- Email: [email protected]
Tissue-Specific Mutations in BRAF and EGFR Necessitate Unique Therapeutic Approaches
Forum
of many critical growth signaling cascades (such as the MAPK and PI3K pathways). Mutations within the KRAS family member represent the second most frequent alteration in cancer, found in 20% of all patients, and 90% of pancreatic cancer, 43% of colorectal cancer (CRC), and 32% of lung adenocarcinoma [1,2]. Mutations typically occur at codons 12, 13, or 61 and the mutational frequency is similar across these three tumor types, with one striking exception (Figure 1A). Lung adenocarci1 Scott A. Foster, nomas show a significant enrichment for Christiaan Klijn,1 and G12C mutations (50% of KRAS G12 Shiva Malek1,* mutations in lung adenocarcinoma are G12C, as opposed to 9% in CRC). This A predominant number of cancers can largely be attributed to G > T transare driven by mutations of key versions associated with smoking [3].
Tissue-Specific Mutations in BRAF and EGFR Necessitate Unique Therapeutic Approaches
growth signaling genes. While it might be expected that the same alterations within a given oncogene would be identified in all tissues, there are clear cases of tissue specificity. Here, we highlight the tissue specificity of BRAF and EGFR alterations and implications for therapeutic targeting. Sustained proliferative growth of tumor cells is often achieved by somatic mutations within components of growth factor signaling pathways. Recent advances in high-throughput sequencing technology have broadened our knowledge of the underlying driver mutations in most common cancers, informing additional therapeutic avenues that could be beneficial for patients. Interesting trends are also observed when comparing mutational spectra across different tumor types. Curiously, the spectrum of mutations often exhibits tissue specificity; both in the context of the target gene and the particular alteration within a given gene, implying either tissue-specific advantages or differing underlying causes of a given alteration. A key component of many of these signaling pathways is RAS, which functions downstream of growth factor receptors (such as EGFR and HER2) and upstream
While these differences can largely be explained for KRAS, another example of tissue-specific mutational spectra occurs within EGFR driver mutations. In the context of EGFR, lung adenocarcinomas are more frequently driven by activating kinase domain (intracellular) mutations (83% of EGFR mutant lung adenocarcinoma samples carry exclusively intracellular mutations), while glioblastomas exhibit an enrichment of EGFR extracellular domain mutations (81% of EGFR mutant glioblastoma samples carry exclusively extracellular domain mutations). Unlike the KRAS context, these tissue-specific mutations in EGFR are harder to explain with single nucleotide transversions, and hence suggest that there is a tissue-selective advantage for enrichment of one mutation class versus another (Figure 1B, C) [1,2,4,5]. Similarly difficult to explain is the striking tissue specificity of mutations with the MAPK pathway component BRAF. BRAF mutant melanomas, thyroid cancer, and CRC are almost exclusively driven by the well-characterized canonical activating mutation V600E (V600 mutations constitute 98% of thyroid, 91% of colorectal and 87% of melanoma BRAF mutations), whereas lung adenocarcinomas show an enrichment of noncanonical kinase domain point mutations
with 77% of samples carrying nonV600E mutations (Figure 1D) [1,2]. Importantly, as with BRAF V600E mutations, these noncanonical BRAF mutations are also largely mutually exclusive with KRAS mutations. Similarly, we recently characterized BRAF mutant pancreatic adenocarcinomas and observed an enrichment for noncanonical mutations, also mutually exclusive with KRAS mutations (Figure 1E) [6]. Interestingly, the spectrum of noncanonical alterations differs from lung, with the majority of pancreatic tumors harboring a previously uncharacterized short, inframe deletion within a critical loop of the BRAF kinase domain. Shortening of this loop (the b3–/C loop, which we referred to as b3–/C deletions) effectively limits the normal structural flexibility of the kinase, locking the kinase in the ‘on’ conformation (Figure 2A). While these tissue-specific enrichments may be difficult to explain based on catalytic activity alone, further investigation is likely to uncover unexpected and interesting biology. One critical outcome of this broad cataloging of unique driver alterations has been to define the sensitivity of specific mutations towards clinical therapies. For BRAF, early direct sequencing efforts defined BRAF V600E as an excellent drug target and led to the clinical success of vemurafenib in metastatic melanoma [7]. Despite this success, V600E-driven colorectal cancers have shown weak response to vemurafenib treatment [8], highlighting yet another level of complexity of tissue specificity. The overall basis for differential sensitivity to vemurafenib appears to reside at the heart of normal MAPK biology. In MAPK signaling, dimerization of inactive RAF monomers (ARAF, BRAF, or CRAF) upon upstream activation results in downstream signaling [9,10]. While the V600E mutation confers dimer-independent activity and sensitivity to vemurafenib in melanoma [11,12] (Figure 2B), V600E colorectal cancers show high EGFR activity and increased
Trends in Cancer, December 2016, Vol. 2, No. 12
699
Q61x
G[12/13]x
Other KRAS
0
Rec..
