Biallelic inactivating mutations and an occult germline mutation ofPTEN in primary cervical carcinomas

FISH analysis of PTEN in endometrial carcinoma. Comparison with SNP arrays and MLPA

AIMS

To check the usefulness of a standardized protocol of PTEN FISH in 31 endometrial carcinomas (ECs) in comparison with SNP array (SNPA), multiplex ligation-dependent probe amplification (MLPA), and immunohistochemistry.

METHODS AND RESULTS

Fluorescence in-situ hybridization analysis showed two PTEN copies in 17 cases, three copies in nine cases, hemizygous deletion in two cases, and diverse cell populations with different PTEN copy number in three cases. A good correlation was seen between FISH and SNPA, particularly in cases with three copies. FISH identified two cases with entire deletion of chromosome 10, but did not identify a focal deletion of PTEN. Five cases with PTEN deletion and duplication of the second allele by SNPA were interpreted as normal by FISH. Concordance between FISH and MLPA was seen in 15 cases with two copies, and in two cases with PTEN deletion. Six cases were interpreted as amplified by MLPA, but showed polyploidy by FISH. FISH was superior to SNPA and MLPA in assessing the tumours with diverse cell populations with different PTEN copies.

CONCLUSIONS

The results show good concordance between FISH, SNPA and MLPA. SNPA was superior in tumours with deletion of one copy and duplication of the second allele. FISH was superior in assessing tumour heterogeneity.

The regulatory roles of phosphatases in cancer

The relevance of potentially reversible post-translational modifications required for controlling cellular processes in cancer is one of the most thriving arenas of cellular and molecular biology. Any alteration in the balanced equilibrium between kinases and phosphatases may result in development and progression of various diseases, including different types of cancer, though phosphatases are relatively under-studied. Loss of phosphatases such as PTEN (phosphatase and tensin homologue deleted on chromosome 10), a known tumour suppressor, across tumour types lends credence to the development of phosphatidylinositol 3-kinase inhibitors alongside the use of phosphatase expression as a biomarker, though phase 3 trial data are lacking. In this review, we give an updated report on phosphatase dysregulation linked to organ-specific malignancies.

Stem cell quiescence acts as a tumour suppressor in squamous tumours

In some organs, adult stem cells are uniquely poised to serve as cancer cells of origin. It is unclear, however, whether tumorigenesis is influenced by the activation state of the adult stem cell. Hair follicle stem cells (HFSCs) act as cancer cells of origin for cutaneous squamous cell carcinoma (SCC) and undergo defined cycles of quiescence and activation. The data presented here show that HFSCs are unable to initiate tumors during the quiescent phase of the hair cycle, indicating that the mechanisms that keep HFSCs dormant are dominant to the gain of oncogenes (Ras) or the loss of tumor suppressors (p53). Furthermore, Pten activity is necessary for quiescence based tumor suppression, as its deletion alleviates tumor suppression without affecting proliferation. These data demonstrate that stem cell quiescence is a form of tumor suppression in HFSCs, and that Pten plays a role in maintaining quiescence in the presence of tumorigenic stimuli.

Clinicopathologic and molecular analysis of a choroidal pigmented schwannoma in the context of a PTEN hamartoma tumor syndrome

PURPOSE

To report the first case of choroidal schwannoma in a patient affected by PTEN hamartoma tumor syndrome (PHTS) and investigate the molecular involvement of the phosphatase and tensin homolog (PTEN) and neurofibromin 2 (NF2) genes in this rare intraocular tumor.

DESIGN

Observational case report.

PARTICIPANT

A 10-year-old girl diagnosed with PHTS.

METHODS

The enucleated specimen underwent histologic, immunohistochemical, and transmission electronic microscopy. The expression of PTEN and NF2 and their protein products were evaluated by reverse transcription-polymerase chain reaction and immunohistochemistry. Somatic mutations of PTEN and NF2, as well as allelic loss, were investigated by direct sequencing of DNA extracted from the tumor. PTEN epigenetic silencing was investigated by pyrosequencing.

