all worm homologs of yeast checkpoint and DNA repair genes. Subsequently, all
these genes were used for two-hybrid analysis to identify interaction partners (Boulton
et al., 2002). The potential checkpoint function of these interaction partners was than
assessed by RNAi (Figure 5). This approach led to the identification of a worm
homolog of the mammalian Bcl-3 oncogene needed for DNA damage signaling
(Boulton et al., 2002). It will be interesting to see whether the mammalian homolog
has a similar defect in DNA damage signaling.
In addition to the checkpoint genes that affect both radiation-induced cell-cycle
arrest and cell death, only one gene-product is known that affects only radiation-
induced cell death. Although it initially evaded detection by conventional homology
searches, bioinformatic approaches using generalized profiles revealed that the worm
genome encodes for a distant homolog of the mammalian p53 tumor suppressor gene
termed cep-1 (C.elegans p53-like) (Figure 5) (Derry et al., 2001; Schumacher et al.,
2001). However, sequence alignments revealed that many of the p53 residues that
are implicated either in DNA binding or in oncogenesis are conserved in cep-1 (Derry
et al., 2001; Schumacher et al., 2001). Unlike mammalian p53 but like Drosophila
p53, cep-1 is not required for DNA damage-induced cell-cycle arrest (Derry et al.,
2001; Schumacher et al., 2001). In addition to its function in cell-cycle arrest, Derry
et al. (2001) showed an involvement of cep-1 in the stress response and some
involvement in meiotic chromosome segregation, as cep-1-mutant animals showed
weak defects in meiotic chromosome segregation. It will be interesting to determine
the transcriptional targets of cep-1 that mediate programmed cell death. Furthermore,
it will be important to determine why only germ cells are able to undergo programmed
cell death in response to radiation.
The DNA damage checkpoint is activated not only after genotoxic insult but also
upon meiotic defects, such as the accumulation of unprocessed meiotic
recombination intermediates. As part of a normal meiotic division, double-strand
breaks, which are needed for meiotic recombination, are generated by the meiotic
endonuclease SPO-11 (for review, see Roeder, 1997). At a following step of meiotic
recombination, double-strand breaks are resected to generate single-stand overhangs.
These single-strand overhangs invade the homologous chromosome via a strand-
exchange reaction that is mediated by the conserved RAD-51 strand-exchange protein
(Roeder, 1997). Worms lacking RAD-51 are thought to accumulate unprocessed
SPO-11-induced double-strand breaks (Rinaldo et al., 1998; Takanami et al., 1998).
These breaks are recognized by the same checkpoint proteins as the proteins sensing
radiation-induced double-strand breaks, and they result in elevated levels of germ-
cell apoptosis (Gartner et al., 2000). The observation that meiotic defects can lead to
increased levels of germ-cell death can be employed in readily isolating mutations
defective in meiotic chromosome pairing and recombination (Pawel Pasierbeck, Josef
Loidl and Anton Gartner, unpublished observation).
The above data indicate conclusively that the DNA damage checkpoint pathways
are conserved throughout evolution (Figure 5 ). However, the apoptotic response to
DNA damage exists only in metazoans, since yeast does not have apoptosis. It is
therefore conceivable that the p53 pathway is the link between the conserved DNA
176 GENETICS OF APOPTOSIS