groups. The frequencies of LOH at chromosomes 6p and 10q were significantly higher in seminoma components in mixed tumors than in pure seminomas (P = .020 and P = .0041, respectively). Immunohistochemical analysis demonstrated a close association between the allelic status of the 10q23 locus and levels of phosphatase and tensin homolog deleted from chromosome 10 protein expression in seminoma (P = .00051). These data indicate that the seminoma, which has a potential to progress to nonseminomatous germ cell tumor, already exhibits several genetic changes including allelic losses of 6p and 10q, unlike pure seminoma.