Neurofibromatosis 2 gene in human colorectal cancer

NF2 Gene Participates in Regulation of the Cell Cycle of Meningiomas by Restoring Spindle Assembly Checkpoint Function and Inhibiting the Binding of Cdc20 Protein to Anaphase Promoting Complex/Cyclosome

BACKGROUND

The neurofibromatosis type 2 (NF2) gene mutation is the leading genetic event in meningiomas, usually accompanied by malignant features. Dysfunction of the spindle assembly checkpoint (SAC) induces tumorigenesis. However, the crosstalk between NF2 and SAC in meningiomas remains unclear.

METHODS

Cell proliferation, invasion, apoptosis, and cell cycle of meningiomas were determined by cell counting kit-8 assay, transwell assay, and flow cytometry, respectively. The expression of SAC in meningioma cells was detected by quantitative real-time polymerase chain reaction and Western blot. The interaction between anaphase promoting complex/cyclosome (APC/C) and cell division cycle 20 (Cdc20) protein in meningioma cells was further explored by co-immunoprecipitation.

RESULTS

We found that the expression of NF2/merlin was low or absent in malignant meningiomas. Overexpression of NF2 suppressed the proliferation and invasion of meningioma cells, prolonged the G₂/M phase, and elevated the expression of SAC proteins at posttranscription. Furthermore, the interaction between APC/C and Cdc20 was inhibited by NF2.

CONCLUSIONS

Our findings suggested that NF2 might restore SAC function by impairing the binding of APC/C and Cdc20, thereby limiting the mitotic rate and inhibiting proliferation of meningiomas.

YAP1 and its fusion proteins in cancer initiation, progression and therapeutic resistance

YAP1 is a transcriptional co-activator whose activity is controlled by the Hippo signaling pathway. In addition to important functions in normal tissue homeostasis and regeneration, YAP1 has also prominent functions in cancer initiation, aggressiveness, metastasis, and therapy resistance. In this review we are discussing the molecular functions of YAP1 and its roles in cancer, with a focus on the different mechanisms of de-regulation of YAP1 activity in human cancers, including inactivation of upstream Hippo pathway tumor suppressors, regulation by intersecting pathways, miRNAs, and viral oncogenes. We are also discussing new findings on the function and biology of the recently identified family of YAP1 gene fusions, that constitute a new type of activating mutation of YAP1 and that are the likely oncogenic drivers in several subtypes of human cancers. Lastly, we also discuss different strategies of therapeutic inhibition of YAP1 functions.

MicroRNA-146b-3p Promotes Cell Metastasis by Directly Targeting NF2 in Human Papillary Thyroid Cancer

Background: MiR-146b has been reported to be overexpressed in papillary thyroid cancer (PTC) tissues and associated with aggressive PTC. MiR-146b is regarded as a relevant diagnostic marker for this type of cancer. MiR-146b-5p has been confirmed to increase cell proliferation by repressing SMAD4. However, detailed functional analysis of another mature form of miR-146b, miR-146b-3p, has not been carried out. This study aimed to identify the differential expression of miR-146b-5p and miR-146b-3p in more aggressive PTC associated with lymph node metastasis, and further elucidate the contribution and mechanism of miR-146b-3p in the process of PTC metastasis. Methods: Expression of miR-146b-5p and miR-146b-3p was assessed in formalin-fixed paraffin-embedded tissue samples from PTC patients, and the relationship with lymph node metastasis was analyzed. A variety of PTC cells, including BHP10-3, BHP10-3SC_mice, and K1 cells, were cultured and treated with miR-146b-5p or miR-146b-3p mimics/inhibitors. The cell migration and invasion abilities were characterized by the real-time cell analyzer assay and Transwell™ assay. PTC xenograft models were used to examine the effect of miR-146b-3p on PTC metastatic ability in vivo. Direct downstream targets of miR-146b-3p were analyzed by luciferase reporter assay and Western blotting. The mechanism by which miR-146b-3p affects cell metastasis was further characterized by co-transfection with merlin, the protein product of the NF2 gene. Results: MiR-146b-5p and miR-146b-3p expression was significantly higher in thyroid cancer tissues and cell lines than in normal thyroid tissue and cells. Moreover, expression of miR-146b-5p and miR-146b-3p was further increased in thyroid metastatic nodes than in thyroid cancer. After overexpression of miR-146b-5p or miR-146b-3p in BHP10-3 or K1 cells, PTC migration and invasion were increased. Notably, miR-146b-3p increased cell migration and invasion more obviously than did miR-146b-5p. Overexpression of miR-146b-3p also significantly promoted PTC tumor metastasis in vivo. Luciferase reporter assay results revealed that NF2 is a downstream target of miR-146b-3p in PTC cells, as miR-146b-3p bound directly to the 3' untranslated region of NF2, thus reducing protein levels of NF2. Overexpression of merlin reversed the enhanced aggressive effects of miR-146b-3p. Conclusions: Overexpression of miR-146b-5p and miR-146b-3p is associated with PTC metastasis. MiR-146b-3p enhances cell invasion and metastasis more obviously than miR-146b-5p through the suppression of the NF2 gene. These findings suggest a potential diagnostic and therapeutic value of these miRNAs in PTC metastasis.

