The contribution of melanocortin-1 receptor (MC1R) gene variants to the development of early-onset melanoma is unknown. Using an Australian population-based, case-control-family study, we sequenced MC1R for 565 cases with invasive cutaneous melanoma diagnosed between ages 18–39 years, 409 unrelated controls and 518 sibling controls. Variants were classified a priori into `R' variants (D84E, R142H, R151C, I155T, R160W, D294H) and `r' variants (all other nonsynonymous variants). We estimated odds ratios (OR) for melanoma using unconditional (unrelated controls) and conditional (sibling controls) logistic regression. The prevalence of having at least one R or r variant was 86% for cases, 73% for unrelated controls and 81% for sibling controls. R151C conferred the highest risk (per allele OR 2.57, 95% confidence interval 1.86–3.56 for the case-unrelated-control analysis and 1.70 (1.12–2.60) for the case-sibling-control analysis). When mutually adjusted, the ORs per R allele were 2.23 (1.77–2.80) and 2.06 (1.47–2.88), respectively from the two types of analysis, and the ORs per r allele were 1.69 (1.33–2.13) and 1.25 (0.88–1.79), respectively. The associations were stronger for men and those with none or few nevi or with high childhood sun exposure. Adjustment for phenotype, nevi and sun exposure attenuated the overall log OR for R variants by approximately 18%, but had lesser influence on r variant risk estimates. MC1R variants explained about 21% of the familial aggregation of melanoma. Some MC1R variants are important determinants of early-onset melanoma. The strength of association with melanoma differs according to the type and number of variants.
MC1R; melanoma; early-onset; phenotype; nevi; sun exposure
BRAF inhibitor (BRAFi) therapy leads to remarkable anti-melanoma responses, but the initial tumor shrinkage is commonly incomplete, providing a nidus for subsequent disease progression. Adaptive signaling may underlie early BRAFi resistance and influence the selection pattern for genetic variants causing late, acquired resistance. We show here that BRAFi (or BRAFi+MEKi) therapy in patients frequently led to rebound p-AKT levels in their melanomas early on treatment. In cell lines, BRAFi treatment led to rebound levels of RTKs (including PDGFRβ), PIP3, pleckstrin homology domain (PHD) recruitment, and p-AKT. PTEN expression limited this BRAFi-elicited PI3K-AKT signaling, which could be rescued by introduction of a mutant AKT1 (Q79K) kown to confer acquired BRAFi resistance. Functionally, AKT1 Q79K conferred BRAFi resistance via amplifying BRAFi-elicited PI3K-AKT signaling. Additionally, MAPK pathway inhibition enhanced clonogenic growth dependency on PI3K or AKT. Thus, adaptive or genetic upregulation of AKT critically participates in melanoma survival during BRAFi therapy.
BRAF; MEK; PI3K; AKT; melanoma; BRAF inhibitors
BRAF inhibitors elicit rapid anti-tumor responses in the majority of patients with V600BRAF mutant melanoma, but acquired drug resistance is almost universal. We sought to identify the core resistance pathways and the extent of tumor heterogeneity during disease progression. We show that MAPK reactivation mechanisms were detected among 70% of disease-progressive tissues, with RAS mutations, mutant BRAF amplification and alternative splicing being most common. We also detected PI3K-PTEN-AKT-upregulating genetic alterations among 22% of progressive melanomas. Distinct molecular lesions, in both core drug escape pathways, were commonly detected concurrently in the same tumor or among multiple tumors from the same patient. Beyond harboring extensively heterogeneous resistance mechanisms, melanoma re-growth emerging from BRAF inhibitor selection displayed branched evolution marked by altered mutational spectra/signatures and increased fitness. Thus, melanoma genomic heterogeneity contributes significantly to BRAF inhibitor treatment failure, implying upfront, co-targeting of two core pathways as an essential strategy for durable responses.
