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

Mutational analysis of tumour suppressor gene NF2 in common solid cancers and acute leukaemias

AIMS

Germline mutation of NF2 gene is a feature of neurofibromatosis type 2 familial cancer syndrome. Also, somatic point mutations of NF2 mutation have been reported in tumours originated from nerve structures. A recent study revealed that NF2 gene was mutated in renal cell carcinoma (RCC) as well, suggesting a possibility that NF2 gene might be somatically mutated in other human cancers. The aim of this study was to explore whether NF2 genes are somatically mutated, and contribute to tumorigenesis in common human cancers.

METHODS

For this, we analysed the entire coding region of NF2 gene in 45 colorectal carcinomas, 45 gastric, 45 breast, 45 lung, 45 hepatocellular (HCC), 45 prostate carcinomas, and 45 acute leukaemias by a single-strand conformation polymorphism assay.

RESULTS

Overall, we found NF2 mutations in one HCC (1/45; 2.2%) (hepatitis B virus-related HCC), one lung carcinoma (1/45; 2.2%) (squamous cell carcinoma), and one acute leukaemia (1/45; 2.2%) (acute myelogenous leukaemia minimally differentiated). All of the mutations were missense mutations that would substitute amino acids in the NF2 protein (p.A238 V, p.A451T and p.R467K).

CONCLUSION

Our data indicate that somatic mutation of NF2 gene is not prevalent in common human cancers, and its mutation somatically occurs in a minor fraction of HCC, lung cancer and acute leukaemia. These data suggest that somatic mutation of NF2 tumour suppressor gene may not play a central role in development of common cancers.

Molecular mechanisms promoting the pathogenesis of Schwann cell neoplasms

Neurofibromas, schwannomas and malignant peripheral nerve sheath tumors (MPNSTs) all arise from the Schwann cell lineage. Despite their common origin, these tumor types have distinct pathologies and clinical behaviors; a growing body of evidence indicates that they also arise via distinct pathogenic mechanisms. Identification of the genes that are mutated in genetic diseases characterized by the development of either neurofibromas and MPNSTs [neurofibromatosis type 1 (NF1)] or schwannomas [neurofibromatosis type 2 (NF2), schwannomatosis and Carney complex type 1] has greatly advanced our understanding of these mechanisms. The development of genetically engineered mice with ablation of NF1, NF2, SMARCB1/INI1 or PRKAR1A has confirmed the key role these genes play in peripheral nerve sheath tumorigenesis. Establishing the functions of the NF1, NF2, SMARCB1/INI1 and PRKAR1A gene products has led to the identification of key cytoplasmic signaling pathways promoting Schwann cell neoplasia and identified new therapeutic targets. Analyses of human neoplasms and genetically engineered mouse models have established that interactions with other tumor suppressors such as TP53 and CDKN2A promote neurofibroma-MPNST progression and indicate that intratumoral interactions between neoplastic and non-neoplastic cell types play an essential role in peripheral nerve sheath tumorigenesis. Recent advances have also provided new insights into the identity of the neural crest-derived populations that give rise to different types of peripheral nerve sheath tumors. Based on these findings, we now have an initial outline of the molecular mechanisms driving the pathogenesis of neurofibromas, MPNSTs and schwannomas. However, this improved understanding in turn raises a host of intriguing new questions.

Merlin/NF2 functions upstream of the nuclear E3 ubiquitin ligase CRL4DCAF1 to suppress oncogenic gene expression

Integrin-mediated activation of PAK (p21-activated kinase) causes phosphorylation and inactivation of the FERM (4.1, ezrin, radixin, moesin) domain-containing protein Merlin, which is encoded by the NF2 (neurofibromatosis type 2) tumor suppressor gene. Conversely, cadherin engagement inactivates PAK, thus leading to accumulation of unphosphorylated Merlin. Current models imply that Merlin inhibits cell proliferation by inhibiting mitogenic signaling at or near the plasma membrane. We have recently shown that the unphosphorylated, growth-inhibiting form of Merlin accumulates in the nucleus and binds to the E3 ubiquitin ligase CRL4(DCAF1) to suppress its activity. Depletion of DCAF1 blocks the hyperproliferation caused by inactivation of Merlin. Conversely, expression of a Merlin-insensitive DCAF1 mutant counteracts the antimitogenic effect of Merlin. Expression of Merlin or silencing of DCAF1 in Nf2-deficient cells induce an overlapping, tumor-suppressive program of gene expression. Mutations present in some tumors from NF2 patients disrupt Merlin's ability to interact with or inhibit CRL4(DCAF1). Lastly, depletion of DCAF1 inhibits the hyperproliferation of Schwannoma cells isolated from NF2 patients and suppresses the oncogenic potential of Merlin-deficient tumor cell lines. Current studies are aimed at identifying the substrates and mechanism of action of CRL4(DCAF1) and examining its role in NF2-dependent tumorigenesis in mouse models. We propose that Merlin mediates contact inhibition and suppresses tumorigenesis by translocating to the nucleus to inhibit CRL4(DCAF1).

