The PTEN tumour suppressor gene and malignant melanoma

Genetic alterations of PTEN in human melanoma

The PTEN gene is one of the most frequently inactivated tumor suppressor genes in sporadic cancers. Inactivating mutations and deletions of the PTEN gene are found in many types of cancers, including melanoma. However, the exact frequency of PTEN alteration in melanoma is unknown. In this study, we comprehensively reviewed 16 studies on PTEN genetic changes in melanoma cell lines and tumor biopsies. To date, 76 PTEN alterations have been reported in melanoma cell lines and 38 PTEN alterations in melanoma biopsies. The rate of PTEN alterations in melanoma cell lines, primary melanoma, and metastatic melanoma is 27.6, 7.3, and 15.2%, respectively. Three mutations were found in both melanoma cell lines and biopsies. These mutations are scattered throughout the gene, with the exception of exon 9. A mutational hot spot is found in exon 5, which encodes the phosphatase activity domain. Evidence is also presented to suggest that numerous homozygous deletions and missense variants exist in the PTEN transcript. Studying PTEN functions and implications of its mutations and other genes could provide insights into the precise nature of PTEN function in melanoma and additional targets for new therapeutic approaches.

The RhoA-ROCK-PTEN pathway as a molecular switch for anchorage dependent cell behavior

The proliferation of anchorage-dependent cells of mesenchymal origin requires the attachment of the cells to substrates. Thus, cells that are poorly attached to substrates exhibit retarded cell cycle progression or apoptotic death. A major disadvantage of most polymers used in tissue engineering is their hydrophobicity; hydrophobic surfaces do not allow cells to attach firmly and, therefore, do not allow normal proliferation rates. In this study, we investigated the molecular mechanism underlying the reduced proliferation rate of cells that are poorly attached to substrates. There was an inverse relationship between the activity of the small GTPase RhoA (RhoA) and the cell proliferation rate. RhoA activity correlated inversely with the strength of cell adhesion to the substrates. The high RhoA activity in the cells poorly attached to substrates caused an increase in the activity of Rho-associated kinase (ROCK), a well-known effector of RhoA that upregulated the activity of phosphatase and tensin homolog (PTEN). The resulting activated PTEN downregulated Akt activity, which is essential for cell proliferation. Thus, the cells that were poorly attached to substrates showed low levels of cell proliferation because the RhoA-ROCK-PTEN pathway was hyperactive. In addition, RhoA activity seemed to be related to focal adhesion kinase (FAK) activity. Weak FAK activity in these poorly attached cells failed to downregulate the high RhoA activity that restrained cell proliferation. Interestingly, reducing the expression of any component of the RhoA-ROCK-PTEN pathway rescued the proliferation rate without physico-chemical surface modifications. Based on these results, we suggest that the RhoA-ROCK-PTEN pathway acts as a molecular switch to control cell proliferation and determine anchorage dependence. In cells that are poorly attached to substrates, its inhibition is sufficient to restore cell proliferation without the need for physico-chemical modification of the material surface.

Therapeutic Implications of Targeting AKT Signaling in Melanoma

Identification of key enzymes regulating melanoma progression and drug resistance has the potential to lead to the development of novel, more effective targeted agents for inhibiting this deadly form of skin cancer. The Akt3, also known as protein kinase B gamma, pathway enzymes regulate diverse cellular processes including proliferation, survival, and invasion thereby promoting the development of melanoma. Accumulating preclinical evidence demonstrates that therapeutic agents targeting these kinases alone or in combination with other pathway members could be effective for the long-term treatment of advanced-stage disease. However, currently, no selective and effective therapeutic agent targeting these kinases has been identified for clinical use. This paper provides an overview of the key enzymes of the PI3K pathway with emphasis placed on Akt3 and the negative regulator of this kinase called PTEN (phosphatase and tensin homolog deleted on chromosome 10). Mechanisms regulating these enzymes, their substrates and therapeutic implications of targeting these proteins to treat melanoma are also discussed. Finally, key issues that remain to be answered and future directions for interested researchers pertaining to this signaling cascade are highlighted.

