Methylation-Independent Silencing of the Tumor Suppressor p15INK4B by CBFb-SMMHC in Acute Myeloid Leukemias with inv(16).

The tumor suppressor INK4B(p15) gene is silenced by CpG island hypermethylation in a majority of acute myeloid leukemias (AML). This silencing can be reversed by the treatment with hypomethylating agents, and these agents are currently being tested for therapeutic intervention. So far, it was not investigated whether or not the INK4B is hypermethylated in all cytogenetic subtypes of AML. Our experiments, which compare levels of INK4B methylation in AML with inv(16), t(8:21) and t(15;17) reveal a strikingly low level of methylation in all leukemias with inv(16). This contrasts with significant levels of DNA methylation in a high proportion of the AML from the other two groups. Surprisingly, even though there is a lack of INK4B methylation in samples from patients with inv(16), expression of the gene is very low when compared to that of PBL from healthy individuals or HL60 cells. Subsequent experiments uncovered a novel mechanism to explain the low level of INK4B expression in the inv(16) AMLs. Overexpression of the aberrant chromosome 16-associated gene CBFb-MYH11 in U937 cells results in failure to induce INK4B in response to vitamin D3. Furthermore, CBFb-SMMHC, encoded by CBFb-MYH11 directly represses transcription from an INK4B promoter in a reporter assay. Electromobility shift assays in the AML-derived cell line ME-1 and U937 cells expressing the fusion gene demonstrate that the repression is due to a change in the composition of the complexes recognizing the binding sites for the transcription regulator CBF. In conclusion, we have found that methylation is not the only way to induce silencing of the tumor suppressor p15^INK4B in AML. In inv(16)-containing AML, loss of gene expression is accomplished by the direct transcriptional repressor activity of CBFβ-SMMHC.