Furin-like
200
0
Rec..
400
GF_recep_IV
600
1.00
0.50
0.25
0.00
1.00
0.75
# mutated 22 22
Skin melanoma
177
Thyroid cancer
242
1210 aa
BRAF mutaon
EGFR - Glioblastoma - TCGA
V600
N581
G469
K601
D594
BRAF deleon
Other BRAF
A289x
20
G589[V/A]
Frequencies of BRAF mutaons Foundaon medicine data
(E)
# mutated
Pancreac cancer 400
GF_recep_IV
600
Pkinase_Tyr
800
Indel - in frame
1000
Truncang
1210 aa
24
BRAF mutaon
V600
BRAF deleon
G469
1.00
200
Missense
Rec..
0.75
0
Furin-like
0.50
Rec..
0.25
0
0.00
# mutaons
Both
Colorectal cancer
Pkinase_Tyr
1000
Extra cellular mutaon
Lung adenocarcinoma
L858R
800
Intra cellular mutaon
Frequencies of BRAF mutaons - TCGA
(D)
0.00
E746_A750del
7
EGFR mutaon
1.00
G13C
G13D
77
0.75
G12V
G12D
# mutated 33
Lung adenocarcinoma Glioblastoma
0.50
G12C
EGFR - Lung adenocarcinoma - TCGA
(B) # mutaons
0.75
123 0.50
Pancreac cancer 0.25
96
0.00
Colorectal cancer
KRAS mutaon
Frequencies of EGFR mutaons - TCGA
(C) # mutated 75
0.25
Frequencies of KRAS mutaons - TCGA
(A) Lung adenocarcinoma
Other BRAF alteraon
Figure 1. Overview of KRAS, EGFR, and BRAF Mutational Patterns in Large Cancer Genomics Data Sets. (A) Proportional bar graph showing the spectrum of codon mutations in KRAS in lung, colorectal and pancreatic cancer in The Cancer Genome Atlas data (TCGA). (B) Lollipop plot showing the distribution of mutations over the EGFR protein as seen in lung cancer or glioblastoma TCGA data. (C) Proportional bar graph of the same data in (B), but categorized as extracellular (before the 621st amino acid) or intracellular (after the 621st amino acid). Samples that presented with mutation in both regions were annotated as ‘both’. (D) Proportional bar graph showing the spectrum of codon mutations in BRAF in lung, colorectal, skin, and thyroid cancer in TCGA data. (E) Overview of BRAF mutations in Foundation Medicine data of pancreatic cancer (n = 1305) [6]. TCGA data was collected from cbioportali and the provisional data sources were used where applicable.
