Lack of evidence of Ki-ras codon 12 mutations in melanocytic lesions

KRAS oncogene mutations suggest a common histogenetic origin for pleomorphic giant cell tumor of the pancreas, osteoclastoma of the pancreas, and pancreatic duct adenocarcinoma

Giant cell neoplasms of the pancreas are rare tumors of uncertain histogenesis. Mutation of the KRAS oncogene is common in typical pancreatic duct adenocarcinoma. We have analyzed DNA from five pancreatic tumors with giant cells for mutations in the KRAS oncogene and found alterations of the second position of codon 12 in each case (four G > A transitions and one G > C transversion). The common mutation pattern in tumors with giant cells and duct adenocarcinoma suggests a common route to malignant transformation and may indicate a shared histogenesis. We also tested 11 cases of malignant fibrous histiocytoma, a histological mimic of pleomorphic giant cell tumor, for mutations in the KRAS oncogene. The absence of KAS mutations in each of the malignant fibrous histiocytomas (MFHs) and in other histologically similar tumors may provide assistance in the differential diagnosis of pleomorphic pancreatic tumors.

Ras gene mutations: a rare event in nonmetastatic primary malignant melanoma

Ras gene mutations have been implicated in the pathogenesis of a variety of human tumors. Mutated ras genes have been isolated from human melanoma cell lines, but subsequent studies indicated that ras gene mutations may be a rare event in melanocytic lesions. Recently, a study reported a high frequency of ras mutations correlated with increasing invasion level. To address this inconsistency in the published data, we analyzed 50 primary melanomas to correlate invasion level, tumor thickness, histologic typing, and body localization with point mutations around codons 12/13/61 of the three ras genes. After micro-dissection of paraffin-embedded tumor tissue, ras gene mutations were analyzed by direct sequencing of tumor DNA amplified by polymerase chain reaction. Only two melanomas exhibited ras gene mutations, one sample containing a transition from A to G at position 2 of N-ras codon 61 and the other exhibiting a transversion from C to A at position 1 and a transition from A to G at position 2 of N-ras codon 61. Both tumors were classified as Clark level IV, with a tumor thickness of 2.5 and 1.2 mm, respectively. Both were typed as superficial spreading melanoma and localized to intermittently sun-exposed body sites. The low frequency of ras mutations in malignant melanoma and the lack of ras mutations in melanoma samples from constantly sun-exposed body sites argue against the hypothesis of ras mutations as a marker of progression in malignant melanoma and the suggestion that ras mutations occur predominantly in melanomas from constantly sun-exposed body sites.

Human malignant melanoma. A genetic disease?

BACKGROUND

Human hereditary malignant melanoma, comprising 5% of all cases of malignant melanoma, occurs in association with other malignancies, predominantly in families with dysplastic nevus syndrome. Additionally, higher incidences of malignant melanoma have been reported in individuals with genetic disorders such as ataxia telangiectasia and xeroderma pigmentosum. The results and observations as reported in the literature on the involvement of oncogenes and chromosomal aberrations in the development of malignant melanoma are reviewed and compared with the authors' own experimental and clinical experience.

RESULTS

Numerous chromosomal regions, as on chromosomes 1 and 9, were altered. The long arm of chromosome 6 was affected in 60% of melanomas. Introduction of a normal copy of chromosome 6 resulted in loss of tumorigenicity in vitro. True melanoma genes were evident in two animal models: the Sinclair swine and the teleost fish Xiphophorus. In the Xiphophorus system, the crossing-conditioned elimination of a tumor suppressor gene led to the uncontrolled activity of a dominantly acting oncogene in certain hybrids. The causative oncogene, Xmrk, encodes a receptor tyrosine kinase closely related to human epidermal growth factor receptor (EGFR). Among the numerous studied human oncogenes, mutations in the extensively investigated ras family are the result rather than the cause of malignant transformation. High expression of nuclear oncogenes simply may be a common feature of rapidly dividing cells. The receptor tyrosine kinase EGFR may be involved in late stage melanoma; the human exon with homology to Xmrk shows elevated transcription levels in 80% of human melanoma metastases. Deletions of the tumor suppressor gene MTS 1 may be important for melanoma formation, whereas deletions of p53 appear to be of minor relevance.

CONCLUSION

Scientific progress in treating and diagnosing malignant melanoma will largely depend on experimental approaches to define relevant genetic changes by functional analysis rather than descriptive phenomenology and correlative observations.