Missense mutations in the NF2 gene result in the quantitative loss of merlin protein and minimally affect protein intrinsic function

Neurofibromatosis type 2 (NF2) is a multiple neoplasia syndrome and is caused by a mutation of the NF2 tumor suppressor gene that encodes for the tumor suppressor protein merlin. Biallelic NF2 gene inactivation results in the development of central nervous system tumors, including schwannomas, meningiomas, ependymomas, and astrocytomas. Although a wide variety of missense germline mutations in the coding sequences of the NF2 gene can cause loss of merlin function, the mechanism of this functional loss is unknown. To gain insight into the mechanisms underlying loss of merlin function in NF2, we investigated mutated merlin homeostasis and function in NF2-associated tumors and cell lines. Quantitative protein and RT-PCR analysis revealed that whereas merlin protein expression was significantly reduced in NF2-associated tumors, mRNA expression levels were unchanged. Transfection of genetic constructs of common NF2 missense mutations into NF2 gene-deficient meningioma cell lines revealed that merlin loss of function is due to a reduction in mutant protein half-life and increased protein degradation. Transfection analysis also demonstrated that recovery of tumor suppressor protein function is possible, indicating that these mutants maintain intrinsic functional capacity. Further, increased expression of mutant protein is possible after treatment with specific proteostasis regulators, implicating protein quality control systems in the degradative fate of mutant tumor suppressor proteins. These findings provide direct insight into protein function and tumorigenesis in NF2 and indicate a unique treatment paradigm for this disorder.

Spinal ependymomas in neurofibromatosis Type 2: a retrospective analysis of 55 patients

OBJECT

The aim of this paper was to define the clinical characteristics of spinal ependymomas associated with neurofibromatosis Type 2 (NF2).

METHODS

The authors retrospectively reviewed the clinical records of patients with NF2 who had imaging findings consistent with ependymomas and were seen at Massachusetts General Hospital between 1994 and 2007. Clinical characteristics of these patients were obtained from hospital records, imaging studies, surgical reports, and pathology reports. Mutational analysis of the NF2 gene was performed in 37 of 44 unrelated patients.

RESULTS

Fifty-five patients met inclusion criteria for the study. The median age at diagnosis of NF2 was 21 years; the median time after diagnosis until identification of ependymomas was 5 years. Multiple ependymomas were present in 58% of patients. The most common site of involvement was the cervical cord or cervicomedullary junction (86% of imaging studies), followed by the thoracic and lumbar cords (62% and 8%, respectively). The majority of patients had no symptoms related to their tumors (42 patients [76%]). After a median follow-up of 50 months, surgery was performed in 11 patients (20%) for symptomatic progression (indications for surgery). Mutational analysis of the NF2 gene detected alterations in 28 (76%) of 37 unrelated patients, with nonsense and frameshift mutations accounting for 64% of detected mutations. The high rate of truncating mutations may help explain the high tumor burden in these patients.

CONCLUSIONS

Neurofibromatosis Type 2-related ependymomas exhibit an indolent growth pattern with tumor progression limited to a minority of patients. The authors believe that surveillance is reasonable for asymptomatic ependymomas, including those with cystic areas that expand the cord. For symptomatic tumors, resection may be warranted depending on age, overall clinical status, and ease of resectability.

The myosin phosphatase targeting protein (MYPT) family: a regulated mechanism for achieving substrate specificity of the catalytic subunit of protein phosphatase type 1δ

