BBA - Reviews on Cancer (v.1756, #2)

The KRAS oncogene: Past, present, and future by Onno Kranenburg (81-82).

Mutant KRAS in aberrant crypt foci (ACF): Initiation of colorectal cancer? by Theresa P. Pretlow; Thomas G. Pretlow (83-96).
Since aberrant crypt foci (ACF) were first described in 1987, they have been the subjects of hundreds of papers; however, the debate continues about their role in colorectal tumorigenesis. This review focuses on the many phenotypic, genetic and epigenetic alterations in ACF that support the hypothesis that ACF are putative precursors of colorectal cancer in both humans and experimental animals. Human ACF, both with and without dysplasia, are monoclonal and display evidence of chromosomal instability. Both of these characteristics are shared by colorectal cancers. While most ACF do not have APC mutations, a large proportion has KRAS mutations and methylated SFRP1 and SFRP2 genes. This epigenetic inactivation gives rise to constitutive Wnt signaling in these putative precursors of colorectal cancer.
Keywords: KRAS; Colorectal cancer; Aberrant crypt foci; Premalignant lesion; Putative precursors of colorectal cancer;

Mutant KRAS in the initiation of pancreatic cancer by Therese Deramaudt; Anil K. Rustgi (97-101).
Pancreatic ductal adenocarcinoma is the most common pancreatic neoplasm. There are approximately 33,000 new cases of pancreatic ductal adenocarcinoma annually in the United States with approximately the same number of deaths. Surgery represents the only opportunity for cure, but this is restricted to early stage pancreatic cancer. Pancreatic ductal adenocarcinoma evolves from a progressive cascade of cellular, morphological and architectural changes from normal ductal epithelium through preneoplastic lesions termed pancreatic intraepithelial neoplasia (PanIN). These PanIN lesions are in turn associated with somatic alterations in canonical oncogenes and tumor suppressor genes. Most notably, early PanIN lesions and almost all pancreatic ductal adenocarcinomas involve mutations in the K-ras oncogene. Thus, it is believed that activating K-ras mutations are critical for initiation of pancreatic ductal carcinogenesis. This has been proven through elegant genetically engineered mouse models in which a Cre-activated K-Ras G12D allele is knocked into the endogenous K-Ras locus and crossed with mice expressing Cre recombinase in pancreatic tissue. As a result, mechanistic insights are now possible into how K-Ras contributes to pancreatic ductal carcinogenesis, what cooperating events are required, and armed with this knowledge, new therapeutic approaches can be pursued and tested.

Control of colorectal metastasis formation by K-Ras by Niels Smakman; Inne H.M. Borel Rinkes; Emile E. Voest; Onno Kranenburg (103-114).
Mutational activation of the K-Ras proto-oncogene is frequently observed during the very early stages of colorectal cancer (CRC) development. The mutant alleles are preserved during the progression from pre-malignant lesions to invasive carcinomas and distant metastases. Activated K-Ras may therefore not only promote tumor initiation, but also tumor progression and metastasis formation. Metastasis formation is a very complex and inefficient process: Tumor cells have to disseminate from the primary tumor, invade the local stroma to gain access to the vasculature (intravasation), survive in the hostile environment of the circulation and the distant microvascular beds, gain access to the distant parenchyma (extravasation) and survive and grow out in this new environment. In this review, we discuss the potential influence of mutant K-Ras on each of these phases. Furthermore, we have evaluated the clinical evidence that suggests a role for K-Ras in the formation of colorectal metastases.
Keywords: Metastasis; K-Ras; Colorectal cancer;

Mutant KRAS, chromosomal instability and prognosis in colorectal cancer by Patrizio Castagnola; Walter Giaretti (115-125).
The RAS gene family provides a global effect on gene expression by encoding small GTP-binding proteins which act as molecular switches connecting extracellular signals with nuclear transcription factors. While wild type RAS proteins are switched off shortly after activation, mutant RAS proteins remain constitutively activated leading to complex interactions among their downstream effectors. For some human tumor types, these interactions were shown to contribute to cancer genesis and progression by inducing changes in cell survival, apoptosis, angiogenesis, invasion and metastasis. This review addresses the controversial link of KRAS mutations in colorectal cancer with chromosomal instability and patient prognosis.
Keywords: KRAS; Prognosis; Colorectal cancer; Chromosomal instability;

K-ras as a target for cancer therapy by Bret B. Friday; Alex A. Adjei (127-144).
The central role K-, H- and N-Ras play in regulating diverse cellular pathways important for cell growth, differentiation and survival is well established. Dysregulation of Ras proteins by activating mutations, overexpression or upstream activation is common in human tumors. Of the Ras proteins, K-ras is the most frequently mutated and is therefore an attractive target for cancer therapy. The complexity of K-ras signaling presents many opportunities for therapeutic targeting. A number of different approaches aimed at abrogating K-ras activity have been explored in clinical trials. Several of the therapeutic agents tested have demonstrated clinical activity, supporting ongoing development of K-ras targeted therapies. However, many of the agents currently being evaluated have multiple targets and their antitumor effects may not be due to K-Ras inhibition. To date, no selective, specific inhibitor of K-ras is available for routine clinical use. In this review, we will summarize the structure and function of K-ras with attention to its role in tumorigenesis and discuss the successes and failures of the various strategies designed to therapeutically target this important oncogene.
Keywords: K-ras; Cancer therapy; Clinical trial; Ras effector; Oncogene;

Mouse models of K-ras-initiated carcinogenesis by Klaus-Peter Janssen; Miguel Abala; Fatima El Marjou; Daniel Louvard; Sylvie Robine (145-154).
Activating mutations of the oncogene K-ras are found in one third of all human cancers. Much of our knowledge on K-ras signal transduction and its influence on tumor initiation and progression comes from in vitro studies with cell lines. However, mouse models of human cancer allow a much more faithful recapitulation of the human disease, and the in vivo perspective is crucial for our understanding of neoplasia. In recent years, several new murine models for K-ras-induced tumorigenesis have been described. They allow new insights into the specific role that oncogenic K-ras proteins play in different solid tumors, and they permit the molecular dissection of the pathways that are initiated by somatic mutations in subsets of cells. Key advances have been made by the use of tissue-specific and inducible control of expression, which is achieved by the Cre/LoxP technology or the tetracycline system. from these sophisticated models, a common picture emerges: The effects of K-ras on tumor initiation depend strongly on the cellular context, and different tissues vary in their susceptibility to K-ras transformation.