BRAF; MAPK; PI3K-PTEN-AKT; clonal heterogeneity; melanoma; acquired BRAF inhibitor resistance
Unprecedented clinical responses have been reported in advanced stage metastatic melanoma patients treated with targeted inhibitors of constitutively activated mutant BRAF, which is present in approximately half of all melanomas. We and others have previously observed an association of elevated nuclear β-catenin with improved survival in molecularly-unselected melanoma patients. This study sought to determine whether levels of Wnt/β-catenin signaling in melanoma tumors prior to treatment might predict patient responses to BRAF inhibitors (BRAFi). We performed automated quantification of β-catenin immunohistochemical expression in pretreatment BRAF-mutant tumors from 32 BRAFi-treated melanoma patients. Unexpectedly, patients with higher nuclear β-catenin in their tumors did not exhibit the survival advantage previously observed in molecularly-unselected melanoma patients who did not receive BRAFi. In cultured melanoma cells treated with long-term BRAFi, activation of Wnt/β-catenin signaling is markedly inhibited, coinciding with a loss of the enhancement of BRAFi-induced apoptosis by WNT3A observed in BRAFi-naïve cells. Together, these observations suggest that long-term treatment with BRAFi can impact the interaction between BRAF/MAPK and Wnt/β-catenin signaling to affect patient outcomes. Studies with larger patient cohorts are required to determine whether nuclear β-catenin expression correlates with clinical responses to BRAFi and to specific mechanisms of acquired resistance to BRAFi. Understanding these pathway interactions will be necessary to facilitate efforts to individualize therapies for melanoma patients.
MAPK inhibitors (MAPKi) are active in BRAF-mutant metastatic melanoma patients, but the extent of response and progression-free survival (PFS) is variable, and complete responses are rare. We sought to examine the patterns of response and progression in patients treated with targeted therapy.
MAPKi-naïve patients treated with combined dabrafenib and trametinib had all metastases ≥5 mm (lymph nodes ≥15 mm in short axis) visible on computed tomography measured at baseline and throughout treatment.
24 patients had 135 measured metastases (median 4.5/patient, median diameter 16 mm). Time to best response (median 5.5 mo, range 1.7–20.1 mo), and the degree of best response (median −70%, range +9 to −100%) varied amongst patients. 17% of patients achieved complete response (CR), whereas 53% of metastases underwent CR, including 42% ≥10 mm. Metastases that underwent CR were smaller than non-CR metastases (median 11 vs 20 mm, P<0.001). PFS was variable among patients (median 8.2 mo, range 2.6–18.3 mo), and 50% of patients had disease progression in new metastases only. Only 1% (1/71) of CR-metastases subsequently progressed. Twelve-month overall survival was poorer in those with a more heterogeneous initial response to therapy than less heterogeneous (67% vs 93%, P = 0.009).
Melanoma response and progression with MAPKi displays marked inter- and intra-patient heterogeneity. Most metastases undergo complete response, yet only a small proportion of patients achieve an overall complete response. Similarly, disease progression often occurs only in a subset of the tumor burden, and often in new metastases alone. Clinical heterogeneity, likely reflecting molecular heterogeneity, remains a barrier to the effective treatment of melanoma patients.
Approximately 50% of melanomas require oncogenic B-RAFV600E signaling for proliferation, survival and metastasis, and the use of highly selective B-RAF inhibitors has yielded remarkable, albeit short term, clinical responses. Re-activation of signaling downstream of B-RAF is frequently associated with acquired resistance to B-RAF inhibitors, and the identification of B-RAF targets may therefore provide new strategies for managing melanoma. In this report, we applied whole genome expression analyses to reveal that oncogenic B-RAFV600E regulates genes associated with epithelial-mesenchymal transition in normal cutaneous human melanocytes. Most prominent was the B-RAF-mediated transcriptional repression of E-cadherin, a keratinocyte-melanoma adhesion molecule whose loss is intimately associated with melanoma invasion and metastasis. Here we identify a link between oncogenic B-RAF, the transcriptional repressor Tbx3 and E-cadherin. We show that B-RAFV600E induces the expression of Tbx3, which potently represses E-cadherin expression in melanocytes and melanoma cells. Tbx3 expression is normally restricted to developmental embryonic tissues, promoting cell motility but is also aberrantly increased in various cancers and has been linked to tumor cell invasion and metastasis. We propose that this B-RAF/Tbx3/E-cadherin pathway plays a critical role in promoting the metastasis of B-RAF mutant melanomas.
