A single base pair polymorphism in the WT1 gene detected by single-strand conformation polymorphism analysis

Mutation analysis of the WT1 gene in myelodysplastic syndromes

The WT1 tumor suppressor gene was examined for mutations in a panel of 44 patients with myelodysplastic syndromes (MDS) including acute myelogenous leukemias (AML) secondary to MDS, using polymerase chain reaction single-strand conformation polymorphism (PCR-SSCP) analysis and sequencing analysis. A WT1 mutation was detected in one out of 17 cases of AML secondary to MDS. This mutation exists upstream of the zinc finger region and is predicted to produce a truncated WT1 protein lacking the zinc finger region. No mutations were detected in 27 MDS patients who had not progressed to AML. This is the first report of analysis for WT1 mutations in a large number of MDS patients, suggesting that WT1 mutations are uncommon in MDS. Abnormalities in this gene may, however, contribute to a small proportion of cases showing progression from MDS into AML.

Dominant-negative mutations of the Wilms' tumour predisposing gene (WT1) are infrequent in CML blast crisis and de novo acute leukaemia

To determine if mutations of the Wilms' tumor predisposing gene (WT1) are associated with haematological malignancies, we have investigated 65 cases of acute leukaemia, including 39 patients in blast crisis of chronic myeloid leukaemia (CML), by amplification of WT1 exons 7, 8 and 9 followed by single-strand conformation polymorphism analysis. WT1 transcripts were detected by RT-PCR in all samples. An exon 7 silent polymorphism (A-->G; Arg 313) was identified in 17 individuals, 5 of whom were homozygous, but no other lesions were found. In 1 sample from a patient with acute lymphoblastic leukaemia a smaller size transcript missing exon 9 was detected; a similar abnormality has been described previously in a patient with Wilms' tumour and the resultant protein shown to act in a dominant-negative manner. No mutations of the exon 9 donor or acceptor splice sites were found in this patient and the basis of the abnormal transcript remains obscure. We conclude that dominant-negative mutations of the zinc finger region of the WT1 gene are uncommon in CML blast crisis. Abnormalities of this gene may, however, contribute to a small proportion of cases of de novo acute leukaemia.

Correlation of germ-line mutations and two-hit inactivation of the WT1 gene with Wilms tumors of stromal-predominant histology

The WT1 gene, located on chromosome 11p13, is mutated in a low number of Wilms tumors (WTs). Germ-line mutations in the WT1 gene are found in patients with bilateral WT and/or associated genital tract malformations (GU). We have identified 19 hemizygous WT1 gene mutations/deletions in 64 patient samples. The histology of the tumors with mutations was stromal-predominant in 13, triphasic in 3, blastemal-predominant in 1, and unknown in 2 cases. Thirteen of 21 patients with stromal-predominant tumors had WT1 mutations and 10 of these were present in the germ line. Of the patients with germ-line alterations, six had GU and a unilateral tumor, two had a bilateral tumor and normal GU tracts, and two had a unilateral tumor and normal GU. Three mutations were tumor-specific and were found in patients with unilateral tumors without GU. These data demonstrate a correlation of WT1 mutations with stromal-predominant histology, suggesting that a germ-line mutation in WT1 predisposes to the development of tumors with this histology. Twelve mutations are nonsense mutations resulting in truncations at different positions in the WT1 protein and only two are missense mutations. Of the stromal-predominant tumors, 67% showed loss of heterozygosity, and in one tumor a different somatic mutation in addition to the germ-line mutation was identified. These data show that in a large proportion of a histopathologically distinct subset of WTs the classical two-hit inactivation model, with loss of a functional WT1 protein, is the underlying cause of tumor development.

DNA recognition by splicing variants of the Wilms' tumor suppressor, WT1

The Wilms' tumor suppressor, WT1, is a zinc finger transcriptional regulator which exists as multiple forms owing to alternative mRNA splicing. The most abundant splicing variants contain a nine-nucleotide insertion encoding lysine, threonine, and serine (KTS) in the H-C link region between the third and fourth WT1 zinc fingers which disrupts binding to a previously defined WT1-EGR1 binding site. We have identified WT1[+KTS] binding sites in the insulin-like growth factor II gene and show that WT1[+KTS] represses transcription from the insulin-like growth factor II P3 promoter. The highest affinity WT1[+KTS] DNA binding sites included nucleotide contacts involving all four WT1 zinc fingers. We also found that different subsets of three WT1 zinc fingers could bind to distinct DNA recognition elements. A tumor-associated, WT1 finger 3 deletion mutant was shown to bind to juxtaposed nucleotide triplets for the remaining zinc fingers 1, 2, and 4. The characterization of novel WT1 DNA recognition elements adds a new level of complexity to the potential gene regulatory activity of WT1. The results also present the possibility that altered DNA recognition by the dominant WT1 zinc finger 3 deletion mutant may contribute to tumorigenesis.

