Paradox of ras Oncogenes in Malignant Melanoma

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)

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

A number of studies have failed to detect point mutations at codon 12 in the Ki-ras gene in melanocytic neoplasms. One recent study, however, has found a high percentage of Ki-ras codon 12 point mutations. In an effort to resolve this difference, the present study examined noncultured melanocytic lesions (i.e., 5 benign nevi, 10 dysplastic nevi, and 8 primary melanomas: 4 in situ and 4 invasive) for point mutations at codon 12 in the first exon of the Ki-ras proto-oncogene using polymerase chain reaction methodology with oligonucleotide hybridization and direct DNA sequencing. The results of this study indicates no detectable mutations in the 12th codon of the first exon of the Ki-ras gene in any premalignant or malignant melanocytic lesion examined.

Apparent absence of a benign precursor lesion: implications for the pathogenesis of malignant melanoma

This review relates concepts derived from the study of chemically induced skin cancer in animal models to the pathogenesis of malignant melanoma in humans. Most chemically induced experimental cancers in animals, including melanomas in rodents, arise within a benign precursor lesion. The initiation-promotion-progression sequence is a central concept in animal models for carcinogenesis. Many human melanomas appear to arise from epidermal melanocytes, with no associated precursor lesion. This article considers why there is no apparent precursor in many human melanomas and the consequences of this absence. Melanocyte physiology and factors that govern escape from defenses such as DNA repair, local tissue environment, and immunity presumably influence melanocyte conversion to melanoma. These factors may determine the absence of a precursor lesion in primary melanomas. In addition, it is possible that some human melanomas arise by cellular mechanisms different from those causing cancer in rodent models. Both molecular and prospective clinical studies will be required to explain this apparent paradox in the pathogenesis of melanoma. A similar approach may help to explain the origin of basal cell carcinoma and perhaps other human cancers that appear to arise directly from normal cells. From a clinical point of view, the absence of an identifiable, benign precursor lesion requires even greater emphasis on melanoma prevention. Research on mechanisms of ultraviolet carcinogenesis indicates that appropriate postexposure treatments may be useful in preventing long-term consequences of sunburn, including melanoma.