Analysis of ras mutations in human melanocytic lesions: activation of the ras gene seems to be associated with the nodular type of human malignant melanoma

We have analyzed the Ha-ras, Ki-ras and N-ras gene for point mutations at codons 12, 13 and 61 via restriction fragment length polymorphism/polymerase chain reaction analysis and subsequent direct sequencing in non-cultured fresh-frozen tissues of 16 superficial spreading melanomas (SSM), 13 nodular malignant melanomas (NMM), 2 lentigo malignant melanomas (LMM), 1 dysplastic nevus, 1 congenital nevus and 5 normal nevi from 38 patients. Mutations were found in 4 melanoma samples, all belonging to the nodular malignant type. Three of them were mutated in N-ras and one in the Ha-ras gene. Mutation in N-ras was also detected in the congenital nevus. All mutations were exclusively located at the first two base pairs of codon 61. No Ki-ras mutation was detected in any lesion. No mutation could be found in SSM and LMM in addition to dysplastic and normal nevi. The frequency of ras mutation in NMM was 31%, whereas in SSM it was 0%. Our study suggests (a) an association between ras mutations (mainly N-ras) and the NMM as a subgroup of human melanoma; (b) that activation of Ki-ras is not involved in the pathogenesis of melanoma. The role of UV radiation in point mutations of ras genes in human melanoma is discussed.

The polymerase chain reaction. Applications in dermatology

Within the space of the last 5 years, application of the revolutionary in vitro method of deoxyribonucleic acid (DNA) amplification known as the polymerase chain reaction (PCR), has become ubiquitous. The rapidly increasing number of clinical and research articles utilizing this technology, both in the dermatologic and general medical literature, requires one to have at least a basic understanding of how the PCR is conducted, what it has to offer, and the potential shortcomings. Such knowledge will hopefully allow a more critical appraisal of an increasingly complex literature. This review aims to describe the methodology and medical applications of this powerful technique with special consideration to the increasing role PCR may have on dermatologic research and practice.

Growth factor independence and growth regulatory pathways in human melanoma development

This review concentrates on growth autonomy of tumor cells in relation to tumor progression. Human malignant melanoma serves as an example for progressive growth factor independence at subsequent stages of tumor progression. Mechanisms by which malignant cells acquire growth factor independence are discussed. In melanoma, deregulation of growth regulatory pathways has been described on four levels: 1) aberrant production of autocrine growth factors that substitute for exogenous growth factors (basic fibroblast growth factor [bFGF]); 2) alterations in the response to negative autocrine growth factors (interleukin [IL]-6 and transforming growth factor [TGF]-beta); 3) overexpression of epidermal growth factor receptors (EGF-R); and 4) alterations of cellular protooncogenes involved in signal transduction (RAS, MYB) and growth suppression (p53). In addition to bFGF and IL-6, multiple other growth factor genes are activated in malignant melanoma cells but not normal melanocytes. These include both chains of platelet-derived growth factor (PDGF), TGF-alpha, IL-1, IL-8, and tumor necrosis factor (TNF)-alpha. Of these, PDGF-B has been investigated in more detail. Melanoma-derived PDGF clearly does not act in a direct autocrine mode, but has important paracrine effects on normal tissue constituents, notably fibroblasts and endothelial cells, that are essential for tumor development in vivo. It is speculated that other melanoma-derived growth factors with as yet undefined functions similarly exert such paracrine or 'indirect' autocrine effects that cannot be sufficiently addressed in studies on cultured cells.

Molecular genetics of human malignant melanoma

Due to a variety of known and unknown control mechanisms, the human genome is remarkably stable when compared to most other species. The long latency periods of most solid tumors, during which the cell undergoes malignant transformation, are presumably due to this stability. The molecular basis responsible for the induction of genetic instability and the resultant biological characteristics manifest in tumor populations is not well understood. The discovery of both oncogenes and tumor suppressor genes, however, has placed the phenomenon of human genome stability on a more solid conceptual footing. These types of genes clearly place multiple barriers to oncogenic transformation, and traversing these barriers apparently requires both time and the accumulation of genetic defects that cannot be corrected. The evolution of neoplasias can, therefore, be predicted to be due to: (1) consistent and progressive loss of tumor suppressor genes; (2) gene amplification, resulting in the over-expression of proteins that aid in tumor progression; (3) gene mutation, which alters the orderly biochemistry of the normal cell; (4) genes that allow a cell like the melanocyte to escape the confining nature of the epidermis and to invade through the dermis into the circulatory and lymphatic systems in order to disseminate itself to other organs (e.g., proteolytic enzymes, enzyme inhibitors, integrins, metastases genes, chemotactic factors etc.); (5) factors, perhaps such as TGF beta 2, that may impact negatively on MHC antigens and confuse host defense mechanisms; and (6) S.O.S.-type genes, which may be expressed as a direct response to the accumulating damage in an attempt to correct the damage, but that may then become part of the problem instead of the solution. The extraordinary plasticity and instability of the genome of a melanoma cell suggests an inordinate amount of genetic flux. In addition to activating and inactivating various genes, this constant shuffling and rearranging of the genome in neoplasms such as MM may be constantly altering gene dose. Cytogenetic and molecular biological studies have been the Rosetta stone for understanding the etiological relevant genetic events in human cancers. Genetic alterations fundamental to the pathology of MM have begun to be defined. Studies designed to understand these perturbations at the biochemical and organismic level are underway.(ABSTRACT TRUNCATED AT 400 WORDS)