Melanocortin-1 receptor (MC1R) gene variants are very common and are associated with melanoma risk, but their contribution to melanoma risk prediction compared with traditional risk factors is unknown. We aimed to 1) evaluate the separate and incremental contribution of MC1R genotype to prediction of early-onset melanoma, and compare this with the contributions of physician-measured and self-reported traditional risk factors, and 2) develop risk prediction models that include MC1R, and externally validate these models using an independent dataset from a genetically similar melanoma population.
Using data from an Australian population-based, case-control-family study, we included 413 case and 263 control participants with sequenced MC1R genotype, clinical skin examination and detailed questionnaire. We used unconditional logistic regression to estimate predicted probabilities of melanoma. Results were externally validated using data from a similar study in England.
When added to a base multivariate model containing only demographic factors, MC1R genotype improved the area under the receiver operating characteristic curve (AUC) by 6% (from 0.67 to 0.73; P < 0.001) and improved the quartile classification by a net 26% of participants. In a more extensive multivariate model, the factors that contributed significantly to the AUC were MC1R genotype, number of nevi and previous non-melanoma skin cancer; the AUC was 0.78 (95% CI 0.75-0.82) for the model with self-reported nevi and 0.83 (95% CI 0.80-0.86) for the model with physician-counted nevi. Factors that did not further contribute were sun and sunbed exposure and pigmentation characteristics. Adding MC1R to a model containing pigmentation characteristics and other self-reported risk factors increased the AUC by 2.1% (P = 0.01) and improved the quartile classification by a net 10% (95% CI 1-18%, P = 0.03).
Although MC1R genotype is strongly associated with skin and hair phenotype, it was a better predictor of early-onset melanoma than was pigmentation characteristics. Physician-measured nevi and previous non-melanoma skin cancer were also strong predictors. There might be modest benefit to measuring MC1R genotype for risk prediction even if information about traditional self-reported or clinically measured pigmentation characteristics and nevi is already available.
MC1R; Risk prediction; Accuracy; Melanoma; Sun exposure; Early-onset; Pigmentation; Nevi
The major factors individually reported to be associated with an increased frequency of CDKN2A mutations are increased number of patients with melanoma in a family, early age at melanoma diagnosis, and family members with multiple primary melanomas (MPM) or pancreatic cancer.
These four features were examined in 385 families with ⩾3 patients with melanoma pooled by 17 GenoMEL groups, and these attributes were compared across continents.
Overall, 39% of families had CDKN2A mutations ranging from 20% (32/162) in Australia to 45% (29/65) in North America to 57% (89/157) in Europe. All four features in each group, except pancreatic cancer in Australia (p = 0.38), individually showed significant associations with CDKN2A mutations, but the effects varied widely across continents. Multivariate examination also showed different predictors of mutation risk across continents. In Australian families, ⩾2 patients with MPM, median age at melanoma diagnosis ⩽40 years and ⩾6 patients with melanoma in a family jointly predicted the mutation risk. In European families, all four factors concurrently predicted the risk, but with less stringent criteria than in Australia. In North American families, only ⩾1 patient with MPM and age at diagnosis ⩽40 years simultaneously predicted the mutation risk.
