Loss of fragile WWOX gene leads to senescence escape and genome instability

Abstract

Induction of DNA damage response (DDR) to ensure accurate duplication of genetic information is crucial for maintaining genome integrity during DNA replication. Cellular senescence is a DDR mechanism that prevents the proliferation of cells with damaged DNA to avoid mitotic anomalies and inheritance of the damage over cell generations. Human WWOX gene resides within a common fragile site FRA16D that is preferentially prone to form breaks on metaphase chromosome upon replication stress. We report here that primary Wwox knockout (Wwox−/−) mouse embryonic fibroblasts (MEFs) and WWOX-knockdown human dermal fibroblasts failed to undergo replication-induced cellular senescence after multiple passages in vitro. Strikingly, by greater than 20 passages, accelerated cell cycle progression and increased apoptosis occurred in these late-passage Wwox−/− MEFs. These cells exhibited γH2AX upregulation and microsatellite instability, indicating massive accumulation of nuclear DNA lesions. Ultraviolet radiation-induced premature senescence was also blocked by WWOX knockdown in human HEK293T cells. Mechanistically, overproduction of cytosolic reactive oxygen species caused p16Ink4a promoter hypermethylation, aberrant p53/p21Cip1/Waf1 signaling axis and accelerated p27Kip1 protein degradation, thereby leading to the failure of senescence induction in Wwox-deficient cells after serial passage in culture. We determined that significantly reduced protein stability or loss-of-function A135P/V213G mutations in the DNA-binding domain of p53 caused defective induction of p21Cip1/Waf1 in late-passage Wwox−/− MEFs. Treatment of N-acetyl-l-cysteine prevented downregulation of cyclin-dependent kinase inhibitors and induced senescence in Wwox−/− MEFs. Our findings support an important role for fragile WWOX gene in inducing cellular senescence for maintaining genome integrity during DDR through alleviating oxidative stress.

Zfra Overrides WWOX in Suppressing the Progression of Neurodegeneration

“…Restoration of the WWOX gene and the resulting protein in cancer cells blocks their growth in vivo and in vitro [ 40 , 41 ]. WWOX protein is not a typical tumor suppressor, as it participates in numerous biological events, including (i) cell survival, proliferation, differentiation, cell cycle regulation, and senescence via complicated signaling pathways [ 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 ], (ii) aging and neurodegeneration [ 22 , 23 , 46 , 47 ], (iii) apoptotic cell death [ 30 , 48 , 49 , 50 , 51 ], (iv) chromosomal DNA stability [ 52 , 53 ], (v) bubbling cell death [ 54 , 55 , 56 ], and (vi) cell-to-cell recognition and migration [ 57 ]. Human newborns lacking the WWOX gene and functional WWOX protein suffer severe neural diseases but do not have spontaneous tumor formation, suggesting WWOX does not fit Knudson’s two-hit hypothesis of tumorigenesis [ 31 , 35 , 45 , 46 ].…”

“…Under physiological conditions, a portion of activated Tyr33-phosphorylated WWOX (pY33-WWOX) localizes on the outer membrane of the mitochondria to support normal physiology [ 28 , 48 , 58 , 59 , 60 , 61 ]. Without WWOX, cells undergo uncontrollable proliferation and die readily [ 43 ]. Under stress conditions such as exposure to UV irradiation and chemotherapeutic drugs, significantly upregulated pY33-WWOX, together with p53, induces mitochondrial apoptosis [ 58 , 59 , 60 , 61 ] or co-relocates to the nucleus to exert cell death [ 30 , 35 , 47 , 48 ].…”