10q deletions in metastatic cutaneous melanoma

Low prevalence of RAS-RAF-activating mutations in Spitz melanocytic nevi compared with other melanocytic lesions

Melanocytic lesions, including Spitz nevi (SN), common benign nevi (CBN) and cutaneous metastatic melanoma (CMM), were analyzed for activating mutations in NRAS, HRAS and BRAF oncogenes, which induce cellular proliferation via the MAP kinase pathway. One of 22 (4.5%) SN tested showed an HRAS G61L mutation. Another lesion, a 'halo' SN, showed a BRAF V600E (T1796A) mutation. BRAF V600E mutations were found in two thirds (20/31) of CBN, while a further 19% (6/31) showed NRAS codon 61 mutations. One third of CMM (10/30) had various BRAF mutations of codon 600, and a further 6% (2/31) showed NRAS codon 61 mutations. Seventeen SN tested for loss of heterozygosity (LOH) at 9p and 10q regions, known to be frequently deleted in melanoma, showed LOH at the 9p loci D9S942 and IFNA. A further lesion was found with low-level microsatellite instability at one locus, D10S214. The low rate of RAS-RAF mutations (2/22, 9.1%) observed in SN suggests that these lesions harbor as yet undetected activating mutations in other components of the RAS-RAF-MEK-ERK-MAPK pathway. Germline DNA from members of 111 multiple-case melanoma families, representing a range of known (CDKN2A) and unknown predisposing gene defects, was analyzed for germline BRAF mutations, but none was found.

Predisposición genética en el melanoma cutáneo

The incidence of cutaneous melanoma has increased worldwide in the last 20 years. Research on potential risk factors, both environmental and genetic, has led us to some new and interesting conclusions. Ultraviolet radiation is clearly the main environmental risk factor for melanoma, but its relationship is complex and controversial. With regard to genetic factors, the discovery of two types of genes was a great advance in further understanding the biology of the melanocyte. CDKN2A (p16) is the prototype of the high-penetrance, low-prevalence gene related to melanoma. This gene has been studied in some families in which several members have been diagnosed with melanoma. In the general population with non-familial melanoma, low-penetrance, high-prevalence genes such as MC1R seem to be more interesting. Studies on the MC1R gene have not only shown its importance in skin and hair pigmentation, but also in the development of melanoma. Functional studies on CDKN2A and MC1R have led us to new and important conclusions. The analysis of data from studies on families, twins and control cases, with the collaboration of several countries, will lead us to new discoveries. For the primary and secondary prevention of this tumor, we must promote public health campaigns on the dangers of sun exposure and the identification of individuals at high risk.

Deletion mapping suggests that the 1p22 melanoma susceptibility gene is a tumor suppressor localized to a 9-Mb interval

Loss of the short arm of chromosome 1 is frequently observed in many tumor types, including melanoma. We recently localized a third melanoma susceptibility locus to chromosome band 1p22. Critical recombinants in linked families localized the gene to a 15-Mb region between D1S430 and D1S2664. To map the locus more finely we have performed studies to assess allelic loss across the region in a panel of melanomas from 1p22-linked families, sporadic melanomas, and melanoma cell lines. Eighty percent of familial melanomas exhibited loss of heterozygosity (LOH) within the region, with a smallest region of overlapping deletions (SRO) of 9 Mb between D1S207 and D1S435. This high frequency of LOH makes it very likely that the susceptibility locus is a tumor suppressor. In sporadic tumors, four SROs were defined. SRO1 and SRO2 map within the critical recombinant and familial tumor region, indicating that one or the other is likely to harbor the susceptibility gene. However, SRO3 may also be significant because it overlaps with the markers with the highest 2-point LOD score (D1S2776), part of the linkage recombinant region, and the critical region defined in mesothelioma. The candidate genes PRKCL2 and GTF2B, within SRO2, and TGFBR3, CDC7, and EVI5, in a broad region encompassing SRO3, were screened in 1p22-linked melanoma kindreds, but no coding mutations were detected. Allelic loss in melanoma cell lines was significantly less frequent than in fresh tumors, indicating that this gene may not be involved late in progression, such as in overriding cellular senescence, necessary for the propagation of melanoma cells in culture.

