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

Sun Exposure and Melanoma, Certainties and Weaknesses of the Present Knowledge

Sun exposure is the main risk factor for cutaneous malignant melanoma (CMM). However, the UV-related pathogenetic mechanisms leading to CMM are far to be fully elucidated. In this paper we will focus on what we still don't fully know about the relationship between UVR and CMM. In particular, we will discuss: the action spectrum of human CMM, how different modalities of exposure (continuous/ intermittent; erythemal/ suberythemal) relate to different CMM variants, the preferential UVR induced DNA mutations observed in different CMM variants, the role of UV-related and UV-unrelated genetic damages in the same melanoma cells. Moreover, we will debate the importance of UVA induced oxidative and anaerobic damages to DNA and other cell structures and the role of melanins, of modulation of innate and acquired immunity, of vitamin D and of chronic exposure to phototoxic drugs and other xenobiotics. A better understanding of these issues will help developing more effective preventative strategies and new therapeutic approaches.

The melanomas: a synthesis of epidemiological, clinical, histopathological, genetic, and biological aspects, supporting distinct subtypes, causal pathways, and cells of origin

Converging lines of evidence from varied scientific disciplines suggest that cutaneous melanomas comprise biologically distinct subtypes that arise through multiple causal pathways. Understanding the respective relationships of each subtype with etiologic factors such as UV radiation and constitutional factors is the first necessary step toward developing refined prevention strategies for the specific forms of melanoma. Furthermore, classifying this disease precisely into biologically distinct subtypes is the key to developing mechanism-based treatments, as highlighted by recent discoveries. In this review, we outline the historical developments that underpin our understanding of melanoma heterogeneity, and we do this from the perspectives of clinical presentation, histopathology, epidemiology, molecular genetics, and developmental biology. We integrate the evidence from these separate trajectories to catalog the emerging major categories of melanomas and conclude with important unanswered questions relating to the development of melanoma and its cells of origin.

Frequencies of BRAF and NRAS mutations are different in histological types and sites of origin of cutaneous melanoma: a meta-analysis

BACKGROUND

There have been conflicting data regarding the prevalence and clinicopathological characteristics of BRAF and NRAS mutations in primary cutaneous melanoma.

OBJECTIVES

To solve this controversy, this study used a meta-analysis to evaluate the frequencies of BRAF and NRAS mutations, and the relationship between these mutations and clinicopathological parameters of cutaneous melanoma.

METHODS

Data from studies published between 1989 and 2010 were combined. The BRAF and NRAS mutations were reported in 36 and 31 studies involving 2521 and 1972 patients, respectively. The effect sizes of outcome parameters were calculated by odds ratios (OR).

RESULTS

BRAF and NRAS mutations were reported in 41% and 18% of cutaneous melanomas, respectively. The mutations were associated with histological subtype and tumour site, but not with age and sex. The BRAF mutation was frequently detected in patients with superficial spreading melanoma (OR=2·021; P<0·001) and in melanomas arising in nonchronic sun-damaged skin (OR=2·043; P=0·001). In contrast, the NRAS mutation was frequently evident in patients with nodular melanoma (OR=1·894; P<0·001) and in melanomas arising in chronic sun-damaged skin (OR=1·887; P=0·018).

CONCLUSIONS

This pooled analysis shows that the incidences of BRAF and NRAS mutations in cutaneous melanomas differ according to histological type and tumour location based on the degree of sun exposure.

Cell death in the skin

The skin is the largest organ of the body and protects the organism against external physical, chemical and biological insults, such as wounding, ultraviolet radiation and micro-organisms. The epidermis is the upper part of the skin that is continuously renewed. The keratinocytes are the major cell type in the epidermis and undergo a specialized form of programmed cell death, called cornification, which is different from classical apoptosis. In keep with this view, several lines of evidence indicate that NF-kB is an important factor providing protection against keratinocyte apoptosis in homeostatic and inflammatory conditions. In contrast, the hair follicle is an epidermal appendage that shows cyclic apoptosis-driven involution, as part of the normal hair cycle. The different cell death programs need to be well orchestrated to maintain skin homeostasis. One of the major environmental insults to the skin is UVB radiation, causing the occurrence of apoptotic sunburn cells. Deregulation of cell death mechanisms in the skin can lead to diseases such as cancer, necrolysis and graft-versus-host disease. Here we review the apoptotic and the anti-apoptotic mechanisms in skin homeostasis and disease.