propensity to reactivate the pathway in structural change caused by the deletion part through RAF dimerization resulting [6] (Figure 2A, D). Thus, while lung and in decreased vemurafenib sensitivity [13]. pancreatic adenocarcinomas are largely driven by noncanonical BRAF mutations, While BRAF V600E results in robust the type of alteration and underlying bioconstitutive kinase activity, the majority chemical behavior enriched in each tisof noncanonical BRAF kinase domain sue differs. Despite these differences, a alterations identified in lung adenocarci- commonality between all of these nonnomas (with the highest frequency of canonical BRAF alterations is vemurafeG469, N581, and D594 mutations) all nib resistance. target residues that contribute to catalytic function of the kinase and are likely Second-generation RAF inhibitors are kinase-inactivating (or weakly activating) being developed that may show benefit mutations. Instead, these mutations to these broader classes of patients. require association with a wild-type Interestingly, while we noted that the lung RAF molecule (functioning as hetero- and pancreatic noncanonical alterations dimers) and show weak vemurafenib show distinct biochemical behavior, sensitivity [12,14,15] (Figure 2C). The these two classes result in a similar strucBRAF b3–/C deletions, which make tural effect on the kinase domain. This up the majority of BRAF mutant pancre- overall structural change, a shift of the atic adenocarcinomas, achieve constitu- /C towards an ‘in’ conformation, is tive kinase activity independent of genetically achieved by shortening of dimerization, but show complete resis- the b3–/C loop in the b3–/C deletions tance to vemurafenib because of the and induced upon dimerization in the lung
700
Trends in Cancer, December 2016, Vol. 2, No. 12
noncanonical alterations (Figure 2C, D). This results in the decreased efficacy of vemurafenib that binds in the /C ‘out’ conformation. Consistent with this structural change, both the b3–/C deletions and most of the noncanonical mutations tested show sensitivity to BRAF inhibitors that bind to the /C ‘in’ conformation [6,12] (Figure 2C, D). Early work to identify and develop therapies selectively targeting certain driver mutations in cancers has led to clinical benefit for many patients. As highlighted with the BRAF mutations, developing next-generation therapies is a complicated problem as both tissue-specific and mutation-specific effects all contribute to the underlying biology and drug sensitivity. As the molecular complexities of specific tumors continue to be defined, development of therapies that take this broader view will be critical to achieve optimal benefit cancer patients.
7. Kim, G. et al. (2014) FDA approval summary: vemurafenib for treatment of unresectable or metastatic melanoma with the BRAFV600E mutation. Clin. Cancer Res. 20, 4994–5000
(A) β3-αC loop
β3-αC loop
αC
β3
β3-αC loop
αC
β3
BRAF
BRAF
αC ‘out’ kinase off
β3-αC deleon kinase on
(C)
αC
β3
BRAFMutant
β3
αC
BRAFMutant
(D)
β3
αC
B/CRAFWildtype
β3
αC
BRAFMutant
β3
αC
Lung adenocarcinoma
Pancreac adenocarcinoma
Predominantly non-canonical Dimeric acvity
Predominantly β3-αC deleon Monomeric acvity
Vemurafenib sensive
Vemurafenib resistant
Vemurafenib resistant
αC ‘in’ inhibitor sensive
αC ‘in’ inhibitor sensive
αC ‘in’ inhibitor sensive
and ‘off’ states. In normal signaling, these shifts are controlled by upstream signaling inputs. In cancer, these conformational dynamics can be hijacked by oncogenic alterations shifting the kinase towards the ‘on’ state. One of these critical shifts is the movement of the /C-helix (labeled /C) from an ‘out’ conformation (shifted away from the b3 strand; labeled b3) in the ‘off’ state inwards (towards the b3 strand) in the ‘on’ state. This shift forms critical interactions between b3 and /C, allowing the proper coordination of ATP required for kinase activity. The ability of the kinase to toggle between these two states is afforded by the flexibility of the b3–/C loop. In the b3–/C deletions, this loop is shortened effectively locking the kinase in the ‘on’ conformation. (B) BRAF mutant melanomas largely consist of BRAF V600E mutations. These mutations confer monomeric kinase activity, allowing the kinase to accommodate both the /C ‘out’ inhibitor vemurafenib (shown in red) and /C ‘in’ inhibitors (shown in green). (C) BRAF mutant lung adenocarcinomas largely consist of BRAF noncanonical kinase domain point mutations. These mutations promote dimerization, which induces the shift of /C towards the ‘in’ conformation. While these alterations show sensitivity to /C ‘in’ inhibitors, they show decreased sensitivity to the /C ‘out’ inhibitor vemurafenib. (D) BRAF mutant pancreatic adenocarcinomas largely consist of BRAF b3–/C deletions. The shortening of this loop results in monomeric kinase activity, but locks the kinase in the /C ‘in’ conformation. These deletions are resistant to the /C ‘out’ inhibitor vemurafenib, while remaining sensitive to /C ‘in’ inhibitors. Model figures were generated with the assistance of Allison Bruce.