MAIN OUTCOME MEASURES

Histopathologic and molecular characterization of a choroidal pigmented schwannoma.

RESULTS

Histopathologic, immunohistochemical, and electron microscopic analysis demonstrated features consistent with a pigmented cellular schwannoma of the choroid. We found no loss of heterozygosity at the genomic level for the PTEN germline mutation and no promoter hypermethylation or other somatic intragenic mutations. However, we observed an approximate 40% reduction of PTEN expression at both the mRNA and the protein level, indicating that the tumor was nonetheless functionally deficient for PTEN. Although DNA sequencing of NF2 failed to identify any pathologic variants, its expression was abolished within the tumor.

CONCLUSIONS

We report the first description of a pigmented choroidal schwannoma in the context of a PHTS. This rare tumor showed a unique combination of reduction of PTEN and absence of NF2 expression.

Allelic loss of 10q23.3, the PTEN gene locus in cervical carcinoma from Northern Indian population

Cervical cancer is one of the most common malignant diseases affecting women worldwide. Studies on loss of heterozygosity have been made for PTEN gene specific microsatellite markers in malignancies like breast, ovary and lungs and the results have shown a significant association. However the role of this gene is not clearly understood in cervical cancer from Indian population. A total of 135 cervical carcinoma tissues samples were analyzed for loss of heterozygosity. DNA was isolated from the samples and their matched control specimens. Polymerase chain reaction was performed using primer specific for two intragenic markers (D10S198 & D10S192) and one marker (D10S541) in flanking region and further electrophoresed on 8% denaturing polyacrylamide gel. Overall, 31 out of 133(23%) informative cases showed loss of heterozygosity in at least one locus in the region examined. The percentage of loss of heterozygosity for these markers ranged from 8% (D10S192) to 13% (D10S198). Loss of heterozygosity was more frequently detected in intragenic region (D10S198 & D10S192) than in flanking region, D10S541 (21% versus 9%). These data argue that PTEN is a tumor suppressor gene whose inactivation may play an important role in the carcinoma of uterine cervix.

Shank-interacting protein-like 1 promotes tumorigenesis via PTEN inhibition in human tumor cells

Inactivation of phosphatase and tensin homolog (PTEN) is a critical step during tumorigenesis, and PTEN inactivation by genetic and epigenetic means has been well studied. There is also evidence suggesting that PTEN negative regulators (PTEN-NRs) have a role in PTEN inactivation during tumorigenesis, but their identity has remained elusive. Here we have identified shank-interacting protein-like 1 (SIPL1) as a PTEN-NR in human tumor cell lines and human primary cervical cancer cells. Ectopic SIPL1 expression protected human U87 glioma cells from PTEN-mediated growth inhibition and promoted the formation of HeLa cell-derived xenograft tumors in immunocompromised mice. Conversely, siRNA-mediated knockdown of SIPL1 expression inhibited the growth of both HeLa cells and DU145 human prostate carcinoma cells in vitro and in vivo in a xenograft tumor model. These inhibitions were reversed by concomitant knockdown of PTEN, demonstrating that SIPL1 affects tumorigenesis via inhibition of PTEN function. Mechanistically, SIPL1 was found to interact with PTEN through its ubiquitin-like domain (UBL), inhibiting the phosphatidylinositol 3,4,5-trisphosphate (PIP3) phosphatase activity of PTEN. Furthermore, SIPL1 expression correlated with loss of PTEN function in PTEN-positive human primary cervical cancer tissue. Taken together, these observations indicate that SIPL1 is a PTEN-NR and that it facilitates tumorigenesis, at least in part, through its PTEN inhibitory function.

