c-KIT is frequently mutated in bilateral germ cell tumours and down-regulated during progression from intratubular germ cell neoplasia to seminoma.

Testicular germ cell tumours (TGCTs) are the most frequent cancer type in young men; 5% of these patients develop a second TGCT in the contralateral testis. The pathogenesis of TGCT is closely linked to primordial germ cells (PGCs) or gonocytes. The receptor tyrosine kinase (c-KIT) is necessary for migration and survival of PGCs and is expressed in intratubular neoplastic germ cells (IGCNUs) and seminomas. We studied the frequency of c-KIT exon 11 and 17 mutations in 155 unilateral (108 seminomas and 47 non-seminomas) and 22 bilateral (18 seminomas, two embryonal carcinomas, two IGCNU) cases. While no mutations were detected in exon 11, the mutation frequency in exon 17 was significantly higher in bilateral (14/22, 63.6%) compared to unilateral TGCT (10/155, 6.4%) (p < 0.001). Different activating mutations (Y823D, D816V, D816H and N822K) were detected in bilateral TGCT. Y823D mutation was identical in both testes in three cases and quantitative pyrosequencing showed that up to 76% of the cells analysed in tumour samples carried this mutation. One bilateral synchronous seminoma revealed a S821F mutation in one testis and a Y823D mutation contralaterally. To study the role of c-KIT in TGCT progression, we compared its expression in 41 seminomas and adjacent IGCNUs. Immunohistochemical analysis revealed that c-KIT expression was significantly reduced in seminomas compared to IGCNUs (p < 0.006) and that there were no significant changes in c-KIT mRNA copy numbers in progressed compared to low-stage seminomas. In summary, our study shows that patients with c-KIT mutations are more prone to develop a bilateral TGCT and suggests that in a portion of bilateral TGCTs, c-KIT mutations occur early during embryonal development, prior to the arrival of PGCs at the genital ridge. Furthermore, our findings show that c-KIT down-regulation occurs during the progression of IGCNU to seminoma.