EDD, the human orthologue of the hyperplastic discs tumour suppressor gene, is amplified and overexpressed in cancer

EDD (E3 isolated by differential display), located at chromosome 8q22.3, is the human orthologue of the Drosophila melanogaster tumour suppressor gene 'hyperplastic discs' and encodes a HECT domain E3 ubiquitin protein-ligase. To investigate the possible involvement of EDD in human cancer, several cancers from diverse tissue sites were analysed for allelic gain or loss (allelic imbalance, AI) at the EDD locus using an EDD-specific microsatellite, CEDD, and other polymorphic microsatellites mapped in the vicinity of the 8q22.3 locus. Of 143 cancers studied, 38 had AI at CEDD (42% of 90 informative cases). In 14 of these cases, discrete regions of imbalance encompassing 8q22.3 were present, while the remainder had more extensive 8q aberrations. AI of CEDD was most frequent in ovarian cancer (22/47 informative cases, 47%), particularly in the serous subtype (16/22, 73%), but was rare in benign and borderline ovarian tumours. AI was also common in breast cancer (31%), hepatocellular carcinoma (46%), squamous cell carcinoma of the tongue (50%) and metastatic melanoma (18%). AI is likely to represent amplification of the EDD gene locus rather than loss of heterozygosity, as quantitative RT-PCR and immunohistochemistry showed that EDD mRNA and protein are frequently overexpressed in breast and ovarian cancers, while among breast cancer cell lines EDD overexpression and increased gene copy number were correlated. These results demonstrate that AI at the EDD locus is common in a diversity of carcinomas and that the EDD gene is frequently overexpressed in breast and ovarian cancer, implying a potential role in cancer progression.

Genotypic analysis of primary and metastatic cutaneous melanoma

Microdissection genotyping was performed on 16 cases of melanoma, including two cutaneous and one lymph node metastases. Three benign nevi were used as controls. Where possible, tumor was microdissected at several sites. Genotyping involved assessment of loss of heterozygosity [LOH]), which was accomplished using a panel of nine polymorphic tetranucleotide microsatellites. Polymerase chain reaction was performed on the normal tissue sample to establish microsatellite heterozygous status. Informative markers were then tested on microdissected lesional tissue and scored for the presence and extent of allelic imbalance (AI). Microsatellite informativeness varied from 33% to 66%. Benign nevi were without AI. All invasive melanomas manifested acquired allelic loss, which involved 75% or 100% of the markers shown to be informative for each subject. Eleven of 13 (84%) primary melanomas demonstrated intratumoral heterogeneity of AI consistent with development of tumor subclones with differing genotypic profiles within thin as well as thick melanomas. Although a consistent pattern did not emerge among the markers, LOH of 9p21 (D9S254) occurred in 60% (9/15) of the cases followed by 40% of cases displaying LOH of 1p34, p53, 10q (MXI1), and 10q23 (D10S520) and 25% with 5q21 (D5S 592) abnormalities. A third of the cases including the metastatic foci demonstrated two different patterns of AI affecting alternative alleles of the same genomic marker within different parts of the melanoma. Two melanomas in situ did not display LOH of any markers in the informative cases although the in situ component in the invasive tumors had allelic losses that were in part similar to the invasive areas. The results of this study support the expanded use of microdissection genotyping and explore other markers to define the unique mutational profile for malignant melanoma that may complement other histologic characteristics of melanoma.

PTEN/MMAC1 expression in melanoma resection specimens

PTEN/MMAC1, a tumour suppressor gene located on chromosome 10q23.3, has been found to be deleted in several types of human malignancies. As the chromosomal region 10q22-qter commonly is affected by losses in melanomas, we addressed this gene as tumour suppressor candidate in melanomas. Investigating PTEN/MMAC1 expression at mRNA level by semi-quantitative reverse transcription-polymerase chain reaction, we did not find a statistically significant down-regulation in melanoma resection specimens in comparison to acquired melanocytic nevi from which melanomas quite often are known to arise. Upon immunohistochemistry, PTEN/MMAC1 protein expression in melanomas was not lost. Sequencing the PTEN/MMAC1 cDNAs in 26 melanoma resection specimens (21 primary melanomas, five metastases), we detected three point mutations and two nucleotide deletions which did not represent genetic polymorphisms. With respect to the predicted protein sequences, all three point mutations were silent whereas the two frame shifts at the extreme C-terminus resulted in a loss of the putative PDZ-targeting consensus sequence. As loss of this motif possibly impairs localization and function of PTEN/MMAC1 in the two corresponding primary tumours, alterations of this tumour suppressor protein may participate in some melanomas.

