Analysis of Mitosis In Acute Myeloid Leukemia Using Live-Cell Imaging.

Abstract 3363

Proper mitotic control is a prerequisite to guarantee the equal distribution of the genetic material onto the two developing daughter cells. A mitotic key regulator is cyclin B. High levels of cyclin B facilitate entry into mitosis whereas its controlled degradation coordinates chromosome separation and cytokinesis. The latter events are coordinated by the anaphase- promoting complex / cyclosome (APC/C), a ubiquitin ligase that couples ubiquitin chains to cyclin B, mediating its proteasomal degradation. The regulation of the APC/C-activity by complex protein networks, such as the spindle assembly checkpoint, therefore presents the basis for an accurate mitosis.

Mitotic errors give rise to daughter cells with an aberrant set of chromosomes and contribute to genetic instability. Genetic instability is a hallmark of cancer cells and plays an important role in the onset and progression of acute myeloid leukemia (AML). In rare cases, de novo AMLs present with multiple cytogenetic aberrations (complex karyotype). However, a larger number of patients develop karyotype deviations in the course of the disease, sometimes even under therapy, which comes along with an adverse prognosis. Understanding the biology that drives the gain and loss of genetic material therefore bears the potential of identifying new therapeutic targets.

We compared a number of lymphoblastic and myeloid cell lines and found AML cell lines to be deficient in arresting at metaphase in the presence of the microtubule-disrupting agent nocodazole. Cyclin B was expressed at much lower levels in the AML cell line Kasumi-1 and did not accumulate following spindle disruption as observed in the lymphoblastic cell line DG-75. We could show that Kasumi-1 cells, when challenged with nocodazole, were not capable of properly maintaining chromatid-cohesion and underwent premature sister chromatid separation. These findings suggest that mitotic control mechanisms do not work tightly enough in AML cells to prevent chromosome separation in the presence of spindle disruption. We applied live-cell imaging to exactly characterize mitotic timing in Kasumi-1 cells at a single cell level. The expression of a GFP-tagged derivative of histone H2 served to visualize the nuclear envelope breakdown and anaphase onset. Detection of the latter events allowed the faithful measurement of mitotic timing. We could find a significant shortening of mitosis in Kasumi-1 cells as compared to the lymphoblastic cell line DG-75. In both AML cell lines and primary AML blasts we identified the spindle assembly checkpoint components BubR1 and Bub1 to be downregulated. Interestingly, re-expression of BubR1 in Kasumi-1 cells led to a significant stabilization of cyclin B on western blots. To address the question whether an increased expression of cyclin B leads to a more pronounced mitotic delay in the presence of spindle-disruption in AML cells is subject of current experiments. It was reported that different cell types can escape from a mitotic block as a consequence of cyclin B degradation. In the literature, this phenomenon was referred to as mitotic slippage and is known to drive genetic instability. To monitor cyclin B turnover and localization at a single cell level, we generated a chimeric cyclin B-molecule, SNAP-cyclin B, which can couple to a suitable fluorochrome in a self-labeling reaction after addition to the growth medium. In this system, the fluorescence intensity reflects the amount of chimeric cyclin B and allows the monitoring of APC/C-dependent proteolysis. In our current approaches we aim at studying cyclin B-turnover at a single cell level in AML cell lines as well as primary leukemia cells by using live-cell imaging before and after BubR1- and Bub1-rescue.

An aberrant cell cycle control is found in most human malignancies and might be an important driving force in leukemogenesis. We hypothesize that BubR1, in concert with different other regulators, might lead to inaccuracies in mitotic control. This hypothesis is underlined by the shortened time to anaphase in Kasumi-1 cells and a decreased expression of cyclin B, both of which are characteristics of BubR1-depletion. Mitotic regulators are already targets in AML therapy and a deeper understanding of mitotic processes in AML might lead to improved approaches.