Immunophenotypic assessment of pure embryonal carcinoma and yolk sac tumour suggests that reprogramming to non‐seminoma occurs outside the spermatogonial niche

Aims: Germ cell neoplasia in situ (GCNIS) is the precursor of GCNIS-derived testicular germ cell tumours, including seminomas and non-seminomas. Most non-seminomas show mixed histology, often including components of seminoma. A small subset presents as pure non-seminomatous germ cell tumors. In these neoplasms, the absence of invasive seminoma. raises the possibility that reprogramming to non-seminoma may occur at an early in-situ stage (i.e., within the spermatogonial niche). This study examined GCNIS associated with pure embryonal carcinoma (EC) and postpubertal-type yolk sac tumour (YST) using immunohistochemistry for transcription factors characteristic of GCNIS/seminoma, EC, and YST phenotypes to determine if reprogramming to non-seminoma occurs within the spermatogonial niche. Methods: GCNIS associated with pure testicular YST and EC was assessed using OCT4, FOXA2 (only YST), CD30 (only EC), SOX2 (only EC), SOX17, and NANOG immunohistochemistry. Adjacent foci of intratubular and invasive tumour were also evaluated, if present. Results: Sixty-seven pure non-seminomas (63 EC, 4 YST) were evaluated. In all cases, GCNIS expressed OCT4, SOX17, and NANOG. GCNIS associated with EC was consistently negative for CD30 and SOX2, and GCNIS associated with YST was consistently negative for FOXA2. Adjacent invasive EC (62/67 cases) and intratubular EC (19/67) showed the classic EC immunophenotype: OCT4+/NANOG+/SOX17–/SOX2+/CD30+. Similarly, invasive YST adjacent to GCNIS was OCT4–/NANOG–/SOX17+/FOXA2+. Conclusions: Assessment of transcription factors involved in the induction and maintenance of EC and YST phenotypes suggests that, in pure non-seminomas, reprogramming occurs outside the spermatogonial niche.