Willmore-Payne Carlynn, Holden Joseph A, Hirschowitz Sharon, Layfield Lester J
Department of Pathology, University of Utah, Salt Lake City, UT 84132, USA.
Hum Pathol. 2006 May;37(5):520-7. doi: 10.1016/j.humpath.2006.01.003.
Activating mutations in BRAF or c-kit have been reported in malignant melanoma. Because the activating mutations are dominant, it has been assumed that they are heterozygous in the affected tumors. To test this, we have carefully examined the DNA sequencing electropherograms on 43 BRAF mutation-positive and 3 c-kit mutation-positive malignant melanomas to determine the ratio of the normal to mutant allele. Of the 43 BRAF mutation-positive tumors, we classified 26 as presumptive heterozygous. Eight cases were indeterminate. Surprisingly, 9 cases appeared to contain an excess of the mutant allele. BRAF fluorescence in situ hybridization on these 9 cases suggested the increased amount of the mutant BRAF allele was due to amplification (2 cases) or chromosome 7 polysomy (7 cases). We have previously described the presence of the c-kit-activating mutation, L576P, in 2 of 100 malignant melanomas. In this report, we have evaluated an additional 53 cases and found 1 additional case that contained the L576P mutation. Evaluation of the DNA sequencing electropherograms from all 3 cases of L576P mutation-positive melanoma suggests a selective loss of the normal allele. Fluorescence in situ hybridization for c-kit on these 3 L576P mutation-positive tumors indicated that one showed slight amplification of the c-kit gene and the other 2 were present in a nonamplified diploid state. These results have important implications concerning the mechanism of oncogenesis in melanoma as well as in the response of the tumor to anticancer drugs targeting BRAF or c-kit.
恶性黑色素瘤中已报道存在BRAF或c-kit的激活突变。由于激活突变是显性的，因此推测在受影响的肿瘤中它们是杂合的。为了验证这一点，我们仔细检查了43例BRAF突变阳性和3例c-kit突变阳性的恶性黑色素瘤的DNA测序电泳图，以确定正常等位基因与突变等位基因的比例。在43例BRAF突变阳性肿瘤中，我们将26例归类为推定杂合子。8例结果不确定。令人惊讶的是，9例似乎含有过量的突变等位基因。对这9例进行的BRAF荧光原位杂交表明，突变BRAF等位基因数量增加是由于扩增（2例）或7号染色体多体性（7例）。我们之前在100例恶性黑色素瘤中的2例中描述了c-kit激活突变L576P的存在。在本报告中，我们又评估了53例，发现另外1例含有L576P突变。对所有3例L576P突变阳性黑色素瘤的DNA测序电泳图评估表明正常等位基因存在选择性缺失。对这3例L576P突变阳性肿瘤进行的c-kit荧光原位杂交表明，1例显示c-kit基因有轻微扩增，另外2例处于非扩增二倍体状态。这些结果对于黑色素瘤的肿瘤发生机制以及肿瘤对靶向BRAF或c-kit的抗癌药物的反应具有重要意义。