Loss of tumor suppressor Merlin results in aberrant activation of Wnt/β-catenin signaling in cancer

The expression of the tumor suppressor Merlin is compromised in nervous system malignancies due to genomic aberrations. We demonstrated for the first time, that in breast cancer, Merlin protein expression is lost due to proteasome-mediated elimination. Immunohistochemical analysis of tumor tissues from patients with metastatic breast cancer revealed characteristically reduced Merlin expression. Importantly, we identified a functional role for Merlin in impeding breast tumor xenograft growth and reducing invasive characteristics. We sought to determine a possible mechanism by which Merlin accomplishes this reduction in malignant activity. We observed that breast and pancreatic cancer cells with loss of Merlin show an aberrant increase in the activity of β-catenin concomitant with nuclear localization of β-catenin. We discovered that Merlin physically interacts with β-catenin, alters the sub-cellular localization of β-catenin, and significantly reduces the protein levels of β-catenin by targeting it for degradation through the upregulation of Axin1. Consequently, restoration of Merlin inhibited β-catenin-mediated transcriptional activity in breast and pancreatic cancer cells. We also present evidence that loss of Merlin sensitizes tumor cells to inhibition by compounds that target β-catenin-mediated activity. Thus, this study provides compelling evidence that Merlin reduces the malignant activity of pancreatic and breast cancer, in part by suppressing the Wnt/β-catenin pathway. Given the potent role of Wnt/β-catenin signaling in breast and pancreatic cancer and the flurry of activity to test β-catenin inhibitors in the clinic, our findings are opportune and provide evidence for Merlin in restraining aberrant activation of Wnt/β-catenin signaling.

A novel alternative splicing isoform of NF2 identified in human Schwann cells

Vestibular schwannoma (VS) is a benign, slow-growing cranial tumor that originates from the hypertrophy of Schwann cells. The majority of sporadic VS are unilateral, and the mechanisms underlying VS tumorigenesis are not fully understood. The human neurofibromin 2 (NF2) gene encodes the tumor suppressor protein merlin and the NF2 transcript can be alternatively spliced to form numerous isoforms. The present study investigated human Schwann cells (HSCs) at the mRNA and protein level to understand the function of the alternative splicing (AS) isoform of NF2. The total RNA of HSCs was isolated and the full-length coding sequence of NF2 was amplified. The amplified products were excised from agarose gels, purified and sequenced. NF2 at a protein level was assayed by immunoprecipitation and western blot analysis. The full-length and spliced NF2 forms were amplified by polymerase chain reaction (PCR) from the HSC complementary DNA and ligated into eukaryotic expression vector pcDNA3.1(+). The plasmids were transfected into the HSC HEI-193 cell line and cell proliferation assays were performed using Cell Counting Kit-8. PCR analysis using HSC total RNA as a template revealed the presence of a shortened NF2 transcript, which was due to splicing at the 3'-end of the NF2 mRNA. Sequence analysis confirmed that this AS isoform omitted exons 11, 12, 13, 14, 15 and 16. Immunoprecipitation and western blot analysis demonstrated that the AS isoform was highly expressed in the HSCs at 38 kDa, while the wild-type (WT) isoform, which was expected at 66 kDa, was undetectable. Transfection and cell proliferation assays revealed that the WT isoform exhibited significant growth inhibition, while the AS isoform did not suppress cell growth. In conclusion, the present study detected AS NF2 isoforms in HSC for the first time, and investigated the function of the principle AS isoform. The present study suggests that although HSCs have an undetectable level of WT isoform of the NF2 protein merlin, they are not merlin-null, since they express the AS isoform. Although the AS merlin isoform has no suppressive effect on cell growth, certain mechanisms may exist that underlie this phenomenon, and this may be associated with the genesis and development of VS.

Role of Merlin/NF2 inactivation in tumor biology

Merlin (Moesin-ezrin-radixin-like protein, also known as schwannomin) is a tumor suppressor protein encoded by the neurofibromatosis type 2 gene NF2. Loss of function mutations or deletions in NF2 cause neurofibromatosis type 2 (NF2), a multiple tumor forming disease of the nervous system. NF2 is characterized by the development of bilateral vestibular schwannomas. Patients with NF2 can also develop schwannomas on other cranial and peripheral nerves, as well as meningiomas and ependymomas. The only potential treatment is surgery/radiosurgery, which often results in loss of function of the involved nerve. There is an urgent need for chemotherapies that slow or eliminate tumors and prevent their formation in NF2 patients. Interestingly NF2 mutations and merlin inactivation also occur in spontaneous schwannomas and meningiomas, as well as other types of cancer including mesothelioma, glioma multiforme, breast, colorectal, skin, clear cell renal cell carcinoma, hepatic and prostate cancer. Except for malignant mesotheliomas, the role of NF2 mutation or inactivation has not received much attention in cancer, and NF2 might be relevant for prognosis and future chemotherapeutic approaches. This review discusses the influence of merlin loss of function in NF2-related tumors and common human cancers. We also discuss the NF2 gene status and merlin signaling pathways affected in the different tumor types and the molecular mechanisms that lead to tumorigenesis, progression and pharmacological resistance.