1
Departments of Discovery Oncology and Bioinformatics, Genentech Inc., South San Francisco, CA, USA *Correspondence: [email protected] (S. Malek). http://dx.doi.org/10.1016/j.trecan.2016.10.015
References 1. Cerami, E. et al. (2012) The cBio cancer genomics portal: an open platform for exploring multidimensional cancer genomics data. Cancer Discov. 2, 401–404
13. Corcoran, R.B. et al. (2012) EGFR-mediated re-activation of MAPK signaling contributes to insensitivity of BRAF mutant colorectal cancers to RAF inhibition with vemurafenib. Cancer Discov. 2, 227–235
15. Heidorn, S.J. et al. (2010) Kinase-dead BRAF and oncogenic RAS cooperate to drive tumor progression through CRAF. Cell 140, 209–221
Melanoma
www.cbioportal.org
12. Yao, Z. et al. (2015) BRAF mutants evade ERK-dependent feedback by different mechanisms that determine their sensitivity to pharmacologic inhibition. Cancer Cell 28, 370–383
BRAFMutant
Figure 2. Activation Mechanism and Inhibitor Sensitivity of Different BRAF Alterations Identified in Varying Tumor Types. (A) Dynamic conformational changes are required for kinases to shift between ‘on’
i
11. Haling, J.R. et al. (2014) Structure of the BRAF-MEK complex reveals a kinase activity independent role for BRAF in MAPK signaling. Cancer Cell 26, 402–413
14. Garnett, M.J. et al. (2005) Wild-type and mutant B-RAF activate C-RAF through distinct mechanisms involving heterodimerization. Mol. Cell 20, 963–969
Predominantly V600E Monomeric acvity
Resources
9. Rajakulendran, T. et al. (2009) A dimerization-dependent mechanism drives RAF catalytic activation. Nature 461, 542–545 10. Weber, C.K. et al. (2001) Active Ras induces heterodimerization of cRaf and BRaf. Cancer Res. 61, 3595–3598
BRAF
αC ‘in’ kinase on
(B)
αC
β3
8. Kopetz, S. et al. (2015) Phase II pilot study of vemurafenib in patients with metastatic BRAF-mutated colorectal cancer. J. Clin. Oncol. 33, 4032–4038
2. Gao, J. et al. (2013) Integrative analysis of complex cancer genomics and clinical profiles using the cBioPortal. Sci. Signal. 6, pl1 3. Dogan, S. et al. (2012) Molecular epidemiology of EGFR and KRAS mutations in 3,026 lung adenocarcinomas: higher susceptibility of women to smoking-related KRASmutant cancers. Clin. Cancer Res. 18, 6169–6177 4. Lee, J.C. et al. (2006) Epidermal growth factor receptor activation in glioblastoma through novel missense mutations in the extracellular domain. PLoS Med. 3, e485 5. Sharma, S.V. et al. (2007) Epidermal growth factor receptor mutations in lung cancer. Nat. Rev. Cancer 7, 169–181 6. Foster, S.A. et al. (2016) Activation mechanism of oncogenic deletion mutations in BRAF. EGFR, and HER2. Cancer Cell 29, 477–493
Forum
Filling the Gap on Caveolin-1 in Liver Carcinogenesis Manuel A. Fernandez-Rojo1,2,* and Grant A. Ramm1,2 Caveolin-1 (CAV1) has emerged as a promoter of proliferation, metastasis, and chemoresistance in hepatoma cells, as well as a marker of poor prognosis in liver cancer. We discuss here current knowledge and future approaches to elucidating the molecular mechanisms underlying CAV1 action during hepatocarcinogenesis and evaluate its potential use in clinical therapies. Liver cancer is one of the major health threats in western populations. Unlike other types of cancers, mortality rates associated with liver cancer have increased dramatically in the past 20 years [1]. This progressive and seemingly
Trends in Cancer, December 2016, Vol. 2, No. 12
701
of many critical growth signaling cascades (such as the MAPK and PI3K pathways). Mutations within the KRAS family member represent the second most frequent alteration in cancer, found in 20% of all patients, and 90% of pancreatic cancer, 43% of colorectal cancer (CRC), and 32% of lung adenocarcinoma [1,2]. Mutations typically occur at codons 12, 13, or 61 and the mutational frequency is similar across these three tumor types, with one striking exception (Figure 1A). Lung adenocarci1 Scott A. Foster, nomas show a significant enrichment for Christiaan Klijn,1 and G12C mutations (50% of KRAS G12 Shiva Malek1,* mutations in lung adenocarcinoma are G12C, as opposed to 9% in CRC). This A predominant number of cancers can largely be attributed to G > T transare driven by mutations of key versions associated with smoking [3].
