Germline mutations of the PTEN gene in Cowden disease, an inherited breast and thyroid cancer syndrome

Identification of PTEN mutations in five families with Bannayan-Zonana syndrome

Germline mutations in PTEN, a putative tumor suppressor gene, has been identified in 2 autosomal dominant inherited hamartoma syndromes, Cowden syndrome (CS) and Bannayan-Zonana syndrome (BZS). While both diseases exhibit distinct phenotypic features, there seems to be a partial clinical overlap between the 2 diseases. To date, 9 families with BZS have been screened for PTEN mutations, of which 5 were found to exhibit mutations in this gene. We report 5 novel germline mutations in the PTEN coding sequence from 5 unrelated families with the BZS phenotype. While all the mutations we identified are novel in BZS, 1003C-->T (nonsense mutation) and 209+5G-->A (putative splice site mutation) have been previously reported in unrelated families with CS and Lhermitte Duclos disease. Interestingly, 1 of the families has an individual with BZS and 1 with CS phenotype, associated with a single PTEN mutation, 885insA. These data support the notion that CS and BZS may be within the spectrum of the same primary disorder.

PTEN/MMAC1 mutations in hepatocellular carcinomas: somatic inactivation of both alleles in tumors

Allelic loss of loci on chromosome 10q occurs frequently in hepatocellular carcinomas. Somatic mutations of the PTEN/MMAC1 gene on this chromosome at 10q23 were recently identified in sporadic cancers of the uterus, brain, prostate and breast. To investigate the potential role of PTEN/MMAC1 gene in the genesis of hepatocellular carcinomas, we examined 96 tumors for allelic loss on 10q and also for subtle mutations anywhere within the coding region of PTEN/MMAC1 gene. Allelic loss was identified in 25 of the 89 (27%) tumors that were informative for polymorphic markers in the region. Somatic mutations were identified in five of those tumors: three frameshift mutations, a 1-bp insertion at codon 83-84 in exon 4 and two 4-bp deletions, both at codon 318-319 in exon 8; two C-to-G transversion mutation, both at -9 bp from the initiation codon in the 5' non-coding region of exon 1. No missense mutation was observed in this panel of tumors. In most of the informative tumors carrying intragenic mutations of one allele, we were able to detect loss of heterozygosity as well. These findings suggest that two alleles of the PTEN/MMAC1 gene may be inactivated by a combination of intragenic point mutation on one allele and loss of chromosomal material on the other allele in some of these tumors.

Regulation of the insulin-like developmental pathway of Caenorhabditis elegans by a homolog of the PTEN tumor suppressor gene

The human PTEN tumor suppressor gene is mutated in a wide variety of sporadic tumors. To determine the function of PTEN in vivo we have studied a PTEN homolog in Caenorhabditis elegans. We have generated a strong loss-of-function allele of the PTEN homolog and shown that the deficient strain is unable to enter dauer diapause. An insulin-like phosphatidylinositol 3-OH kinase (PI3'K) signaling pathway regulates dauer-stage entry. Mutations in either the daf-2 insulin receptor-like (IRL) gene or the age-1 encoded PI3'K catalytic subunit homolog cause constitutive dauer formation and also affect the life span, brood size, and metabolism of nondauer animals. Strikingly, loss-of-function mutations in the age-1 PI3'K and daf-2 IRL genes are suppressed by loss-of-function mutations in the PTEN homolog. We establish that the PTEN homolog is encoded by daf-18, a previously uncloned gene that has been shown to interact genetically with the DAF-2 IRL AGE-1 PI3'K signaling pathway. This interaction provides clear genetic evidence that PTEN acts to antagonize PI3'K function in vivo. Given the conservation of the PI3'K signaling pathway between C. elegans and mammals, the analysis of daf-18 PTEN mutant nematodes should shed light on the role of human PTEN in the etiology of metabolic disease, aging, and cancer.

Multiple inositol polyphosphate phosphatase: evolution as a distinct group within the histidine phosphatase family and chromosomal localization of the human and mouse genes to chromosomes 10q23 and 19

Multiple inositol polyphosphate phosphatase is the only enzyme known to hydrolyze the abundant metabolites inositol pentakisphosphate and inositol hexakisphosphate. We have previously demonstrated that the chick homolog of multiple inositol polyphosphate phosphatase, designated HiPER1, has a role in growth plate chondrocyte differentiation. The relationship of these enzymes to intracellular signaling is obscure, and as part of our investigation we have examined the murine ((MMU)Minpp1) and human ((HSA)MINPP1) homologs. Northern blot analysis demonstrated expression of ((MMU)Minpp1 in a variety of mouse tissues, comparable to the expression of other mammalian homologs, but less restricted than the expression of HiPER1 in chick. A purified (MMU)Minpp1 fusion protein cleaved phosphate from inositol (1,3,4,5)-tetrakisphosphate and para-nitrophenyl phosphate. When the presumptive active site histidine was altered to alanine by site-directed mutagenesis, enzyme activity was abolished, confirming the classification of (MMU)Minpp1 as a histidine phosphatase. The amino acid sequences of the murine and human MINPP proteins share >80% identity with the rat enzyme and >56% identity with HiPER1, with conservation of the C-terminal consensus sequence that retains proteins in the endoplasmic reticulum. The intron/exon structure of the mammalian (MMU)Minpp1 and (HSA)MINPP1 genes is also conserved compared to the chick HiPER1 gene. Sequence analysis of plant and fruit fly MINPP homologs supports the hypothesis that the MINPP enzymes constitute a distinct evolutionary group within the histidine phosphatase family. We have mapped (HSA)MINPP1 to human chromosome 10q23 by fluorescence in situ hybridization, YAC screening, and radiation hybrid mapping. This assignment places (HSA)MINPP1 in a region of chromosome 10 that is frequently mutated in human cancers and places (HSA)MINPP1 proximal to the tumor suppressor PTEN, which maps to 10q23.3. Using a radiation hybrid panel, we localized (MMU)Minpp1 to a region of mouse chromosome 19 that includes the murine homolog of Pten. The evolutionary conservation of this novel enzyme within the inositol polyphosphate pathway suggests a significant role for multiple inositol polyphosphate phosphatase throughout higher eukaryotes.

