Appropriate repair of DNA lesions and the inhibition of DNA repair activities at telomeres are critical to prevent genomic instability. By fuelling the generation of genetic alterations and by compromising cell viability, genomic instability is a driving force in cancer and aging 1, 2 . Here we identify MAD2L2 (also known as MAD2B or REV7) through functional genetic screening as a novel factor controlling DNA repair activities at mammalian telomeres. We show that MAD2L2 accumulates at uncapped telomeres and promotes non-homologous end-joining (NHEJ)-mediated fusion of deprotected chromosome ends and genomic instability. MAD2L2 depletion causes elongated 3′ telomeric overhangs, implying that MAD2L2 inhibits 5′ end-resection. End-resection blocks NHEJ while committing to homology-directed repair (HDR) and is under control of 53BP1, RIF1 and PTIP 3 . Consistent with MAD2L2 promoting NHEJ-mediated telomere fusion by inhibiting 5′ end-resection, knockdown of the nucleases CTIP or EXO1 partially restores telomere-driven genomic instability in MAD2L2-depleted cells. Control of DNA repair by MAD2L2 is not limited to telomeres. MAD2L2 also accumulates and inhibits end-resection at irradiation (IR)-induced DNA double-strand breaks (DSBs) and promotes end-joining of DSBs in multiple settings, including during immunoglobulin class switch recombination (CSR). These activities of MAD2L2 depend on ATM kinase activity, RNF8, RNF168, 53BP1 and RIF1, but not

Distinguishing the primary from secondary effects and compensatory mechanisms is of crucial importance in understanding adult-onset neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). Transgenic mice that overexpress the G93A mutation of the human Cu-Zn superoxide dismutase 1 gene (hSOD1 G93A mice) are a commonly used animal model of ALS. Whole-cell patch-clamp recordings from neurons in acute slice preparations from neonatal wild-type and hSOD1 G93A mice were made to characterize functional changes in neuronal activity. Hypoglossal motoneurons (HMs) in postnatal day 4 (P4)-P10 hSOD1 G93A mice displayed hyperexcitability, increased persistent Na ϩ current (PC Na ), and enhanced frequency of spontaneous excitatory and inhibitory transmission, compared with wild-type mice. These functional changes in neuronal activity are the earliest yet reported for the hSOD1 G93A mouse, and are present 2-3 months before motoneuron degeneration and clinical symptoms appear in these mice. Changes in neuronal activity were not restricted to motoneurons: superior colliculus interneurons also displayed hyperexcitability and synaptic changes (P10 -P12). Furthermore, in vivo viral-mediated GFP (green fluorescent protein) overexpression in hSOD1 G93A HMs revealed precocious dendritic remodeling, and behavioral assays revealed transient neonatal neuromotor deficits compared with controls. These findings underscore the widespread and early onset of abnormal neural activity in this mouse model of the adult neurodegenerative disease ALS, and suggest that suppression of PC Na and hyperexcitability early in life might be one way to mitigate or prevent cell death in the adult CNS.

Loss of telomere protection causes natural chromosome ends to become recognized by DNA-damage response and repair proteins. These events result in ligation of chromosome ends with dysfunctional telomeres, thereby causing chromosomal aberrations on cell division. The control of these potentially dangerous events at deprotected chromosome ends with their unique telomeric chromatin configuration is poorly understood. In particular, it is unknown to what extent bulky modification of telomeric chromatin is involved. Here we show that uncapped telomeres accumulate ubiquitylated histone H2A in a manner dependent on the E3 ligase RNF8. The ability of RNF8 to ubiquitylate telomeric chromatin is associated with its capacity to facilitate accumulation of both 53BP1 and phospho-ATM at uncapped telomeres and to promote non-homologous end-joining of deprotected chromosome ends. In line with the detrimental effect of RNF8 on uncapped telomeres, depletion of RNF8, as well as of the E3 ligase RNF168, reduces telomere-induced genome instability. This indicates that, besides suppressing tumorigenesis by mediating repair of DNA double-strand breaks, RNF8 and RNF168 might enhance cancer development by aggravating telomere-induced genome instability.

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