Nuclear PTEN levels and G2 progression in melanoma cells

The phosphatase and tensin homolog (PTEN) exerts its function, in part, by negatively regulating the well-known phosphatidylinositol-3-kinase/AKT signaling pathway. Previous histological work has suggested that alterations in the nuclear/cytoplasmic compartmentalization of PTEN may play a role in the development and progression of melanoma. In this study, we examined the nuclear/cytoplasmic compartmentalization of PTEN in melanoma cell lines and its correlation with the cell cycle. Studies were performed in melanoma cells lines using classic cell biological techniques. In contrast to breast cancer cell lines, we found that increased levels of nuclear PTEN levels correlate with G2 rather than with G1 arrest. In WM164 and SKmel28 cells, overexpression of PTEN protein did not significantly increase the number of cells in the G2 phase. Differential CDC2 phosphorylation levels in cells that overexpressed PTEN compared with those where PTEN was downregulated suggest some involvement of PTEN in G2 checkpoint regulation. The data suggest that although nuclear PTEN levels correlate with the G2 phase, the role of PTEN in modulating G2/M arrest is not limiting. Further, the specific cell cycle phase regulated by nuclear PTEN is cell-type dependent. Taken together, our observations suggest that in melanoma, nuclear PTEN is involved in G2 progression possibly through the modulation of CDC2, opening up a new arena for investigation.

The PTEN-AKT3 signaling cascade as a therapeutic target in melanoma

Melanocytes undergo extensive genetic changes during transformation into aggressive melanomas. These changes deregulate genes whose aberrant activity promotes the development of this disease. The phosphoinositide-3-kinase (PI3K) and mitogen-activated protein (MAP) kinase pathways are two key signaling cascades that have been found to play prominent roles in melanoma development. These pathways relay extra-cellular signals via an ordered series of consecutive phosphorylation events from cell surface throughout the cytoplasm and nucleus regulating diverse cellular processes including proliferation, survival, invasion and angiogenesis. It is generally accepted that therapeutic agents would need to target these two pathways to be an effective therapy for the long-term treatment of advanced-stage melanoma patients. This review provides an overview of the PI3 kinase pathway focusing specifically on two members of the pathway, called PTEN and Akt3, which play important roles in melanoma development. Mechanisms leading to deregulation of these two proteins and therapeutic implications of targeting this signaling cascade to treat melanoma are detailed in this review.

Mutational spectrum of conserved regions of TP53 and PTEN genes in Kangri cancer (of the skin) in the Kashmiri population

Kangri cancer is a unique, thermally induced squamous cell carcinoma (SCC) of the skin that develops due to persistent use of a Kangri (a brazier) by the Kashmiri people to combat the cold temperature during winter. Unlike classical UV-induced SCC of the skin, Kangri cancer appears on the legs and abdomen. Its common features are erythematous patches, recurrence and metastasis. In the absence of any molecular etiology, we made a preliminary attempt to estimate the nature and frequency of mutations in the TP53 and PTEN genes in Kangri cancer patients from Kashmir. PCR-SSCP analysis followed by direct sequencing revealed that TP53 mutations account for 40% (12/30) of sporadic Kangri cancer patients and that PTEN mutations account for only 6.6% (2/30). There were 16 mutations in TP53 exons 5 and 7, found in 12 patients. They consisted of 11 substitutions (7 transitions, 3 transversions and 1 double-base) and 5 insertions. The 11 substitutions represent 8 distinct missense mutations, 3 of which were silent mutations. The mutations detected in the PTEN gene consisted of one insertion and one C>T transition. This high percentage of TP53 mutations (especially A>G) showed a statistically significant association with age and positive lymph node status. Our results indicate that TP53 is a predominant target of chronic hyperthermia in the development of Kangri cancer in the moderate risk Kashmiri population. The differences in the TP53 mutation spectrum of UV-induced SCC of the skin and Kangri cancer are probably due to the nature of the respective environmental carcinogens. The study also suggests that TP53 may function as a potential molecular marker and prognostic tool, at least in a subset of sporadic Kangri tumors.