A splicing variant of Merlin promotes metastasis in hepatocellular carcinoma

Merlin, which is encoded by the tumour suppressor gene Nf2, plays a crucial role in tumorigenesis and metastasis. However, little is known about the functional importance of Merlin splicing forms. In this study, we show that Merlin is present at low levels in human hepatocellular carcinoma (HCC), particularly in metastatic tumours, where it is associated with a poor prognosis. Surprisingly, a splicing variant of Merlin that lacks exons 2, 3 and 4 (^Δ2–4Merlin) is amplified in HCC and portal vein tumour thrombus (PVTT) specimens and in the CSQT2 cell line derived from PVTT. Our studies show that ^Δ2–4Merlin interferes with the capacity of wild-type Merlin to bind β-catenin and ERM, and it is expressed in the cytoplasm rather than at the cell surface. Furthermore, ^Δ2–4Merlin overexpression increases the expression levels of β-catenin and stemness-related genes, induces the epithelium–mesenchymal-transition phenotype promoting cell migration in vitro and the formation of lung metastasis in vivo. Our results indicate that the ^Δ2–4Merlin variant disrupts the normal function of Merlin and promotes tumour metastasis.

Merlin plays a crucial role as a tumour suppressor in liver tumorigenesis. Here, the authors show that a splicing variant of Merlin that lacks exons 2,3 and 4 (^Δ2–4Merlin) is highly expressed in hepatocarcinoma and promotes tumour metastasis by interfering with the binding of wild-type Merlin to ß-catenin.

Functional interdependence of NHE1 and merlin in human melanoma cells

Upregulation of the Na(+)/H(+) exchanger isoform 1 (NHE1) has been correlated with tumor malignancy. In contrast, moesin-radixin-ezrin-like protein (merlin) is a tumor suppressor that protects from cancerogenesis. Merlin is highly related to the members of the ezrin, radixin, and moesin (ERM) protein family that are directly attached to and functionally linked with NHE1. In addition, merlin inhibits the MAPK cascade and the Rho-GTPases known to activate NHE1 activity. The present study investigates whether NHE1 expression and activity affect merlin or, conversely, whether merlin has an impact on NHE1 in human melanoma (MV3) cells. Indeed, features of merlin-deficient MV3 cells point to a functional link: merlin-deficient cells showed a decreased NHE1 expression and, paradoxically, an increase in NHE1 activity as measured upon cytosolic acidification (NH4Cl prepulse method). Loss of merlin also led to an elevated cell motility that could be further increased by NHE1 overexpression, whereas NHE1 overexpression alone had no effect on migration. In contrast, neither NHE1 expression nor its activity had an impact on merlin expression. These results suggest a novel tumor suppressor function of merlin in melanoma cells: the inhibition of the proto-oncogenic NHE1 activity, possibly including its downstream signaling pathways.

Molecular insights into NF2/Merlin tumor suppressor function

The FERM domain protein Merlin, encoded by the NF2 tumor suppressor gene, regulates cell proliferation in response to adhesive signaling. The growth inhibitory function of Merlin is induced by intercellular adhesion and inactivated by joint integrin/receptor tyrosine kinase signaling. Merlin contributes to the formation of cell junctions in polarized tissues, activates anti-mitogenic signaling at tight-junctions, and inhibits oncogenic gene expression. Thus, inactivation of Merlin causes uncontrolled mitogenic signaling and tumorigenesis. Merlin's predominant tumor suppressive functions are attributable to its control of oncogenic gene expression through regulation of Hippo signaling. Notably, Merlin translocates to the nucleus where it directly inhibits the CRL4(DCAF1) E3 ubiquitin ligase, thereby suppressing inhibition of the Lats kinases. A dichotomy in NF2 function has emerged whereby Merlin acts at the cell cortex to organize cell junctions and propagate anti-mitogenic signaling, whereas it inhibits oncogenic gene expression through the inhibition of CRL4(DCAF1) and activation of Hippo signaling. The biochemical events underlying Merlin's normal function and tumor suppressive activity will be discussed in this Review, with emphasis on recent discoveries that have greatly influenced our understanding of Merlin biology.

Loss of NF2/Merlin expression in advanced sporadic colorectal cancer

PURPOSE

NF2/Merlin was first identified through its association with neurofibromatosis type 2 (NF2). However, accumulating evidence suggests a more general involvement in tumorigenesis and, in particular, a broader role in tumor suppression. The aim of this study was to examine NF2/Merlin involvement in sporadic colorectal cancer.

METHODS

This study is the first to examine the role of NF2/Merlin in sporadic colorectal cancer through LOH analysis at the NF2 locus and mRNA expression analysis via quantitative RT-PCR of total NF2, NF2 isoform I and II. In addition, Merlin protein expression was assessed by immunohistochemistry and Western blotting.

