WILMS’ TUMOUR GENE 1 PROTEIN (WT1) – AN EFFECTOR IN LEUKEMOGENESIS?

Abstract: Wilms’ tumour gene 1 (WT1) encodes a zinc-finger transcription factor functioning as a key regulator in organ development. WT1 was first identified as a tumour suppressor gene due to its inactivation in Wilms’ tumour cases, a childhood kidney cancer. In adult tissues WT1 expression is restricted to few organs, but various forms of cancers express high WT1 levels, suggesting an oncogenic potential for WT1. In normal hematopoiesis WT1 is expressed in a small subset of early progenitor cells, with declining levels as maturation occurs into mature blood cells. However, most primary leukemias express high levels of WT1 at diagnosis and this is an independent predictor of adverse outcome. In paper I, I demonstrate that WT1 expression is induced by tyrosine kinase signalling from BCR/ABL1 via the PI3K/Akt pathway. Chronic myeloid leukemia (CML) cells with forced expression of WT1 showed enhanced resistance to apoptosis induced by the ABL1 tyrosine kinase inhibitor, imatinib, further proposing an oncogenic function for WT1. Interferon regulatory factor 8 (IRF8), is absent or expressed at very low levels in leukemia. In paper II, IRF8 was shown to be a direct target gene of WT1, repressed by WT1 both in human hematopoietic progenitor cells and in leukemic cell lines. Furthermore, a strong anti-correlation between WT1 mRNA and IRF8 mRNA levels was observed when analyzed in a compiled compendium of expression datasets from primary leukemia samples. WT1 was retrovirally transduced in CD34+ progenitor cells to investigate the function of high WT1 levels in these cells. An oligonucleotide array revealed N-myc downstream regulated gene 2 (NDRG2), as an upregulated target gene to WT1. NDRG2 has later been identified as one of the genes in the HSC signature, which may suggest an indirect impact on HSC self-renewal capacity mediated by WT1 via regulation of NDRG2. In paper IV a delayed maturation of CD34+ human progenitor cells with forced expression of a WT1-mutant, lacking the entire zinc-finger domain and termed WT1(delZ), was observed. Enhanced and prolonged proliferation, increased clonogenic growth and differentiation towards an erythroid phenotype of WT1(delZ) expressing cells, suggest an oncogenic gain-of-function for mutated WT1, as compared to wild-type WT1. WT1 mutations are found in AML and T-ALL and are predominantly heterozygous frameshift mutations affecting exon 7, resulting in premature stop codons, thus encoding truncated WT1, similar to the WT1(delZ) protein. AML patients with WT1 mutations have higher frequency of resistence to chemotherapy with worse disease-free and overall survival rate. In summary, the abovementioned experimental findings, thoroughly discussed in my thesis, in association with the prevalence of high wild-type WT1 levels and WT1-mutations found in leukemias indicate an oncogenic effector role for WT1 in leukemogenesis.