Functional and therapeutic significance of Akt deregulation in malignant melanoma

Identification of specific genes or signaling pathways involved in development of melanoma could lead to new therapies that target and correct these defects. Recent studies have revealed deregulation of the Akt signaling pathway occurring in 43-67% of melanomas. Akt kinase family members, Akt1/PKBalpha, Akt2/PKBbeta and Akt3/PKBgamma, share extensive structural similarity and perform common as well as unique functions within cells. The Akt signaling cascade initiates at the cell surface when growth factors or other extracellular stimuli activate phosphoinositide 3-kinase (PI3K). Activated PI3K generates a lipid second messenger, phosphatidylinositol-3,4,5-trisphosphate (PIP3), causing translocation of Akt to the plasma membrane where it becomes phosphorylated and activated. The balance of cellular PIP3 is regulated primarily by a phosphatase called PTEN that reduces PIP3 levels thereby lowering Akt activity. In melanomas, decreased PTEN activity elevates PIP3 levels resulting in Akt activation. Active Akt then phosphorylates downstream cellular proteins that promote melanoma cell proliferation and survival. Recently, Akt3 was discovered to be the predominant isoform activated in sporadic melanomas. Levels of activity increased during melanoma progression with metastatic melanomas having the highest activity. Although mechanisms of Akt3 activation remain to be fully characterized, overexpression of Akt3 and decreased PTEN activity play important roles in this process. Targeted reduction of Akt3 activity decreased survival of melanoma tumor cells leading to inhibition of tumor development, which may be therapeutically effective for shrinking tumors in melanoma patients. This review surveys recent developments in Akt deregulation in melanoma and its potential as a selective therapeutic target in patients in the advanced stages of this disease.

PTEN expression in normal skin, acquired melanocytic nevi, and cutaneous melanoma

BACKGROUND

Although various studies have shown mutations of the tumor suppressor gene, PTEN/MMAC1, in primary, metastatic, and cultured cutaneous melanoma specimens, little is known about the pattern of PTEN protein expression in early melanocytic tumor progression.

OBJECTIVE

To further investigate the role of PTEN in melanocytic tumor development.

METHODS

We assessed the level and distribution of PTEN in normal skin, 39 acquired melanocytic nevi, and 30 primary cutaneous melanomas, including lentigo malignas, by immunostaining.

RESULTS

We found high levels of PTEN expression in cutaneous muscles, nerves, and muscular arteries, and moderate-to-high amounts of PTEN in the epidermis, follicular epithelium, and sebaceous and eccrine glands. PTEN staining in cutaneous lymphatics, dermal and periadnexal adventitial fibroblasts, and chondrocytes were variably absent. Junctional melanocytes and chondrocytes frequently exhibited preferential nuclear staining. We found uniformly strong PTEN expression in the cytoplasm of almost all benign and dysplastic nevi. However, there was some evidence of nuclear PTEN loss even in the benign melanocytic proliferations. In addition, out of 30 primary cutaneous melanomas and lentigo malignas, we detected diffuse expression of PTEN in 11 (37%) tumors, widespread loss of PTEN in 11 (37%) tumors and mixed PTEN expression in 8 (27%) lesions. In the primary cutaneous melanomas, PTEN was largely localized to the cytoplasm.

CONCLUSIONS

The presence of PTEN in benign melanocytic tumors and the absence of PTEN in a significant proportion of primary cutaneous melanomas support a role for PTEN loss in the pathogenesis of melanoma.