E6 variants of human papillomavirus 18 differentially modulate the protein kinase B/phosphatidylinositol 3-kinase (akt/PI3K) signaling pathway

Intra-type genome variations of high risk Human papillomavirus (HPV) have been associated with a differential threat for cervical cancer development. In this work, the effect of HPV18 E6 isolates in Akt/PKB and Mitogen-associated protein kinase (MAPKs) signaling pathways and its implication in cell proliferation were analyzed. E6 from HPV types 16 and 18 are able to bind and promote degradation of Human disc large (hDlg). Our results show that E6 variants differentially modulate hDlg degradation, rebounding in levels of activated PTEN and PKB. HPV18 E6 variants are also able to upregulate phospho-PI3K protein, strongly correlating with activated MAPKs and cell proliferation. Data was supported by the effect of E6 silencing in HPV18-containing HeLa cells, as well as hDlg silencing in the tested cells. Results suggest that HPV18 intra-type variations may derive in differential abilities to activate cell-signaling pathways such as Akt/PKB and MAPKs, directly involved in cell survival and proliferation.

Expression of PTEN in the progression of cervical neoplasia and its relation to tumor behavior and angiogenesis in invasive squamous cell carcinoma

BACKGROUND AND OBJECTIVES

Loss of PTEN expression has been associated with tumor progression and adverse patient outcome. The purpose of this study was to evaluate PTEN expression in the successive steps of progression in cervical neoplasia and to determine its correlation with tumor angiogenesis and clinicopathologic features in squamous cell carcinoma of the uterine cervix.

METHODS

Immunohistochemical staining with anti-PTEN antibody was performed in a total of 160 patients with 12 normal cervical epithelium, 63 cervical intraepithelial neoplasia (33 CIN I, 30 CIN III), and 85 cervical squamous cell carcinomas. Microvessels were immunohistochemically labeled with an antibody for CD34. Computerized image analysis was used to evaluate microvessel density (MVD).

RESULTS

Reduced PTEN expression progressively increased along the continuum from normal epithelium to squamous cell carcinoma (P < 0.01). There was no significant correlation between PTEN expression and MVD. On univariate analysis, stage and reduced PTEN expression were significant prognostic factors for both disease-free and overall survival. However, stage was the only independent prognostic factor by multivariate analysis.

CONCLUSIONS

Our results suggest that tumor progression in the cervical epithelium is accompanied by loss of PTEN protein expression. Reduced PTEN expression is not associated with tumor angiogenesis of squamous cell carcinoma of the uterine cervix.

Control of phosphatase and tensin homolog (PTEN) gene expression in normal and neoplastic thyroid cells

The lipid phosphatase, phosphatase and tensin homolog (PTEN), is a key element in controlling cell growth and survival and has a well established role as tumor suppressor protein in many neoplasia. Several data indicate that silencing of PTEN gene expression may be relevant in follicular thyroid cell transformation. Thus, in the present study regulation of PTEN gene expression in thyroid cells was investigated. Cotransfection experiments indicated that in normal FRTL-5 rat thyroid cells, PTEN promoter activity was increased by overexpression of the transcription factor early growth response protein-1 (Egr-1). Moreover, Western blot experiments indicated that when Egr-1 expression was up-regulated by treating FRTL-5 cells with H2O2, an increase in PTEN expression was also observed. TSH induced opposite modifications on PTEN and Egr-1 protein levels. Moreover, acute or chronic TSH stimulation determined distinct effects. In fact, acute TSH stimulation (30 and 60 min) induced a decrease in PTEN, but an increase in Egr-1 protein levels. These effects were cAMP dependent; in fact, they were mimicked by forskolin. A chronic TSH treatment (5 d) stimulated PTEN protein expression, whereas Egr-1 protein was down-regulated. In contrast to normal thyroid cells, when the thyroid tumor cell lines ARO and BCPAP were exposed to H2O2, neither Egr-1 nor PTEN protein levels were increased. Acute stimulation of ARO and BCPAP cells with forskolin increased Egr-1, but not PTEN, protein levels. Therefore, thyroid tumor cell lines show alteration of PTEN gene expression regulation. RT-PCR experiments performed on human thyroid tumors showed that the absence of Egr-1 mRNA is always paralleled by the absence of PTEN mRNA. Thus, modification of the Egr-1-dependent mechanisms may play a role in the silencing of PTEN gene expression occurring during thyroid cell transformation.

