Guikema, J. E. | bioRxiv Science

bioRxiv2020

DNA polymerase β prevents AID-instigated mutagenic non-canonical mismatch DNA repair

In B cells, the error-prone repair of activation-induced cytidine deaminase (AID)-induced lesions in immunoglobulin variable genes cause somatic hypermutation (SHM) of antibody genes. Due to clonal selection in the germinal centers (GC) this active mutation process provides the molecular basis for antibody affinity maturation. AID deaminates cytosine (C) to create uracil (U) in DNA. Typically, the short patch base excision repair (spBER) effectively restores genomic U lesions. We here demonstrate that GC B cells actively degrade DNA polymerase {beta} (Pol{beta}), resulting in the inactivation of the gap-filling step of spBER. Consequently, lesions instigated by AID, and likely other base damages, are channeled towards mutagenic non-canonical mismatch repair (mncMMR), responsible for the vast majority of mutations at adenine and thymine (A:T) bases. Apparently, GC B cells prohibit faithful spBER, thereby favoring A:T mutagenesis during SHM. Lastly, our data suggest that the loss of Pol{beta} relates to hypoxia that characterizes the GC microenvironment.

immunology↗

bioRxiv2019

Targeting AKT elicits tumor suppressive functions of FOXO transcription factors and GSK3 kinase in Multiple Myeloma

The phosphatidylinositide-3 kinases (PI3K) and the downstream mediator AKT drive survival and proliferation of multiple myeloma (MM) cells and several AKT inhibitors are currently being tested in clinical trials for MM patients. AKT inhibition has pleiotropic effects, and the key aspects that determine therapeutic efficacy are not fully clear. Therefore, we investigated the antimyeloma mechanism(s) of AKT inhibition. Among the various downstream AKT targets are Forkhead box O (FOXO) transcription factors, and we demonstrate that they are crucial for changes in gene expression upon AKT inhibition. Based on gene expression profiling we defined an AKT-induced FOXO-dependent gene set that has prognostic significance in a large cohort of MM patients, where low FOXO activity correlates with inferior survival. We show that cell cycle exit and cell death of MM cells after AKT inhibition required FOXO. In addition, glycogen synthase kinase 3 (GSK3), a negatively regulated AKT substrate, proved to be pivotal to induce cell death and to inhibit cell cycle progression after AKT inhibition. Finally, we demonstrate that FOXO and GSK3 induced cell death by increasing the turnover of the myeloid cell leukemia 1 (MCL1) protein. In concordance, the AKT inhibitor MK2206 greatly sensitized MM cells for the MCL1 inhibitor S63845. Thus, our results indicate that FOXO and GSK3 are crucial mediators of the antimyeloma effects of AKT inhibition, and suggest combination therapies that may have therapeutic potential in MM.\n\nKEYPOINTSO_LIFOXO transcription factors and the GSK3 kinase are pivotal tumor suppressors downstream of AKT inhibition in MM cells.\nC_LIO_LIFOXO and GSK3 activation after AKT inhibition leads to a decrease in MCL1 levels in MM cells resulting in cell death.\nC_LI

cancer biology↗