Detection of high‐mobility group proteins A1 and A2 represents a valid diagnostic marker in post‐pubertal testicular germ cell tumours

The high-mobility group A (HMGA) non-histone chromosomal proteins HMGA1 and HMGA2 are architectural factors. They are abundantly expressed during embryogenesis and in most malignant neoplasias, whereas their expression is low or absent in normal adult tissues. Their over-expression is known to have a causal role in cellular neoplastic transformation. Previous studies from our group have shown that their expression is restricted to specific germinal cells. In this study we have evaluated, by immunohistochemistry, the expression of HMGA1 and HMGA2 in a series of post-pubertal testicular tumours of different histological types, including 30 seminomas, 15 teratomas, 15 embryonal carcinomas and 10 mixed germinal tumours with a prominent yolk sac tumour component. HMGA1 protein expression was detected in all seminomas and embryonal carcinomas analysed, but not in teratomas or yolk sac carcinomas. Conversely, HMGA2 was present only in embryonal carcinomas and yolk sac carcinomas, but not in seminomas or teratomas. The immunohistochemical data were further confirmed by Western blot and, at the mRNA level, by RT-PCR analyses. These findings indicate that HMGA1 and HMGA2 are differently expressed with respect to the state of differentiation of testicular germ cell tumours (TGCTs), with over-expression of both proteins in pluripotential embryonal carcinoma cells and loss of expression of HMGA1 in yolk sac tumours and of both proteins in the mature adult tissue of teratoma areas. Therefore, the different profiles of HMGA1 and HMGA2 protein expression could represent a valuable diagnostic tool in some cases in which the histological differential diagnosis is problematic.