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Raf proteins and cancer: B-Raf is identified as a mutational target
Kathryn E Mercer1, Catrin A Pritchard
1Department of Biochemistry, University of Leicester, University Road, LE1 7RH, Leicester, UK.
Abstract:
A recent report has shown that activating mutations in the BRAF gene are present in a large percentage of human malignant melanomas and in a proportion of colon cancers. The vast majority of these mutations represent a single nucleotide change of T-A at nucleotide 1796 resulting in a valine to glutamic acid change at residue 599 within the activation segment of B-Raf. This exciting new discovery is the first time that a direct association between any RAF gene and human cancer has been reported. Raf proteins are also indirectly associated with cancer as effectors of activated Ras proteins, oncogenic forms of which are present in approximately one-third of all human cancers. BRAF and RAS mutations are rarely both present in the same cancers but the cancer types with BRAF mutations are similar to those with RAS mutations. This has been taken as evidence that the inappropriate regulation of the downstream ERKs (the p42/p44 MAP kinases) is a major contributing factor in the development of these cancers. Recent studies in mice with targeted mutations of the raf genes have confirmed that B-Raf is a far stronger activator of ERKs than its better studied Raf-1 homologue, even in cell types in which the protein is barely expressed. The explanation for this lies in a number of key differences in the regulation of B-Raf and Raf-1 activity. Constitutive phosphorylation of serine 445 of B-Raf leads to this protein having a higher basal kinase activity than Raf-1. Phosphorylation of threonine 598 and serine 601 within the activation loop of B-Raf at the plasma membrane also regulates its activity. The V599E mutation is thought to mimic these phosphorylations, resulting in a protein with high activity, leading to constitutive ERK activation. B-Raf now provides a critical new target to which drugs for treating malignant melanoma can be developed and, with this in mind, it is now important to gain clear insight into the biochemical properties of this relatively little characterised protein.
Insights
Activating BRAF mutations drive melanoma and colon cancers. The V599E mutation in BRAF mimics phosphorylation, causing constitutive ERK activation and highlighting BRAF as a key cancer target.
Area of Science:
- Oncology
- Molecular Biology
- Genetics
Background:
- Activating mutations in the BRAF gene are frequently found in malignant melanomas and colon cancers.
- BRAF mutations are associated with cancer, distinct from but similar to RAS mutations, suggesting a role in ERK pathway dysregulation.
- B-Raf is a potent activator of ERKs (extracellular signal-regulated kinases), a pathway implicated in cancer development.
Purpose of the Study:
- To investigate the direct association between RAF gene mutations and human cancers.
- To understand the biochemical properties and regulatory differences between B-Raf and Raf-1.
- To establish B-Raf as a critical therapeutic target for malignant melanoma.
Main Methods:
- Analysis of BRAF gene mutations in human cancer samples (melanoma, colon cancer).
- Comparison of B-Raf and Raf-1 activity and regulation through biochemical studies.
- Investigating the role of specific phosphorylation sites (Serine 445, Threonine 598, Serine 601) and the V599E mutation.
Main Results:
- A specific T-A nucleotide change at position 1796 (V599E mutation) in BRAF is common in melanoma and colon cancer.
- B-Raf exhibits higher basal kinase activity than Raf-1 due to constitutive phosphorylation at Serine 445.
- The V599E mutation mimics activating phosphorylations, leading to constitutive ERK activation.
Conclusions:
- BRAF mutations, particularly V599E, are directly implicated in human cancer development.
- Understanding B-Raf's unique regulatory mechanisms is crucial for its therapeutic targeting.
- B-Raf represents a significant new target for developing drugs against malignant melanoma.