Affiliations of authors: Departments of Dermatology (JLM, BCB), Laboratory Medicine (JF, ANJ, DGA, DP), and Pathology (BCB), Cancer Research Institute (JF, ANJ, DGA), Comprehensive Cancer Center (HP, ANJ, DGA, DP, BCB), University of California, San Francisco, San Francisco; Department of Pathology, Memorial Sloan-Kettering Cancer Center, New York, NY (KB); Department of Dermatology, Kumamoto University School of Medicine, Kumamoto, Japan (TK, TO).
Correspondence to: Boris C. Bastian, MD, Department of Dermatology and Pathology, University of California, San Francisco, 2340 Sutter St., Rm. N461, San Francisco, CA 94115 (e-mail: bastian{at}cc.ucsf.edu)
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() |
---|
All specimens were embedded in paraffin and were retrieved from the Departments of Pathology at both the University of California, San Francisco, and the Memorial Sloan-Kettering Cancer Center and from the Department of Dermatology, Kumamoto University School of Medicine, Kumamoto, Japan. The melanoma specimens were selected to represent the four groups of primary invasive melanomas and to contain no more than 50% normal cells by histologic examination (Table 1). The skin was separated into glabrous skin (i.e., skin devoid of hair follicles, such as palms, soles, and subungual sites) and skin with hair follicles (i.e., the remainder of the skin, which for simplicity we refer to hereafter as skin). Melanomas on the skin were further distinguished by whether signs of chronic sun damage (chronic sun damage versus nonchronic sun damage) were found, as reflected by the presence or absence of severe solar elastosis (i.e., a dermis with a homogenous blue-gray discoloration) in sections stained with hematoxylineosin. The fourth group consisted of melanomas that arose on mucosal membranes. All tumors except for those on the mucosa had a median thickness of 3.6 mm (range = 115 mm); mucosal tumors had a median thickness of 5 mm (range = 2.485 mm).
|
To determine whether the BRAF mutation rate is associated with solar exposure patterns, we further stratified the nonchronic sun-damage group of melanomas by anatomic location. We assumed that melanomas on the trunk arise from skin that is typically subjected to intermittent sun exposure and that those on the extremities have a more continuous exposure pattern. Within the nonchronic sun-damage group, BRAF mutations were associated with melanomas of the trunk (15 [65%] of 23 specimens) compared with melanomas of the extremities or face (5 [33%] of 15 specimens) (P = .096, two-sided Fishers exact test). In concordance with published data (4), patients in our study with nonchronic sun damage whose melanoma arose on the trunk were younger than patients with melanomas arising on the extremities or face. To assess a possible influence of patient age, we compared the age distributions between melanomas with mutations and melanomas without mutations among those patients with nonchronic sun damage whose tumors occurred on the trunk. No difference in age was detected between patients with and without BRAF mutations among that relatively homogeneous group (P = .74, two-sided Wilcoxon rank sum test). Twenty-seven of the melanomas were from Japanese patients; 21 of the 27 melanomas were located on glabrous skin. No difference was observed between the frequency of mutations in Japanese patients and in the remaining patients.
The BRAF gene resides on chromosome 7q, which is frequently gained in melanoma (7). We analyzed specimens from 68 case patients (19 with mutations) with array comparative genomic hybridization (8,9) to determine whether the mutant BRAF allele was preferentially gained. Nine tumors had an increased 7q copy number and had gained a BRAF mutation. We estimated the ratio of mutant to normal BRAF sequences in each of these tumors by quantitatively comparing the forward and reverse sequencing traces to those of melanoma cell lines that contained only normal or mutant BRAF alleles. Fluorescence in situ hybridization analysis indicated that the tumors were near diploid. A near-diploid tumor containing 50% normal cells and a mutation in one of two alleles would be expected to have a mutant to normal allele ratio of 0.25, and such a tumor with an additional mutant allele would have a ratio of 0.4. In seven of the nine specimens with increased 7q copy number and BRAF mutations, the mutant-to-normal allele ratio was more than 0.5, which, after adjusting for the normal cell content of the specimens, indicates a gain of the mutant BRAF allele. In the other two specimens, the results do not clearly indicate which allele was gained. Thus, BRAF mutations are one of the factors that drive selection for the frequent gain of chromosome 7q in melanoma. However, because the distal chromosome 7q, including the BRAF region, was gained in 15 tumor specimens in which no mutations were found, additional genes are likely to be involved in this selection. Further studies are required to determine the effect of increased dosage of a mutated BRAF allele on the activation levels of mitogen-activated protein kinase and on melanocyte transformation.