Death receptors and apoptosis

Interactions between death receptors from the tumor necrosis factor superfamily and their ligands play a crucial role in the development and the integrity of the epidermis. The major consequence resulting from death receptor targeting is apoptosis. Evidence for dysregulation of death receptor signaling associated with the pathogenesis of selected cutaneous diseases, including toxic epidermal necrolysis, graft versus host disease, and skin cancer, are reviewed herein.

Preponderance of the oncogenic V599E and V599K mutations in the B-raf kinase domain is enhanced in melanoma lymph node metastases

Downstream of Ras, the serine/threonine kinase B-raf has been reported to be mutated, amongst other carcinomas, in a substantial subset of primary melanomas, with a preponderance of mutations within the kinase domain, including the activating V599E and V599K transitions. We investigated a representative series of 54 resection specimens of melanoma lymph node metastases for the presence of mutations within the activation segment (exon 15) of the B-raf kinase domain by polymerase chain reaction (PCR) and single-strand conformational polymorphism (SSCP) gel electrophoresis. Sequencing of cloned PCR-SSCP amplicons resulted in 24 (44%) samples harbouring somatic mutations, which is not significantly different from the mutation frequency found in recently investigated primary cutaneous melanomas (Deichmann M, Thome M, Benner A, Näher H. B-raf exon 15 mutations are common in primary melanoma resection specimens but not associated with clinical outcome. Oncology 2004; 66:411-419). The activating mutation T1796A was present in 20 (83%) of these resection specimens, followed in frequency by the oncogenic g1795A mutation in five (21%) cases. With regard to the B-raf protein sequence, the acidic amino acid transitions V599E and V599K were predicted in 15 (62%) and five (21%) of the 24 positive metastases, respectively. The detection of mutations at this hot spot codon was significantly associated with subsequent visceral metastasis (P=0.03; Fisher's exact test). During the transition from primary melanomas (see reference above) to lymph node metastases, the spectrum of B-raf mutations narrows significantly (P=0.00047). The oncogenic B-raf mutations V599E and V599K, as early events in melanocyte transformation, persist throughout metastasis with important prognostic implications.

Preponderance of the oncogenic V599E and V599K mutations in B-raf kinase domain is enhanced in melanoma cutaneous/subcutaneous metastases

BACKGROUND

Downstream of Ras, the serine/threonine kinase B-raf has been reported to be mutated, among other carcinomas, in a substantial subset of primary melanomas with a preponderance of mutations within the kinase domain including the activating V599E and V599K transitions.

METHODS

We here investigated a representative series of 60 resection specimens of cutaneous and subcutaneous melanoma metastases for the presence of mutations within the activation segment (exon 15) of the B-raf kinase domain by polymerase chain reaction (PCR) and single-strand conformation polymorphism (SSCP) gel electrophoresis.

RESULTS

Sequencing of cloned PCR-SSCP amplicons resulted in 24 (40%) samples harbouring somatic mutations which is not exceeding the mutation frequency in recently investigated primary melanomas. The activating mutation T1796A was present in 24/60 (40%) resection specimens, followed in frequency by the oncogenic g1795A mutation in 8/60 (13%) cases. As to the B-raf protein sequence, the acidic amino acid transitions V599E and V599K were predicted in 19/60 (32%) and 6/60 (10%) cases, respectively, but were not associated with enhanced risk for subsequent metastasis in patients' follow up. In comparison to the primary melanomas that we recently investigated, the spectrum of predicted B-raf protein mutations narrowed significantly in the cutaneous/subcutaneous metastases. Unexpectedly, V599 and V599E mutations were absent in cutaneous/subcutaneous metastases derived from acrolentiginous melanomas as preceding primary tumours.

CONCLUSION

During transition from primary melanomas towards cutaneous/subcutaneous metastases, the spectrum of predicted B-raf mutations narrows significantly. Focusing on the V599E and V599K, these oncogenic mutations are likely to affect melanocyte-specific pathways controlling proliferation and differentiation.

Single genetic mutations can account for melanocytic naevi

BACKGROUND

The nature of melanocytic naevi is unknown notwithstanding their considerable significance for clinician and pathologist and despite the wealth of existing knowledge about melanocyte biology.

