Research Focus "Control of the G1-to S-phase transition"
Cells generally decide during the G1 phase whether or not to advance into a division cycle. As a first step in identifying controls that govern cell-cycle entry, we set out to identify genes that regulate G1 progression in C. elegans.
We identified cyd-1 Cyclin D and cdk-4 Cdk4/6 mutations in a screen for genes essential for S phase entry. We used these mutations to establish that lin-35 Rb, the single member of the retinoblastoma gene family in C. elegans, acts as a negative regulator of G1 progression, most likely downstream of cyd-1 Cyclin D and cdk-4 CDK4/6. Moreover, we found that cki-1, which encodes a CDK inhibitor of the Cip/Kip family, acts in parallel to lin-35 Rb to inhibit G1 progression.
The lin-35 Rb gene was originally identified as a synthetic Multivulva (synMuv) gene. These genes form redundant classes, A, B and C, that prevent abnormal induction of the vulval cell fate. We examined other synMuv genes and identified cell-cycle functions for a subset of the class B genes. This includes efl-1 E2F, which negatively regulates cell-cycle entry, and dpl-1 DP, which appeared to act both as a positive and negative regulator. In addition, we identified a negative G1 regulatory function for lin-9 ALY, as well as lin-15B and lin-36, which encode novel proteins. Our data are consistent with lin-35 Rb, efl-1 and lin-36 acting in a common pathway or complex that negatively regulates G1 progression.
These and other results have demonstrated that G1 control in C. elegans involves mechanisms that are closely related to those used in humans. Thus, studies in C. elegans can contribute to understanding the in vivo functions and interactions of known G1 regulators. In addition, genetic approaches in C. elegans may identify novel G1 regulators. In forward genetic screens, we have identified several loci that contain critical negative regulators of cell-cycle entry. Surprisingly, one of the negative regulators identified is the phosphatase CDC-14, which appears to promote accumulation of the CDK inhibitor CKI-1 Kip1 (Saito et al. Nature Cell Biol. 2004).
In addition to forward genetics, we perform high throughput reverse genetic screens, using the feeding RNAi library generated in the Vidal lab (Rual et al. Genome Research 2004. see outline below).
In these screens we have identified a number of genes that are synthetic lethal when combined with lin-35 Rb inactivation. As the Rb pathway is inactivated in nearly all human cancers, proteins that are particularly important for the survival of cells lacking a functional Rb pathway should be attractive targets for anti-cancer therapeutics.