PTEN inactivation is rare in melanoma tumours but occurs frequently in melanoma cell lines

Deletions detected in cytogenetic and loss of heterozygosity (LOH) studies indicate that at least one tumour suppressor gene maps to the long arm of chromosome 10. Previous deletion mapping studies have observed LOH on 10q in about 30% of melanomas analysed. The PTEN gene, mapping to chromosome band 10q23.3, encodes a protein with both lipid and protein phosphatase activity. Somatic mutations and deletions in have been detected in a variety of cell lines and tumours, including melanoma samples. We performed mutation analyses and extensive allelic loss studies to investigate the role this gene plays in melanoma pathogenesis. We found that a total of 34 out of 57 (60%) melanoma cell lines carried hemizygous deletions of chromosome 10q encompassing the PTEN locus. A further three cell lines carried smaller deletions excluding PTEN. Inactivation of both PTEN alleles by exon-specific homozygous deletion or mutation was observed in 13 out of 57 (23%) melanoma cell lines. The mutation spectrum observed does not indicate an important role for ultraviolet radiation in the genesis of these mutations, and evidence from three cell lines supports the acquisition of PTEN aberrations in culture. Ten out of 49 (20%) matched melanoma tumour/normal samples harboured hemizygous deletions of either the whole chromosome or most of the long arm. Mutations within were detected in only one of the 10 tumours demonstrating LOH at 10q23 that were analysed. These results suggest that PTEN inactivation may be important for the propagation of melanoma cells in culture, and that another chromosome 10 tumour suppressor gene may be important for melanoma pathogenesis.

The HtrA1 serine protease is down-regulated during human melanoma progression and represses growth of metastatic melanoma cells

Differential gene expression of cell lines derived from a malignant melanoma or its autologous lymph node metastasis using cDNA arrays indicated down-regulation of PRSS11, a gene encoding the serine protease HtrA1, a homolog of the Escherichia coli protease HtrA, in the metastatic line. Stable PRSS11 overexpression in the metastatic cell line strongly inhibited proliferation, chemoinvasion and Nm23-H1 protein expression in vitro, as well as cell growth in vivo in nu/nu mice. A polyclonal anti-HtrA1 serum demonstrated a significantly higher expression in primary melanomas when compared to unrelated metastatic lesions in a human melanoma tissue array, and down-modulation of HtrA1 expression in autologous lymph node melanoma metastases in seven out of 11 cases examined. These results suggest that down-regulation of PRSS11 and HtrA1 expression may represent an indicator of melanoma progression.

Molecular genetic analysis of malignant melanomas for aberrations of the WNT signaling pathway genes CTNNB1, APC, ICAT and BTRC

Aberrant activation of the Wnt signaling pathway has been reported in different human tumor types, including malignant melanomas. We investigated 37 malignant melanomas (15 primary tumors and 22 metastases) for alterations of 4 genes encoding members of this pathway, i.e., CTNNB1 (beta-catenin gene, 3p22.1), APC (adenomatous polyposis coli gene, 5q22.2), BTRC (beta-transducin repeat-containing protein gene, 10q24.3) and ICAT (inhibitor of beta-catenin and Tcf-4, 1p36.2). Mutational analysis of CTNNB1 identified somatic mutations in 1 primary melanoma and 1 melanoma metastasis from 2 different patients (5%). Both mutations affected the N-terminal degradation box of beta-catenin, which is important for the regulation of beta-catenin homeostasis. Another primary melanoma carried a somatic APC missense mutation within the known mutation cluster region in exon 15. Fourteen tumors (40%) showed LOH at microsatellite markers on 1p36. None of the tumors had lost both copies of the ICAT gene, but 1 melanoma metastasis carried a somatic point mutation altering the translation start codon of ICAT. Real-time RT-PCR showed markedly reduced ICAT transcript levels (<or=20% relative to normal skin and benign melanocytic nevi) in 28/36 malignant melanomas (78%), including 13/14 tumors with LOH on 1p36. Allelic loss on 10q was detected in 15 tumors (44%). We found neither mutations nor complete loss of expression of the BTRC gene in our melanoma series. Taken together, our results indicate that the Wnt pathway may be altered in malignant melanomas by different mechanisms, including rare somatic mutations in CTNNB1, APC or ICAT, as well as low or absent expression of ICAT transcripts.