OBJECTIVES

To investigate how far a simple mutational model can explain the clinical and pathological features of melanocytic naevi, in particular their pattern of onset and frequency.

METHODS

I have constructed a model of the development of the adult melanocyte population from a single stem cell. The total cutaneous melanocyte population in a human adult is already known, as well as the range of spontaneous mutation rates at a given gene site. For each cycle of mitosis during the post stem-cell expansion of the melanocyte population, I calculate the accumulated number of cells likely to be mutated at a particular (although unknown) gene site. The results are interpreted in the light of a hypothesis that each of these mutant melanocytes will go on to form a melanocytic naevus. Comparisons are made with neurofibromas, occurring in type 1 neurofibromatosis and as sporadic lesions.

RESULTS

A single genetic mutation in melanocyte precursors is found to be sufficient to explain the clinical and pathological features of melanocytic naevi.

CONCLUSIONS

I propose that melanocytic naevi are a consequence of single spontaneous genetic mutations which inevitably occur during the development of the adult population of cutaneous melanocytes.

B-raf Exon 15 Mutations Are Common in Primary Melanoma Resection Specimens but Not Associated with Clinical Outcome

Downstream of Ras, the serine/threonine kinase B-raf has recently been reported to be mutated, among other carcinomas, in a majority of melanoma cell lines with a preponderance of mutations within the kinase domain including the activating V599E transition. We therefore investigated a representative number of 50 primary melanoma resection specimens for the presence of mutations within the activation segment (exon 15) of the B-raf kinase domain. Applying polymerase chain reaction and single-strand conformation polymorphism gel electrophoresis, followed by DNA cloning and sequencing, we found 19 cases (38%) to harbor somatic B-raf exon 15 mutations. With respect to the B-raf protein sequence, the V599E mutation was predicted in 63% of these positive melanomas, followed in frequency by the V599K transition (31%). Detection of B-raf exon 15 mutations or prediction of the activating mutation V599E were not statistically associated with the risk for subsequent metastasis in the follow-up of patients. Altogether, the B-raf oncogene is affected in a substantial subset of melanoma resection specimens. As B-raf alterations possibly affect melanocyte-specific pathways controlling proliferation and differentiation, activation of this oncogene may contribute to the development of melanoma.

Frequent alterations of Ras signaling pathway genes in sporadic malignant melanomas

Ras signaling is important for the intracellular transduction of mitogenic stimuli from activated growth factor receptors. We have investigated 37 sporadic malignant melanomas (15 primary cutaneous melanomas and 22 melanoma metastases) and 6 melanoma cell lines for mutations in the 3 Ras genes NRAS, KRAS and HRAS. All tumors and cell lines were additionally analyzed for mutation and expression of BRAF, which encodes a Ras-regulated serine/threonine kinase with oncogenic properties, as well as for expression of RASSF1A, which encodes a Ras-binding protein with tumor suppressor properties. Mutational analyses identified somatic NRAS mutations in 2 primary melanomas, 4 melanoma metastases and 2 cell lines. One melanoma metastasis showed a somatic KRAS mutation whereas HRAS mutations were not detected. Eight primary melanomas, 6 melanoma metastases and 4 melanoma cell lines carried BRAF mutations affecting the known hot-spot codon 599. None of the tumors or cell lines with BRAF mutation demonstrated NRAS or KRAS mutations. Real-time reverse transcription-PCR showed that 8 melanomas (3 primary tumors, 5 melanoma metastases) had reduced RASSF1A transcript levels of < or =50% relative to benign melanocytic nevi and normal skin. Three melanoma cell lines lacked detectable RASSF1A transcripts. The RASSF1A gene promoter was hypermethylated in these 3 cell lines as well as in 6 of 8 melanomas with reduced RASSF1A mRNA levels. Treatment of the cell lines with 5-aza-2'-deoxycytidine and trichostatin A resulted in demethylation of the RASSF1A promoter and re-expression of RASSF1A transcripts. Most tumors and all cell lines with RASSF1A promoter methylation additionally carried BRAF or NRAS mutations, suggesting a synergistic effect of these aberrations on melanoma growth. Taken together, 57% of the investigated melanomas and 100% of the melanoma cell lines carried mutations in either NRAS, KRAS or BRAF. In addition, 22% of the melanomas and 50% of the cell lines showed reduced RASSF1A transcript levels. Thus, alterations of Ras pathway genes are of paramount importance in the pathogenesis of sporadic melanomas.