Long-chain L 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency does not appear to be the primary cause of lipid myopathy in patients with Bannayan-Riley-Ruvalcaba syndrome (BRRS)

In order to test the hypothesis that long-chain L 3-hydroxyacyl-coenzyme A dehydrogenase (LCHAD) deficiency is associated with the lipid myopathy and muscle carnitine deficiency observed in Bannayan-Riley-Ruvalcaba syndrome (BRRS), we studied the enzyme activity in cultured skin fibroblasts from three generations of a family with a clear dominant inheritance of BRRS. Enzyme activities were normal while the germline PTEN missense mutation P246L segregated with BRRS in this family. No PTEN mutations were identified in the original patient with BRRS and LCHAD deficiency. These data suggest that the previously reported case of LCHAD and BRRS either represents the coincidental concurrence of two rare genetic events or that a gene other than PTEN is related to LCHAD and BRRS.

Transcriptional analysis of the PTEN/MMAC1 pseudogene, psiPTEN

PTEN/MMAC1 is a recently characterized tumor suppressor. A pseudogene derived from the human PTEN/MMAC1 phosphatase, psiPTEN, has been reported. Recent analysis of the pseudogene revealed conflicting results about the expression of psiPTEN. In this study, we show that the PTEN/MMAC1 pseudogene is actively transcribed in all cells and tissues examined. In some cases, pseudogene transcripts were found to represent as much as 70% of the total PTEN/MMAC1 RNA. As psiPTEN is transcribed, there is a potential for misinterpretation of PTEN/MMAC1 mutations when RT-PCR techniques are used, as well as potential for a psiPTEN-encoded translation product. Although we were unable to detect a pseudogene protein product in the cell lines examined, a baculovirus produced GST pseudogene fusion protein exhibited phosphatase activity comparable to wild type. The results of this study, taken together, indicate the potential complication of PTEN/MMAC1 molecular analysis caused by the expression of psiPTEN.

Somatic mutation of the Peutz-Jeghers syndrome gene, LKB1/STK11, in malignant melanoma

Mutations in LKB1/STK11, a gene mapping to chromosome 19p13.3 and encoding a widely expressed serine/threonine kinase, were recently identified as the cause of Peutz-Jeghers syndrome. Despite the hamartomatous polyps and increased cancer risk associated with this syndrome, somatic alterations in LKB1/STK11 have not been identified in human tumours. Prompted by another feature of the syndrome, lentigines of the lips and oral mucosa, we evaluated the status of LKB1/STK11 expression, deletion, and mutation in cell lines and tumour samples from 35 patients with sporadic malignant melanoma. Two somatic mutations were identified, a nonsense mutation (Glu170Stop) causing exon skipping and intron retention, and a missense mutation (Asp194Tyr) affecting an invariant residue in the catalytic subunit of LKB1/STK11. Our data suggest that LKB1/STK11 may contribute to tumorigenesis in a small fraction of malignant melanomas.

Regulation of G1 progression by the PTEN tumor suppressor protein is linked to inhibition of the phosphatidylinositol 3-kinase/Akt pathway

PTEN/MMAC1 is a tumor suppressor gene located on chromosome 10q23. Inherited PTEN/MMAC1 mutations are associated with a cancer predisposition syndrome known as Cowden's disease. Somatic mutation of PTEN has been found in a number of malignancies, including glioblastoma, melanoma, and carcinoma of the prostate and endometrium. The protein product (PTEN) encodes a dual-specificity protein phosphatase and in addition can dephosphorylate certain lipid substrates. Herein, we show that PTEN protein induces a G1 block when reconstituted in PTEN-null cells. A PTEN mutant associated with Cowden's disease (PTEN;G129E) has protein phosphatase activity yet is defective in dephosphorylating inositol 1,3,4,5-tetrakisphosphate in vitro and fails to arrest cells in G1. These data suggest a link between induction of a cell-cycle block by PTEN and its ability to dephosphorylate, in vivo, phosphatidylinositol 3,4,5-trisphosphate. In keeping with this notion, PTEN can inhibit the phosphatidylinositol 3,4, 5-trisphosphate-dependent Akt kinase, a downstream target of phosphatidylinositol 3-kinase, and constitutively active, but not wild-type, Akt overrides a PTEN G1 arrest. Finally, tumor cells lacking PTEN contain high levels of activated Akt, suggesting that PTEN is necessary for the appropriate regulation of the phosphatidylinositol 3-kinase/Akt pathway.

Mutation in the PTEN/MMAC1 gene in archival low grade and high grade gliomas

The PTEN gene, located on 10q23.3, has recently been described as a candidate tumour suppressor gene that may be important in the development of advanced cancers, including gliomas. We have investigated mutation in the PTEN gene by direct sequence analysis of PCR products amplified from samples microdissected from 19 low grade (WHO Grade I and II) and 27 high grade (WHO grade III and IV) archival, formalin-fixed, paraffin-embedded gliomas. Eleven genetic variants in ten tumours have been identified. Eight of these are DNA sequence changes that could affect the encoded protein and were present in 0/2 pilocytic astrocytomas, 0/2 oligoastrocytomas, 0/1 oligodendroglioma, 0/14 astrocytomas, 3/13 (23%) anaplastic astrocytomas and 5/14 (36%) glioblastomas. PTEN mutations were found exclusively in high grade gliomas; this finding was statistically significant. Only two of the PTEN genetic variants have been reported in other studies; two of the genetic changes are in codons in which mutations have not been found previously. The results of this study indicate that mutation in the PTEN gene is present only in histologically more aggressive gliomas, may be associated with the transition from low histological grade to anaplasia, but is absent from the majority of high grade gliomas.

Genetic pathways in colorectal and other cancers

Cells from cancers show aberrant behaviour such as unrestrained growth, invasion into adjacent tissue and metastasis. All these features of cancer cell behaviour can be explained in terms of genetic changes and the functional impact of these changes. In this review, colorectal cancer (CRC) is examined as a classical example of multistep carcinogenesis. First there is an overview which shows that cancers develop by a process of somatic evolution. This gives rise to preferred genetic pathways of tumorigenesis. The factors which may influence the development and ultimate choice of genetic pathways are then examined. Next, CRC is studied as a specific disease and the putative genetic pathways are described. The mutations that comprise these pathways and the possible functional sequelae of these are explored. The review concludes with a look at those avenues which may further elucidate the natural history of CRC and lead to improved therapy.