Nuclear PTEN expression and clinicopathologic features in a population-based series of primary cutaneous melanoma

Germline mutations of the PTEN tumor-suppressor gene, on 10q23, cause Cowden syndrome, an inherited hamartoma syndrome with a high risk of breast, thyroid and endometrial carcinomas and, some suggest, melanoma. To date, most studies which strongly implicate PTEN in the etiology of sporadic melanomas have depended on cell lines, short-term tumor cultures and noncultured metastatic melanomas. The only study which reports PTEN protein expression in melanoma focuses on cytoplasmic expression, mainly in metastatic samples. To determine how PTEN contributes to the etiology or the progression of primary cutaneous melanoma, we examined cytoplasmic and nuclear PTEN expression against clinical and pathologic features in a population-based sample of 150 individuals with incident primary cutaneous melanoma. Among 92 evaluable samples, 30 had no or decreased cytoplasmic PTEN protein expression and the remaining 62 had normal PTEN expression. In contrast, 84 tumors had no or decreased nuclear expression and 8 had normal nuclear PTEN expression. None of the clinical features studied, such as Clark's level and Breslow thickness or sun exposure, were associated with cytoplasmic PTEN expressional levels. An association with loss of nuclear PTEN expression was indicated for anatomical site (p = 0.06) and mitotic index (p = 0.02). There was also an association for melanomas to either not express nuclear PTEN or to express p53 alone, rather than both simultaneously (p = 0.02). In contrast with metastatic melanoma, where we have shown previously that almost two-thirds of tumors have some PTEN inactivation, only one-third of primary melanomas had PTEN silencing. This suggests that PTEN inactivation is a late event likely related to melanoma progression rather than initiation. Taken together with our previous observations in thyroid and islet cell tumors, our data suggest that nuclear-cytoplasmic partitioning of PTEN might also play a role in melanoma progression.

Chromosomal imbalances in primary and metastatic melanomas revealed by comparative genomic hybridization

Characteristic genetic changes underlying the metastatic progression of malignant melanoma is incompletely understood. The goal of our study was to explore specific chromosomal alterations associated with the aggressive behavior of this neoplasm. Comparative genomic hybridization was performed to screen and compare genomic imbalances present in primary and metastatic melanomas. Sixteen primary and 12 metastatic specimens were analyzed. We found that the pattern of chromosomal aberrations is similar in the two subgroups; however, alterations present only in primary and/or metastatic tumors were also discovered. The mean number of genetic changes was 6.3 (range 1-14) in primary and 7.8 (range 1-16) in metastatic lesions. Frequent losses involved 9p and 10q, whereas gains most often occurred at 1q, 6p, 7q, and 8q. Distinct, high-level amplifications were mapped to 1p12-p21 and 1p22-p31 in both tumor types. Amplification of 4q12-q13.1, 7q21.3-qter and 8q23-qter were detected only in primary tumors. The 20q13-qter amplicon was present in a metastatic tumor. The number of genetic alterations were significantly higher in primary tumors which developed metastases within one year after the surgery compared to tumors without metastasis during this time period. Fluorescence in situ hybridization with centromeric and locus-specific probes was applied to validate CGH results on a subset of tumors. Comparison of FISH and CGH data gave good correlation. The aggressive behavior of melanoma is associated with accumulation of multiple genetic alterations. Chromosome regions, which differ in the primary and metastatic lesions, may represent potential targets to identify metastases-related chromosomal alterations.

Detection of invasion-related chromosomal changes in highly and weakly invasive melanoma cell clones by a modified comparative genomic hybridization approach

Invasion is a key step in the systemic spread of tumour cells. The aim of this study was to investigate whether specific chromosomal aberrations in melanoma occur during acquisition of a strongly invasive phenotype. We previously selected highly and weakly invasive cell clones from the human melanoma cell line Mel Im. Both cell clones retained a stable phenotype over more than 40 passages, revealing a five-fold difference in their invasive potential in vitro. Direct comparative genomic hybridization (CGH) (modified CGH) of the two cell clones was used as a powerful tool to screen for different chromosomal aberrations in both clones. Standard CGH and fluorescence in situ hybridization (FISH) was performed to verify the results of this improved technique. In general, the CGH pattern showed a high degree of identity consistent with the fact that the cell lines represent subclones of the same cell line. However, a few defined changes between the two cell clones were observed, including loss of 1q21-qter, 4q, 11p14-pter, 19q and 20p in the highly invasive cell clone. Two of the regions (1q and 11p) have already been suggested to be involved in melanoma progression, whereas changes in the others have not been detected before. In summary, our direct CGH approach proved to be suitable for fast and direct comparison of two cell types and allowed the identification of two known and three novel chromosomal changes involved in the acquisition of a strongly invasive melanoma cell phenotype.

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.