The variation in CDKN2A mutations for the four features across continents is consistent with the lower melanoma incidence rates in Europe and higher rates of sporadic melanoma in Australia. The lack of a pancreatic cancer–CDKN2A mutation relationship in Australia probably reflects the divergent spectrum of mutations in families from Australia versus those from North America and Europe. GenoMEL is exploring candidate host, genetic and/or environmental risk factors to better understand the variation observed.
; multiple primary melanomas; pancreatic cancer
BRAF inhibitors (BRAFi) induce antitumor responses in nearly 60% of patients with advanced V600E/KBRAF melanomas. Somatic activating MEK1 mutations are thought to be rare in melanomas, but their potential concurrence with V600E/KBRAF may be selected for by BRAFi. We sequenced MEK1/2 exon 3 in melanomas at baseline and upon disease progression. Of 31 baseline V600E/KBRAF melanomas, 5 (16%) carried concurrent somatic BRAF/MEK1 activating mutations. Three of 5 patients with BRAF/MEK1 double-mutant baseline melanomas showed objective tumor responses, consistent with the overall 60% frequency. No MEK1 mutation was found in disease progression melanomas, except when it was already identified at baseline. MEK1-mutant expression in V600E/KBRAF melanoma cell lines resulted in no significant alterations in p-ERK1/2 levels or growth-inhibitory sensitivities to BRAFi, MEK1/2 inhibitor (MEKi), or their combination. Thus, activating MEK1 exon 3 mutations identified herein and concurrent with V600E/KBRAF do not cause BRAFi resistance in melanoma.
As BRAF inhibitors gain widespread use for treatment of advanced melanoma, bio-markers for drug sensitivity or resistance are urgently needed. We identify here concurrent activating mutations in BRAF and MEK1 in melanomas and show that the presence of a downstream mutation in MEK1 does not necessarily make BRAF–mutant melanomas resistant to BRAF inhibitors.
Resistance to therapy with BRAF kinase inhibitors is associated with reactivation of the mitogen-activated protein kinase (MAPK) pathway. To address this problem, we conducted a phase 1 and 2 trial of combined treatment with dabrafenib, a selective BRAF inhibitor, and trametinib, a selective MAPK kinase (MEK) inhibitor.
In this open-label study involving 247 patients with metastatic melanoma and BRAF V600 mutations, we evaluated the pharmacokinetic activity and safety of oral dabrafenib (75 or 150 mg twice daily) and trametinib (1, 1.5, or 2 mg daily) in 85 patients and then randomly assigned 162 patients to receive combination therapy with dabrafenib (150 mg) plus trametinib (1 or 2 mg) or dabrafenib monotherapy. The primary end points were the incidence of cutaneous squamous-cell carcinoma, survival free of melanoma progression, and response. Secondary end points were overall survival and pharmacokinetic activity.
Dose-limiting toxic effects were infrequently observed in patients receiving combination therapy with 150 mg of dabrafenib and 2 mg of trametinib (combination 150/2). Cutaneous squamous-cell carcinoma was seen in 7% of patients receiving combination 150/2 and in 19% receiving monotherapy (P = 0.09), whereas pyrexia was more common in the combination 150/2 group than in the monotherapy group (71% vs. 26%). Median progression-free survival in the combination 150/2 group was 9.4 months, as compared with 5.8 months in the monotherapy group (hazard ratio for progression or death, 0.39; 95% confidence interval, 0.25 to 0.62; P<0.001). The rate of complete or partial response with combination 150/2 therapy was 76%, as compared with 54% with monotherapy (P = 0.03).
Dabrafenib and trametinib were safely combined at full monotherapy doses. The rate of pyrexia was increased with combination therapy, whereas the rate of proliferative skin lesions was nonsignificantly reduced. Progression-free survival was significantly improved. (Funded by GlaxoSmithKline; ClinicalTrials.gov number, NCT01072175.)