RESULTS

NF2 LOH was detected in 20.0 % of heterozygous cases and was found to be more frequent in tumors larger than 5 cm in diameter (p = 0.041) and in tumors with a less differentiated phenotype (p = 0.027). No differences were observed in total NF2 and NF2 isoform I/isoform II mRNA expression between the tumors and their corresponding normal mucous tissues. NF2 isoform II was the most predominant isoform in all samples analyzed. mRNA expression levels of total NF2 and isoforms I and II were significantly lower in poorly differentiated tumors (p = 0.033, p = 0.036 and p = 0.044, respectively). Weak Merlin immunostaining was more frequent in poorly differentiated tumors (p = 0.034) and tumors classified as Dukes' C (p = 0.023). A distinct pattern of Merin phosphorylation was observed in tumors compared to normal mucous tissues.

CONCLUSION

Our data indicate that NF2/Merlin may serve as a potential target in the management of colorectal cancer.

Tumor suppressor NF2/Merlin is a microtubule stabilizer

Cancer-associated mutations in oncogene products and tumor suppressors contributing to tumor progression manifest themselves, at least in part, by deregulating microtubule-dependent cellular processes that play important roles in many cell biological pathways, including intracellular transport, cell architecture, and primary cilium and mitotic spindle organization. An essential characteristic of microtubules in the performance of these varied cell processes is their ability to continuously remodel, a phenomenon known as dynamic instability. It is therefore conceivable that part of the normal function of certain cancer-causing genes is to regulate microtubule dynamic instability. Here, we report the results of a high-resolution live-cell image-based RNA interference screen targeting a collection of 70 human tumor suppressor genes to uncover cancer genes affecting microtubule dynamic instability. Extraction and computational analysis of microtubule dynamics from EB3-GFP time-lapse image sequences identified the products of the tumor suppressor genes NF1 and NF2 as potent microtubule-stabilizing proteins. Further in-depth characterization of NF2 revealed that it binds to and stabilizes microtubules through attenuation of tubulin turnover by lowering both rates of microtubule polymerization and depolymerization as well as by reducing the frequency of microtubule catastrophes. The latter function appears to be mediated, in part, by inhibition of hydrolysis of tubulin-bound GTP on the growing microtubule plus end.

Merlin: a tumour suppressor with functions at the cell cortex and in the nucleus

Inhibition of proliferation by cell-to-cell contact is essential for tissue organization, and its disruption contributes to tumorigenesis. The FERM domain protein Merlin, encoded by the NF2 tumour suppressor gene, is an important mediator of contact inhibition. Merlin was thought to inhibit mitogenic signalling and activate the Hippo pathway by interacting with diverse target-effectors at or near the plasma membrane. However, recent studies highlight that Merlin pleiotropically affects signalling by migrating into the nucleus and inducing a growth-suppressive programme of gene expression through its direct inhibition of the CRL4DCAF1 E3 ubiquitin ligase. In addition, Merlin promotes the establishment of epithelial adhesion and polarity by recruiting Par3 and aPKC to E-cadherin-dependent junctions, and by ensuring the assembly of tight junctions. These recent advances suggest that Merlin acts at the cell cortex and in the nucleus in a similar, albeit antithetic, manner to the oncogene β-catenin.

Merlin: the wizard requires protein stability to function as a tumor suppressor

Neurofibromatosis type 2 (NF2), characterized by tumors of the nervous system, is a result of functional loss of the NF2 gene. The NF2 gene encodes Merlin (moesin-ezrin-radixin-like protein), an ERM (Ezrin, Radixin, Moesin) protein family member. Merlin functions as a tumor suppressor through impacting mechanisms related to proliferation, apoptosis, survival, motility, adhesion, and invasion. Several studies have summarized the tumor intrinsic mutations in Merlin. Given the fact that tumor cells are not in isolation, but rather in an intricate, mutually sustaining synergy with their surrounding stroma, the dialog between the tumor cells and the stroma can potentially impact the molecular homeostasis and promote evolution of the malignant phenotype. This review summarizes the epigenetic modifications, transcript stability, and post-translational modifications that impact Merlin. We have reviewed the role of extrinsic factors originating from the tumor milieu that influence the availability of Merlin inside the cell. Information regarding Merlin regulation could lead to novel therapeutics by stabilizing Merlin protein in tumors that have reduced Merlin protein expression without displaying any NF2 genetic alterations.

