Maintaining genomic integrity is critical to avoid life-threatening disorders, such as premature aging, neurodegeneration and cancer. A multiprotein cascade operates at sites of DNA double-strand breaks (DSBs) to recognize, signal and repair damage. RNF168 (ring-finger nuclear factor) contributes to this emerging pathway of several E3 ubiquitin ligases that perform sequential ubiquitylations on damaged chromosomes, chromatin modifications essential for aggregation of repair complexes at the DSB sites. Here, we report the clinical and cellular phenotypes associated with a newly identified homozygous nonsense mutation in the RNF168 gene of a patient with a syndrome mimicking ataxia-telangiectasia. The mutation eliminated both of RNF168's ubiquitin-binding motifs, thus blocking progression of the ubiquitylation cascade and retention of repair proteins including tumor suppressors 53BP1 and BRCA1 at DSB sites, consistent with the observed defective DNA damage checkpoints/repair and pronounced radiosensitivity. Rapid screening for RNF168 pathway deficiency was achieved by scoring patients' lymphoblastoid cells for irradiation-induced nuclear foci containing 53BP1, a robust assay we propose for future diagnostic applications. The formation of radiation-induced DSB repair foci was rescued by ectopic expression of wild-type RNF168 in patient's cells, further causally linking the RNF168 mutation with the pathology. Clinically, this novel syndrome featured ataxia, telangiectasia, elevated alphafetoprotein, immunodeficiency, microcephaly and pulmonary failure and has implications for the differential diagnosis of autosomal recessive ataxias. The human genome is continuously exposed to free radicals, ionizing radiation and other genotoxic agents that cause DNA double-strand breaks (DSBs).1-3 Over one hundred genes are involved in recognition, signaling and repairing DSBs in order to maintain genomic stability and avoid accumulation of disease-predisposing mutations.2 At the cellular level, functions of the DNA damage sensors, signal transducers and effectors ultimately modulate the cell-fate decisions under genotoxic stress conditions, including decisions whether to undergo cell death or survive, or possibly take the route toward oncogenic transformation. [2][3][4] At the level of the whole organism, mutations in a subset of such genes are known to predispose to diverse types of cancer, neurodegenerative disorders, premature aging and/or immunodeficiency syndromes.2-4 On the other hand, the majority of the DNA damage response (DDR) genes are neither completely characterized nor linked to human disease phenotypes. 2-5The RNF168 gene is located on chromosome 3q29 and encodes a 571 amino acid protein that is a member of the 'ring-finger nuclear factor' (RNF) family of ubiquitin ligases.These enzymes modify functional properties of other proteins by covalent attachment of ubiquitin polypeptides, and emerging evidence suggests that this type of modification has a key role in the hierarchically structured genome surveillance pathw...

Purpose: Discovery of agents that protect or mitigate normal tissue from radiation injury during radiotherapy, accidents, or terrorist attacks is of importance. Specifically, bone marrow insufficiency, with possible infection due to immunosuppression, can occur after total body irradiation (TBI) or regional irradiation and is a major component of the acute radiation syndrome. The purpose of this study was to identify novel radioprotectors and mitigators of the hematopoietic system. Experimental Design: High-throughput screening of small-molecule libraries was done using viability of a murine lymphocyte line as a readout with further validation in human lymphoblastoid cells. The selected compounds were then tested for their ability to counter TBI lethality in mice. Results: All of two major classes of antibiotics, tetracyclines and fluoroquinolones, which share a common planar ring moiety, were radioprotective. Furthermore, tetracycline protected murine hematopoietic stem/progenitor cell populations from radiation damage and allowed 87.5% of mice to survive when given before and 35% when given 24 h after lethal TBI. Interestingly, tetracycline did not alter the radiosensitivity of Lewis lung cancer cells. Tetracycline and ciprofloxacine also protected human lymphoblastoid cells, reducing radiation-induced DNA double-strand breaks by 33% and 21%, respectively. The effects of these agents on radiation lethality are not due to the classic mechanism of free radical scavenging but potentially through activation of the Tip60 histone acetyltransferase and altered chromatin structure. Conclusions: Tetracyclines and fluoroquinolones can be robust radioprotectors and mitigators of the hematopoietic system with potential utility in anticancer radiotherapy and radiation emergencies. (Clin Cancer Res 2009;15(23):7238-45)

Nevi are a main risk factor for malignant melanoma, and most nevi develop in childhood. This study examined the relationship between vacations and nevi in 681 White children born in 1998 who were lifetime residents of Colorado. Vacation histories were assessed through telephone interviews of parents, whereas nevus and phenotypic characteristics were assessed through skin exams at age 7. Multiple linear and logistic regression were used to assess the influence of vacations on counts of nevi <2 mm in size and the presence of any nevi z2 mm after controlling for other variables. Each waterside vacation z1 year before the exam at age 7 was found to be associated with a 5% increase in nevi <2 mm. Waterside vacations <1 year before the skin exam were not related to nevus count (<2 mm); regardless of timeframe, waterside vacations were not related to the presence of nevi z2 mm. UV dose received on waterside vacations, number of days spent on waterside vacations, and nonwaterside vacations were not significantly related to nevi <2 or z2 mm. These results suggest that there is a lag of at least 1 year in the development of new nevi after vacation sun exposure. It appears that a threshold dose of UV exposure is received quickly on each waterside vacation. Parents of young children should exercise caution in selection of vacation locations to reduce melanoma risk. (Cancer Epidemiol Biomarkers Prev 2009;18(2):454 -63)