DNA recognition by splicing variants of the Wilms' tumor suppressor, WT1

The Wilms' tumor suppressor, WT1, is a zinc finger transcriptional regulator which exists as multiple forms owing to alternative mRNA splicing. The most abundant splicing variants contain a nine-nucleotide insertion encoding lysine, threonine, and serine (KTS) in the H-C link region between the third and fourth WT1 zinc fingers which disrupts binding to a previously defined WT1-EGR1 binding site. We have identified WT1[+KTS] binding sites in the insulin-like growth factor II gene and show that WT1[+KTS] represses transcription from the insulin-like growth factor II P3 promoter. The highest affinity WT1[+KTS] DNA binding sites included nucleotide contacts involving all four WT1 zinc fingers. We also found that different subsets of three WT1 zinc fingers could bind to distinct DNA recognition elements. A tumor-associated, WT1 finger 3 deletion mutant was shown to bind to juxtaposed nucleotide triplets for the remaining zinc fingers 1, 2, and 4. The characterization of novel WT1 DNA recognition elements adds a new level of complexity to the potential gene regulatory activity of WT1. The results also present the possibility that altered DNA recognition by the dominant WT1 zinc finger 3 deletion mutant may contribute to tumorigenesis.

Molecular genetic analysis of chromosome 11p in familial Wilms tumour

In the family reported here, a mother and both of her children developed a Wilms tumour, and all three tumours were of the relatively rare monomorphous epithelial histopathological subtype. Using restriction fragment length polymorphism analysis, both sibs were shown to inherit the same maternal allele from the 11p13 region but different maternal alleles from the 11p15 region. Using a combination of single-strand conformation polymorphism (SSCP) and polymerase chain reaction (PCR) sequencing techniques, no mutations were identified in the WT1 tumour-suppressor gene from the 11p13 region, but a novel polymorphism was identified in exon 1. mRNA expression studies using the insulin-like growth factor II (IGF-II) gene, located in 11p15, showed that there was no relaxation of imprinting at this locus. There was also no evidence of loss of heterozygosity on the long arm of chromosome 16. These findings indicate that the WT1 and IGF-II genes, together with the long arm of chromosome 16, are not directly implicated in tumorigenesis in this Wilms family, but that a recombination event has occurred on the short arm of chromosome 11.

Molecular mechanisms possibly affecting WT1 function in human ovarian tumors

The frequent allelic deletions observed on the short arm of chromosome 11 in ovarian tumors suggest that the WT1 gene, a proposed tumor-suppressor gene located on chromosome 11p13 and expressed in the human fetal genitourinary system, may contribute to the development of ovarian neoplasms. Structural and sequence analysis of the entire coding portions of the WT1 gene did not reveal any abnormalities in the 20 ovarian tumor specimens (13 of which showed 11p13 allelic deletions) and 5 cell lines which we analyzed. These findings invalidate the hypothesis that the WT1 gene functions as a classical tumor-suppressor gene in ovarian tumorigenesis and suggest that a different recessive oncogene may be "exposed" by the observed 11p13 allelic deletions. Expression analysis showed that the WT1 gene was transcriptionally active in all the tumors tested, but considerable variations in the mRNA levels were found. This apparent variability, which should be confirmed at the cellular level in the tumor specimens, was also observed in the ovarian tumor-cell lines. Finally, WT1 expression data were evaluated in conjunction with immunohistochemical data on p53. The possible functional effects of altered WT1 mRNA expression in ovarian tumors are discussed, taking into account the potential WT1/p53 protein interaction.

Infrequent mutation of the WT1 gene in 77 Wilms' Tumors

Homozygous deletions in Wilms' tumor DNA have been a key step in the identification and isolation of the WT1 gene. Several additional loci are also postulated to contribute to Wilms' tumor formation. To assess the frequency of WT1 alterations we have analyzed the WT1 locus in a panel of 77 Wilms' tumors. Eight tumors showed evidence for large deletions of several hundred or thousand kilobasepairs of DNA, some of which were also cytogenetically detected. Additional intragenic mutations were detected using more sensitive SSCP analyses to scan all 10 WT1 exons. Most of these result in premature stop codons or missense mutations that inactivate the remaining WT1 allele. The overall frequency of WT1 alterations detected with these methods is less than 15%. While some mutations may not be detectable with the methods employed, our results suggest that direct alterations of the WT1 gene are present in only a small fraction of Wilms' tumors. Thus, mutations at other Wilms' tumor loci or disturbance of interactions between these genes likely play an important role in Wilms' tumor development.