The development of acquired drug resistance hampers the long-term success of B-RAF inhibitor (B-RAFi) therapy for melanoma patients. Here we show V600EB-RAF copy number gain as a mechanism of acquired B-RAFi resistance in four out of twenty (20%) patients treated with B-RAFi. In cell lines, V600EB-RAF over-expression and knockdown conferred B-RAFi resistance and sensitivity, respectively. In V600EB-RAF amplification-driven (vs. mutant N-RAS-driven) B-RAFi resistance, ERK reactivation is saturable, with higher doses of vemurafenib down-regulating pERK and re-sensitizing melanoma cells to B-RAFi. These two mechanisms of ERK reactivation are sensitive to the MEK1/2 inhibitor AZD6244/selumetinib or its combination with the B-RAFi vemurafenib. In contrast to mutant N-RAS-mediated V600EB-RAF bypass, which is sensitive to C-RAF knockdown, V600EB-RAF amplification-mediated resistance functions largely independently of C-RAF. Thus, alternative clinical strategies may potentially overcome distinct modes of ERK reactivation underlying acquired B-RAFi resistance in melanoma.
Sunbed use is associated with increased risk of melanoma. Younger people might be more susceptible to the carcinogenic effects of ultraviolet radiation. We investigated the association between sunbed use and risk of early-onset cutaneous malignant melanoma. From the Australian Melanoma Family Study, a multi-centre, population-based, case-control-family study, we analysed data for 604 cases diagnosed between ages 18 and 39 years and 479 controls. Data were collected by interview. Associations were estimated as odds ratios (ORs) using unconditional logistic regression, adjusting for age, sex, city, education, family history, skin colour, usual skin response to sunlight, and sun exposure. Compared with having never used a sunbed, the OR for melanoma associated with ever-use was 1.41 (95% confidence interval (CI) 1.01-1.96), and 2.01 (95% CI 1.22-3.31) for more than 10 lifetime sessions (Ptrend 0.01 with cumulative use). The association was stronger for earlier age at first use (Ptrend 0.02). The association was also stronger for melanoma diagnosed when aged 18-29 years (OR for more than 10 lifetime sessions = 6.57, 95% CI 1.41-30.49) than for melanoma diagnosed when 30-39 years (OR 1.60, 95% CI 0.92-2.77; Pinteraction 0.01). Among those who had ever used a sunbed and were diagnosed between 18-29 years of age, three quarters (76%) of melanomas were attributable to sunbed use. Sunbed use is associated with increased risk of early-onset melanoma, with risk increasing with greater use, an earlier age at first use and for earlier onset disease.
sunbed; artificial tanning; melanoma; risk factor; early-onset
We report a genome-wide association study of melanoma, conducted by GenoMEL, of 2,981 cases, of European ancestry, and 1,982 study-specific controls, plus a further 6,426 French and UK population controls, all genotyped for 317,000 or 610,000 SNPs. The analysis confirmed previously known melanoma susceptibility loci. The 7 novel regions with at least one SNP with p<10−5 and further local imputed or genotyped support were selected for replication using two other genome-wide studies (from Australia and Houston, Texas). Additional replication came from UK and Dutch case-control series. Three of the 7 regions replicated at p<10−3: an ATM missense polymorphism (rs1801516, overall p=3.4×10−9); a polymorphism within MX2 (rs45430, p=2.9×10−9) and a SNP adjacent to CASP8 (rs13016963, p=8.6×10−10). A fourth region near CCND1 remains of potential interest, showing suggestive but inconclusive evidence of replication. Unlike the previously known regions, the novel loci showed no association with nevus or pigmentation phenotypes in a large UK case-control series.
