Targeting DNA replication before it starts: Cdc7 as a therapeutic target in p53 mutant Her2 and triple negative breast cancer

Based on protein expression profiles of core regulatory proteins involved in the G₁-S and G₂-M phase transitions, we have identified three distinct cell cycle phenotypes in a series of 200 breast cancers: a G₀ out-of-cycle state (18% of cases); a G₁ arrested/delayed state (24% cases); and accelerated S-G₂-M phase progression (58% of cases). The accelerated cell cycle progression phenotype had a higher risk of relapse when compared with G₀ and G₁-delayed/arrested phenotypes (HR = 3.90 (95% CI = 1.81 to 8.4), P < 0.001) and was associated with Her2 and triple negative subtypes (P < 0.001). High-grade tumours with the G₁-delayed/arrested phenotype showed an identical low risk of relapse compared with well-differentiated G₀ tumours. In addition to its prognostic significance, the cell cycle phenotype also impacts on individualised therapeutic decisions. Only patients showing the actively cycling, aggressive cell cycle phenotype are likely to benefit from conventional chemotherapeutic S-phase-directed or M-phase-directed agents or from the new generation of targeted cell cycle inhibitors that are now entering clinical trials.

The DNA replication initiation factor Cdc7 is an emerging anticancer target. Cdc7 inhibition results in an abortive S phase and potent cancer cell killing. Specificity is based on normal cells undergoing a reversible G₁ arrest following Cdc7 inhibition due to activation of a novel cell cycle checkpoint that is lost or impaired in cancer cells. Our analysis of the molecular circuitry underlying this replication origin activation checkpoint reveals that G₁ arrest is dependent on three nonredundant checkpoint axes coordinated through the Forkhead transcription factor FoxO3a and p53. We show that only breast cancers displaying the accelerated cell cycle phenotype express elevated Cdc7 levels and are therefore highly represented in p53 mutant Her2-subtype and triple negative tumours. Breast cancers of the luminal subtype expressing low levels of Cdc7 undergo a cytostatic G₁ arrest after Cdc7 inhibition due to their p53 wild-type status, a checkpoint response mimicking untransformed cells. In contrast, Her2 and triple negative tumours show a marked response to Cdc7 inhibitors with potent cancer-cell-specific killing as a result of overexpression of the target protein and a result of impairment of the origin activation checkpoint due to p53 lesions.

We have thus defined a new therapy and a means of assessing response.