Absence of ras gene mutations in UV-induced malignant melanomas correlates with a dermal origin of melanocytes in Monodelphis domestica

The South American opossum, Monodelphis domestica, has been used as a model system to study ultraviolet (UV)-induced genetic alterations that lead to the development of melanoma. Suckling young of Monodelphis exposed to multiple doses of UVB radiation can develop benign or malignant melanomas later as adults. Point mutations predominantly at codon 61 of the N-ras gene have been found in melanomas from sun-exposed body sites in humans. To determine if similar mutations are associated with UV-induced melanoma in Monodelphis, the nucleotide sequence of a Monodelphis N-ras cDNA was determined, and the occurrence of ras mutations in melanomas from UV-irradiated opossums was investigated. Single-strand conformation polymorphism analysis revealed no mutations in either the Monodelphis N-ras or H-ras genes in any of 24 primary malignant melanoma samples analyzed in this study. The disparate association of ras mutations with melanoma in humans and Monodelphis may be explained by differences in nucleotide sequence at codon 61 of the N-ras gene as well as differences in skin architecture between the two species. These results support the contention that a mutationally activated N-ras gene contributes to the vertical growth phase, which is specific to the progression of malignant melanoma in humans.

Defective death receptor signaling as a cause of tumor immune escape

Death receptors are a subgroup of TNF-receptor family members that can trigger caspase-8 activation and apoptosis upon interaction with their selective ligands. One of the death receptors, Fas (CD95) and its ligand is critically involved in the regulation of immune homeostasis and effectorfunction. Fas-mediated cell death is a major pathway of cytolytic T-cell-mediated death that is involved in specific killing of tumor cells. Recent investigations summarized herein have shown that defective Fas-signaling due to receptor downregulation or dysfunction, or intracellular inhibition by FLIP (FLICE inhibitory protein) can interfere with Fas-mediated tumor cell death, and thereby favor tumor immune escape.

The protein phosphatase 2A subunit Bgamma gene is identified to be differentially expressed in malignant melanomas by subtractive suppression hybridization

Several genes implicated in the development of various malignancies appear to be of minor relevance in melanoma. We therefore aimed to find a tumour suppressor candidate involved in this malignancy by comparing gene expression in uncultured primary melanoma specimens with those in acquired melanocytic naevi, from which quite often melanomas are known to arise. Applying the subtractive suppression hybridization technique, we generated a subtracted library of candidate genes downregulated in melanoma. Among the cDNA fragments identical to known genes, this library included a cDNA fragment 630 bp in length that is identical to the gene for the human protein phosphatase 2A (PP2A) regulatory subunit B (B56) gamma isoform (PP2A-Bgamma, PPP2R5C). On further evaluation of 15 primary melanoma and 16 acquired melanocytic naevus tissue specimens from independent patients using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis, expression of this gene was found to be suppressed in melanomas compared with naevi; the difference was statistically significant. As PP2A is known to be a major cellular serine-threonine phosphatase, and has been implicated not only in the regulation of cell growth and division but also in the control of gene transcription and growth factor signal transduction, alterations in the pattern of the regulatory subunits may affect substrate specificity and subcellular localization of the PP2A holoenzyme in melanoma cells.

Management of malignant melanoma: new developments in immune and gene therapy

Thus far, the use of classical anti-cancer treatment modalities had only rarely a beneficial impact on the prognosis of patients with metastatic melanoma. We as physicians have therefore the obligation as well as the chance to develop and test new therapeutic strategies. Our growing knowledge about the genetic basis of melanoma provides one platform to fulfil this task. Another one comes from our increasing understanding of the molecular and cellular mechanisms involved in the induction/modulation of immune responses, as well as the progress made in the field of identification of melanoma antigens, and allows for the development of a new generation of vaccines. The aim of this article is to discuss several of these new concepts towards the use of immune and gene therapy of melanoma.

