Molecular characterization of the cancer susceptibility protein wrap53? in Cajal body formation and DNA repair

Abstract: WRAP53? is a multifaceted protein involved in several biological processes including Cajal body maintenance, cancer cell survival and DNA damage repair. By directing factors to Cajal bodies and DNA double-strand breaks (DSBs), WRAP53? facilitates site-specific interactions necessary for proper biological responses. The Cajal body is a subnuclear organelle implicated in cellular processes such as splicing machinery maturation and telomere maintenance. In Paper I, we reveal that WRAP53? is an essential structural component of Cajal bodies. Furthermore, WRAP53? is required for the intracellular targeting of factors to this site. WRAP53? associates with the survival of motor neuron (SMN) complex in the cytoplasm, mediates its nuclear import and subsequent Cajal body accumulation. In addition, we find that the interaction between WRAP53? and SMN is disrupted in the severe neurodegenerative disorder spinal muscular atrophy, suggesting clinical relevance of WRAP53?-mediated SMN transport. In Paper II, we study the relationship between WRAP53? expression and cancer cell survival. We demonstrate that WRAP53? is overexpressed in a panel of different cancer cell lines in comparison to primary cells. WRAP53? depletion results in massive induction of cancer cell death, whereas normal human fibroblasts are largely insensitive to WRAP53? knockdown. The cell death associated with WRAP53? silencing occurs via the intrinsic mitochondrial pathway as demonstrated by Bax/Bak activation, loss of mitochondrial membrane potential and release of cytochrome c. Finally, we show that high WRAP53? expression levels correlate with poor prognosis and radioresistance of head and neck cancer patients. In Paper III, we establish WRAP53? as a novel player in the DNA damage response. We show that WRAP53? rapidly and transiently localizes to DNA DSBs in an ATM- and PARP-dependent manner. WRAP53? binds the E3 ligase RNF8 and facilitates its interaction with MDC1, which is essential for the downstream recruitment of repair proteins 53BP1, BRCA1 and RAD51 to damaged sites. Knockdown of WRAP53? results in deficient DNA DSB repair, whereas WRAP53? overexpression enhances repair efficiency and provides resistance to DNA damaging agents. Furthermore, reduced expression of WRAP53? is related to decreased ovarian cancer patient survival. In summary, our data identify WRAP53? as a novel structural and regulatory component of Cajal bodies as well as an important factor in carcinogenesis and DNA repair.