WWOX : A fragile tumor suppressor

WWOX, the WW domain-containing oxidoreductase gene at chromosome region 16q23.3-q24.1, spanning chromosomal fragile site FRA16D, encodes the 46 kDa Wwox protein, a tumor suppressor that is lost or reduced in expression in a wide variety of cancers, including breast, prostate, ovarian, and lung. The function of Wwox as a tumor suppressor implies that it serves a function in the prevention of carcinogenesis. Indeed, in vitro studies show that Wwox protein interacts with many binding partners to regulate cellular apoptosis, proliferation, and/or maturation. It has been reported that newborn Wwox knockout mice exhibit nascent osteosarcomas while Wwox(+/-) mice exhibit increased incidence of spontaneous and induced tumors. Furthermore, absence or reduction of Wwox expression in mouse xenograft models results in increased tumorigenesis, which can be rescued by Wwox re-expression, though there is not universal agreement among investigators regarding the role of Wwox loss in these experimental models. Despite this proposed tumor suppressor function, the overlap of the human WWOX locus with FRA16D sensitizes the gene to protein-inactivating deletions caused by replication stress. The high frequency of deletions within the WWOX locus in cancers of various types, without the hallmark protein inactivation-associated mutations of "classical" tumor suppressors, has led to the proposal that WWOX deletions in cancers are passenger events that occur in early cancer progenitor cells due to fragility of the genetic locus, rather than driver events which provide the cancer cell a selective advantage. Recently, a proposed epigenetic cause of chromosomal fragility has suggested a novel mechanism for early fragile site instability and has implications regarding the involvement of tumor suppressor genes at chromosomal fragile sites in cancer. In this review, we provide an overview of the evidence for WWOX as a tumor suppressor gene and put this into the context of fragility associated with the FRA16D locus.