Approximately 5% to 10% of melanoma may be hereditary in nature, and about 2% of melanoma can be specifically attributed to pathogenic germline mutations in cyclin-dependent kinase inhibitor 2A (CDKN2A). To appropriately identify the small proportion of patients who benefit most from referral to a genetics specialist for consideration of genetic testing for CDKN2A, we have reviewed available published studies of CDKN2A mutation analysis in cohorts with invasive, cutaneous melanoma and found variability in the rate of CDKN2A mutations based on geography, ethnicity, and the type of study and eligibility criteria used. Except in regions of high melanoma incidence, such as Australia, we found higher rates of CDKN2A positivity in individuals with 3 or more primary invasive melanomas and/or families with at least one invasive melanoma and two or more other diagnoses of invasive melanoma and/or pancreatic cancer among first- or second-degree relatives on the same side of the family. The work summarized in this review should help identify individuals who are appropriate candidates for referral for genetic consultation and possible testing.
CDKN2A; familial; genetic counseling; genetic testing; hereditary; melanoma; p16
So far, two familial melanoma genes have been identified, accounting for a minority of genetic risk in families. Mutations in CDKN2A account for approximately 40% of familial cases1, and predisposing mutations in CDK4 have been reported in a very small number of melanoma kindreds2. To identify other familial melanoma genes, here we conducted whole-genome sequencing of probands from several melanoma families, identifying one individual carrying a novel germline variant (coding DNA sequence c.G1075A; protein sequence p.E318K; rs149617956) in the melanoma-lineage-specific oncogene microphthalmia-associated transcription factor (MITF). Although the variant co-segregated with melanoma in some but not all cases in the family, linkage analysis of 31 families subsequently identified to carry the variant generated a log odds ratio (lod) score of 2.7 under a dominant model, indicating E318K as a possible intermediate risk variant. Consistent with this, the E318K variant was significantly associated with melanoma in a large Australian case–control sample. Likewise, it was similarly associated in an independent case–control sample from the United Kingdom. In the Australian sample, the variant allele was significantly over-represented in cases with a family history of melanoma, multiple primary melanomas, or both. The variant allele was also associated with increased naevus count and non-blue eye colour. Functional analysis of E318K showed that MITF encoded by the variant allele had impaired sumoylation and differentially regulated several MITF targets. These data indicate that MITF is a melanoma-predisposition gene and highlight the utility of whole-genome sequencing to identify novel rare variants associated with disease susceptibility.
Discovering and understanding genetic risk factors for melanoma and their interactions with phenotype, sun exposure, and other risk factors could lead to new strategies for melanoma control. This paper describes the Australian Melanoma Family Study, which uses a multicenter, population-based, case-control-family design. From 2001 to 2005, the authors recruited 1,164 probands including 629 cases with histopathologically confirmed, first-primary cutaneous melanoma diagnosed before age 40 years, 240 population-based controls frequency matched for age, and 295 spouse/friend controls. Information on lifetime sun exposure, phenotype, and residence history was collected for probands and nearly 4,000 living relatives. More than 3,000 subjects donated a blood sample. Proxy-reported information was collected for childhood sun exposure and deceased relatives. Important features of this study include the population-based, family-based design; a focus on early onset disease; probands from 3 major cities differing substantially in solar ultraviolet exposure and melanoma incidence; a population at high risk because of high ultraviolet exposure and susceptible pigmentation phenotypes; population-based, spouse/friend, and sibling controls; systematic recruitment of relatives of case and control probands; self and parent reports of childhood sun exposure; and objective clinical skin examinations. The authors discuss methodological and analytical issues related to the study design and conduct, as well as the potentially novel insights the study can deliver.
case-control studies; environmental exposure; family; genetic predisposition to disease; melanoma; risk factors
The endothelin system is implicated in the pathogenesis of melanoma. We evaluated the effects of bosentan - a dual endothelin receptor antagonist - in patients receiving first-line dacarbazine therapy for stage IV metastatic cutaneous melanoma in a phase 2, proof-of-concept study.
