et al., 2000). When propagated over several generations, mrt-2 homozygous worms
accumulate progressively shorter telomeres, leading to genomic instability and to the
accumulation of chromosomal end-to-end fusion, and ultimately resulting in sterility
(Ahmed and Hodgkin, 2000). Positional cloning of mrt-2 revealed that this gene is
the worm homolog of the Schizzosaccharomyces pombe rad1 and the Saccharomyces
cerevisiae rad17 checkpoint genes. rad1/rad17 has previously been shown to be
involved in yeast DNA damage checkpoints (Ahmed and Hodgkin, 2000). In yeast,
checkpoint signaling involves a group of proteins needed for the recognition of DNA
damage and for the transduction of the damage signal to effect cell-cycle arrest
(Figure 5 ) (for reviews, see Caspari and Carr, 1999; Foiani et al., 2000; Murakami
and Nurse, 2000; Rhind and Russell, 2000; Walworth, 2000; Abraham, 2001; Wahl
and Carr, 2001). Schizzosaccharomyces pombe Rad9p, Radlp, and Huslp form a
complex that structurally resembles a PCNA sliding DNA clamp, and has recently
been shown, in vivo, to associate to DNA close to double-strand breaks in a Rad17p-
dependent manner (Kondo et al., 2001; Melo et al., 2001). In addition, the S.
cerevisiae homolog of mammalian ATM/ATR Mec1 also localizes to damaged DNA,
and its kinase activity is activated in response to DNA damage. The DNA damage
signal is then relayed via the scMec1p/spRad3p/ATM/ATR kinases to CHK1 and
CHK2 kinases, which cause cell-cycle arrest via phosphorylation of key cell-cycle
proteins. Given that mrt-2 is involved in radiation-induced apoptosis, it was
reasonable to assume that worm homologs of yeast checkpoint genes are also defective
in DNA damage checkpoint responses. RNAi inactivation of these candidate worm
checkpoint genes led to the inactivation of radiation-induced cell-cycle arrest
(Boulton et al., 2002) (Figure 5 ). However, defects in radiation in induced
programmed cell death could be induced only at a low penetrance, probably because
these checkpoint genes could be only partially inhibited in meiotic pachytene cells
by RNAi (Boulton et al., 2002). Generation of true checkpoint gene mutation will
therefore be necessary to assign an unambiguous function to genes in radiation-
induced programmed cell death.
rad-5 was the first conserved checkpoint gene whose function in the DNA damage
checkpoint was defined in the C. elegans system (Hartman and Herman, 1982). rad-5
is an essential gene, and the two known rad-5 mutations are temperature-sensitive
lethals (Hartman and Herman, 1982). Cloning of the C. elegans rad-5 revealed that
this gene is related to S. cerevisiae tel-2, an essential gene shown to be involved in
telomere-length regulation (Ahmed et al., 2001). rad-5 function in telomere
regulation is less clear, as telomere length in rad-5 mutants fluctuates but does not
become progressively either longer or shorter (Ahmed et al., 2001). Interestingly, the
checkpoint gene rad-5 was found to be allelic with clk-2 (Ahmed et al., 2001). clk
genes were found to slow the worm’s development and to extend its lifespan
(Lakowski and Hekimi, 1996; Benard et al., 2001; Lim et al., 2001). In clk-2(qm37),
this lifespan extension is only weak, and it is unclear whether this effect is merely an
indirect consequence of the slow-growth phenotype of clk-2 (qm37) mutation.
A recent functional genomics-based approach has led to the identification of
additional genes involved in worm checkpoint responses. Boulton et al. (2002) cloned
PROGRAMMED CELL DEATH IN C.ELEGANS 175