Tissue-Specific Mutations in BRAF and EGFR Necessitate Unique Therapeutic Approaches
growth signaling genes. While it might be expected that the same alterations within a given oncogene would be identified in all tissues, there are clear cases of tissue specificity. Here, we highlight the tissue specificity of BRAF and EGFR alterations and implications for therapeutic targeting. Sustained proliferative growth of tumor cells is often achieved by somatic mutations within components of growth factor signaling pathways. Recent advances in high-throughput sequencing technology have broadened our knowledge of the underlying driver mutations in most common cancers, informing additional therapeutic avenues that could be beneficial for patients. Interesting trends are also observed when comparing mutational spectra across different tumor types. Curiously, the spectrum of mutations often exhibits tissue specificity; both in the context of the target gene and the particular alteration within a given gene, implying either tissue-specific advantages or differing underlying causes of a given alteration. A key component of many of these signaling pathways is RAS, which functions downstream of growth factor receptors (such as EGFR and HER2) and upstream
While these differences can largely be explained for KRAS, another example of tissue-specific mutational spectra occurs within EGFR driver mutations. In the context of EGFR, lung adenocarcinomas are more frequently driven by activating kinase domain (intracellular) mutations (83% of EGFR mutant lung adenocarcinoma samples carry exclusively intracellular mutations), while glioblastomas exhibit an enrichment of EGFR extracellular domain mutations (81% of EGFR mutant glioblastoma samples carry exclusively extracellular domain mutations). Unlike the KRAS context, these tissue-specific mutations in EGFR are harder to explain with single nucleotide transversions, and hence suggest that there is a tissue-selective advantage for enrichment of one mutation class versus another (Figure 1B, C) [1,2,4,5]. Similarly difficult to explain is the striking tissue specificity of mutations with the MAPK pathway component BRAF. BRAF mutant melanomas, thyroid cancer, and CRC are almost exclusively driven by the well-characterized canonical activating mutation V600E (V600 mutations constitute 98% of thyroid, 91% of colorectal and 87% of melanoma BRAF mutations), whereas lung adenocarcinomas show an enrichment of noncanonical kinase domain point mutations
with 77% of samples carrying nonV600E mutations (Figure 1D) [1,2]. Importantly, as with BRAF V600E mutations, these noncanonical BRAF mutations are also largely mutually exclusive with KRAS mutations. Similarly, we recently characterized BRAF mutant pancreatic adenocarcinomas and observed an enrichment for noncanonical mutations, also mutually exclusive with KRAS mutations (Figure 1E) [6]. Interestingly, the spectrum of noncanonical alterations differs from lung, with the majority of pancreatic tumors harboring a previously uncharacterized short, inframe deletion within a critical loop of the BRAF kinase domain. Shortening of this loop (the b3–/C loop, which we referred to as b3–/C deletions) effectively limits the normal structural flexibility of the kinase, locking the kinase in the ‘on’ conformation (Figure 2A). While these tissue-specific enrichments may be difficult to explain based on catalytic activity alone, further investigation is likely to uncover unexpected and interesting biology. One critical outcome of this broad cataloging of unique driver alterations has been to define the sensitivity of specific mutations towards clinical therapies. For BRAF, early direct sequencing efforts defined BRAF V600E as an excellent drug target and led to the clinical success of vemurafenib in metastatic melanoma [7]. Despite this success, V600E-driven colorectal cancers have shown weak response to vemurafenib treatment [8], highlighting yet another level of complexity of tissue specificity. The overall basis for differential sensitivity to vemurafenib appears to reside at the heart of normal MAPK biology. In MAPK signaling, dimerization of inactive RAF monomers (ARAF, BRAF, or CRAF) upon upstream activation results in downstream signaling [9,10]. While the V600E mutation confers dimer-independent activity and sensitivity to vemurafenib in melanoma [11,12] (Figure 2B), V600E colorectal cancers show high EGFR activity and increased
Trends in Cancer, December 2016, Vol. 2, No. 12
699
Q61x
G[12/13]x
Other KRAS
0
Rec..