Overexpression of ezrin inactivates NF2 tumor suppressor in glioblastoma

Glioblastoma is a frequent brain malignancy with a dismal prognosis. The molecular changes causing its aggressive phenotype are under investigation. We report that the cytoskeletal-related proteins neurofibromatosis type 2 (NF2) and ezrin have opposite yet interdependent activities in glioblastoma growth. We show that NF2 is absent in approximately one-third of glioblastoma cell lines and tumors, and that it suppresses growth when expressed in cells. Although ezrin overexpression was previously observed in glioblastoma, we show here that ezrin enhanced cell proliferation and anchorage-independent growth but only in cells expressing NF2. Ezrin interacted and delocalized NF2 from the cortical compartment releasing its inhibition on Rac1. By using swap NF2-ezrin molecules, we identified that the opposite effects on cell growth of NF2 and ezrin depend on their amino-terminal FERM domain. The subcellular cortical localization appeared important for NF2 suppressive activity. In contrast, the ability of ezrin to enhance growth or complex NF2 did not depend on the molecular conformation or subcellular localization. In conclusion, these studies show 2 mechanisms for NF2 inactivation in glioblastoma: (i) decreased protein expression and (ii) increasing dosages of ezrin that disable NF2 by intermolecular association and aberrant intracellular recruitment.

Shedding light on Merlin's wizardry

Inactivation of the tumor suppressor Merlin, encoded by the NF2 (Neurofibromatosis type 2) gene, contributes to malignant conversion in many cell types. Merlin is an Ezrin-Radixin-Moesin protein and localizes underneath the plasma membrane at cell-cell junctions and other actin-rich sites. Recent studies indicate that Merlin mediates contact inhibition of proliferation by blocking recruitment of Rac to the plasma membrane. In mitogen-stimulated cells, p21-activated kinase phosphorylates Ser518 in the C-terminus of Merlin, inactivating the growth suppressive function of the protein. Furthermore, the myosin phosphatase MYPT1-PP1delta, has been identified as a direct activator of Merlin and its inhibition has been linked to malignant transformation. Finally, studies in the fruit fly Drosophila melanogaster have revealed that Merlin functions together with the band 4.1 protein Expanded to promote [corrected] the endocytosis of many signaling receptors, limiting [corrected] their accumulation at the plasma membrane, and to activate [corrected] the Hippo signaling pathway. Here, we review these recent findings and their relevance to the tumor suppressor function of Merlin.

Mutational spectrum of the NF2 gene: a meta-analysis of 12 years of research and diagnostic laboratory findings

The NF2 tumor suppressor gene on chromosome 22 is a member of the protein 4.1 family of cytoskeletal elements. A number of single- and multiple-tumor phenotypes have been linked to alterations of NF2 since its characterization in 1993. We present a meta-analysis of 967 constitutional and somatic NF2 alterations from 93 published reports, along with 59 additional unpublished events identified in our laboratory and 115 alterations identified in clinical samples submitted to the Massachusetts General Hospital (MGH) Neurogenetics DNA Diagnostic Laboratory. In total, these sources defined 1,070 small genetic changes detected primarily by exon scanning, 42 intragenic changes of one whole exon or larger, and 29 whole gene deletions and gross chromosomal rearrangements. Constitutional single-exon events (N=422) were significantly more likely to be nonsense or splice site changes than somatic events (N=533), which favored frameshift changes (chi(2) test; P<0.001). Somatic events also differed markedly between tumors of different pathology, most significantly in the tendency of somatic events in meningiomas to lie within the 5' FERM domain of the transcript (Fisher's exact test; P<0.01 in comparison to schwannomas) with a complete absence of mutations in exons 14 and 15. There was no statistically significant difference in mutation type or exon distribution between published constitutional events and those found by the clinical laboratory. Less than 10% of all published and unpublished small alterations are nontruncating (N=63) and these changes are clustered in exons 2 and 3, suggesting that this region may be especially crucial to tumor suppressor activity in the protein.

Genetic and epigenetic alterations of the candidate tumor-suppressor gene MYO18B, on chromosome arm 22q, in colorectal cancer

Allelic imbalance (AI) on chromosome arm 22q has been detected in 20%-40% of colorectal cancers, suggesting that this chromosome arm has a tumor-suppressor gene involved in colorectal carcinogenesis. Recently, we isolated a candidate tumor-suppressor gene, MYO18B, at 22q12.1, that is deleted, mutated, and hypermethylated in more than 50% of lung cancers. In the present study, we analyzed genetic and epigenetic alterations of the MYO18B gene in colorectal cancers. AI at the MYO18B locus was detected in 16 of 43 (40%) informative cases. Mutations of the MYO18B gene were detected in 2 of 11 (18%) cell lines and 1 of 47 (2%) surgical specimens. Nine of 11 (82%) cell lines showed reduced MYO18B expression, which was restored in all 9 by treatment with 5-aza-2'-deoxycytidine and/or trichostatin A (TSA). Although hypermethylation of the promoter CpG island for MYO18B was not detected, a significant correlation was observed between the level of MYO18B expression and the level of acetylation of histones H3 and H4 in 6 cell lines with and without TSA treatment. Thus, it was suggested that MYO18B is inactivated in a considerable fraction of colorectal cancers by several mechanisms, especially silencing by histone deacetylation and/or AI. Furthermore, restoration of MYO18B expression in colorectal cancer cell lines HT29 and DLD-1 suppressed anchorage-independent growth, whereas it did not affect the growth rate in vitro. These results suggest that genetic and epigenetic inactivation of the MYO18B gene play an important role in colorectal carcinogenesis.

