KRAS

KRAS Background

The KRAS gene encodes a small GTPase transductor protein called KRAS, which was first identified as an oncogene in the Kirsten RAt Sarcoma virus ^[1]. The KRAS proteins have two splice variants: KRASA and KRASB. KRASA is expressed in a tissue-specific and developmentally restricted fashion, whereas KRASB is ubiquitously expressed. KRAS relays signals from outside the cell to the cell's nucleus, instructing the cell to grow and divide (proliferate) or to mature and take on specialized functions (differentiate ^[2]. As a GTPase, KRAS is activated when bound to GTP, whereas it is inactive in GDP-bound form. Following EGF binding to its receptor and activation of tyrosine kinases, the KRAS protein becomes activated by binding to GTP, transducing the activation signal to the nucleus by MAPKs and PI3K/AKT-mediated cascades ^[3]. Specifically, the active state of the KRAS protein is facilitated by binding to the Grb2 protein, which interacts with the SH3 domains of the SOS protein, a member of the nucleotide exchange factor family. In the GTP state, KRAS can activate downstream proteins and regulate cell transformation. As a result of these effects, KRAS elicits changes in the cytoskeleton and consequently affects cell shape, adhesion, and migration, triggering cell differentiation, growth, chemotaxis, and apoptosis ^[4][5]. KRAS mutations rarely cause NS with the NS-causing mutations resulting in increased signaling of the Ras/MAPK pathway. Although most clinical geneticists would agree that individuals with a KRAS mutation have a clinical phenotype consistent with NS, some individuals have a more severe phenotypic manifestation consistent with CFC (see below). HCM has been seen in patients with KRAS mutations and clinical features of both CS and CFC.

[1] Tsuchida N, Ryder T, et al. Nucleotide sequence of the oncogene encoding p21 transforming protein of Kirsten murine sarcoma virus [J]. Science. 1982, 217 (4563): 937-939.
[2] J. P. McGrath, D. J. Capon, et al. Structure and organization of the human Ki-ras proto-oncogene and a related processed pseudogene [J]. Nature, 1983, vol. 304, no. 5926, pp. 501-506.
[3] Normanno N, Tejpar S, et al. Implications for KRAS status and EGFR-targeted therapies in metastatic CRC. Nat Rev Clin Oncol. 2009 Sep; 6(9):519-27.
[4] D. Esser, B. Bauer, et al. Structure determination of the ras-binding domain of the Ral-specific guanine nucleotide exchange factor Rlf [J]. Biochemistry, 1998, vol. 37, no. 39, pp. 13453-13462.
[5] J. Zuber, O. I. Tchernitsa, et al. A genome-wide survey of RAS transformation targets [J]. Nature Genetics, 2000, vol. 24, no. 2, pp. 144-152.