38.13:
The Ras Gene
Eukaryotic cells employ numerous intracellular and extracellular signaling cascades to respond appropriately to the external stimuli as well as to maintain their normal physiological functions.
These signaling cascades are controlled by a diverse set of intracellular regulatory proteins, such as the Ras proteins.
Ras is a family of GTPases involved in the regulation of signaling pathways that control cell growth and proliferation.
These proteins are anchored to the cytoplasmic side of the plasma membrane, and transit between two states – the GTP bound active state, and the GDP bound inactive state.
In a healthy cell, an appropriate signaling molecule activates the tyrosine kinase receptor present on the cell surface.
The active receptor helps to recruit adapter proteins and Ras guanine nucleotide exchange factors or Ras GEFs, which act as a link between the receptor and the Ras protein.
The Ras GEFs then replace the GDP bound to the inactive RAS protein with a GTP, switching it into an active state.
The active Ras protein then further transmits the signal via a signaling cascade to activate the mitogen-activated protein kinase or MAP kinase.
The active MAP kinase phosphorylates transcription factors that turn on the genes encoding growth-promoting proteins and facilitate controlled cell growth and division.
However, Ras is a proto-oncogene that, upon hyperactivation can lead to uncontrolled proliferation of cells.
In normal situations, the cell employs special proteins called Ras GTPase-activating proteins or GAPs that can hydrolyze the GTP bound to the active Ras and transform it back to the inactive state.
But in the case of a Ras mutation, the altered protein may not allow Ras GAPs to hydrolyze the GTP.
This effectively turns RAS into a hyperactive protein which is constitutively active, even in the absence of external stimuli, and continuously transmits growth signals to the downstream signaling molecules. The result is uncontrolled cell growth and proliferation.
Mutations in the Ras genes have implications in many human cancers, especially colorectal cancer.
38.13:
The Ras Gene
The Ras-gene-encoded proteins are regulators of signaling pathways controlling cell proliferation, differentiation, or cell survival. The Ras-gene family in humans constitutes three primary members—the HRas, NRas, and KRas. These genes code for four functionally distinct yet closely related proteins—the HRas, NRas, KRas4A, and KRas4B. The involvement of mutant Ras genes in human cancer was first discovered in 1982 and is among the most common causes of human tumorigenesis.
Ras is a superfamily of small GTPase proteins that facilitate the transmission of signals by continually cycling between an active GTP-bound form and an inactive GDP-bound form, thereby acting as molecular switches. The exchange of GDP with GTP, facilitated by the guanine nucleotide exchange factors or GEFs, turns ‘on’ the Ras proteins. The GTPase-activating proteins or GAPs catalyze the hydrolysis of GTP to GDP, turning ‘off’ the Ras proteins. An active Ras-GTP binds and activates its downstream effector molecules involved in cell growth and proliferation signaling.
Some specific point mutations, such as in codons 12, 13, or 61 in the Ras genes, result in the production of significantly impaired proteins. These mutations can affect the overall GTPase activity in the Ras proteins or impair the GAP sensitivity of the proteins. The absence of GTP hydrolysis locks the proteins into a constitutively active state. The mutant Ras proteins continually transmit signals to downstream effector molecules in the pathway, even in the absence of external stimuli, triggering the uncontrolled proliferation of cells.
Ras mutations can be found in up to 30 percent of all the human tumors screened, most commonly in colorectal carcinoma, non-small cell lung carcinoma, and pancreatic ductal adenocarcinoma. Mutations in K-ras locus are found in about 25-30 percent of tumor samples, N-ras mutations in about 8 percent of tumors, and H-ras mutations in only about 3 percent of tumors.
Suggested Reading
- Castellano, Esther, and Eugenio Santos. "Functional Specificity of Ras Isoforms: So Similar but so Different." Genes & Cancer 2, no. 3 (2011): 216-231. [Source]
- Hobbs, G Aaron, Channing J Der, and Kent L Rossman. "RAS Isoforms and Mutations in Cancer at a Glance." Journal of Cell Science 129, no. 7 (2016): 1287-1292. [Source]
- Prior, Ian A, Paul D Lewis, and Carla Mattos. "A Comprehensive Survey of Ras Mutations in Cancer." Cancer Research 72, no. 10 (2012): 2457-2467. [Source]