Eligible patients had metastatic cutaneous melanoma naïve to chemotherapy or immunotherapy, no central nervous system involvement, and serum lactate dehydrogenase <1.5 × upper limit of normal. Treatment comprised bosentan 500 mg twice daily or matching placebo, in addition to dacarbazine 1000 mg/m2 every three weeks. Eighty patients were randomized (double-blind) and 38 in each group received study treatment. Median time to tumor progression (primary endpoint) was not significantly different between the two groups (placebo, 2.8 months; bosentan, 1.6 months; bosentan/placebo hazard ratio, 1.144; 95% CI, 0.717-1.827; p = 0.5683). Incidences of most adverse events and clinically relevant increases in hepatic transaminases were similar between treatment groups although hemoglobin decrease to >8 and ≤ 10 g/dL and ≤ 8 g/dL was more common in the bosentan group.
In patients receiving dacarbazine as first-line chemotherapy for metastatic melanoma, the addition of high-dose bosentan had no effect on time to tumor progression or other efficacy parameters. There were no unexpected safety findings.
This study is registered in ClinicalTrials.gov under the unique identifier NCT01009177.
We report a genome-wide association study of melanoma conducted by the GenoMEL consortium based on 317k tagging SNPs for 1650 genetically-enriched cases (from Europe and Australia) and 4336 controls and subsequent replication in 1149 genetically-enriched cases and 964 controls and a population-based case-control study of 1163 cases and 903 controls. The genome-wide screen identified five regions with genotyped or imputed SNPs reaching p < 5×10−7; three regions were replicated: 16q24 encompassing MC1R (overall p=2.54×10−27 for rs258322), 11q14-q21 encompassing TYR (p=2.41×10−14 for rs1393350) and 9p21 adjacent to MTAP and flanking CDKN2A (p=4.03×10−7 for rs7023329). MC1R and TYR are associated with pigmentation, freckling and cutaneous sun sensitivity, well-recognised melanoma risk factors, while the 9p21 locus is novel for common variants associated with melanoma. Despite wide variation in allele frequency, these genetic variants show notable homogeneity of effect across populations of European ancestry living at different latitudes and contribute independently to melanoma risk.
We conducted a genome-wide association pooling study for cutaneous melanoma and performed validation in samples totalling 2019 cases and 2105 controls. Using pooling we identified a novel melanoma risk locus on chromosome 20 (rs910873, rs1885120), with replication in two further samples (combined P <1 × 10-15). The odds ratio is 1.75 (1.53, 2.01), with evidence for stronger association in early onset cases.
senescence acts as a barrier against tumour formation and has been
implicated as the mechanism preventing the transformation of benign
melanocytic lesions that frequently harbour oncogenic B-RAF or N-RAS mutations.
the present study we systematically assessed the relative importance
of the tumour suppressor proteins p53, p21Waf1, pRb
and p16INK4a in mediating oncogene-induced senescence in human
We now show
that oncogenic N-RAS induced senescence in melanocytes is
associated with DNA damage, a potent DNA damage response and the activation
of both the p16INK4a/pRb and p53/p21Waf1 tumour
suppressor pathways. Surprisingly neither the
pharmacological inhibition of the DNA damage response pathway nor silencing of
p53 expression had any detectable impact on oncogene-induced senescence in
human melanocytes. Our data indicate that the pRb pathway is
the dominant effector of senescence in these cells, as its specific
inactivation delays the onset of senescence and weakens oncogene-induced
proliferative arrest. Furthermore, we show that although both p16INK4a
and p21Waf1 are upregulated in response to N-RASQ61K,
the activities of these CDK inhibitors are clearly distinct and only the
loss of p16INK4a weakens senescence. We propose that the ability
of p16INK4a to inhibit the cyclin D-dependent kinases and DNA
replication, functions not shared by p21Waf1, contribute to its
role in senescence. Thus, in melanocytes with oncogenic signalling only p16INK4a
can fully engage the pRb pathway to alter chromatin structure and
silence the genes that are required for proliferation.
