Aberrant CDK1 Activity in S Phase Induces Chromosomal Instability by Increasing Mitotic Microtubule Polymerisation Rates in Colorectal Cancer Cells
von Ann-Kathrin Schmidt
Datum der mündl. Prüfung:2021-04-13
Betreuer:Prof. Dr. Holger Bastians
Gutachter:Prof. Dr. Holger Bastians
Gutachter:Dr. Ruth Geiss-Friedlander
EnglischWhole chromosomal instability (W-CIN), one of the major hallmarks of human cancer, is defined as the perpetual gain or loss of whole chromosomes during mitosis leading to the development of heterogeneous karyotypes. W-CIN is associated with tumourigenesis, tumour progression, therapy resistance, and poor prognosis. Therefore, it is of great interest to elucidate the molecular mechanisms underlying W-CIN in order to understand genome instability driven tumorigenesis and tumour progression. Recently, our lab has shown that W-CIN is triggered by increased microtubule polymerisation rates during mitosis which promote the formation of lagging chromosomes in anaphase, a mitotic error frequently observed in cancer cells. The results presented in this study show that the concomitant loss of the transcription factors p53 and p73 increases mitotic microtubule polymerisation rates and, consequently, induces W-CIN. Reduced expression of CDKN1A, a target gene of p53 and p73 encoding for the CDK inhibitor p21CIP1, causes the mitotic defects observed upon p53/p73 depletion. Importantly, increased CDK1 activity, which is the result of loss of the p73/p53-p21CIP1 pathway, is responsible for the observed chromosome missegregation. Unexpectedly, increased microtubule polymerisation rates and W-CIN are the result of elevated CDK1 activity specifically during S phase and not during mitosis. This suggests that increased CDK1 activity might interfere with DNA replication which leads to subsequent mitotic chromosome missegregation. Accordingly, normal mitotic microtubule growth rates and chromosome segregation were restored by nucleoside supplementation or inhibition of CDC7, a kinase involved in the firing of DNA replication origins. These results indicate that replication stress and increased origin firing are indeed triggered by increased CDK1 activity. In line with this, inhibition of the kinase ATR, a negative regulator of CDK1 in S phase, also increases mitotic microtubule growth rates and the incidence of lagging chromosomes. In summary, this study shows that loss of the p73/p53-p21CIP1 tumour suppressor pathway in colorectal cancer cells causes abnormal microtubule polymerisation rates in mitosis and W-CIN by unleashing CDK1 activity during S phase of the cell cycle, which might trigger increased origin firing during DNA replication. Thus, the present study provides new insights into the mechanisms promoting the generation of chromosomal instability in human cancer cells.
Keywords: Cancer; Chromosomal Instability; Microtubule; Mitosis; CDK1; p53; replication stress; origin firing