The wilms tumor 1 gene (WT1) encodes a transcriptional regulator involved in normal hematopoietic development. The role of WT1 in acute leukemia has been underscored by the finding of WT1 overexpression in subsets of patients (pts) associated with an increased relapse risk. In addition mutations of WT1 have been found in about 10–15% of acute myeloid leukemia (AML) pts and have recently shown to predict inferior survival. Thus far, larger studies have not yet determined the frequency and impact of WT1 mutations in acute T-lymphoblastic leukemia (T-ALL). Herein, we have analyzed WT1 mutations and WT1 mRNA expression levels in a large cohort of T-ALL including 239 newly diagnosed adult pts treated on the GMALL protocols 0699 and 0703. Diagnostic bone marrow specimens were studied for WT1 mutations by DNA sequencing. In addition, samples were immunophenotyped, and mRNA expression of the molecular markers HOX11, HOX11L2, ERG, BAALC, as well as WT1 were determined by real-time RT-PCR. Twenty (8%) of the 239 analyzed T-ALL pts had WT1 mutations (WT1mut) [20 pts had mutations in exon 7 (WT1mut7), with 2 pts having coexisting mutations in exon 9 (WT1mut9)]. WT1mut7 were frameshift or nonsense mutations predicted to result in a truncated WT1 protein, whereas WT1mut9 were missense mutations leading to single amino-acid substitutions. WT1mut and WT1 wildtype (WTwt) pts did not significantly differ with respect to clinical parameters at diagnosis (e. g. age, leukocyte count, and sex). WT1mut cases were characterized by immature features such as an early immunophenotype (45% of WT1mut showed an early T-ALL immunophenotype as compared to only 25% of WT1wt), and WT1mut also showed higher levels of CD34 expression as determined by flow cytometry (WT1mut median: 46% vs. WT1wt median: 2 %; P=0.03). Moreover, WT1mut had significantly higher WT1 mRNA expression levels [WT1mut median: 0.05 (range: 0–0.395) vs. WT1wt median: 0 (range: 0–0.15); P<0.001]. Significant differences were not observed in the complete remission rate nor overall survival or relapse free-survival (RFS) between WT1mut and WT1wt pts. However, in the standard risk group of thymic T-ALL 80% (4/5) of WT1mut relapsed as compared to 28% (25/89) of WT1wt thymic pts [P=0.01; RFS at 18 months: 20% (SE: ±18) for thymic WT1mut vs. 82% (SE: ±4) for thymic WT1wt pts; P=0.008]. In conclusion, in adult T-ALL WT1 mutations are present in 8% of newly diagnosed pts and are located in the same region as reported in AML expected to impair the DNA binding ability of the WT1 protein. Similar to findings in AML, WT1mut cases are characterized by immature features pinpointing to a genetic hit in hematopoietic progenitors likely harboring bilineage potential. The prognostic implications of WT1 mutations in standard risk thymic T-ALL will have to be further validated in independent studies and may in future direct molecularly-based treatment stratification.

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