Death receptors in cutaneous biology and disease

Death receptors are a growing family of transmembrane proteins that can detect the presence of specific extracellular death signals and rapidly trigger cellular destruction by apoptosis. Expression and signaling by death receptors and their respective ligands is a tightly regulated process essential for key physiologic functions in a variety of organs, including the skin. Several death receptors and ligands, Fas and Fas ligand being the most important to date, are expressed in the skin and have proven to be essential in contributing to its functional integrity. Recent evidence has shown that Fas-induced keratinocyte apoptosis in response to ultraviolet light, prevents the accumulation of pro-carcinogenic p53 mutations by deleting ultraviolet-mutated keratinocytes. Further- more, there is strong evidence that dysregulation of Fas expression and/or signaling contributes to the pathogenesis of toxic epidermal necrolysis, acute cutaneous graft versus host disease, contact hypersensitivity and melanoma metastasis. With these new developments, strategies for modulating the function of death receptor signaling pathways have emerged and provided novel therapeutic possibilities. Specific blockade of Fas, for example with intravenous immunoglobulin preparations that contain specific anti-Fas antibodies, has shown great promise in the treatment of toxic epidermal necrolysis and may also be useful in the treatment acute graft versus host disease. Likewise, induction of death signaling by ultraviolet light can lead to hapten-specific tolerance, and gene transfer of Fas ligand to dendritic cells can be used to induce antigen specific tolerance by deleting antigen-specific T cells. Further developments in this field may have important clinical implications in cutaneous disease.

Melanoma genetics: an update with focus on the CDKN2A(p16)/ARF tumor suppressors

Investigative interest in atypical nevi and familial melanoma has contributed to the identification of several candidate melanoma loci within the human genome. Molecular defects in both tumor suppressor genes and oncogenes have been pathogenically linked to melanoma in recent studies. Of the loci currently characterized, the major gene resides on chromosome 9p and encodes a tumor suppressor designated p16. This gene, which is also known as CDKN2A, is either mutated or deleted in a large majority of melanoma cell lines, as well as in many uncultured melanoma cells and in the germline of melanoma kindreds. A novel aspect of the p16 locus is that it encodes not just one but two separate gene products that are transcribed in alternative reading frames. Both products function as negative regulators of cell cycle progression. The p16 protein itself executes its effects by competitively inhibiting cyclin-dependent kinase 4, which is a factor necessary for cellular progression through a major regulatory transition of the cell division cycle. Inherited and acquired deletions or point mutations in the p16 gene increase the likelihood that potentially mutagenic DNA damage will escape repair before cell division. Notably, the second product of the locus, ARF (for alternative reading frame), regulates cell growth through independent effects on the p53 pathway. Although there is little evidence that ARF by itself is involved in the pathogenesis of melanoma, deletions at the p16 locus disable two separate pathways that control cell growth. These recent advances open up the possibility of genetic testing for melanoma susceptibility in the setting of familial melanoma and suggest novel therapeutic strategies for melanoma based on gene therapy or small molecule mimicry targeted to the correction of defects in the p16 regulatory pathway. (J Am Acad Dermatol 2000;42:705-22.)

LEARNING OBJECTIVE

At the conclusion of this learning activity, participants should be familiar with the historical aspects of melanoma genetics and should have a greater understanding of the CDKN2A(p16)/ARF tumor suppressor genes.

Homozygous deletion of the p16INK4a and the p15INK4b tumour suppressor genes in a subset of human sporadic cutaneous malignant melanoma

Chromosome 9p21 is frequently deleted in malignant melanoma, and one familial malignant melanoma gene has been linked to 9p21-22. Recently, the cyclin D-dependent kinase inhibitors (CDKIs) p16INK4a and p15INK4b have been localized within chromosome 9p21, and the presence of p16INK4a point mutations has been demonstrated in familial melanoma and melanoma cell lines in vitro. To analyse the role of these CDKIs in sporadic human cutaneous non-metastatic malignant melanoma, we examined 36 primary tumour specimens representing different stages of melanoma progression and their corresponding normal skin samples for the three mechanisms of CDKI inactivation described so far. Homozygous codeletion of the p16INK4a and the p15INK4b gene was detected by Southern blot analysis in two tumour samples. By direct sequencing of polymerase chain reaction (PCR)-amplified microdissected genomic DNA; no somatic or germline p16INK4a point mutations or small deletions were detected in the remaining 34 tumour samples; one individual exhibited the previously described germline codon 148 (Ala-->Thr) polymorphism. In these tumour specimens, comparative semiquantitative reverse PCR analysis of p16INK4a transcript levels revealed no evidence for repression of p16INK4a gene transcription and thus for p16INK4a promoter inactivation by DNA methylation. These results indicate homozygous p16INK4a and p15INK4b loss to occur in a subset of cutaneous melanomas and suggest, in view of the frequent loss of heterozygosity on chromosome 9p21, the presence of another tumour suppressor gene within this chromosomal region.

