Cell cycle progression is controlled by the activity of cyclin-dependent kinases (CDKs) in association with their cyclin subunit. Extracellular cues are thought to regulate cyclin D kinases, which are involved in progression through G1 phase. The active cyclin D/CDK complex targets the negative G1 regulators Rb and possibly the Cip/Kip family of CKIs (CDK inhibitors). Importantly, tumor cells generally show defects in G1 control caused by inactivation of the 'Rb pathway' and/or other mechanisms. We use C. elegans as a model system for G1 control studies. Animals lacking the positive G1 regulators
cyd-1 cyclin D1 and
cdk-4 CDK4 fail in postembryonic cell divisions and growth [Park and Krause, 1999; Boxem and van den Heuvel, 2001]. These defects are suppressed by inactivation of
lin-35 Rb, in agreement with results in mammals. However, candidate null mutations in
lin-35 rescue the
cyd-1 and
cdk-4 phenotypes only partially. This indicates that
cyd-1/cdk-4 acts not only to inactivate
lin-35 Rb, and may have additional targets. A candidate target is the negative G1 regulator
cki-1 Cip/Kip. Loss of
cki-1 function acts synergistically with
lin-35 inactivation, in suppressing the
cyd-1 and
cdk-4 phenotypes. These results indicate that
lin-35 and
cki-1 act in parallel pathways, functioning downstream or in parallel to
cyd-1/cdk-4. We are using a forward genetic approach to identify additional G1 control genes. Double mutant
cyd-1 lin-35 animals develop into adults that are fully penetrant sterile. Screening for suppression of the sterility of
cyd-1 lin-35 worms has identified the recessive mutation
he121. The
he121 mutation suppresses the cell division defects of
cyd-1 and
cdk-4 mutants, suggesting that the gene mutated functions downstream or in parallel to
cyd-1/cdk-4. Complete rescue is dependent on
lin-35 inactivation, indicating that the
he121 mutation alters a pathway separate from
lin-35. As the negative G1 regulator
cki-1 defines the only other known pathway parallel to
lin-35 in cell cycle control, we are testing whether
he121 alters the
cki-1 pathway or a novel G1 control pathway. Preliminary mapping data place
he121 on chromosome II in a region different from
cki-1. Together, these results indicate that the
he121 mutation defines an interesting negative G1 regulator, distinct from
lin-35 and
cki-1. Hence, we will characterize this gene further to understand its contribution to G1 control in worms, and possibly in humans.