Furin-like
200
0
Rec..
400
GF_recep_IV
600
1.00
0.50
0.25
0.00
1.00
0.75
# mutated 22 22
Skin melanoma
177
Thyroid cancer
242
1210 aa
BRAF mutaon
EGFR - Glioblastoma - TCGA
V600
N581
G469
K601
D594
BRAF deleon
Other BRAF
A289x
20
G589[V/A]
Frequencies of BRAF mutaons Foundaon medicine data
(E)
# mutated
Pancreac cancer 400
GF_recep_IV
600
Pkinase_Tyr
800
Indel - in frame
1000
Truncang
1210 aa
24
BRAF mutaon
V600
BRAF deleon
G469
1.00
200
Missense
Rec..
0.75
0
Furin-like
0.50
Rec..
0.25
0
0.00
# mutaons
Both
Colorectal cancer
Pkinase_Tyr
1000
Extra cellular mutaon
Lung adenocarcinoma
L858R
800
Intra cellular mutaon
Frequencies of BRAF mutaons - TCGA
(D)
0.00
E746_A750del
7
EGFR mutaon
1.00
G13C
G13D
77
0.75
G12V
G12D
# mutated 33
Lung adenocarcinoma Glioblastoma
0.50
G12C
EGFR - Lung adenocarcinoma - TCGA
(B) # mutaons
0.75
123 0.50
Pancreac cancer 0.25
96
0.00
Colorectal cancer
KRAS mutaon
Frequencies of EGFR mutaons - TCGA
(C) # mutated 75
0.25
Frequencies of KRAS mutaons - TCGA
(A) Lung adenocarcinoma
Other BRAF alteraon
Figure 1. Overview of KRAS, EGFR, and BRAF Mutational Patterns in Large Cancer Genomics Data Sets. (A) Proportional bar graph showing the spectrum of codon mutations in KRAS in lung, colorectal and pancreatic cancer in The Cancer Genome Atlas data (TCGA). (B) Lollipop plot showing the distribution of mutations over the EGFR protein as seen in lung cancer or glioblastoma TCGA data. (C) Proportional bar graph of the same data in (B), but categorized as extracellular (before the 621st amino acid) or intracellular (after the 621st amino acid). Samples that presented with mutation in both regions were annotated as ‘both’. (D) Proportional bar graph showing the spectrum of codon mutations in BRAF in lung, colorectal, skin, and thyroid cancer in TCGA data. (E) Overview of BRAF mutations in Foundation Medicine data of pancreatic cancer (n = 1305) [6]. TCGA data was collected from cbioportali and the provisional data sources were used where applicable.