The mammalian MYPT family consists of the products of five genes, denoted MYPT1, MYPT2, MBS85, MYPT3 and TIMAP, which function as targeting and regulatory subunits to confer substrate specificity and subcellular localization on the catalytic subunit of type 1δ protein serine/threonine phosphatase (PP1cδ). Family members share several conserved domains, including an RVxF motif for PP1c binding and several ankyrin repeats that mediate protein-protein interactions. MYPT1, MYPT2 and MBS85 contain C-terminal leucine zipper domains involved in dimerization and protein-protein interaction, whereas MYPT3 and TIMAP are targeted to membranes via a C-terminal prenylation site. All family members are regulated by phosphorylation at multiple sites by various protein kinases; for example, Rho-associated kinase phosphorylates MYPT1, MYPT2 and MBS85, resulting in inhibition of phosphatase activity and Ca(2+) sensitization of smooth muscle contraction. A great deal is known about MYPT1, the myosin targeting subunit of myosin light chain phosphatase, in terms of its role in the regulation of smooth muscle contraction and, to a lesser extent, non-muscle motile processes. MYPT2 appears to be the key myosin targeting subunit of myosin light chain phosphatase in cardiac and skeletal muscles. MBS85 most closely resembles MYPT2, but little is known about its physiological function. Little is also known about the physiological role of MYPT3, although it is likely to target myosin light chain phosphatase to membranes and thereby achieve specificity for substrates involved in regulation of the actin cytoskeleton. MYPT3 is regulated by phosphorylation by cAMP-dependent protein kinase. TIMAP appears to target PP1cδ to the plasma membrane of endothelial cells where it serves to dephosphorylate proteins involved in regulation of the actin cytoskeleton and thereby control endothelial barrier function. With such a wide range of regulatory targets, MYPT family members have been implicated in diverse pathological events, including hypertension, Parkinson's disease and cancer.

Molecular characterization of the NF2 gene in Korean patients with neurofibromatosis type 2: a report of four novel mutations

BACKGROUND

Neurofibromatosis type 2 (NF2) is an autosomal dominant syndrome caused by the NF2 tumor suppressor gene. However, the NF2 mutation characteristics in Korean patients are not sufficiently understood. In this study, we conducted a comprehensive mutational analysis in 7 Korean NF2 patients by performing direct sequencing and gene-dosage assessment.

METHODS

We analyzed all exons and flanking regions of NF2 by direct sequencing and screened the deletions or duplications involving NF2 by multiplex ligation-dependent probe amplification.

RESULTS

Four novel NF2 mutations, including 2 splice-site mutations (c.364-1G>A and c.886-3C>G), 1 frameshift mutation (c.524delA), and 1 missense mutation (c.397T>C; p.Cys133Arg), were identified in our patients. No large deletion or duplication was identified in our series. Subsequently, we identified an abnormal splicing product by using reverse transcription-PCR and direct sequencing in 2 patients with a novel splice-site mutation. The missense mutation c.397T>C was predicted to have harmful effects on protein function.

CONCLUSIONS

The detection rate of NF2 mutations in Korean patients (57%) is similar to those in other populations. Our results provided a greater insight into the mutational spectrum of the NF2 gene in Korean subjects.

Merlin/NF2 suppresses tumorigenesis by inhibiting the E3 ubiquitin ligase CRL4(DCAF1) in the nucleus

Current models imply that the FERM domain protein Merlin, encoded by the tumor suppressor NF2, inhibits mitogenic signaling at or near the plasma membrane. Here, we show that the closed, growth-inhibitory form of Merlin accumulates in the nucleus, binds to the E3 ubiquitin ligase CRL4(DCAF1), and suppresses its activity. Depletion of DCAF1 blocks the promitogenic effect of inactivation of Merlin. Conversely, enforced expression of a Merlin-insensitive mutant of DCAF1 counteracts the antimitogenic effect of Merlin. Re-expression of Merlin and silencing of DCAF1 implement a similar, tumor-suppressive program of gene expression. Tumor-derived mutations invariably disrupt Merlin's ability to interact with or inhibit CRL4(DCAF1). Finally, depletion of DCAF1 inhibits the hyperproliferation of Schwannoma cells from NF2 patients and suppresses the oncogenic potential of Merlin-deficient tumor cell lines. We propose that Merlin suppresses tumorigenesis by translocating to the nucleus to inhibit CRL4(DCAF1).

Large intragenic deletions of the NF2 gene: breakpoints and associated phenotypes

In this study, the breakpoints of six large intragenic deletions in the NF2 gene are determined, which had initially been detected by multiplex ligation-dependent probe amplification. While one breakpoint occurred within an exon, the remaining 11 lied in the corresponding flanking introns. Two of the deletions were most likely caused by nonallelic homologous recombination between Alu sequences, while the other four appeared to be the result of nonhomologous endjoining, possibly facilitated by rearrangement-promoting elements at the junctions in some cases. The clinical features of patients with large intragenic deletions and individuals with mutations affecting single or multiple nucleotides of the NF2 gene are relatively similar. However, patients with deletions of the 3' exons 15 and 16 of the NF2 gene did exhibit milder phenotypes, especially with respect to the age of disease onset.