Mutation of Pten/Mmac1 in mice causes neoplasia in multiple organ systems

Pten/Mmac1+/- heterozygous mice exhibited neoplasms in multiple organs including the endometrium, liver, prostate, gastrointestinal tract, thyroid, and thymus. Loss of the wild-type allele was detected in neoplasms of the thymus and liver. Surprisingly, tumors of the gastrointestinal epithelium developed in association with gut lymphoid tissue. Tumors of the endometrium, thyroid, prostate, and liver were not associated with lymphoid tissue and appeared to be highly mitotic. In addition, these mice have nonneoplastic hyperplasia of lymph nodes that was caused by an inherited defect in apoptosis detected in B cells and macrophages. Examination of peripheral lymphoid tissue including lymphoid aggregates associated with polyps revealed that the normal organization of B and T cells was disrupted in heterozygous animals. Taken together, these data suggest that PTEN is a regulator of apoptosis and proliferation that behaves as a "landscaper" tumor suppressor in the gut and a "gatekeeper" tumor suppressor in other organs.

A physical, transcript, and deletion map of chromosome region 12p12.3 flanked by ETV6 and CDKN1B: hypermethylation of the LRP6 CpG island in two leukemia patients with hemizygous del(12p)

FISH analyses and loss of heterozygosity studies have delineated a commonly deleted region in hematological malignancies flanked by ETV6 and CDKN1B on chromosome 12p12.3. The same chromosomal region is also a target for deletions in certain solid tumors. As an initial step toward the cloning of a potential tumor suppressor gene at 12p12.3, we mapped the ETV6-CDKN1B region physically using bacterial artificial chromosome (BAC) and P1-derived clone (PAC) contigs. The 1.2-Mb high-resolution, contiguous map extends from D12S1095 to D12S929 and consists of 19 PACs and 20 BACs. Pulsed-field gel electrophoresis experiments confirmed the integrity of the clone-based map and identified six CpG islands in the region. A transcript map was generated by performing hybridization selection experiments with the genomic clones, by evaluating known 12p ESTs for their presence in the contig, and by sequence analysis of CpG islands in the region. Altogether evidence was gathered for the presence of the recently published LRP6 gene and at least seven other new genes in this chromosomal region. The CLAPS3 gene, mapped between D12S391 and D12S358, was reassigned to chromosome 5 since genomic sequencing demonstrated the chromosome 12p sequence to be a pseudogene. Polymorphic CA repeats were identified approximately every 100 kb, which will support future analysis of loss of heterozygosity in tumors. Fluorescence in situ hybridization analysis of leukemia patients with del(12p) further refined the commonly deleted segment to 600 kb between ETV6 and D12S358, which apparently excludes CDKN1B. Methylation changes of the CpG islands in the ETV6-CDKN1B interval were assessed by Southern analysis for leukemia patients with hemizygous 12p deletions. A "de novo" methylation was detected only at the LRP6 CpG island in 2 of 22 leukemia patients tested and was confirmed by methylation-sensitive PCR and sequencing. The genomic structure of LRP6 was elucidated to allow screening for inactivating mutations, but only intragenic polymorphisms were identified. Hypermethylation of CpG islands associated with gene promoters is reported as a common mechanism for gene silencing and tumor suppressor inactivation. Therefore the consequences of the LRP6 CpG island methylation and its role in the observed phenotype need further investigation.

Severe Lhermitte-Duclos disease with unique germline mutation of PTEN

Germline mutations in the PTEN gene have recently been identified in some individuals with Cowden disease (CD), Lhermitte-Duclos disease (LDD), and Bannayan-Zonana syndrome. We report on a patient with CD and LDD in whom a unique de novo germline missense mutation is present in the PTEN gene. Direct sequence analysis detected a transitional change (T-->C) at nucleotide 335, resulting in substitution of the amino acid proline for leucine. The mutation is in exon 5, which has been proposed as a "hot-spot" for germline mutations. Comparison of this patient's clinical course with the previously reported cases of CD and LDD shows more extensive and more severe clinical findings than reported previously. Findings in this patient contribute to the current understanding of germline PTEN mutations and clinical outcome.

Over-representation of a germline RET sequence variant in patients with sporadic medullary thyroid carcinoma and somatic RET codon 918 mutation

The aetiology of sporadic medullary thyroid carcinoma is unknown. About 50% harbour a somatic mutation at codon 918 of RET (M918T). To investigate whether other RET sequence variants may be associated with or predispose to the development of sporadic medullary thyroid carcinoma, we analysed genomic DNA from the germline and corresponding tumour from 50 patients to identify RET sequence variants. In one patient, tumour DNA showed a novel somatic 12 bp in-frame deletion in exon 15. More interestingly, we found that the rare polymorphism at codon 836 (c.2439C > T; S836S) occurred at a significantly higher frequency than that in control individuals without sporadic medullary thyroid carcinoma (Fisher's exact test, P = 0.03). Further, among the nine evaluable cases with germline c.2439C/T, eight also had the somatic M918T mutation in MTC DNA which was more frequent than in patients with the more common c.2439C/C (89% vs 40%, respectively; Fisher's exact test, P = 0.01). These findings suggest that the rare sequence variant at codon 836 may somehow play a role in the genesis of sporadic medullary thyroid carcinoma.

Phenotypic analysis of human glioma cells expressing the MMAC1 tumor suppressor phosphatase

MMAC1, also known as PTEN or TEP-1, was recently identified as a gene commonly mutated in a variety of human neoplasias. Sequence analysis revealed that MMAC1 harbored sequences similar to those found in several protein phosphatases. Subsequent studies demonstrated that MMAC1 possessed in vitro enzymatic activity similar to that exhibited by dual specificity phosphatases. To characterize the potential cellular functions of MMAC1, we expressed wild-type and several mutant variants of MMAC1 in the human glioma cell line, U373, that lacks endogenous expression. While expression of wild-type MMAC1 in these cells significantly reduced their growth rate and saturation density, expression of enzymatically inactive MMAC1 significantly enhanced growth in soft agar. Our observations indicate that while wild-type MMAC1 exhibits activities compatible with its proposed role as a tumor suppressor, cellular expression of MMAC1 containing mutations in the catalytic domain may yield protein products that enhance transformation characteristics.

