Cancer represents a pathological manifestation of uncontrolled cell division; therefore, it has long been anticipated that our understanding of the basic principles of cell cycle control would result in effective cancer therapies. In particular, cyclin-dependent kinases (CDKs) that promote transition through the cell cycle were expected to be key therapeutic targets because many tumorigenic events ultimately drive proliferation by impinging on CDK4 or CDK6 complexes in the G1 phase of the cell cycle. Moreover, perturbations in chromosomal stability and aspects of S phase and G2/M control mediated by CDK2 and CDK1 are pivotal tumorigenic events. Translating this knowledge into successful clinical development of CDK inhibitors has historically been challenging, and numerous CDK inhibitors have demonstrated disappointing results in clinical trials. Here, we review the biology of CDKs, the rationale for therapeutically targeting discrete kinase complexes and historical clinical results of CDK inhibitors. We also discuss how CDK inhibitors with high selectivity (particularly for both CDK4 and CDK6), in combination with patient stratification, have resulted in more substantial clinical activity.

Pancreatic ductal adenocarcinoma (PDA) has a dismal prognosis and insights into both disease etiology and targeted intervention are needed. A total of 109 micro-dissected PDA cases were subjected to whole-exome sequencing. Microdissection enriches tumour cellularity and enhances mutation calling. Here we show that environmental stress and alterations in DNA repair genes associate with distinct mutation spectra. Copy number alterations target multiple tumour suppressive/oncogenic loci; however, amplification of MYC is uniquely associated with poor outcome and adenosquamous subtype. We identify multiple novel mutated genes in PDA, with select genes harbouring prognostic significance. RBM10 mutations associate with longer survival in spite of histological features of aggressive disease. KRAS mutations are observed in >90% of cases, but codon Q61 alleles are selectively associated with improved survival. Oncogenic BRAF mutations are mutually exclusive with KRAS and define sensitivity to vemurafenib in PDA models. High-frequency alterations in Wnt signalling, chromatin remodelling, Hedgehog signalling, DNA repair and cell cycle processes are observed. Together, these data delineate new genetic diversity of PDA and provide insights into prognostic determinants and therapeutic targets.

In animal cells, duplication of centrosomes and DNA is coordinated. Since CDK2/cyclin E triggers initiation of both events, activation of CDK2/cyclin E is thought to link these two events. We identified nucleophosmin (NPM/B23) as a substrate of CDK2/cyclin E in centrosome duplication. NPM/B23 associates specifically with unduplicated centrosomes, and NPM/B23 dissociates from centrosomes by CDK2/cyclin E-mediated phosphorylation. An anti-NPM/B23 antibody, which blocks this phosphorylation, suppresses the initiation of centrosome duplication in vivo. Moreover, expression of a nonphosphorylatable mutant NPM/ B23 in cells effectively blocks centrosome duplication. Thus, NPM/B23 is a target of CDK2/cyclin E in the initiation of centrosome duplication.

Competing interests C.J.D. is on the Scientific Advisory Board of Mirati Therapeutics. A.C.K. has financial interests in Vescor Therapeutics, LLC. A.C.K. is an inventor on patents pertaining to KRAS-regulated metabolic pathways, redox control pathways in pancreatic cancer, targeting GOT1 as a therapeutic approach, and the autophagic control of iron metabolism. A.C.K. is on the Scientific Advisory Board of Cornerstone/Rafael Pharmaceuticals.

The cyclin D1 proto-oncogene exercises powerful control over the mechanisms that regulate the mitotic cell cycle, and excessive cyclin D1 expression and/or activity is common in human cancers. Although somatic mutations of the cyclin D1 locus are rarely observed, mounting evidence demonstrates that a specific polymorphism of cyclin D1 (G/A870) and a protein product of a potentially related alternate splicing event (cyclin D1b) may influence cancer risk and outcome. Herein, we review the epidemiological and functional literatures that link these alterations of cyclin D1 to human tumor development and progression.

