**2. BRAF**

The RAF protein is made of three conserved regions: CR1, CR2 and CR3. CR1 and CR2 are situated in the N terminus. CR1 acts as the main binding domain for RAS; CR2 is the regulatory domain. CR3 is situated in the C terminus and functions as the catalytic kinase domain. CR3 contains two regions important for RAF activation: the activation segment and the regulatory region [5]. Of the RAF family of protein kinases, BRAF is the most frequently mutated and remains the most potent activator of MEK.

The BRAF protooncogene, which encodes for the BRAF protein kinase, is located on chromosome 7 (q34) and is composed of 18 exons. There have been more than thirty BRAF mutations identified to date, occurring in various frequencies. The most common is BRAF V600E mutation (MT), which corresponds to a thymine to adenine transversion at position 1799, resulting in the substitution of valine by glutamate at position 600 of the protein [5]. This lies within the activating segment of the kinase domain. It renders BRAF constitutionally active, increasing kinase activity relative to BRAF wild-type (WT) by 10 times [6]. Because of this, co-mutations in the MAPK signalling cascade offers no selective advantage for developing tumours and therefore BRAF mutations are mutually exclusive with KRAS or NRAS mutations [7].

The V600E mutation accounts for more than 85% of BRAF mutations in melanoma, more than 50% of the mutations in non-small cell lung cancer and more than 95% of mutations in cholangiocarcinoma and hairy cell leukaemia. It accounts for more than 90% of BRAF mutations in colorectal cancer (CRC) [8]. Other BRAF mutations include R461I, I462S, G463E, G463V, G465A, G465E, G465V, G468A, G468E, N580S, E585K, D593V, F594L, G595R, L596V, T598I, V599D, V599E (V600E), V599K, V599R, V600K, and A727V [9].

**37**

*BRAF Mutation and Its Importance in Colorectal Cancer*

**3. Prevalence and clinical features of BRAF MT CRC**

chromosomal instability leads to neoplastic progression [20].

*MLH1* results in microsatellite instability (MSI) in sporadic CRC [24].

BRAF mutations have been found in 7–10% of patients with metastatic CRC [7, 10]. BRAF MT CRC has been associated with a particular phenotype in multiple studies and meta-analysis and specifically pertaining to the BRAF V600E mutation. BRAF tumours are more prevalent in women and in patients >70 years of age. BRAF is not associated with age at diagnosis of less than 60 years [11]. BRAF mutation is more prevalent in proximal colon tumours and is rarely found in the left colon [7]. Histopathology also differs, with 60% of BRAF MT tumours being poorly differentiated and a higher rate of mucinous pathology [12]. There is an association with larger primary tumours. BRAF MT CRC is also associated with a high rate of peritoneal metastases and less lung and liver-limited disease [13–15]. In contrast, most non-V600 mutations were more likely to be lower grade and left-sided tumours with a greater overall survival [16, 17], except for codon 601/597 mutations which behave similarly to V600E MT CRC [18].

The pathogenesis of CRC is a heterogeneous and complex process. The classic model of adenoma-carcinoma sequence was initially described by Vogelstein and accounts for approximately 80% of sporadic CRC [19]. Mutation of the tumour suppressor gene, APC, occurs early in the process and additional mutations and

The serrated neoplastic pathway is an alternative model of CRC pathogenesis with distinct morphologic and molecular characteristics. It is estimated about 20% of CRC develop via this pathway. These lesions develop from aberrant crypt foci and hyperplastic polyps (HP) into traditional serrated adenoma (TSA) and sessile serrated adenoma (SSA), with malignant potential. BRAF mutation occurs early in the pathway, shown to be present in HP, hyperplastic adenomas and SSA [21].

SSA are also characterised by the CpG island methylator phenotype (CIMP) [22]. A cytosine nucleotide followed by a guanine nucleotide (CpG dinucleotide) can be found in dense clusters (CpG islands) in the promoter regions of approximately half of all genes [23]. Aberrant hypermethylation of these CpG islands can lead to silencing of tumour suppressor genes that, in turn, lead to carcinogenesis. CIMP can be described as high, low or negative. Hypermethylation of the mismatch repair gene

MSI is implicated in 15% of sporadic CRC and >95% of Hereditary Non Polyposis Colorectal Cancer (HNPCC), also known as Lynch syndrome. It is caused by deficiency of the DNA mismatch repair (MMR) system, composed of multiple interacting proteins including MSH2, MLH1. The majority of sporadic MSI high CRC is due to the hypermethylation of the mismatch repair gene *MLH1* [25]. Sporadic MSI high CRC is also associated with BRAF mutation. BRAF mutations have been observed in 30–50% of MSI high CRC compared with 10% in microsatellite stable tumours [26, 27].

Germline mutations in 1 of 4 mismatch repair genes (MLH1, MSH2, MSH6 and PMS2) account for the majority of cases of HNPCC. BRAF mutations rarely occur in patients

BRAF MT CRC is strongly associated with inferior survival compared with BRAF WT disease. Randomised control trials of first line treatment of metastatic CRC demonstrate differences in OS of up to 12 months, shown in **Table 1**.

*DOI: http://dx.doi.org/10.5772/intechopen.82571*

**4. The serrated neoplastic pathway**

with germline mutations in MMR genes [28].

**5. Prognostic significance of BRAF mutation**

*Advances in the Molecular Understanding of Colorectal Cancer*

The RAF protein is made of three conserved regions: CR1, CR2 and CR3. CR1 and CR2 are situated in the N terminus. CR1 acts as the main binding domain for RAS; CR2 is the regulatory domain. CR3 is situated in the C terminus and functions as the catalytic kinase domain. CR3 contains two regions important for RAF activation: the activation segment and the regulatory region [5]. Of the RAF family of protein kinases, BRAF is the most frequently mutated and remains the most potent

The BRAF protooncogene, which encodes for the BRAF protein kinase, is located on chromosome 7 (q34) and is composed of 18 exons. There have been more than thirty BRAF mutations identified to date, occurring in various frequencies. The most common is BRAF V600E mutation (MT), which corresponds to a thymine to adenine transversion at position 1799, resulting in the substitution of valine by glutamate at position 600 of the protein [5]. This lies within the activating segment of the kinase domain. It renders BRAF constitutionally active, increasing kinase activity relative to BRAF wild-type (WT) by 10 times [6]. Because of this, co-mutations in the MAPK signalling cascade offers no selective advantage for developing tumours and therefore BRAF mutations are mutually exclusive with KRAS or NRAS mutations [7]. The V600E mutation accounts for more than 85% of BRAF mutations in melanoma, more than 50% of the mutations in non-small cell lung cancer and more than 95% of mutations in cholangiocarcinoma and hairy cell leukaemia. It accounts for more than 90% of BRAF mutations in colorectal cancer (CRC) [8]. Other BRAF mutations include R461I, I462S, G463E, G463V, G465A, G465E, G465V, G468A, G468E, N580S, E585K, D593V, F594L, G595R, L596V, T598I, V599D, V599E

**36**

(V600E), V599K, V599R, V600K, and A727V [9].

**2. BRAF**

*The MAPK pathway [80].*

**Figure 1.**

activator of MEK.