The genetics of breast cancer susceptibility

Following the genomic localization and subsequent identification of the breast cancer susceptibility genes, BRCA1 and BRCA2, the basic patterns of cancer risk associated with mutations in these genes have been defined. In addition, preliminary insights into the prevalence of mutations and their contributions to cancer incidence have been acquired. Features of breast and other cancers that develop in these genetic syndromes have now been investigated and shown to differ from sporadic versions of the same neoplasms. However, several areas are complex and require further clarification. There remain discrepancies between published cancer risk estimates. Furthermore, there may be variation in cancer risk between different mutations in the same gene and there is preliminary evidence that genetic and nongenetic influences may modify risks. Finally, it is probable that the genes underlying a substantial component of susceptibility to breast cancer remain to be identified.

Analysis of the 10q23 chromosomal region and the PTEN gene in human sporadic breast carcinoma

We examined a panel of sporadic breast carcinomas for loss of heterozygosity (LOH) in a 10-cM interval on chromosome 10 known to encompass the PTEN gene. We detected allele loss in 27 of 70 breast tumour DNAs. Fifteen of these showed loss limited to a subregion of the area studied. The most commonly deleted region was flanked by D10S215 and D10S541 and encompasses the PTEN locus. We used a combination of denaturing gradient gel electrophoresis and single-strand conformation polymorphism analyses to investigate the presence of PTEN mutations in tumours with LOH in this region. We did not detect mutations of PTEN in any of these tumours. Our data show that, in sporadic breast carcinoma, loss of heterozygosity of the PTEN locus is frequent, but mutation of PTEN is not. These results are consistent with loss of another unidentified tumour suppressor in this region in sporadic breast carcinoma.

Symptomatic colonic polyps in childhood: not so benign

BACKGROUND

The clinical spectrum of symptomatic polyps and the frequency of familial polyposis is not well defined in children. In the present study, a series of children with juvenile polyposis coli (JPC) and non-JPC polyps were studied.

METHODS

Children with symptomatic colonic polyps and negative family history of polyps were ascertained by review of endoscopic records. Juvenile polyposis coli was defined as 10 or more juvenile polyps or any juvenile polyp in a relative of an index case of JPC. Polyps were tested for Ki-ras mutations, p53 overexpression, and aneuploidy.

RESULTS

Seventy-eight children (age range, 0.4-18 years) were identified, all evaluated for lower gastrointestinal bleeding. Nine (12%) had JPC, 66 (84%) had isolated juvenile polyps, and 3 (4%) had other types of polyps. The JPC and non-JPC groups were similar in age (p = 0.4) and symptom duration (p = 0.3). The JPC group had more polyps (p = 0.0001), and greater likelihood of anemia (p = 0.01), polyps with adenomatous change (p = 0.03), and right-colon polyps (p = 0.001). In three of eight JPC families, polyps were identified in asymptomatic first-degree relatives. No abnormalities in Ki-ras, p53, or aneuploidy were identified.

CONCLUSIONS

Juvenile polyposis coli is common in children with symptomatic polyps, and is associated with anemia, right-colon polyps, and adenomas. The risk of polyps and of colorectal cancer in relatives of persons with JPC requires further study.

The genetic basis of breast cancer and its clinical implications

While it has long been recognized that a proportion of breast cancer cases are the result of an inherited familial predisposition, precise knowledge of the underlying genetic processes has been lacking. Recent advances in molecular biology, however, have shown that hereditary breast cancer may eventuate as a result of mutations on several specific gene loci including BRCA1, BRCA2, ATM gene, PTEN and p53. Several other less frequently occurring predisposition genes such as the androgen receptor gene (AR), the HNPCC genes and the oestrogen receptor gene may also be involved, but to a lesser extent. Overall, approximately 5-10% of all breast cancers are thought to involve one of these inherited predisposition genes, with BRCA1 and BRCA2 being responsible for as much as 90% of this group. Because of the complex nature of genetic testing, mutation analysis is not presently routinely available outside genetic counselling clinics. In this review the current knowledge and role of each predisposition gene is outlined and the management implications of genetic testing for members of breast cancer families for both affected and non-affected members are discussed. The need to provide comprehensive counselling for women with an inherited predisposition to breast cancer has seen the evolution of the familial cancer clinic, involving a multidisciplinary specialist team approach. Familial cancer clinics will provide individuals with information about their risk of developing breast cancer and offer advice regarding further management strategies. It is important that surgeons, who have traditionally played a key role in breast cancer treatment, remain cognizant of these advances in genetic molecular biology, and in so doing continue to remain key participants in the conduct of breast cancer management.

The current social, political, and medical role of genetic testing in familial breast and ovarian carcinomas

Few advances in medical science have yielded as much publicity and controversy as discoveries in genetics. Moving quickly from the bench to the bedside, genetic testing for inherited susceptibility to breast and ovarian cancer has had a significant impact on our paradigms for decisions about the treatment and prevention of disease. Assessment of cancer risk is developing into a distinct discipline, with rapidly evolving genetic technologies and models for estimating an individual's risk of cancer. Exciting developments in chemoprevention of breast cancer demonstrate the potential to offer a broader range of options for decreasing cancer risk. This article will consider recent advances in the understanding of cancer genetics, and describe the state-of-the-art in terms of management of individuals with inherited susceptibility to breast and ovarian cancer.

Exclusion of a major role for the PTEN tumour-suppressor gene in breast carcinomas

PTEN is a novel tumour-suppressor gene located on chromosomal band 10q23.3. This region displays frequent loss of heterozygosity (LOH) in a variety of human neoplasms including breast carcinomas. The detection of PTEN mutations in Cowden disease and in breast carcinoma cell lines suggests that PTEN may be involved in mammary carcinogenesis. We here report a mutational analysis of tumour specimens from 103 primary breast carcinomas and constitutive DNA from 25 breast cancer families. The entire coding region of PTEN was screened by single-strand conformation polymorphism (SSCP) analysis and direct sequencing using intron-based primers. No germline mutations could be identified in the breast cancer families and only one sporadic carcinoma carried a PTEN mutation at one allele. In addition, all sporadic tumours were analysed for homozygous deletions by differential polymerase chain reaction (PCR) and for allelic loss using the microsatellite markers D10S215, D10S564 and D10S573. No homozygous deletions were detected and only 10 out of 94 informative tumours showed allelic loss in the PTEN region. These results suggest that PTEN does not play a major role in breast cancer formation.

