Downregulation of a Candidate Myeloid Tumor Suppressor Gene Isolated from 7q21 Induces Abnormal Mitosis Due to Insufficient Centrosome Maturation.

To isolate myeloid tumor suppressor gene(s) in 7q, we searched microdeletions in a region spanning 21.7 Mb within 7q21.2–7q31.1 using a microarray-based CGH system. By investigation of 21 childhood myeloid leukemia patients with normal karyotype, we identified a common microdeletion cluster spanning approximately 120 Kb in the 7q21.3 subband. Eight (38%) patients shared this microdeletion, which was not detected in normal individuals. Real-time quantitative PCR revealed that this region is also deleted in 9 (29%) out of 31 adult RAEB and AML patients. Database search revealed that this region contains three hypothetical genes. Among them, we chose one previously uncharacterized gene for further investigation and named Miki (mitotic kinetics regulator) for the function of its gene product, described below. Immunoblot analysis revealed high levels of Miki expression in most lymphoid leukemia cell lines, while half of myeloid leukemia cell lines expressed Miki at reduced levels. In six leukemia lines carrying monosomy 7, expression levels were generally low. Miki co-localized with the Golgi apparatus in the interphase and with centrosomes and spindles in the mitotic phase. To test the function of Miki, we used si-RNAs to downregulate Miki expression in HeLa and K562 cells, both of which show basically normal metaphase and nuclear morphology. Cells expressing Miki at reduced levels showed insufficient maturation and disturbed positioning of centrosomes, resulting in unorganized spindles including loss of spindle tension, curled and fragile spindles, or even completely disturbed spindle formation. Time-lapse observation revealed prometaphase and/or metaphase delay with unaligned or even totally scattered chromosomes in prometaphase in virtually all cells in the mitotic phase. As a result, cells underwent pre-anaphase arrest and exited mitosis in the absence of chromosome segregation or terminated mitosis by cell death. In the interphase, there were many cells with chromatid bridges and/or bi- or tri-nuclear or even multinuclear cells with micronuclei that resembled pathological cells routinely observed in the bone marrow pictures of MDS patients. Interestingly, myeloid cell lines with low Miki expression, including those with monosomy 7, generally showed abnormal mitosis such as scattered chromosomes and abnormal nuclear morphologies (multi-nuclear cells with small nuclei) at higher frequency than cell lines expressing Miki at high levels. Moreover, induction of Miki restores normal mitosis in leukemia cells with monosomy 7. Miki was poly(ADP-ribosyl)ated (PARsylated) in late G2 to M phase by tankyrase-1, one of PAR polymerase (PARP), and tankyrase-1 activity was required for the binding of Miki to spindles and centrosomes. These data suggest that loss of Miki gene contributes to the development and progression of MDS by disturbing mitosis.