propensity to reactivate the pathway in structural change caused by the deletion part through RAF dimerization resulting [6] (Figure 2A, D). Thus, while lung and in decreased vemurafenib sensitivity [13]. pancreatic adenocarcinomas are largely driven by noncanonical BRAF mutations, While BRAF V600E results in robust the type of alteration and underlying bioconstitutive kinase activity, the majority chemical behavior enriched in each tisof noncanonical BRAF kinase domain sue differs. Despite these differences, a alterations identified in lung adenocarci- commonality between all of these nonnomas (with the highest frequency of canonical BRAF alterations is vemurafeG469, N581, and D594 mutations) all nib resistance. target residues that contribute to catalytic function of the kinase and are likely Second-generation RAF inhibitors are kinase-inactivating (or weakly activating) being developed that may show benefit mutations. Instead, these mutations to these broader classes of patients. require association with a wild-type Interestingly, while we noted that the lung RAF molecule (functioning as hetero- and pancreatic noncanonical alterations dimers) and show weak vemurafenib show distinct biochemical behavior, sensitivity [12,14,15] (Figure 2C). The these two classes result in a similar strucBRAF b3–/C deletions, which make tural effect on the kinase domain. This up the majority of BRAF mutant pancre- overall structural change, a shift of the atic adenocarcinomas, achieve constitu- /C towards an ‘in’ conformation, is tive kinase activity independent of genetically achieved by shortening of dimerization, but show complete resis- the b3–/C loop in the b3–/C deletions tance to vemurafenib because of the and induced upon dimerization in the lung
700
Trends in Cancer, December 2016, Vol. 2, No. 12
noncanonical alterations (Figure 2C, D). This results in the decreased efficacy of vemurafenib that binds in the /C ‘out’ conformation. Consistent with this structural change, both the b3–/C deletions and most of the noncanonical mutations tested show sensitivity to BRAF inhibitors that bind to the /C ‘in’ conformation [6,12] (Figure 2C, D). Early work to identify and develop therapies selectively targeting certain driver mutations in cancers has led to clinical benefit for many patients. As highlighted with the BRAF mutations, developing next-generation therapies is a complicated problem as both tissue-specific and mutation-specific effects all contribute to the underlying biology and drug sensitivity. As the molecular complexities of specific tumors continue to be defined, development of therapies that take this broader view will be critical to achieve optimal benefit cancer patients.
7. Kim, G. et al. (2014) FDA approval summary: vemurafenib for treatment of unresectable or metastatic melanoma with the BRAFV600E mutation. Clin. Cancer Res. 20, 4994–5000
(A) β3-αC loop
β3-αC loop
αC
β3
β3-αC loop
αC
β3
BRAF
BRAF
αC ‘out’ kinase off
β3-αC deleon kinase on
(C)
αC
β3
BRAFMutant
β3
αC
BRAFMutant
(D)
β3
αC
B/CRAFWildtype
β3
αC
BRAFMutant
β3
αC
Lung adenocarcinoma
Pancreac adenocarcinoma
Predominantly non-canonical Dimeric acvity
Predominantly β3-αC deleon Monomeric acvity
Vemurafenib sensive
Vemurafenib resistant
Vemurafenib resistant
αC ‘in’ inhibitor sensive
αC ‘in’ inhibitor sensive
αC ‘in’ inhibitor sensive
and ‘off’ states. In normal signaling, these shifts are controlled by upstream signaling inputs. In cancer, these conformational dynamics can be hijacked by oncogenic alterations shifting the kinase towards the ‘on’ state. One of these critical shifts is the movement of the /C-helix (labeled /C) from an ‘out’ conformation (shifted away from the b3 strand; labeled b3) in the ‘off’ state inwards (towards the b3 strand) in the ‘on’ state. This shift forms critical interactions between b3 and /C, allowing the proper coordination of ATP required for kinase activity. The ability of the kinase to toggle between these two states is afforded by the flexibility of the b3–/C loop. In the b3–/C deletions, this loop is shortened effectively locking the kinase in the ‘on’ conformation. (B) BRAF mutant melanomas largely consist of BRAF V600E mutations. These mutations confer monomeric kinase activity, allowing the kinase to accommodate both the /C ‘out’ inhibitor vemurafenib (shown in red) and /C ‘in’ inhibitors (shown in green). (C) BRAF mutant lung adenocarcinomas largely consist of BRAF noncanonical kinase domain point mutations. These mutations promote dimerization, which induces the shift of /C towards the ‘in’ conformation. While these alterations show sensitivity to /C ‘in’ inhibitors, they show decreased sensitivity to the /C ‘out’ inhibitor vemurafenib. (D) BRAF mutant pancreatic adenocarcinomas largely consist of BRAF b3–/C deletions. The shortening of this loop results in monomeric kinase activity, but locks the kinase in the /C ‘in’ conformation. These deletions are resistant to the /C ‘out’ inhibitor vemurafenib, while remaining sensitive to /C ‘in’ inhibitors. Model figures were generated with the assistance of Allison Bruce.