Mutation analysis of EP300 in colon, breast and ovarian carcinomas

The putative tumour suppressor gene EP300 is located on chromosome 22q13 which is a region showing frequent loss of heterozygosity (LOH) in colon, breast and ovarian cancers. We analysed 203 human breast, colon and ovarian primary tumours and cell lines for somatic mutations in EP300. LOH across the EP300 locus was detected in 38% of colon, 36% of breast, and 49% of ovarian primary tumours but no somatic mutations in EP300 were identified in any primary tumour. Analysis of 17 colon, 11 breast, and 11 ovarian cancer cell lines identified truncating mutations in 4 colon cancer cell lines (HCT116, HT29, LIM2405 and LIM2412). We confirmed the presence of a previously reported frameshift mutation in HCT116 at codon 1699 and identified a second frameshift mutation at codon 1468. Bi-allelic inactivation of EP300 was also detected in LIM2405 that harbours an insC mutation at codon 927 as well an insA mutation at codon 1468. An insA mutation at codon 1468 was identified in HT29 and a CGA>TGA mutation at codon 86 was identified in LIM2412. Both these lines were heterozygous across the EP300 locus and western blot analysis confirmed the presence of an apparently wild-type protein. Our study has established that genetic inactivation of EP300 is rare in primary colorectal, breast and ovarian cancers. In contrast, mutations are common among colorectal cancer cell lines with 4/17 harbouring homozygous or heterozygous mutations. The rarity of EP300 mutations among these tumour types that show a high frequency of LOH across 22q13 may indicate that another gene is the target of the loss. It is possible that bi-allelic inactivation of EP300 is not necessary and that haploinsufficiency is sufficient to promote tumorigenesis. Alternatively, silencing of EP300 may be achieved by epigenetic mechanisms such as promoter methylation.

Loss of heterozygosity on long arm of chromosome 22 in sporadic colorectal carcinoma

AIM: The loss of heterozygosity (LOH) on tumor suppressor genes is believed to play a key role in carcinogenesis of colorectal cancer. In this study, we analyzed the LOH at 5 loci on the long arm of chromosome 22 in sporadic colorectal cancer to identify additional loci involved in colorectal tumorigenesis.

METHODS: Five polymorphic microsatellite markers were analyzed in 83 cases of colorectal and normal DNA by PCR. PCR products were eletrophoresed on an ABI 377 DNA sequencer; Genescan 3.1 and Genotype 2.1 software were used for LOH scanning and analysis. Comparison between LOH frequency and clinicopathological data were performed by χ² test. P < 0.05 was considered as statistically significant.

RESULTS: The average LOH frequency on chromosome 22q was 28.38%. The region between markers D22S280 and D22S274 (22q12.2-q13.33) exhibited relatively high LOH frequency. The two highest LOH loci with frequencies of 35.09% and 34.04% was identified on D22S280 (22q12.2-12.3) and D22S274 (22q13.32-13.33).8 cases showed LOH at all informative loci, suggesting that one chromosome 22q had been completely lost. On D22S274 locus, LOH frequency of rectal cancer was 50% (9/18), which was higher than that of proximal colon cancer (12%, 2/17) (P = 0.018). The frequency of distal colon cancer was 42% (5/12), also higher than that of proximal colon cancer. But there was no statistical significance. Putting both the tumors in distal colon and rectum together into consideration, the frequency, 47% (14/30), was higher than that of proximal colon cancer (P = 0.015), suggesting the mechanism of carcinogenisis was different in both groups.

CONCLUSIONS: This study provided evidence for the involvement of putative tumor suppressor genes related to the sporadic colorectal carcinoma on chromosome 22q. The tumor-suppressor-gene (s) might locate on the 22q12.2-12.3 and/or 22q13.32-13.33.

ERM-Merlin and EBP50 protein families in plasma membrane organization and function

The ezrin-radixin-moesin (ERM) family of proteins have emerged as key regulatory molecules in linking F-actin to specific membrane proteins, especially in cell surface structures. Merlin, the product of the NF2 tumor suppressor gene, has sequence similarity to ERM proteins and binds to some of the same membrane proteins, but lacks a C-terminal F-actin binding site. In this review we discuss how ERM proteins and merlin are negatively regulated by an intramolecular association between their N- and C-terminal domains. Activation of at least ERM proteins can be accomplished by C-terminal phosphorylation in the presence of PIP2. We also discuss membrane proteins to which ERM and merlin bind, including those making an indirect linkage through the PDZ-containing adaptor molecules EBP50 and E3KARP. Finally, the function of these proteins in cortical structure, endocytic traffic, signal transduction, and growth control is discussed.

