Three different human RAS genes have been identified: KRAS (homologous to the oncogene from the Kirsten rat sarcoma virus), HRAS (homologous to the oncogene from the Harvey rat sarcoma virus), and NRAS (first isolated from a human neuroblastoma). The different RAS genes are highly homologous but functionally distinct; the degree of redundancy remains a topic of investigation (reviewed in Pylayeva-Gupta et al. 2011). RAS proteins are small GTPases which cycle between inactive guanosine diphosphate (GDP)-bound and active guanosine triphosphate (GTP)-bound forms. RAS proteins are central mediators downstream of growth factor receptor signaling and therefore are critical for cell proliferation, survival, and differentiation. RAS can activate several downstream effectors, including the PI3K-AKT-mTOR pathway, which is involved in cell survival, and the RAS-RAF-MEK-ERK pathway, which is involved in cell proliferation (Figure 1).

RAS has been implicated in the pathogenesis of several cancers. Activating mutations within the RAS gene result in constitutive activation of the RAS GTPase, even in the absence of growth factor signaling. The result is a sustained proliferation signal within the cell.

Specific RAS genes are recurrently mutated in different malignancies. KRAS mutations are particularly common in colon cancer, lung cancer, and pancreatic cancer (for reviews see Karnoub and Weinberg 2008 and Schubbert, Shannon, and Bollag 2007).

Figure 1. Schematic of the MAPK and PI3K pathways. Growth factor binding to receptor tyrosine kinase results in activation of the MAPK signaling pathway (RAS-RAF-MEK-ERK) and the PI3K pathway (PI3K-AKT-mTOR). The letter "K" within the schema denotes the tyrosine kinase domain.

Related Pathways

• MAP kinase signaling