BRAF mutations have been reported to occur in a large majority of nevi (10), suggesting that melanomas with a mutation may arise from preexisting nevi. Among the patients with a melanoma located on the skin, 11 patients had an associated melanocytic nevus microscopically and 35 did not (only those in which the entire width of the lesion could be evaluated microscopically were included in this analysis). Six (55%) of 11 nevus-associated melanomas and 15 (43%) of 35 unassociated melanomas had a BRAF mutation (P = .73, two-sided Fishers exact test). Thus, not all melanomas with an associated melanocytic nevus may arise from melanocytes with BRAF mutations. Importantly, there was no association between BRAF mutation status and outcome, as determined by metastasis or death from disease in 89 patients for whom clinical follow-up information (average follow-up time = 37 months) was available (data not shown).
In summary, our findings demonstrate that BRAF mutations are most frequent in melanoma types for which epidemiologic data suggest a pathogenetic role of intermittent sun exposure as opposed to chronic sun exposure (4,10). These melanomas occur predominantly on the trunk, are typically of the superficial spreading melanoma or nodular melanoma type, and occur earlier in life than melanomas on other sites. However, the relationship of BRAF mutations and sun exposure is complex. First, as previously noted (6), the mutations do not have the standard UVB signature. Second, mutation frequencies are low in anatomic areas that receive the lowest (e.g., mucosa) and the highest (e.g., chronic sun damage) sun exposure. Further studies are therefore necessary to determine whether exposure to UVA, other indirect mechanisms affected by sun exposure, or anatomic variation of melanocyte susceptibility contribute to the BRAF mutation spectrum in melanoma.
The association of BRAF mutation frequency with anatomic site and sun-exposure patterns supports the concept of distinct genetic paths of melanoma development, as suggested by a previous study (11). The frequent occurrence of BRAF mutations in the most common type of melanoma raises the possibility that specific BRAF inhibitors (12) may be useful therapeutic agents for this type of disease.
![]() |
NOTES |
---|
![]() ![]() ![]() ![]() |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() |
---|
1 Elwood JM. Melanoma and sun exposure. Semin Oncol 1996;23:65066.[ISI][Medline]
2 Elwood JM, Gallagher RP. Body site distribution of cutaneous malignant melanoma in relationship to patterns of sun exposure. Int J Cancer 1998;78:27680.[CrossRef][ISI][Medline]
3 Clark WH Jr, Elder DE, Van Horn M. The biologic forms of malignant melanoma. Hum Pathol 1986;17:44350.[ISI][Medline]
4 Holman CD, Armstrong BK, Heenan PJ. Relationship of cutaneous malignant melanoma to individual sunlight-exposure habits. J Natl Cancer Inst 1986;76:40314.[ISI][Medline]
5 McGovern VJ. The nature of melanoma. A critical review. J Cutan Pathol 1982;9:6181.[ISI][Medline]
6 Davies H, Bignell GR, Cox C, Stephens P, Edkins S, Clegg S, et al. Mutations of the BRAF gene in human cancer. Nature 2002;417:94954.[CrossRef][ISI][Medline]
7 Bastian BC, LeBoit PE, Hamm H, Bröcker EB, Pinkel D. Chromosomal gains and losses in primary cutaneous melanomas detected by comparative genomic hybridization. Cancer Res 1998;58:21705.[Abstract]
8 Snijders AM, Nowak N, Segraves R, Blackwood S, Brown N, Conroy J, et al. Assembly of microarrays for genome-wide measurement of DNA copy number. Nat Genet 2001;29:2634.[CrossRef][ISI][Medline]
9 Veltman JA, Fridlyand J, Pejavar S, Olshen AB, Korkola JE, DeVries S, et al. Array-based comparative genomic hybridization for genome-wide screening of DNA copy number in bladder tumors. Cancer Res 2003;63:287280.
10 Bulliard JL. Site-specific risk of cutaneous malignant melanoma and pattern of sun exposure in New Zealand. Int J Cancer 2000;85:62732.[CrossRef][ISI][Medline]
11 Bastian BC, Kashani-Sabet M, Hamm H, Godfrey T, Moore DH II, Bröcker EB, et al. Gene amplifications characterize acral melanoma and permit the detection of occult cells in the surrounding skin. Cancer Res 2000;60:196873.
12 Lyons JF, Wilhelm S, Hibner B, Bollag G. Discovery of a novel Raf kinase inhibitor. Endocr Relat Cancer 2001;8:21925.
Manuscript received April 14, 2003; revised October 1, 2003; accepted October 14, 2003.
This article has been cited by other articles in HighWire Press-hosted journals:
Correspondence about this Article
![]() |
||||
|
Oxford University Press Privacy Policy and Legal Statement |