Allelic loss of 14q and 22q, NF2 mutation, and genetic instability occur independently of c-kit mutation in gastrointestinal stromal tumor

Gastrointestinal stromal tumor (GIST) is the most common mesenchymal tumor of the gastrointestinal tract. Since c-kit mutation occurs only in one-third of GIST, there might be other molecular mechanisms. Loss of heterozygosity (LOH), microsatellite instability (MSI) and NF2 gene mutation were investigated in 22 GISTs (9 low-risk and 13 high-risk tumors). LOH and MSI were evaluated using 41 markers on 21 chromosomal arms, and NF2 gene mutation was examined by PCR-SSCP. High frequency of LOH was observed on 14q (9 / 19, 47%), and 22q (17 / 22, 77%). The frequencies were similar in low-risk and high-risk tumors, and were unrelated with gastric or intestinal origin. Two other abnormalities, additional LOH on other chromosomes and MSI at more than two loci, were characteristic of the high-risk tumors (P < 0.05). NF2 gene mutation was identified in two cases showing 22q-LOH (8 bp deletion on the splice donor site of exon 7, and 1 bp insertion at position 432 of exon 4, which resulted in nonsense mutation). There was no significant correlation between these results and c-kit gene mutation, which was observed in 8 of 22 tumors. Suppressor genes on 14q and 22q may be involved, independently of c-kit gene mutation, in the development of GIST. NF2 contributes as a tumor suppressor in a small subset of GIST. These abnormalities are presumably followed by increased genetic instability.

Refinement of an ovarian cancer tumour suppressor gene locus on chromosome arm 22q and mutation analysis of CYP2D6, SREBP2 and NAGA

Loss of heterozygosity on chromosome 22q was detected in 53% of 123 ovarian carcinomas, suggesting the presence of at least one tumour suppressor gene. We have refined the location of one possible tumour suppressor gene to the region between the microsatellite markers D22S299 and CYP2D. Located within this region are the genes SREBP2 (sterol regulatory element binding protein 2) and NAGA (N-acetyl-alpha-D-galactosaminidase). Investigation of the coding exons of these genes by single stranded conformational polymorphism/heteroduplex analysis failed to identify any somatic genetic alterations in 57 ovarian tumours which exhibited LOH on 22q13. The CYP2D gene locus straddles the distal boundary of the candidate region. Germline variants of the active CYP2D6 gene with differing abilities to metabolise specific substrates have been implicated in the development of various cancers. Comparison of the frequency of the two common germline mutations among 258 ovarian tumours and 231 non-cancer controls did not reveal any significant differences between the two groups. This suggests that the known polymorphic variants of CYP2D6 are not involved in ovarian cancer predisposition. We also conclude that neither NAGA nor SREBP2 are likely to be mutated in ovarian carcinomas.

Use of crypt isolation to determine loss of heterozygosity of multiple tumor suppressor genes in colorectal carcinoma

Analysis of loss of heterozygosity (LOH) is very important in the study of tumor suppressor genes. However, accurate LOH analysis of tumor suppressor genes is difficult because of dilution by contaminating non-tumor DNA. Thus, enrichment of tumor DNA is required to accurately determine LOH of the tumor. We developed a new application of the fluorescent polymerase chain reaction by coupling it with crypt isolation to accurately assess the incidence of LOH of tumor suppressor genes in 45 colorectal carcinomas. LOH was observed at p53 in 26 of 37 tumors (70.3%), at APC in 13 of 35 (37.1%), at DCC in 16 of 25 (64.0%), at NF-2 in 5 of 23 (21.7%), and at nm23 H-1 in 7 of 30 (23.3%). We could clearly determine LOH of these genes because the crypt isolation technique was used. Although the incidence of LOH at each of these loci, as determined by using this technique, was similar to that obtained in previous studies using conventional methods, this method provides a simpler, more accurate way to assess LOH. In addition, the morphology of the samples can be analyzed before genetic analysis.

A unique method for mutation analysis of tumor suppressor genes in colorectal carcinomas using a crypt isolation technique

BACKGROUND

Contamination of nontumor tissue makes genetic analysis difficult. For this reason, it is important to obtain pure tumor tissue to ensure accurate genetic analysis.

OBJECTIVE

To accurately assess the incidence of mutation of tumor suppressor genes (p53: exon 5-8; APC: mutated cluster region; NF-2 gene: all exons) in 45 colorectal carcinomas.

METHODS

We developed an application of the polymerase chain reaction-single-strand conformation polymorphism and DNA sequence by coupling them with crypt isolation.

RESULTS

Mutations of p53 and APC genes were found in 24 and 22 of 45 colorectal carcinomas, respectively. No mutation of the NF-2 gene was observed in this cancer. Single-strand conformation polymorphism using a crypt isolation technique showed a clear migrating band and no false-positive data.

CONCLUSIONS

The crypt isolation technique is a useful method for accurately analyzing genetic alterations. Furthermore, our proposed method confirmed the morphological findings obtained before the genetic analysis.

Mapping of a target region of allelic loss to a 0.5-cM interval on chromosome 22q13 in human colorectal cancer

BACKGROUND & AIMS

Chromosomal allelic losses have a varying frequency in colorectal cancer. The aim of this study was to define the target region of allelic loss on chromosome 22q in human colorectal carcinogenesis.