Dysplastic gangliocytoma (Lhermitte-Duclos disease) associated with Cowden disease: report of a case and review of the literature for the genetic relationship between the two diseases

We report a case of dysplastic gangliocytoma of the cerebellum (Lhermitte-Duclos disease, LDD). The patient also had cutaneous and mucosal hamartomas, adenomatous goiter, bilateral breast tumors, and gastrointestinal polyposis, indicating the diagnosis of Cowden disease (CD), the familial hamartoma syndrome. This was a rare sporadic case without any family history of CD, though CD is considered to be an autosomal dominant hereditary disease. Based on a thorough review of the previously reported cases, it is reasonable to consider that CD is inherited in autosomal dominant fashion through a CD gene (PTEN) containing a germline mutation, and that the occurrence of LDD is predicted on an additional somatic hit on the remaining normal CD allele or another unknown gene.

Microsatellite instability in thyroid tumours and tumour-like lesions

Fifty-one thyroid tumours and tumour-like lesions were analysed for instability at ten dinucleotide microsatellite loci and at two coding mononucleotide repeats within the transforming growth factor beta (TGF-beta) type II receptor (TbetaRII) and insulin-like growth factor II (IGF-II) receptor (IGFIIR) genes respectively. Microsatellite instability (MI) was detected in 11 out of 51 cases (21.5%), including six (11.7%) with MI at one or two loci and five (9.8%) with MI at three or more loci (RER+ phenotype). No mutations in the TbetaRII and IGFIIR repeats were observed. The overall frequency of MI did not significantly vary in relation to age, gender, benign versus malignant status and tumour size. However, widespread MI was significantly more frequent in follicular adenomas and carcinomas than in papillary and Hürthle cell tumours: three out of nine tumours of follicular type (33.3%) resulted in replication error positive (RER+), versus 1 out of 29 papillary carcinomas (3.4%, P = 0.01), and zero out of eight Hürthle cell neoplasms. Regional lymph node metastases were present in five MI-negative primary cancers and resulted in MI-positive in two cases.

MMDB: Entrez's 3D structure database

The three dimensional structures for representatives of nearly half of all protein families are now available in public databases. Thus, no matter which protein one investigates, it is increasingly likely that the 3D structure of a homolog will be known and may reveal unsuspected structure-function relationships. The goal of Entrez's 3D-structure database is to make this information accessible and usable by molecular biologists (http://www.ncbi.nlm.nih.gov/Entrez). To this end Entrez provides two major analysis tools, a search engine based on sequence and structure 'neighboring' and an integrated visualization system for sequence and structure alignments. From a protein's sequence 'neighbors' one may rapidly identify other members of a protein family, including those where 3D structure is known. By comparing aligned sequences and/or structures in detail, using the visualization system, one may identify conserved features and perhaps infer functional properties. Here we describe how these analysis tools may be used to investigate the structure and function of newly discovered proteins, using the PTEN gene product as an example.

The PTEN/MMAC1 tumor suppressor phosphatase functions as a negative regulator of the phosphoinositide 3-kinase/Akt pathway

The PTEN/MMAC1 phosphatase is a tumor suppressor gene implicated in a wide range of human cancers. Here we provide biochemical and functional evidence that PTEN/MMAC1 acts a negative regulator of the phosphoinositide 3-kinase (PI3-kinase)/Akt pathway. PTEN/MMAC1 impairs activation of endogenous Akt in cells and inhibits phosphorylation of 4E-BP1, a downstream target of the PI3-kinase/Akt pathway involved in protein translation, whereas a catalytically inactive, dominant negative PTEN/MMAC1 mutant enhances 4E-BP1 phosphorylation. In addition, PTEN/MMAC1 represses gene expression in a manner that is rescued by Akt but not PI3-kinase. Finally, higher levels of Akt activation are observed in human prostate cancer cell lines and xenografts lacking PTEN/MMAC1 expression when compared with PTEN/MMAC1-positive prostate tumors or normal prostate tissue. Because constitutive activation of either PI3-kinase or Akt is known to induce cellular transformation, an increase in the activation of this pathway caused by mutations in PTEN/MMAC1 provides a potential mechanism for its tumor suppressor function.

The akt kinase: molecular determinants of oncogenicity

The serine-threonine kinase Akt is a downstream target of phosphoinositide 3-kinase (PI 3-kinase); it is activated by the phosphoinositide 3-phosphate-dependent kinases PDK1 and PDK2. Certain mutated forms of Akt induce oncogenic transformation in chicken embryo fibroblast cultures and hemangiosarcomas in young chickens. This ability to transform cells depends on localization of Akt at the plasma membrane and on the kinase activity of Akt. A transdominant negative form of Akt interferes with oncogenic transformation induced by the p3k oncogene, which codes for an activated form of PI 3-kinase. Akt is therefore an essential mediator of p3k-induced oncogenicity.