Retinoblastoma (RB; encoded by RB1) is a tumor suppressor that is frequently disrupted in tumorigenesis and acts in multiple cell types to suppress cell cycle progression. The role of RB in tumor progression, however, is poorly defined. Here, we have identified a critical role for RB in protecting against tumor progression through regulation of targets distinct from cell cycle control. In analyses of human prostate cancer samples, RB loss was infrequently observed in primary disease and was predominantly associated with transition to the incurable, castration-resistant state. Further analyses revealed that loss of the RB1 locus may be a major mechanism of RB disruption and that loss of RB function was associated with poor clinical outcome. Modeling of RB dysfunction in vitro and in vivo revealed that RB controlled nuclear receptor networks critical for tumor progression and that it did so via E2F transcription factor 1-mediated regulation of androgen receptor (AR) expression and output. Through this pathway, RB depletion induced unchecked AR activity that underpinned therapeutic bypass and tumor progression. In agreement with these findings, disruption of the RB/E2F/nuclear receptor axis was frequently observed in the transition to therapy resistance in human disease. Together, these data reveal what we believe to be a new paradigm for RB function in controlling prostate tumor progression and lethal tumor phenotypes. IntroductionRetinoblastoma (RB; encoded by RB1), a tumor suppressor protein, is a critical negative regulator of tumor development. RB prevents tumorigenesis by suppressing cell cycle progression (1). However, the role of RB in tumor progression is poorly understood, and the clinical importance of RB loss during this process has not been well considered. Here, we identified a clinically relevant function for RB in tumor progression, manifest through control of hormone signaling networks.The function of RB in cell cycle control has been well described (1). Conditions favoring cell cycle arrest induce RB hypophosphorylation and activation. Active RB binds to promoters of genes required for S-phase entry (e.g., CCNA2 and MCM7) and, through association with the SWI/SNF complex and corepressor molecules (e.g., Sin3B), elicits transcriptional corepression. Many RB target genes are positively regulated by activator E2F transcription factors, supporting the current model that RB acts by suppressing E2F-mediated transcriptional activation. Indeed, the minimal transcriptional repression and tumor suppression domain of RB contains the E2F binding motif. E2F-independent functions of RB have been identified (2), but the contribution of these functions to tumor suppression is uncertain. Thus, contemporary views of RB suggest that the protein prevents cell cycle deregulation and tumor development through suppression of activator E2Fs.

The retinoblastoma tumour suppressor (RB) is a crucial regulator of cell-cycle progression that is invoked in response to a myriad of anti-mitogenic signals. It has been hypothesized that perturbations of the RB pathway confer a synonymous proliferative advantage to tumour cells; however, recent findings demonstrate context-specific outcomes associated with such lesions. Particularly, loss of RB function is associated with differential response to wide-ranging therapeutic agents. Thus, the status of this tumour suppressor may be particularly informative in directing treatment regimens.

A hallmark of cancer is the deregulation of cell-cycle machinery, ultimately facilitating aberrant proliferation that fuels tumorigenesis and disease progression. Particularly, in breast cancers, cyclin D1 has a crucial role in the development of disease. Recently, a highly specific inhibitor of CDK4/6 activity (PD-0332991) has been developed that may have efficacy in the treatment of breast cancer. To interrogate the utility of PD-0332991 in treating breast cancers, therapeutic response was evaluated on a panel of breast cancer cell lines. These analyses showed that the chronic loss of Rb is specifically associated with evolution to a CDK4/6-independent state and, ultimately, resistance to PD-0332991. However, to interrogate the functional consequence of Rb directly, knockdown experiments were performed in models that represent immortalized mammary epithelia and multiple subtypes of breast cancer. These studies showed a highly specific role for Rb in mediating the response to CDK4/6 inhibition that was dependent on transcriptional repression manifest through E2F, and the ability to attenuate CDK2 activity. Acquired resistance to PD-03322991 was specifically associated with attenuation of CDK2 inhibitors, indicating that redundancy in CDK functions represents a determinant of therapeutic failure. Despite these caveats, in specific models, PD-0332991 was a particularly effective therapy, which induced Rb-dependent cytostasis. Combined, these findings indicate the critical importance of fully understanding cell-cycle regulatory pathways in directing the utilization of CDK inhibitors in the clinic.