METHODS

Fifty-seven pairs of matched normal colonic mucosa and tumor specimens from patients with colorectal cancer, as well as 15 colon cancer-derived cell lines, were genotyped using 15 microsatellite markers spanning chromosome 22q. A potential candidate gene was analyzed by a single-strand conformation polymorphism (SSCP)/direct DNA sequencing approach.

RESULTS

After excluding 7 tumors with evidence of microsatellite instability, allelic loss was observed in 11 of the informative tumors (22%), 5 of which exhibited losses in all informative loci. The remaining 6 tumors showed variable patterns of partial allelic loss on chromosome 22q, thereby localizing a minimal region of allelic deletion between markers D22S1171 and D22S928. Physical mapping showed that this interval was 0.57 cM consisting of approximately 425 kilobases. Database searches identified the NBK/BIK gene, a proapoptotic BCL-2 family member, as a candidate gene in that region. However, SSCP/sequencing analysis excluded mutations of this gene.

CONCLUSIONS

This study provides evidence for the involvement of putative tumor-suppressor gene(s) on chromosome 22q in human colorectal carcinogenesis. The identification of a 0.5-cM interval serves as the basis for the isolation of such a gene by positional cloning.

Structure-function relationships in the ezrin family and the effect of tumor-associated point mutations in neurofibromatosis 2 protein

Ezrin, radixin and moesin (ERM proteins) link cell adhesion molecules to the cytoskeleton, modulate cell morphology and cell growth and are involved in Rho-mediated signal transduction. Merlin, the tumor suppressor in neurofibromatosis 2, is a diverged member of the ezrin family, but its function is at least partially similar to the ERM proteins. In the N-domain, the ezrin family belongs to the band 4.1 superfamily. Secondary structure predictions made separately for the ezrin and band 4.1-tyrosine phosphatase families give a similar pattern for the homologous N-domains, indicating that both families have a similar binding site for the integral membrane proteins. The alpha-domain shows a strong coiled-coil prediction, that can be involved in the protein dimerization. The C-terminal actin-binding site in the ERM proteins and the actin-binding helix in the villin headpiece have a common amino acid motif. In merlin, the published tumor-associated single amino acid mutations in the N-domain are located in the conserved sites, and they affect mainly the predicted helices and strands, indicating that these mutations cause the disease primarily by disturbing the protein structure. In the alpha- and C-domains, some of the mutations break the helical structures. Some known mutations are observed at a site potentially interacting with cell adhesion molecules. We will also discuss the implications of the evolutionary information and the actin-binding models in the ezrin family.

Frequency and distribution of NF2 mutations in schwannomas

Sporadic and inherited schwannomas were scanned for the nature, frequency, and distribution of mutations in the NF2 locus encoding the merlin tumor suppressor protein on 22q. Of 58 tumors, 47% displayed loss of heterozygosity for NF2, leaving a total of 89 NF2 alleles to be examined. Pathogenic alterations were identified in 62 of these alleles, including 36 frameshifts with premature termination, 14 nonsense mutations, and 12 changes presumed to affect splicing. Effects of ten of the latter were confirmed in the NF2 transcript and indicated that activation of cryptic splice sites in coding sequence is another frequent mechanism leading to truncation of merlin. The mutations were relatively evenly distributed across both the protein 4.1 superfamily (exons 1-9) and the alpha-helical (exons 10-15) domains of merlin, but they did not occur at all in exons 16 and 17, which encode the protein's alternative COOH-termini. The data support the "two-hit" tumor suppressor model for formation of schwannomas and indicate that loss of merlin function can be achieved by truncation at various locations in the protein. However, the absence of mutations in exons 16 and 17 suggests that an inactivating mutation affecting only one of the merlin's alternative termini may not be sufficient to eliminate tumor suppressor function.

Neuropathology and molecular genetics of neurofibromatosis 2 and related tumors

Neurofibromatosis 2 (NF2) is an uncommon, autosomal dominant disorder in which patients are predisposed to neoplastic and dysplastic lesions of Schwann cells (schwannomas and schwannosis), meningeal cells (meningiomas and meningioangiomatosis) and glial cells (gliomas and glial hamartomas). Clinical and genetic criteria that distinguish NF2 from neurofibromatosis 1 have allowed more accurate assignment of specific pathological features to NF2. The NF2 tumor suppressor gene on chromosome 22q12 encodes a widely expressed protein, named merlin, which may link the cytoskeleton and cell membrane. Germline NF2 mutations in NF2 patients and somatic NF2 mutations in sporadic schwannomas and meningiomas have different mutational spectra, but most NF2 alterations result in a truncated, inactivated merlin protein. In NF2 patients, specific mutations do not necessarily correlate with phenotypic severity, although grossly truncating alterations may result in a more severe phenotype. In schwannomas, NF2 mutations are common and may be necessary for tumorigenesis. In meningiomas, NF2 mutations occur more commonly in fibroblastic than meningothelial subtypes, and may cluster in the first half of the gene. In addition, in meningiomas, a second, non-NF2 meningioma locus is probably also involved. Future efforts in NF2 research will be directed toward elucidating the role of merlin in the normal cell and the sequelae of its inactivation in human tumors.