Roles And Mechanism Of CUL4B In DNA Double-strand Break Repair

Cell is the basic unit of life. DNA (Deoxyribonucleic acid) is the most important genetic substance of the cell, the integrity of which is necessary for cell division, physiological activities and cellular processes. During the life processes, DNA suffers from endogenous and extraneous genotoxic insults. Double-strand breaks (DSB), in which both strands in the double helix break at the same or adjacent location is considered as the most dangerous DNA lesion.To preserve genomic integrity, cells employ a complex DNA damage response system which contains sensors, transducers and effectors. After the sensors detect the DNA lesion, the downstream effectors will be activated by the transducers. Different lesions can activate different DNA repair system. For example, alkylated bases will activate Direct Repair (DR), and pyrimidine dimmers will activate nucleotide excision repair (NER). Homologous recombination (HR) and nonhomologous end-joining (NHEJ) are the two principal pathways that mediate repair of DSBs in eukaryotic cells.Ubiquitin activating enzyme El, ubiquitin conjugating enzyme E2and ubiquitin ligase E3mediate ubiquitin modification and degradation of proteins. E3ubiquitin ligases mediate almost all cell processes including cell cycle, signal transduction, DNA repair.Cullin family is a component of cullin-RING ubiquitin ligases (CRLs), which functions as a scaffold. It was reported that mutations in the CUL4B gene which belongs to the Cullin gene family resulted in X-linked mental retardation syndrome. Recent study in our lab showed that deficiency of CUL4B causes a defect of cell proliferation, accumulation of Cyclin E and reduction of the cell division cycle-6(cdc6) and the Cyclin-dependent kinases-2(CDK2).Several studies suggested that CUL4participate in DNA damage repair induced by UV radiation. To investigating the roles of CUL4B in DNA double-stranded break repair, we performed the plate clone formation experiment, the comet electrophoresis experiment and the immune-fluorescence experiment. The results showed that RNAi of CUL4B resulted in down-regulation of DNA double-stranded break repair. While CUL4B may not involve in the classic NHEJ pathway, the Western Blot experiments demonstrated that deficiency of CUL4B caused a reduction of chromatin-bound RAD51protein that is involved in HR pathway. These results suggested that CUL4B may involve in the HR pathway by regulating the chromatin assembly of RAD51.Taken together, this study revealed that CUL4B was involved in the DSB repair by regulating the chromatin assembly of RAD51.We demonstrated a new function of CUL4B in DSB repair, providing a new potential target of tumor treatment, and a scientific basis for tumor radiation therapy and chemical therapy.