Thyroid pathology in four patients with Cowden's disease

Cowden's disease (multiple hamartoma syndrome) is a rare genodermatosis, which carries an increased risk of malignancy, especially breast and thyroid carcinoma. Thyroid disease is the most common internal manifestation of the syndrome, but the histological features in benign cases have hitherto been relatively poorly described. Thyroidectomy specimens from four patients with Cowden's disease have been reported in our laboratory in recent years. A number of rather distinctive features were common to all, raising the possibility of a distinctive 'Cowden's thyroid' phenotype.

PTEN signaling pathways in melanoma

Phosphatase and tensin homolog deleted in from chromosome ten (PTEN), initially also known as mutated in multiple advanced cancers or TGF-beta-regulated and epithelia cell-enriched phosphatase, is a tumor suppressor gene that is mutated in a large fraction of human melanomas. A broad variety of human cancers carry PTEN alterations, including glioblastomas, endometrial, breast, thyroid and prostate cancers. The PTEN protein has at least two biochemical functions: it has both lipid phosphatase and protein phosphatase activity. The lipid phosphatase activity of PTEN decreases intracellular PtdIns(3,4,5)P(3) level and downstream Akt activity. Cell-cycle progression is arrested at G1/S, mediated at least partially through the upregulation of the cyclin-dependent kinase inhibitor p27. In addition, agonist-induced apoptosis is mediated by PTEN, through the upregulation of proapoptotic machinery involving caspases and BID, and the downregulation of antiapoptotic proteins such as Bcl2. The protein phosphatase activity of PTEN is apparently less central to its involvement in tumorigenesis. It is involved in the inhibition of focal adhesion formation, cell spreading and migration, as well as the inhibition of growth factor-stimulated MAPK signaling. Therefore, the combined effects of the loss of PTEN lipid and protein phosphatase activity may result in aberrant cell growth and escape from apoptosis, as well as abnormal cell spreading and migration. In melanoma, PTEN loss has been mostly observed as a late event, although a dose-dependent loss of PTEN protein and function has been implicated in early stages of tumorigenesis as well. In addition, loss of PTEN and oncogenic activation of RAS seem to occur in a reciprocal fashion, both of which could cooperate with CDKN2A loss in contribution to melanoma tumorigenesis.

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.

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.

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.

Synchronous high-risk melanoma and lymphoid neoplasia

Large population-based studies have shown a significant association between melanoma and lymphoid neoplasia, particularly non-Hodgkin's lymphoma (NHL) and chronic lymphocytic leukaemia (CLL), that is independent of any treatment received for the initial tumour. This study examines the presentation, diagnosis, treatment and progress of three patients who developed advanced melanoma concurrently with a lymphoid neoplasm (one NHL, two CLLs), in order to illustrate their association, discuss common aetiological factors and examine possible therapeutic options. As it is the melanoma rather than the lymphoid neoplasm that represents the bigger threat to overall survival, initial treatment should be targeted towards this cancer. However, because of the interplay between the diseases and the possible side-effects of the various treatments, the choice of adjuvant therapy requires careful consideration. Immunosuppression associated with chemotherapy may permit a more aggressive course for the melanoma, while locoregional radiotherapy is contraindicated following lymph node dissections. As immunotherapy is of benefit in the treatment of melanoma and has also been recently shown to be effective in the management of lymphoid neoplasia, we instituted interferon-alpha as adjuvant therapy for these patients, thereby utilizing a single agent to treat the dual pathologies. The three patients have now been followed-up for 6 months without evidence of disease recurrence or progression.

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.

