Methylation-Independent Silencing of the Tumor Suppressor

The tumor suppressor gene INK4b (p15) is silenced by CpG island hypermethylation in most acute myelogenous leukemias (AML), and this epigenetic phenomenon can be reversed by treatment with hypomethylating agents. Thus far, it was not investigated whether INK4b is hypermethylated in all cytogenetic subtypes of AML. A comparison of levels of INK4b methylation in AML with the three most common cytogenetic alterations, inv(16), t(8;21), and t(15;17), revealed a strikingly low level of methylation in all leukemias with inv(16) compared with the other types. Surprisingly, the expression level of INK4b in inv(16)+ AML samples was low and comparable with that of the other subtypes. An investigation into an alternative mechanism of INK4b silencing determined that the loss of INK4b expression was caused by inv(16)-encoded core binding factor B-smooth muscle myosin heavy chain (CBFB-SMMHC). The silencing was manifested in an inability to activate the normal expression of INK4b RNA as shown in vitamin D3–treated U937 cells expressing CBFB-SMMHC. CBFB-SMMHC was shown to displace RUNX1 from a newly determined CBF site in the promoter of INK4b . Importantly, this study (a) establishes that the gene encoding the tumor suppressor p15 is a target of CBFB-SMMHC, a finding relevant to the leukemogenesis process, and (b) indicates that, in patients with inv(16)-containing AML, reexpression from the INK4b locus in the leukemia would not be predicted to occur using hypomethylating drugs. [Cancer Res 2007; 67(3):992–1000]