Allelotyping nephrogenic rests: putative precursor lesions of Wilms tumors

Perilobar nephrogenic rests and chromosome 22

Perilobar nephrogenic rests (NR) are precursor lesions that may display genetic changes similar to their associated Wilms tumor (WT). Two patients presented with WT, both with perilobar NR and 1 with bilateral, multifocal metachronous WT. Both patients' WT displayed monosomy 22 as the predominant cytogenetic change, and the constitutional cytogenetic analysis was normal. The purpose of our study was to identify at what stage in the morphologic progression from NR to WT the monosomy 22 occurred by using a fluorescent in situ hybridization probe for chromosome 22 in the subtypes of perilobar NR and WT present in both cases. Section and core fluorescent in situ hybridization with a chromosome 22 probe was performed on formalin-fixed, paraffin-embedded tissues containing WT and perilobar NR. We also performed fluorescent-based microsatellite analysis on some of the WT in the bilateral case to determine whether there was a preferential loss of the same allele of chromosome 22. Sclerotic and dormant perilobar NR showed a rate of monosomy 22 of only approximately 30%, but this increased to approximately 50% in hyperplastic and adenomatous NR. Monosomy 22 was present in 60%-80% of nuclei in WT. Microsatellite analysis showed loss of homozygosity, with preferential loss of the same allele of chromosome 22 in the tumors examined. There are differences in the rate of detection of monosomy 22 in perilobar NR and WT, suggesting loss of chromosome 22 in the progression of perilobar NR to WT in a subset of tumors.

Sequential WT1 and CTNNB1 mutations and alterations of beta-catenin localisation in intralobar nephrogenic rests and associated Wilms tumours: two case studies

BACKGROUND

Intralobar nephrogenic rests (ILNRs) are precursor lesions for Wilms tumours and are associated with WT1 gene mutations. ILNR-associated Wilms tumours have a co-clustering of WT1 and beta-catenin (CTNNB1) mutations and unique histological features characterised by a stromal-predominant histology.

AIM

To determine the order in which WT1 and CTNNB1 mutations occur to understand the ILNR-Wilms tumour sequence.

METHODS

Of nine Wilms tumours with WT1 and CTNNB1 mutations, three ILNRs lesions in two Wilms tumours were available for analysis of WT1 and CTNNB1 mutations using microdissection. Immunohistochemistry was also performed to investigate how the mutations in beta-catenin alter the localisation in Wilms tumour development.

RESULTS

WT1 mutations were present in the ILNRs, however CTNNB1 mutations were absent. Immunohistochemistry for WT1 confirmed inactivation of WT1 in both ILNRs and Wilms tumours. Both the ILNRs and the associated Wilms tumours had similar immunostaining patterns for beta-catenin in the blastemal and epithelial components. Although rhabdomyoblasts were not included in ILNRs, the associated Wilms tumours showed rhabdomyogenic differentiation with a positive beta-catenin nuclear staining.

CONCLUSIONS

The results suggest that CTNNB1 mutation is a later event in Wilms tumourigenesis. CTNNB1 mutations might be associated with rhabdomyogenesis.

Clonality and loss of heterozygosity of WT genes are early events in the pathogenesis of nephroblastomas

Nephrogenic rests (NRs), putative precursor lesions of nephroblastomas (Wilms' tumors), are found in 25% to 40% of kidneys presenting with nephroblastomas. Nephroblastomas are clonal tumors that, according to a genetic multistep model, are thought to arise as subclonal proliferations from NRs by accumulating genetic alterations. Different candidate genes for the pathogenesis of nephroblastomas have been identified, including those at chromosomes 11p13 (WT1 gene), 11p15 (WT2 gene), and 16q (WT3 gene). We investigated clonality and loss of heterozygosity (LOH) at these loci in different subtypes of NR. After microdissection under microscopic control, we analyzed a highly polymorphic locus of the human androgen receptor gene (HUMARA) for nonrandom X-inactivation of genomic DNA using a methylation-sensitive restriction enzyme to investigate clonality. Out of 14 patients, we found that 1 case each of adenomatous and hyperplastic NR and 2 of 7 cases of sclerosing NR were monoclonal. Five patients were noninformative. We assessed LOH at chromosomes 11p13, 11p15, and 16q by analyzing polymorphic gene loci at these regions. One hyperplastic NR and the corresponding tumor showed LOH at 11p13 and 11p15; 1 sclerosing NR and the corresponding tumor exhibited LOH at chromosome 16q. We demonstrate for the first time that sclerosing NRs can exhibit genetic alterations found in nephroblastomas, namely monoclonality and LOH at the WT gene loci. The histological morphology is no different between NRs with these genetic alterations and NRs without them. We conclude that these genetic changes are early events in the multistep genetic pathogenesis of nephroblastomas; however, they do not seem to fully determine a malignant potential of NR.