Inherited breast cancer: an emerging picture

A role for BRCA1 and BRCA2 in the control of genome integrity easily fits a tumor suppressor model. It is well established that mutations in DNA repair genes lead to genomic instability (138). Genomic instability may directly lead to tumorigenesis by allowing for the accumulation of mutations in key cell cycle regulators (139). The studies summarized here suggest that BRCA1, BRCA2, RAD51. and BARD1 function as a biochemical complex. This complex apparently plays a role in one or more of the DNA damage response pathways. Experimental data suggest that BRCA1 and BRCA2 function as regulators of transcription. These observations highlight some of the fundamental questions that remain to be addressed in the study of the biology of these genes. Are the DNA repair and transcriptional regulatory functions of BRCA1 and BRCA2 related? BRCA1 and BRCA2 may maintain the integrity of the genome by regulating expression of genes directly involved in this process. Alternatively, if the functions are not related, which is required for suppression of tumorigenesis? Researchers also are grappling with another paradox. If BRCA1 and BRCA2 are ubiquitously expressed, why do mutations in BRCA1 and BRCA2 lead specifically to tumors primarily of the breast and ovary, as well as a limited number of other tissues to a lesser degree? Nothing to date has been revealed that would explain how alteration of the transcriptional regulatory function and or the DNA repair function ascribed to BRCA1 and BRCA2 would result in tumor specificity as both of these functions are essential to a broad spectrum of tissues. It is possible that BRCAI and BRCA2 may regulate genes expressed only in the breast and ovary. Similarly, there may be unidentified BRCA1 and BRCA2 co-factors that are active only in the breast and ovary and, therefore, are critical to tumorigenesis. All breast cancer is genetic, although only a small fraction of cases are attributable to inherited genetic predisposition. Most breast cancer is due to genetic alterations that are specific to breast epithelial cells, many of which remain unknown. Integration of genetic approaches into research designed to elucidate biological pathways of breast cancer tumorigenesis will ultimately lead to new information critical to the development of new tools for the diagnosis and treatment of disease.

Identification of a novel PTEN mutation (L139X) in a patient with Cowden disease and Sjögren's syndrome

Cowden disease is an autosomal dominant disorder associated with an increased risk of breast, thyroid, and skin cancer in which germline mutations in a candidate tumour suppressor gene (PTEN) have been identified previously. Sjögren's syndrome is a chronic inflammatory and autoimmune disorder of exocrine glands for which the genetic basis is unknown. This report describes a novel PTEN mutation (L139X) in a patient with Cowden disease and Sjögren's syndrome. This observation raises the possibility of a link between mutations in the PTEN gene and Sjögren's syndrome.

A gene predisposing to familial thyroid tumors with cell oxyphilia maps to chromosome 19p13.2

Familial nonmedullary thyroid cancer (FNMTC) is a clinical entity characterized by a phenotype more aggressive than that of its sporadic counterpart. Families with recurrence of nonmedullary thyroid cancer (NMTC) have been repeatedly reported in the literature, and epidemiological data show a very high relative risk for first-degree relatives of probands with thyroid cancer. The transmission of susceptibility to FNMTC is compatible with autosomal dominant inheritance with reduced penetrance, or with complex inheritance. Cases of benign thyroid disease are often found in FNMTC kindreds. We report both the identification of a new entity of FNMTC and the mapping of the responsible gene, named "TCO" (thyroid tumors with cell oxyphilia), in a French pedigree with multiple cases of multinodular goiter and NMTC. TCO was mapped to chromosome 19p13.2 by linkage analysis with a whole-genome panel of microsatellite markers. Interestingly, both the benign and malignant thyroid tumors in this family exhibit some extent of cell oxyphilia, which, until now, had not been described in the FNMTC. These findings suggest that the relatives of patients affected with sporadic NMTC with cell oxyphilia should be carefully investigated.

Tumor suppressor PTEN inhibits integrin- and growth factor-mediated mitogen-activated protein (MAP) kinase signaling pathways

The tumor suppressor PTEN dephosphorylates focal adhesion kinase (FAK) and inhibits integrin-mediated cell spreading and cell migration. We demonstrate here that expression of PTEN selectively inhibits activation of the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) pathway. PTEN expression in glioblastoma cells lacking the protein resulted in inhibition of integrin-mediated MAP kinase activation. Epidermal growth factor (EGF) and platelet-derived growth factor (PDGF)- induced MAPK activation were also blocked. To determine the specific point of inhibition in the Ras/Raf/ MEK/ERK pathway, we examined these components after stimulation by fibronectin or growth factors. Shc phosphorylation and Ras activity were inhibited by expression of PTEN, whereas EGF receptor autophosphorylation was unaffected. The ability of cells to spread at normal rates was partially rescued by coexpression of constitutively activated MEK1, a downstream component of the pathway. In addition, focal contact formation was enhanced as indicated by paxillin staining. The phosphatase domain of PTEN was essential for all of these functions, because PTEN with an inactive phosphatase domain did not suppress MAP kinase or Ras activity. In contrast to its effects on ERK, PTEN expression did not affect c-Jun NH2-terminal kinase (JNK) or PDGF-stimulated Akt. Our data suggest that a general function of PTEN is to down-regulate FAK and Shc phosphorylation, Ras activity, downstream MAP kinase activation, and associated focal contact formation and cell spreading.

The lipid phosphatase activity of PTEN is critical for its tumor supressor function

Since their discovery, protein tyrosine phosphatases have been speculated to play a role in tumor suppression because of their ability to antagonize the growth-promoting protein tyrosine kinases. Recently, a tumor suppressor from human chromosome 10q23, called PTEN or MMAC1, has been identified that shares homology with the protein tyrosine phosphatase family. Germ-line mutations in PTEN give rise to several related neoplastic disorders, including Cowden disease. A key step in understanding the function of PTEN as a tumor suppressor is to identify its physiological substrates. Here we report that a missense mutation in PTEN, PTEN-G129E, which is observed in two Cowden disease kindreds, specifically ablates the ability of PTEN to recognize inositol phospholipids as a substrate, suggesting that loss of the lipid phosphatase activity is responsible for the etiology of the disease. Furthermore, expression of wild-type or substrate-trapping forms of PTEN in HEK293 cells altered the levels of the phospholipid products of phosphatidylinositol 3-kinase and ectopic expression of the phosphatase in PTEN-deficient tumor cell lines resulted in the inhibition of protein kinase (PK) B/Akt and regulation of cell survival.