Oncogene-induced senescence; melanocytes; p53; pRb; p16; p21
CDKN2A/p16INK4a is frequently altered in human cancers and it is the most important melanoma susceptibility gene identified to date. p16INK4a inhibits pRb phosphorylation and induces cell cycle arrest, which is considered its main tumour suppressor function. Nevertheless, additional activities may contribute to the tumour suppressor role of p16INK4a and could help explain its specific association with melanoma predisposition. To identify such functions we conducted a yeast-two-hybrid screen for novel p16INK4a binding partners.
We now report that p16INK4a interacts with the chromatin remodelling factor BRG1. We investigated the cooperative roles of p16INK4a and BRG1 using a panel of cell lines and a melanoma cell model with inducible p16INK4a expression and BRG1 silencing. We found evidence that BRG1 is not required for p16INK4a-induced cell cycle inhibition and propose that the p16INK4a-BRG1 complex regulates BRG1 chromatin remodelling activity. Importantly, we found frequent loss of BRG1 expression in primary and metastatic melanomas, implicating this novel p16INK4a binding partner as an important tumour suppressor in melanoma.
This data adds to the increasing evidence implicating the SWI/SNF chromatin remodelling complex in tumour development and the association of p16INK4a with chromatin remodelling highlights potentially new functions that may be important in melanoma predisposition and chemoresistance.
The p16INK4a-Rb tumour suppressor pathway is required for the initiation and maintenance of cellular senescence, a state of permanent growth arrest that acts as a natural barrier against cancer progression. Senescence can be overcome if the pathway is not fully engaged, and this may occur when p16INK4a is inactivated. p16INK4a is frequently altered in human cancer and germline mutations affecting p16INK4a have been linked to melanoma susceptibility. To characterize the functions of melanoma-associated p16INK4a mutations, in terms of promoting proliferative arrest and initiating senescence, we utilized an inducible expression system in a melanoma cell model. We show that wild-type p16INK4a promotes rapid cell cycle arrest that leads to a senescence programme characterized by the appearance of chromatin foci, activation of acidic β-galactosidase activity, p53 independence and Rb dependence. Accumulation of wild-type p16INK4a also promoted cell enlargement and extensive vacuolization independent of Rb status. In contrast, the highly penetrant p16INK4a variants, R24P and A36P failed to arrest cell proliferation and did not initiate senescence. We also show that overexpression of CDK4, or its homologue CDK6, but not the downstream kinase, CDK2, inhibited the ability of wild-type p16INK4a to promote cell cycle arrest and senescence. Our data provide the first evidence that p16INK4a can initiate a CDK4/6-dependent autonomous senescence programme that is disabled by inherited melanoma-associated mutations.
CDK4; CDK6; melanoma; naevi; p16INK4a; senescence
Metastatic cutaneous melanoma is highly resistant to cytotoxic drugs, and this contributes to poor prognosis. In vivo studies on the chemosensitivity of metastatic melanoma are rare and hampered by poor response rates to systemic chemotherapeutics. Patients who undergo isolated limb infusion (ILI) with cytotoxic drugs show high response rates and are, therefore, a good cohort for studying chemosensitivity in vivo. We used tumors from patients who underwent ILI to study the role of melanoma tumor-suppressor genes and oncogenes on melanoma chemosensitivity. Prospectively acquired tumors from 30 patients who subsequently underwent ILI with melphalan and actinomycin-D for metastatic melanoma were investigated for mRNA expression levels of p14ARF, p16INK4a, and MITFm. The mutation status of B-RAF, N-RAS, and PTEN were also determined. A high percentage of tumors had activating mutations in either B-RAF (15/30) or N-RAS (10/30) and only two tumors carried altered PTEN. High expression of p16INK4a and absence of an activating B-RAF mutation independently predicted response to treatment. Further, inducible expression of p16INK4a sensitized a melanoma cell line to death induced by melphalan or actinomycin-D. This study shows that high expression of p16INK4a or the absence of activated B-RAF correlates with in vivo response of melanoma to cytotoxic drugs.