1
Departments of Discovery Oncology and Bioinformatics, Genentech Inc., South San Francisco, CA, USA *Correspondence: [email protected] (S. Malek). http://dx.doi.org/10.1016/j.trecan.2016.10.015
References 1. Cerami, E. et al. (2012) The cBio cancer genomics portal: an open platform for exploring multidimensional cancer genomics data. Cancer Discov. 2, 401–404
13. Corcoran, R.B. et al. (2012) EGFR-mediated re-activation of MAPK signaling contributes to insensitivity of BRAF mutant colorectal cancers to RAF inhibition with vemurafenib. Cancer Discov. 2, 227–235
15. Heidorn, S.J. et al. (2010) Kinase-dead BRAF and oncogenic RAS cooperate to drive tumor progression through CRAF. Cell 140, 209–221
Melanoma
www.cbioportal.org
12. Yao, Z. et al. (2015) BRAF mutants evade ERK-dependent feedback by different mechanisms that determine their sensitivity to pharmacologic inhibition. Cancer Cell 28, 370–383
BRAFMutant
Figure 2. Activation Mechanism and Inhibitor Sensitivity of Different BRAF Alterations Identified in Varying Tumor Types. (A) Dynamic conformational changes are required for kinases to shift between ‘on’
i
11. Haling, J.R. et al. (2014) Structure of the BRAF-MEK complex reveals a kinase activity independent role for BRAF in MAPK signaling. Cancer Cell 26, 402–413
14. Garnett, M.J. et al. (2005) Wild-type and mutant B-RAF activate C-RAF through distinct mechanisms involving heterodimerization. Mol. Cell 20, 963–969
Predominantly V600E Monomeric acvity
Resources
9. Rajakulendran, T. et al. (2009) A dimerization-dependent mechanism drives RAF catalytic activation. Nature 461, 542–545 10. Weber, C.K. et al. (2001) Active Ras induces heterodimerization of cRaf and BRaf. Cancer Res. 61, 3595–3598
BRAF
αC ‘in’ kinase on
(B)
αC
β3
8. Kopetz, S. et al. (2015) Phase II pilot study of vemurafenib in patients with metastatic BRAF-mutated colorectal cancer. J. Clin. Oncol. 33, 4032–4038
2. Gao, J. et al. (2013) Integrative analysis of complex cancer genomics and clinical profiles using the cBioPortal. Sci. Signal. 6, pl1 3. Dogan, S. et al. (2012) Molecular epidemiology of EGFR and KRAS mutations in 3,026 lung adenocarcinomas: higher susceptibility of women to smoking-related KRASmutant cancers. Clin. Cancer Res. 18, 6169–6177 4. Lee, J.C. et al. (2006) Epidermal growth factor receptor activation in glioblastoma through novel missense mutations in the extracellular domain. PLoS Med. 3, e485 5. Sharma, S.V. et al. (2007) Epidermal growth factor receptor mutations in lung cancer. Nat. Rev. Cancer 7, 169–181 6. Foster, S.A. et al. (2016) Activation mechanism of oncogenic deletion mutations in BRAF. EGFR, and HER2. Cancer Cell 29, 477–493
Forum
Filling the Gap on Caveolin-1 in Liver Carcinogenesis Manuel A. Fernandez-Rojo1,2,* and Grant A. Ramm1,2 Caveolin-1 (CAV1) has emerged as a promoter of proliferation, metastasis, and chemoresistance in hepatoma cells, as well as a marker of poor prognosis in liver cancer. We discuss here current knowledge and future approaches to elucidating the molecular mechanisms underlying CAV1 action during hepatocarcinogenesis and evaluate its potential use in clinical therapies. Liver cancer is one of the major health threats in western populations. Unlike other types of cancers, mortality rates associated with liver cancer have increased dramatically in the past 20 years [1]. This progressive and seemingly
Trends in Cancer, December 2016, Vol. 2, No. 12
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