Germline PTEN mutations in Cowden syndrome-like families

Cowden syndrome (CS) or multiple hamartoma syndrome (MIM 158350) is an autosomal dominant disorder with an increased risk for breast and thyroid carcinoma. The diagnosis of CS, as operationally defined by the International Cowden Consortium, is made when a patient, or family, has a combination of pathognomonic major and/or minor criteria. The CS gene has recently been identified as PTEN, which maps at 10q23.3 and encodes a dual specificity phosphatase. PTEN appears to function as a tumour suppressor in CS, with between 13-80% of CS families harbouring germline nonsense, missense, and frameshift mutations predicted to disrupt normal PTEN function. To date, only a small number of tumour suppressor genes, including BRCA1, BRCA2, and p53, have been associated with familial breast or breast/ovarian cancer families. Given the involvement of PTEN in CS, we postulated that PTEN was a likely candidate to play a role in families with a "CS-like" phenotype, but not classical CS. To answer these questions, we gathered a series of patients from families who had features reminiscent of CS but did not meet the Consortium Criteria. Using a combination of denaturing gradient gel electrophoresis (DGGE), temporal temperature gel electrophoresis (TTGE), and sequence analysis, we screened 64 unrelated CS-like subjects for germline mutations in PTEN. A single male with follicular thyroid carcinoma from one of these 64 (2%) CS-like families harboured a germline point mutation, c.209T-->C. This mutation occurred at the last nucleotide of exon 3 and within a region homologous to the cytoskeletal proteins tensin and auxilin. We conclude that germline PTEN mutations play a relatively minor role in CS-like families. In addition, our data would suggest that, for the most part, the strict International Cowden Consortium operational diagnostic criteria for CS are quite robust and should remain in place.

PTEN Gene Alterations in Lymphoid Neoplasms

Recently, a novel phosphatase designated PTEN/MMAC1/TEP1 and located on chromosome 10q23.3 has been implicated as a new tumor suppressor gene in human cancer. Allelic loss and mutation of this gene has been reported in epithelial derived tumors, including breast cancer and prostate cancer, and in glioblastoma multiforme. The present study was designed to evaluate the potential involvement of PTEN in the pathogenesis of lymphoid neoplasms. We analyzed 27 hematopoietic cell lines (representing a variety of lymphoid lineages), 65 primary lymphoid tumors (including 24 lymphoblastic leukemia/lymphoma [LBL], 30 large B-cell lymphoma [LBCL], 7 Burkitt’s lymphoma [BL], and 4 anaplastic large cell lymphoma [ALCL]), and 25 nonmalignant lymph node controls. Gene deletion and gross rearrangement were evaluated using Southern blot analysis, and mutations were studied by polymerase chain reaction (PCR)-single-strand conformation polymorphism (SSCP) (PCR-SSCP) and sequencing. Six of 27 cell lines (22.2%) and 3 of 65 primary lymphomas (4.6%) contained alterations of this gene. A large homozygous deletion spanning exons 2 through 5 was detected in one LBL cell line, and two insertions potentially resulting in premature termination, were detected in a second LBL cell line. Nonconservative nucleotide variations were found in two other cell lines (one LBCL and one BL) and in one primary case of LBCL. In addition, two other cell lines (one BL and one myeloma) and two primary lymphomas, both LBCL, contained small deletions within intron 7. These deletions mapped to a poly-T–rich tract just 5′ to the intron 7/exon 8 spice site. Their significance is unclear, as they may represent polymorphisms. Overall, our results suggest that abnormalities of the PTEN gene can contribute to pathogenesis in a small percentage of malignant lymphomas.

This is a US government work. There are no restrictions on its use.

Mutations in DPC4 (SMAD4) cause juvenile polyposis syndrome, but only account for a minority of cases

Juvenile polyps are present in a number of Mendelian disorders, sometimes in association only with gastrointestinal cancer [juvenile polyposis syndrome (JPS)] and sometimes as part of known syndromes (Cowden, Gorlin and Banayan-Zonana) in association with developmental abnormalities, dysmorphic features or extra-intestinal tumours. Recently, a gene for JPS was mapped to 18q21.1 and the candidate gene DPC4 (SMAD4) was shown to carry frameshift mutations in some JPS families. We have analysed eight JPS families for linkage to DPC4. Overall, there was no evidence for linkage to DPC4; linkage could be excluded in two of the eight pedigrees and was unlikely in two others. We then tested these eight families and a further 13 familial and sporadic JPS cases for germline mutations in DPC4. Just one germline DPC4 mutation was found (in a familial JPS patient from a pedigree unsuitable for linkage analysis). Like all three previously reported germline mutations, this variant occurred towards the C-terminus of the DPC4 protein. However, our patient's mutation is a missense change (R361C); somatic missense mutations in DPC4 have been reported previously in tumours. We therefore confirm DPC4 as a cause of JPS, but show that there is considerable remaining, uncharacterized genetic heterogeneity in this disease.

PTEN Gene Alterations in Lymphoid Neoplasms

Recently, a novel phosphatase designated PTEN/MMAC1/TEP1 and located on chromosome 10q23.3 has been implicated as a new tumor suppressor gene in human cancer. Allelic loss and mutation of this gene has been reported in epithelial derived tumors, including breast cancer and prostate cancer, and in glioblastoma multiforme. The present study was designed to evaluate the potential involvement of PTEN in the pathogenesis of lymphoid neoplasms. We analyzed 27 hematopoietic cell lines (representing a variety of lymphoid lineages), 65 primary lymphoid tumors (including 24 lymphoblastic leukemia/lymphoma [LBL], 30 large B-cell lymphoma [LBCL], 7 Burkitt’s lymphoma [BL], and 4 anaplastic large cell lymphoma [ALCL]), and 25 nonmalignant lymph node controls. Gene deletion and gross rearrangement were evaluated using Southern blot analysis, and mutations were studied by polymerase chain reaction (PCR)-single-strand conformation polymorphism (SSCP) (PCR-SSCP) and sequencing. Six of 27 cell lines (22.2%) and 3 of 65 primary lymphomas (4.6%) contained alterations of this gene. A large homozygous deletion spanning exons 2 through 5 was detected in one LBL cell line, and two insertions potentially resulting in premature termination, were detected in a second LBL cell line. Nonconservative nucleotide variations were found in two other cell lines (one LBCL and one BL) and in one primary case of LBCL. In addition, two other cell lines (one BL and one myeloma) and two primary lymphomas, both LBCL, contained small deletions within intron 7. These deletions mapped to a poly-T–rich tract just 5′ to the intron 7/exon 8 spice site. Their significance is unclear, as they may represent polymorphisms. Overall, our results suggest that abnormalities of the PTEN gene can contribute to pathogenesis in a small percentage of malignant lymphomas.

This is a US government work. There are no restrictions on its use.

