Unique and redundant roles of SOX2 and SOX17 in regulating the germ cell tumour fate

Abstract

Embryonal carcinomas (ECs) and seminomas are testicular germ cell tumours. ECs display expression of SOX2, while seminomas display expression of SOX17. In somatic differentiation, SOX17 drives endodermal cell fate. However, seminomas lack expression of endoderm markers, but show features of pluripotency. Here, we use ChIP-sequencing to report and compare the binding pattern of SOX17 in seminoma-like TCam-2 cells to SOX2 in EC-like 2102EP cells and SOX17 in somatic cells. In seminoma-like cells, SOX17 was detected at canonical (SOX2/OCT4), compressed (SOX17/OCT4) and other SOX family member motifs. SOX17 directly regulates TFAP2C, PRDM1 and PRDM14, thereby maintaining latent pluripotency and supressing somatic differentiation. In contrast, in somatic cells canonical motifs are not bound by SOX17. In sum only 10% of SOX17 binding sites overlap in seminoma-like and somatic cells. This illustrates that binding site choice is highly dynamic and cell type specific. Deletion of SOX17 in seminoma-like cells resulted in loss of pluripotency, marked by a reduction of OCT4 protein level and loss of alkaline phosphatase activity. Further, we found that in EClike cells SOX2 regulates pluripotency-associated genes, predominantly by partnering with OCT4. In conclusion, SOX17 (in seminomas) functionally replaces SOX2 (in ECs) to maintain expression of the pluripotency cluster.

Molecular and epigenetic pathogenesis of germ cell tumors

“…Jostes et al. [ 45 ] finally illuminated the different roles of SOX2 and SOX17 in TGCT fate. ChIP-seq data revealed that in TCam-2 cells, SOX17 also bound to both motifs (canonical and compressed), resulting in an overlap of SOX2 and SOX17 target genes.…”

“…Therefore, pluripotency factors can be regulated by SOX17 as well as SOX2, indicating an overlap in their function as transcriptional regulators ( Table 1 ). However, SOX2 also binds to genes upregulated in embryonic stem cells like GDF3, LEFTY2, SALL4, TP53, as well as SOX2 and OCT4 itself, creating a regulating loop, which was not found for SOX17 binding sites [ 45 ]. Further, comparing SOX17 binding sites in TCam-2 and differentiated ESC, only showed an overlap of 10%, where, out of all genes bound by SOX17 in TCam-2, 66% were only in TCam-2, but not found in the somatic lineages.…”

“…Further, comparing SOX17 binding sites in TCam-2 and differentiated ESC, only showed an overlap of 10%, where, out of all genes bound by SOX17 in TCam-2, 66% were only in TCam-2, but not found in the somatic lineages. PRDM1 and TFAP2C were found within these specific genes for TCam-2 [ 45 ], which are known to be transactivated by SOX17 during PGC specification [ 21 , 46 ], further demonstrating high similarity between SE and PGCs. The canonical motif was not found in hits of differentiated lineages, suggesting the canonical motif being specific for SE and possibly PGC.…”

“…The canonical motif was not found in hits of differentiated lineages, suggesting the canonical motif being specific for SE and possibly PGC. This concludes that DNA accessibility plays an important role in availability of target genes to SOX17 [ 45 ]. Considering the dramatic epigenetic changes, like DNA demethylation and histone modification during GC development, the accessibility of SOX17 to its target genes might be much easier in PGCs compared to somatic cells, making it possible for SOX17 to regulate factors of GC fate and pluripotency.…”

“…In addition, Jostes et al. [ 45 ] could identify genes bound by SOX2 in EC cell line 2102EP, but not in ESC, concluding that MLEC, PIM2, CD99L2, and APOBEC3F are genes which might be involved in GC tumorigenesis. The same applies to cancer related MYC and IGF1, which are bound by SOX17 only in TCam-2 cells.…”