Epigenetic PTEN silencing in malignant melanomas without PTEN mutation

A tumor suppressor gene at 10q 23.3, designated PTEN, encoding a dual specificity phosphatase with lipid and protein phosphatase activity, has been shown to play an important role in the pathogenesis of a variety of human cancers. Germline mutations in PTEN cause Cowden syndrome (CS), which is characterized by multiple hamartomas and a high risk of breast and thyroid cancers. Frequent loss of heterozygosity at 10q is found in both early and advanced-stage sporadic melanomas; however, mutations or deletions in PTEN are detected mainly in melanoma cell lines. In this study, we examined PTEN expression in 34 unselected sporadic melanomas (4 primary melanomas, 30 metastases) using immunohistochemistry and correlated this with the results of structural studies of this gene. Immunostaining of 34 melanoma samples revealed no PTEN expression in 5 (15%) and low PTEN expression in 17 (50%), whereas the rest of the tumors (35%) had high levels of expression. Hemizygous deletion was found in 32% of the tumors but neither intragenic PTEN mutation nor biallelic deletion was found in any of the samples. Of the 5 melanomas showing no PTEN expression, 4 had no mutation or deletion of PTEN. Of the 13 tumors having weak PTEN immunoreactivity and informative loss of heterozygosity results, 6 had evidence of hemizygous allelic loss of PTEN while the remaining 7 had intact PTEN. These results strongly support PTEN as a major tumor suppressor on 10q involved in melanoma tumorigenesis and suggest an epigenetic mechanism of biallelic functional inactivation not previously observed in other cancers where PTEN might be involved.

Expression of PTEN in PTEN-deficient multiple myeloma cells abolishes tumor growth in vivo

Biochemical abnormalities associated with the development of multiple myeloma have been difficult to define especially in terms of demonstrating an in vivo effect of suspected lesions. Herein, we have identified such a defect associated with lack of expression of PTEN, a cellular phosphatase involved in the regulation of phosphatidylinositol phosphates (PIP's). In myeloma cells, PIP's are required for phosphorylation of Akt, a key event leading to inhibition of apoptosis. Loss of PTEN results in a failure to de-phosphorylate PIP's and a corresponding increase in Akt phosphorylation. OPM-2 cells lacking PTEN expression have the highest level of Akt phosphorylation of eight lines examined. Loss of PTEN was found to be associated with a 630 bp deletion corresponding to amino acids 56 - 267. Ectopic expression of wild type PTEN in OPM-2 cells inhibited Akt phosphorylation which was correlated with an increase in apoptosis. The in vivo relevance of loss of PTEN expression was demonstrated by injecting control and wild type PTEN transfected OPM-2 cells into SCID mice. Tumors arose at an incidence of 100% in controls, but only 50% (and of smaller size and longer latency) in low PTEN expressing clones. Importantly, clones expressing high levels of PTEN failed to produce tumors even at five times the latency period of controls. These results demonstrate that PTEN deletion/mutation is responsible for in vivo growth of this tumor and suggests that PTEN regulation may play an important role in tumor development in a subset of multiple myeloma patients. Oncogene (2000) 19, 4091 - 4095

Mutation and allelic loss of the PTEN/MMAC1 gene in primary and metastatic melanoma biopsies

The PTEN/MMAC1 gene on chromosome 10q23 encodes a lipid phosphatase with tumor-suppressive properties. Germline PTEN/MMAC1 mutations have been implicated as the predisposing factor in Cowden disease and other hamartoma syndromes, and somatic mutations and deletions have been identified in a wide range of human cancers, including 30-40% of metastatic melanoma cell lines. To study further the possible role of PTEN/MMAC1 in the pathogenesis and progression of malignant melanoma, we examined uncultured specimens from 16 primary and 61 metastatic tumors from 67 patients. Denaturing gradient gel electrophoresis was used to analyze systematically the coding region of PTEN/MMAC1 and revealed mutations in four of the metastatic samples (7%). Sequence analysis of the mutants identified a 1 bp frameshift insertion, a 2 bp frameshift deletion, an 11 bp frameshift deletion, and a single base substitution resulting in the generation of a premature stop codon. Analysis of two intragenic polymorphisms showed allelic loss in three of eight informative primary tumors (38%) and in 18 of 31 metastatic tumors (58%). One of the mutant cases showed allelic loss, suggesting that both PTEN/MMAC1 alleles were inactivated in this tumor. Altogether, these results suggest that mutation and deletion of PTEN/MMAC1 may contribute to the development and progression of malignant melanoma.