Mutations of PTEN in patients with Bannayan-Riley-Ruvalcaba phenotype

We report three new mutations in PTEN, the gene responsible for Cowden disease in five patients with Bannayan-Riley-Ruvalcaba syndrome from three unrelated families. This finding confirms that Cowden disease, a dominant cancer predisposing syndrome, and Bannayan-Riley-Ruvalcaba syndrome, which includes macrocephaly, multiple lipomas, intestinal hamartomatous polyps, vascular malformations, and pigmented macules of the penis, are allelic disorders at the PTEN locus on chromosome 10q.

Somatic mutations of the PTEN tumor suppressor gene in sporadic follicular thyroid tumors

The PTEN (MMAC1/TEP1) tumor suppressor gene was recently isolated and mapped to human chromosome band 10q23. Homozygous deletions and mutations of PTEN were observed in cell lines and sporadic cancers of the breast, kidney, and central nervous system. Germline mutations in PTEN were recently found in Cowden disease, an autosomal dominant inherited syndrome, previously mapped to chromosome bands 10q22-23. This disease is associated with a wide variety of malignancies and hamartomas of ectodermal, mesodermal, and endodermal origin. The most common neoplasms in Cowden disease patients arise in the breast, skin, and thyroid (follicular subtype). To determine the involvement of PTEN in sporadic follicular thyroid tumors, we first analyzed sporadic follicular adenomas and carcinomas for deletions of the PTEN gene. Loss of heterozygosity was found in 7/26 (27%) follicular carcinomas and 2/27 (7%) follicular adenomas, one of which was a small hemizygous deletion (approximately 3 cm). Sequence analysis of the entire PTEN coding region revealed two mutations in carcinomas with 10q loss. Our findings suggest that the PTEN tumor suppressor gene is occasionally inactivated in sporadic follicular thyroid tumors.

High cancer susceptibility and embryonic lethality associated with mutation of the PTEN tumor suppressor gene in mice

BACKGROUND

Germ-line and sporadic mutations in the tumor suppressor gene PTEN (also known as MMAC or TEP1), which encodes a dual-specificity phosphatase, cause a variety of cancers such as Cowden disease, glioblastoma, endometrial carcinoma and prostatic cancer. PTEN is widely expressed, and Cowden disease consistently affects various organ systems, suggesting that the PTEN protein must have an important, although as yet poorly understood, function in cellular physiology.

RESULTS

Homozygous mutant mice lacking exons 3-5 of the PTEN gene (mPTEN3-5) had severely expanded and abnormally patterned cephalic and caudal regions at day 8.5 of gestation. Embryonic death occurred by day 9.5 and was associated with defective chorio-allantoic development. Heterozygous mPTEN3-5 mice had an increased incidence of tumors, especially T-cell lymphomas; gamma-irradiation reduced the time lapse of tumor formation. DNA analysis of these tumors revealed the deletion of the mPTEN gene due to loss of heterozygosity of the wild-type allele. Tumors associated with loss of heterozygosity in mPTEN showed elevated phosphorylation of protein kinase B (PKB, also known as Akt kinase), thus providing a functional connection between mPTEN and a murine proto-oncogene (c-Akt) involved in the development of lymphomas.

CONCLUSIONS

The mPTEN gene is fundamental for embryonic development in mice, as mPTEN3-5 mutant embryos died by day 9.5 of gestation, with patterning defects in cephalic and caudal regions and defective placentation. Heterozygous mice developed lymphomas associated with loss of heterozygosity of the wild-type mPTEN allele, and tumor appearance was accelerated by gamma-irradiation. These lymphomas had high levels of activated Akt/PKB, the protein product of a murine proto-oncogene with anti-apoptotic function, associated with thymic lymphomas. This suggests that tumors associated with mPTEN loss of heterozygosity may arise as a consequence of an acquired survival advantage. We provide direct evidence of the role of mPTEN as a tumor suppressor gene in mice, and establish the mPTEN mutant mouse as an experimental model for investigating the role of PTEN in cancer progression.

Negative regulation of PKB/Akt-dependent cell survival by the tumor suppressor PTEN

PTEN is a tumor suppressor with sequence homology to protein tyrosine phosphatases and the cytoskeletal protein tensin. mPTEN-mutant mouse embryos display regions of increased proliferation. In contrast, mPTEN-deficient immortalized mouse embryonic fibroblasts exhibit decreased sensitivity to cell death in response to a number of apoptotic stimuli, accompanied by constitutively elevated activity and phosphorylation of protein kinase B/Akt, a crucial regulator of cell survival. Expression of exogenous PTEN in mutant cells restores both their sensitivity to agonist-induced apoptosis and normal pattern of PKB/Akt phosphorylation. Furthermore, PTEN negatively regulates intracellular levels of phosphatidylinositol (3,4,5) trisphosphate in cells and dephosphorylates it in vitro. Our results show that PTEN may exert its role as a tumor suppressor by negatively regulating the PI3'K/PKB/Akt signaling pathway.

Inherited macrocephaly-hamartoma syndromes

Recent discoveries in the molecular biology of the phosphatase and tensin homolog (PTEN) locus in the q22-23 region of chromosome 10 prove and/or suggest that several syndromes previously considered to be clinically and genetically distinct entities should actually be unified into a single entity. This conclusion is most secure for the Cowden and "Bannayan-Zonana" phenotypes, but almost certainly should also include the "Riley-Ruvalcaba" and Lhermitte-Duclos phenotypes as well benign familial macrocephaly and external hydrocephalus. The clinical and molecular data supporting this unification are presented along with a proposal for new nomenclature-the PTEN MATCHS (macrocephaly, autosomal dominant, thyroid disease, cancer, hamartomata, skin abnormalities) syndrome-based on the observed clinical abnormalities.

Some neoplasms and some hamartomatous syndromes: genetic considerations

This is the first of three articles on modern genetic concepts of a number of syndromes and disorders. About 1% of cancer mutations arise in the germline and produce a variety of neoplasms and hamartomatous syndromes. However, upward of 10-15% of all cancers have a major inherited component, although many of these are still enigmatic. The genetic basis is presented and following a review of many neoplasms and hamartomatous syndromes, the RET proto-oncogene is discussed as an example.