Frequent inactivation of PTEN by promoter hypermethylation in microsatellite instability-high sporadic colorectal cancers

Loss of PTEN tumor suppressor function is observed in tumors of breast, prostate, thyroid, and endometrial origin. Allelic losses in the proximity of the PTEN locus (10q23) also occur in sporadic colorectal cancers (CRCs), but biallelic inactivation of this site has not been frequently demonstrated. We hypothesized that alternative mechanisms of PTEN allelic inactivation, such as promoter hypermethylation, might be operative in CRC and that PTEN inactivation may be related to recognized forms of genomic instability. We characterized a cohort of 273 sporadic CRCs by determining their microsatellite instability (MSI) status. Of these, 146 cancers were examined for PTEN promoter methylation by methylation-specific PCR. Mutations at the poly(A)6 repeat sequences in PTEN exons 7 and 8 and deletions at the 10q23 locus were also identified using microsatellite analysis. The presence of PTEN protein was determined by immunostaining, and the results were correlated with the promoter methylation status. We observed that PTEN promoter hypermethylation was a frequent occurrence in MSI-high (MSI-H) tumors (19.1% of MSI-H versus 2.2% of MSI-low/microsatellite stable tumors; P = 0.002). A PTEN mutation or a deletion event was present in 60% of the tumors with promoter region hypermethylation. Hypermethylation of the PTEN promoter correlated significantly with either decreased or complete loss of PTEN protein expression (P = 0.004). This is the first demonstration of PTEN inactivation as a result of promoter hypermethylation in MSI-H sporadic CRCs. These data suggest that this silencing mechanism plays a major role in PTEN inactivation and, in colon cancer, may be more important than either allelic losses or inactivating mutations. The significant correlation of PTEN hypermethylation with MSI-H tumors further suggests that PTEN is an additional important "target" of methylation along with the hMLH1 gene in the evolution of MSI-H CRCs and also confers the "second hit" in the biallelic inactivation mechanism for some proportion of tumors.

PTEN mutations are common in sporadic microsatellite stable colorectal cancer

The tumour suppressor gene PTEN, located at chromosome sub-band 10q23.3, encodes a dual-specificity phosphatase that negatively regulates the phosphatidylinositol 3'-kinase (PI3 K)/Akt-dependent cellular survival pathway. PTEN is frequently inactivated in many tumour types including glioblastoma, prostate and endometrial cancers. While initial studies reported that PTEN gene mutations were rare in colorectal cancer, more recent reports have shown an approximate 18% incidence of somatic PTEN mutations in colorectal tumours exhibiting microsatellite instability (MSI+). To verify the role of this gene in colorectal tumorigenesis, we analysed paired normal and tumour DNA from 41 unselected primary sporadic colorectal cancers for PTEN inactivation by mutation and/or allelic loss. We now report PTEN gene mutations in 19.5% (8/41) of tumours and allele loss, including all or part of the PTEN gene, in a further 17% (7/41) of the cases. Both PTEN alleles were affected in over half (9/15) of these cases showing PTEN genetic abnormalities. Using immunohistochemistry, we have further shown that all tumours harbouring PTEN alterations have either reduced or absent PTEN expression and this correlated strongly with later clinical stage of tumour at presentation (P=0.02). In contrast to previous reports, all but one of the tumours with PTEN gene mutations were microsatellite stable (MSI-), suggesting that PTEN is involved in a distinct pathway of colorectal tumorigenesis that is separate from the pathway of mismatch repair deficiency. This work therefore establishes the importance of PTEN in primary sporadic colorectal cancer.

