or by ␥-irradiation revealed an extreme sensitivity and a high genomic instability to both agents. Following whole body ␥-irradiation (8 Gy) mutant mice died rapidly from acute radiation toxicity to the small intestine. Mice-derived PARP ؊/؊ cells displayed a high sensitivity to MNU exposure: a G 2 ͞M arrest in mouse embryonic fibroblasts and a rapid apoptotic response and a p53 accumulation were observed in splenocytes. Altogether these results demonstrate that PARP is a survival factor playing an essential and positive role during DNA damage recovery.To protect their genome from the deleterious consequences of accumulation of unrepaired or misrepaired lesions, cells have developed an intricate DNA damage surveillance network. Through its function as a single-stranded breaks detector, poly(ADP-ribose) polymerase [PARP; NAD ϩ ADP-ribosyltransferase; NAD ϩ : poly(adenosine-diphosphate-D-ribosyl)-acceptor ADP-D-ribosyltransferase, EC 2.4.2.30], a nuclear enzyme, participates to this basic process (1). PARP (113 kDa) has a modular organization (2): a N-terminal DNA-binding domain that acts as a molecular nick-sensor, encompassing two zinc-finger motifs (3) and a bipartite nuclear location signal (4), a central region bearing the auto-poly(ADP-ribosylation) sites which serves to regulate PARP-DNA interactions and a C-terminal catalytic domain involved in the nick-binding dependent poly(ADP-ribose) synthesis (5). The x-ray crystallographic structure of this domain has been recently solved revealing a surprising structural homology between the active site of PARP and that of bacterial mono-ADP-ribosylating toxins despite weak sequence homology (6).Although the physiological role of PARP is still much debated, recent molecular and genetic approaches including expression of either a dominant-negative mutant (7-10) or antisense (11) have clearly revealed the implication of PARP in the maintenance of the genomic integrity in the base excision repair pathway (7)(8)(9)(10)12). To elucidate its function we disrupted the mouse PARP gene by homologous recombination and exposed the PARP-deficient mice and derived cells to various genotoxins. MATERIALS AND METHODSGene Targeting in Embryonic Stem Cells and Generation of Mice. Mouse PARP was isolated from a 129SVJ strain genomic library. The targeting vector was constructed using a 9-kb EcoRI fragment extending from intron 2 to 7 by inserting PGK-neo (phosphoglycerate kinase promoter followed by the neo gene) in the BamHI site of the 4th exon and herpes simplex virus thymidine kinase followed by the TK gene (HSV-Tk) in the XhoI site outside the sequence of the targeting vector. Following electroporation, embryonic stem cells were selected in 200 g⅐ml Ϫ1 G418 and 2 mM of gancyclovir. A positive clone microinjected into C57BL͞6 blastocysts (13) gave rise to chimaeric offspring, which in turn were mated with C57BL͞6.

Cancer genomes exhibit numerous deletions, some of which inactivate tumor suppressor genes and/or correspond to unstable genomic regions, notably common fragile sites (CFSs). However, 70%-80% of recurrent deletions cataloged in tumors remain unexplained. Recent findings that CFS setting is cell-type dependent prompted us to reevaluate the contribution of CFS to cancer deletions. By combining extensive CFS molecular mapping and a comprehensive analysis of CFS features, we show that the pool of CFSs for all human cell types consists of chromosome regions with genes over 300 kb long, and different subsets of these loci are committed to fragility in different cell types. Interestingly, we find that transcription of large genes does not dictate CFS fragility. We further demonstrate that, like CFSs, cancer deletions are significantly enriched in genes over 300 kb long. We now provide evidence that over 50% of recurrent cancer deletions originate from CFSs associated with large genes.

The karyotypes of more than 60 species of Primates are studied and compared, with the use of almost all existing banding techniques. There is a very close analogy of chromosome banding between the Simians studied and man. The quantitative or qualitative variations detected all involve the heterochromatin. It is very likely that all the euchromatin (nonvariable R and Q bands) is identical in all the species. Approximately 70% of the bands are common to the Simians and to the Lemurs (Prosimians). In the remaining 30%, technical difficulties prevented a valuable comparison, but this does not exclude the possibility that a complete analogy may exist. Thus, it is very likely that chromosomal evolutions of the Simians, and probably of all the Primates, has occurred without duplication or deficiency of the euchromatin. Approximately 150 rearrangements could be identified and related to the human chromosomes. The types of rearrangement vary from one group (suborder, family, genus) to another. For instance, Robertsonian translocations are preponderant among the Lemuridae (44/57), but are nonexistent among the Pongidae. Chromosome fissions are very frequent amng the Cercopithecidae (10/23), but were not found elsewhere, and pericentric inversions are preponderant in the evolution of Pongidae and man (17/28). This suggest that the chromosomal evolution may be directed by the genic constitution (favouring the occurrence of a particular type of rearrangement, by enzymatic reaction), by the chromosomal morphology (the probability that Robertsonian translocation will be formed depends at least partially on the number of acrocentrics), and by the reproductive behaviour of the animals. Reconstitution of the sequence of the chromosomal rearrangements allowed us to propose a fairly precise genealogy of many Primates, giving the positions of the Catarrhines, the Platyrrhines, and the Prosimians. It was also possible to reconstruct the karyotypes of ancestors that died out several dozen million years ago. The possible role of chromosomal rearrangements in evolution is discussed. It appears necessary to consider different categories of rearrangements separately, depending on their behaviour. The 'nonfavoured' rearrangements, such as pericentric inversions, need to occur in an isolated small population for implanting, by an equivalent of genic derivation. The 'favoured' rearrangements, e.g., Robertsonian translocations, may occur and diffuse in panmictic populations, and accumulate. Their role of gametic barrier could be much more progressive. For discrimination between these two categories, it was necessary to differentiate the selective advantage or disadvantage of the rearrangement itself. It was not possible to show that chromosomal rearrangements play a direct role in modification of the phenotype by position effect. Comparison of the rearrangement that have occurred during evolution and those detected in the human population shows a strong correlation for some of them...