Cooperative effects of INK4a and ras in melanoma susceptibility in vivo

The familial melanoma gene (INK4a/MTS1/CDKN2) encodes potent tumor suppressor activity. Although mice null for the ink4a homolog develop a cancer-prone condition, a pathogenetic link to melanoma susceptibility has yet to be established. Here we report that mice with melanocyte-specific expression of activated H-rasG12V on an ink4a-deficient background develop spontaneous cutaneous melanomas after a short latency and with high penetrance. Consistent loss of the wild-type ink4a allele was observed in tumors arising in ink4a heterozygous transgenic mice. No homozygous deletion of the neighboring ink4b gene was detected. Moreover, as in human melanomas, the p53 gene remained in a wild-type configuration with no observed mutation or allelic loss. These results show that loss of ink4a and activation of Ras can cooperate to accelerate the development of melanoma and provide the first in vivo experimental evidence for a causal relationship between ink4a deficiency and the pathogenesis of melanoma. In addition, this mouse model affords a system in which to identify and analyze pathways involved in tumor progression against the backdrop of genetic alterations encountered in human melanomas.

Advances in dermatologic surgery

The major advances in dermatologic surgery for this period reside in the areas of diagnosis and management of cutaneous melanoma, laser surgery, tumescent liposuction, and tumorigenesis. A closer look at these advances is presented.

Expression of p53 protein in melanoma progression

p53, A tumor suppressor gene, has been documented as the most frequently mutated gene in human cancers including non-melanoma skin tumors. It has been controversial whether the p53 gene mutation plays a major role for melanoma genesis. To examine the role of p53 in human malignant melanoma carcinogenesis, we performed immunohistochemical analysis using anti-p53 antibodies (CM-1 and DO-7) in microwaved paraffin sections. When cases having more than 1% reactive cells were regarded as positive, immunohistochemical analysis revealed that in primary melanomas 14 of 51 (27%) were positive with CM-1 or 15 of 51 (29%) were positive with DO-7. Tumor thickness of primary melanomas in p53 positive cases was significantly thicker than that in p53 negative cases. In metastatic melanomas, 35 of 41 (85%) lymph node metastases were positive with either antibody and in skin metastases 16 of 28 (57%) lesions with CM-1 or 18 of 28 (64%) lesions with DO-7 were positive. The mean percentages of reactive cells were 2.3% in primary lesions and 4.9% in metastases. The incidence of positivity was significantly higher in metastases than primary lesions. In 10 cases examined, with both primary and metastatic melanoma, 3 cases were negative in both lesions and 1 case was positive in both lesions, while 6 cases were negative in the primary lesions and positive only in metastatic lesions. Four Spitz nevi, 6 dysplastic nevi and 11 common nevi were all negative. These data suggest that the expression of p53 protein may be a late event in melanoma progression.

Melanosomal and lysosomal alterations in murine melanocytes following transfection with the v-rasHa oncogene

Melanomas exhibiting mutated ras genes are frequently invasive and amelanotic. Transfecting melanocytes with ras oncogenes causes transformation and a loss of visible pigmentation. We analyzed murine melanocytes rendered amelanotic by transfection with the v-rasHa oncogene. Consistent with previous reports, tyrosinase and tyrosinase-related protein-1 (TRP-1) were not expressed by transformed cells. In addition, lack of expression of TRP-2 and the product of the silver locus was documented. Levels of melanosomal matrix antigens, the pink-eyed dilution locus protein and lysosome-associated membrane protein-1 were markedly reduced. Residual matrix antigens were localized by immunofluorescence to large vacuoles distributed peri-nuclearly in transfected cells. Electron microscopy demonstrated the absence of typical melanosomes and the presence of large vacuolar structures, also in a peri-nuclear distribution. Although levels of lysosomal hydrolases, such as beta-glucuronidase and cathepsin D, were diminished, marked elevations were observed in the expression of cathepsins B and L, 2 thiol proteases implicated in the acquisition of invasiveness. Our data demonstrate that transfection of melanocytes with v-rasHa is sufficient to disrupt the biogenesis of melanosomes and to up-regulate thiol protease synthesis, providing insights into the amelanotic and invasive nature of melanomas exhibiting mutations in ras genes.