PTEN: one gene, many syndromes

PTEN, on 10q23.3, encodes a major lipid phosphatase which signals down the phosphoinositol-3-kinase/Akt pathway and effects G1 cell cycle arrest and apoptosis. Germline PTEN mutations have been found to occur in 80% of classic Cowden syndrome (CS), 60% of Bannayan-Riley-Ruvalcaba syndrome (BRRS), up to 20% of Proteus syndrome (PS), and approximately 50% of a Proteus-like syndrome (PSL). CS is a heritable multiple hamartoma syndrome with a high risk of breast, thyroid, and endometrial carcinomas. BRRS is a congenital autosomal dominant disorder characterized by megencephaly, developmental delay, lipomatosis, and speckled penis. PS and PSL had never been associated with risk of malignancy. Finding germline PTEN mutations in patients with BRRS, PS, and PSL suggests equivalent risks of developing malignancy as in CS with implications for medical management. The mutational spectra of CS and BRRS overlap, with many of the mutations occurring in exons 5, 7, and 8. Genotype-phenotype association analyses have revealed that the presence of germline PTEN mutations is associated with breast tumor development, and that mutations occurring within and 5' of the phosphatase motif were associated with multi-organ involvement. Pooled analysis of PTEN mutation series of CS and BRRS occurring in the last five years reveals that 65% of CS-associated mutations occur in the first five exons encoding the phosphatase domain and the promoter region, while 60% of BRRS-associated mutations occur in the 3' four exons encoding mainly the C2 domain. Somatic PTEN mutations occur with a wide distribution of frequencies in sporadic primary tumors, with the highest frequencies in endometrial carcinomas and glioblastoma multiform. Several mechanisms of PTEN inactivation occur in primary malignancies derived from different tissues, but a favored mechanism appears to occur in a tissue-specific manner. Inappropriate subcellular compartmentalization and increased/decreased proteosome degradation may be two novel mechanisms of PTEN inactivation. Further functional work could reveal more effective means of molecular-directed therapy and prevention.

Cowden's disease: clinical and molecular genetic findings in a patient with a novel PTEN germline mutation

We report a 54-year-old woman with Cowden's disease (CD) who was found to carry a novel germline mutation in the PTEN gene. The mutation (c.334C-->G) introduced a splice donor site within exon 5 that caused the expression of an aberrant transcript lacking 159 nucleotides corresponding to codons 112-164. Clinically, the patient showed multiple benign hamartomatous lesions of the skin, papillomatosis of the lips and oral mucosa, polyposis coli and bilateral fibrocystic disease of the breast. In addition, she developed different types of malignant neoplasms, including bilateral carcinomas of the breast and malignant melanomas of the skin. Molecular genetic analysis of a benign skin hamartoma and an invasive ductal breast carcinoma revealed loss of heterozygosity (LOH) at microsatellite markers on chromosome 10 in the carcinoma but not in the hamartoma. The breast carcinoma additionally carried a somatic TP53 point mutation (c.466C-->G; R156G) that was associated with LOH on 17p and nuclear p53 protein accumulation. Taken together, our findings indicate that benign hamartomas in CD may develop without loss of the second (wild-type) PTEN allele, whereas the pathogenesis of malignant tumours, such as breast carcinomas, appears to require the complete inactivation of Pten as well as further alterations such as the loss of p53-dependent growth control.

PTEN mutational spectra, expression levels, and subcellular localization in microsatellite stable and unstable colorectal cancers

PTEN on 10q23.3 encodes a dual-specificity phosphatase that negatively regulates the phosphoinositol-3-kinase/Akt pathway and mediates cell-cycle arrest and apoptosis. Germline PTEN mutations cause Cowden syndrome and a range of several different hamartoma-tumor syndromes. Hereditary nonpolyposis colon cancer (HNPCC) syndrome is characterized by germline mutations in the mismatch repair (MMR) genes and by microsatellite instability (MSI) in component tumors. Although both colorectal carcinoma and endometrial carcinoma are the most frequent component cancers in HNPCC, only endometrial cancer has been shown to be a minor component of Cowden syndrome. We have demonstrated that somatic inactivation of PTEN is involved in both sporadic endometrial cancers and HNPCC-related endometrial cancers but with different mutational spectra and different relationships to MSI. In the current study, we sought to determine the relationship of PTEN mutation, 10q23 loss of heterozygosity, PTEN expression, and MSI status in colorectal cancers (CRCs). Among 11 HNPCC CRCs, 32 MSI+ sporadic cancers, and 39 MSI- tumors, loss of heterozygosity at 10q23.3 was found in 0%, 8%, and 19%, respectively. Somatic mutations were found in 18% (2 of 11) of the HNPCC CRCs and 13% (4 of 32) of the MSI+ sporadic tumors, but not in MSI- cancers (P = 0.015). All somatic mutations occurred in the two 6(A) coding mononucleotide tracts in PTEN, suggestive of the etiological role of the deficient MMR. Immunohistochemical analysis revealed 31% (14 of 45) of the HNPCC CRCs and 41% (9 of 22) of the MSI+ sporadic tumors with absent or depressed PTEN expression. Approximately 17% (4 of 23) of the MSI- CRCs had decreased PTEN expression, and no MSI- tumor had complete loss of PTEN expression. Among the five HNPCC or MSI+ sporadic CRCs carrying frameshift somatic mutations with immunohistochemistry data, three had lost all PTEN expression, one showed weak PTEN expression levels, and one had mixed tumor cell populations with weak and moderate expression levels. These results suggest that PTEN frameshift mutations in HNPCC and sporadic MSI+ tumors are a consequence of mismatch repair deficiency. Further, hemizygous deletions in MSI- CRCs lead to loss or reduction of PTEN protein levels and contribute to tumor progression. Finally, our data also suggest that epigenetic inactivation of PTEN, including differential subcellular compartmentalization, occurs in CRCs.

