Stratification of Wilms tumor by genetic and epigenetic analysis

Abstract

Somatic defects at five loci, WT1, CTNNB1, WTX, TP53 and the imprinted 11p15 region, are implicated in Wilms tumor, the commonest childhood kidney cancer. In this study we analysed all five loci in 120 Wilms tumors. We identified epigenetic 11p15 abnormalities in 69% of tumors, 37% were H19 epimutations and 32% were paternal uniparental disomy (pUPD). We identified mutations of WTX in 32%, CTNNB1 in 15%, WT1 in 12% and TP53 in 5% of tumors. We identified several significant associations: between 11p15 and WTX (P=0.007), between WT1 and CTNNB1 (P less than 0.001), between WT1 and pUPD 11p15 (P=0.01), and a strong negative association between WT1 and H19 epimutation (P less than 0.001). We next used these data to stratify Wilms tumor into three molecular Groups, based on the status at 11p15 and WT1. Group 1 tumors (63%) were defined as 11p15-mutant and WT1-normal; a third also had WTX mutations. Group 2 tumors (13%) were WT1-mutant. They either had 11p15 pUPD or were 11p15-normal. Almost all had CTNNB1 mutations but none had H19 epimutation. Group 3 tumors (25%) were defined as 11p15-normal and WT1-normal and were typically normal at all five loci (P less than 0.001). We also identified a novel clinical association between H19 epimutation and bilateral disease (P less than 0.001). These data provide new insights into the pattern, order, interactions and clinical associations of molecular events in Wilms tumor.

Figure 1. Schematic diagram of the overlapping distributions of molecular abnormalities at 11p15, WTX, WT1 and CTNNB1 in Wilms tumor

The percentage of sporadic tumors with each abnormality in our series is indicated in brackets. CTNNB1 mutations occur predominantly in tumors with WT1 mutations. WTX mutations occur predominantly in tumors with 11p15 defects. WT1-mutant tumors often have pUPD 11p15 while H19 epimutation, the other class of 11p15 defect, is not seen in this context. WTX mutations are infrequent in tumors with WT1 or CTNNB1 mutations. (pUPD: paternal uniparental disomy)

Figure 2. Schematic diagram of the molecular events in tumors in Group 1 and Group 2

Group 1 tumors are defined as having 11p15 defects in the absence of a WT1 mutation. Group 1 is subdivided into Group 1A, with H19 epimutation, and Group 1B, with pUPD 11p15. In Group 1 tumors, an 11p15 defect occurs as the likely first event, occurring either constitutionally or somatically. Approximately 30% of Group 1 tumors undergo WTX mutation, while mutations in TP53 and CTNNB1 mutation are infrequent. No additional event is currently identifiable in the majority of Group 1 tumors. Group 2 tumors are defined as having WT1 mutations. In Group 2 tumors, monoallelic WT1 mutation is the likely first event, occurring either constitutionally or somatically. Group 2 tumors in which the mutation targets the paternally-derived WT1 allele frequently next undergo somatic recombination resulting in UPD 11p and causing loss of the wild-type WT1 allele and pUPD 11p15. The remainder undergo mutation or deletion of the wild-type WT1 allele and retain normal 11p15 status. The large majority (~90%) of Group 2 tumors undergo CTNNB1 mutation, while WTX mutation is less common. (pUPD: paternal uniparental disomy; UPD: uniparental disomy)