Frequent loss of PTEN expression is linked to elevated phosphorylated Akt levels, but not associated with p27 and cyclin D1 expression, in primary epithelial ovarian carcinomas

PTEN (MMAC1/TEP1), a tumor suppressor gene on chromosome subband 10q23.3, is variably mutated and/or deleted in a variety of human cancers. Germline mutations in PTEN, which encode a dual-specificity phosphatase, have been implicated in at least two hamartoma tumor syndromes that exhibit some clinical overlap, Cowden syndrome and Bannayan-Riley-Ruvalcaba syndrome. Among several series of ovarian cancers, the frequency of loss of heterozygosity (LOH) of markers flanking and within PTEN, is approximately 30 to 50%, and the somatic intragenic PTEN mutation frequency is <10%. In this study, we screened primary adenocarcinomas of the ovary for LOH of polymorphic markers within and flanking the PTEN gene and for intragenic mutations of the PTEN gene and compared them to PTEN expression using immunohistochemistry. Furthermore, we sought to detect the expression of the presumed downstream targets of PTEN, such as P-Akt, p27, and cyclin D1 by immunohistochemistry. LOH at 10q23 was observed in 29 of 64 (45%) cases. Of the 117 samples, 6 somatic intragenic PTEN mutations, 1 germline mutation, and 1 novel polymorphism were found in 7 (6%) patients. Immunostaining of 49 ovarian cancer samples revealed that 13 (27%) were PTEN immunostain-negative, 25 (51%) had reduced staining, and the rest (22%) were PTEN expression-positive. Among the 44 informative tumors assessed for 10q23 LOH and PTEN immunostaining, there was an association between 10q23 LOH and decreased or absent staining (P = 0.0317). Of note, there were five (11%) tumors with neither mutation nor deletion that exhibited no PTEN expression and 10 (25%) others without mutation or deletion but had decreased PTEN expression. Among the 49 tumors available for immunohistochemistry, 28 (57%) showed P-Akt-positive staining, 24 (49%) had decreased p27 staining, and cyclin D1 was overexpressed in 35 (79%) cases. In general, P-Akt expression was inversely correlated with PTEN expression (P = 0.0083). These data suggest that disruption of PTEN by several mechanisms, allelic loss, intragenic mutation, or epigenetic silencing, all contribute to epithelial ovarian carcinogenesis, and that epigenetic silencing is a significant mechanism. The Akt pathway is prominently involved, but clearly not in all cases. Surprisingly, despite in vitro demonstration that p27 and cyclin D1 lies downstream of PTEN and Akt, there was no correlation between p27 and cyclin D1 expression and PTEN or P-Akt status. Thus, in vivo, although PTEN and Akt play a prominent role in ovarian carcinogenesis, p27 and cyclin D1 might not be the primary downstream targets. Alternatively, these observations could also suggest that pathways involving other than Akt, p27 and cyclin D1 that lie downstream of PTEN play roles in ovarian carcinogenesis.