Neurofibromatose Typ 2

Neurofibromatose Typ 2 (NF2) ist eine genetische Erkrankung, die autosomal-dominant vererbt wird und durch multiple Tumoren des Nervensystems gekennzeichnet ist. Bilaterale vestibuläre Schwannome, bekannt als Akustikusneurinome, sind das Hauptmerkmal der Erkrankung und treten bei etwa 90% der Betroffenen auf. Sie beeinträchtigen das Hörvermögen der Patienten und führen nicht selten zur Ertaubung. Ebenso häufig (>90%) treten bei NF2 spinale Tumoren (Schwannome, Meningiome und Ependymome) auf, die in etwa 1/3 der Fälle zu neurologischen Defiziten führen. Daneben finden sich zerebrale Schwannome und Meningiome als typische NF2-assoziierte Tumormanifestationen. Ferner kennzeichnen Polyneuropathie und okuläre Abnormalitäten das Krankheitsbild. Die genetische Ursache der NF2 ist eine heterozygote Inaktivierung des NF2-Tumorsuppressorgens, welches auf Chromosom 22q lokalisiert ist. Die Inzidenz der NF2 beträgt etwa 1:25.000 Geburten, wobei 50–80% der Patienten keine familiäre Belastung aufweisen. Bei Patienten mit Neumutationen (de novo) wurde eine Mosaikbildung mit überraschend hoher Frequenz (25–30%) nachgewiesen. Der vorliegende Beitrag beschreibt Klinik und Genetik der NF2 sowie aktuelle Entwicklungen in der medikamentösen Therapie.

Neurofibromatosis 2 [Bilateral acoustic neurofibromatosis, central neurofibromatosis, NF2, neurofibromatosis type II]

Neurofibromatosis 2 is a dominantly inherited tumor predisposition syndrome caused by mutations in the NF2 gene on chromosome 22. Affected individuals inevitably develop schwannomas typically affecting both vestibular nerves leading to deafness. Rehabilitation with brainstem implants is improving this outcome. Schwannomas also occur on other cranial nerves, on spinal nerve roots, and on peripheral nerves. Meningiomas and ependymomas are other tumor features. In excess of 50% of patients represent new mutations and as many as one third are mosaic for the underlying disease causing mutation. Although truncating mutations (nonsense and frameshifts) are the most frequent germline event and cause the most severe disease, single and multiple exon deletions are common. A strategy for detection of the latter is vital for a sensitive analysis. NF2 represents a difficult management problem with most patients facing substantial morbidity and reduced life expectancy. Surgery remains the focus of current management, although watchful waiting and occasionally radiation treatment have a role. In the future, the development of tailored drug therapies aimed at the genetic level are likely to provide huge improvements for this devastating, life limiting condition.

Somatic EGFR mutations and efficacy of tyrosine kinase inhibitors in NSCLC

Early clinical studies of tyrosine kinase inhibitors (TKIs) that target the EGFR in patients with advanced non-small-cell lung cancer (NSCLC) showed that some patients experienced rapid, durable, complete or partial responses. These data were the basis for attempts to identify specific subgroups of patients who would further benefit from these agents. The discovery of somatic mutations in EGFR that correlated with sensitivity to TKIs identified a plausible explanation for these observations. Clinical and pathological factors such as female sex, never having smoked, Asian origin and adenocarcinoma histology correlate with the presence of EGFR mutations and objective responses to TKIs in patients with NSCLC. Recent studies in metastatic colorectal cancer highlighted that somatic mutations in KRAS represent a negative predictor of response to anti-EGFR monoclonal antibodies; KRAS mutations also represent an important mechanism of resistance to TKIs in NSCLC. Many large clinical studies are currently investigating the predictive and prognostic value of EGFR mutational status and other candidate biomarkers. We summarize the literature and present an overview of the field of anti-EGFR therapy in NSCLC, focusing on the influence of somatic EGFR mutations on selection of patients for TKI therapy and the influence of EGFR pathway regulation.

Neurofibromatosis type 2 (NF2): a clinical and molecular review

Neurofibromatosis type 2 (NF2) is a tumour-prone disorder characterised by the development of multiple schwannomas and meningiomas. Prevalence (initially estimated at 1: 200,000) is around 1 in 60,000. Affected individuals inevitably develop schwannomas, typically affecting both vestibular nerves and leading to hearing loss and deafness. The majority of patients present with hearing loss, which is usually unilateral at onset and may be accompanied or preceded by tinnitus. Vestibular schwannomas may also cause dizziness or imbalance as a first symptom. Nausea, vomiting or true vertigo are rare symptoms, except in late-stage disease. The other main tumours are schwannomas of the other cranial, spinal and peripheral nerves; meningiomas both intracranial (including optic nerve meningiomas) and intraspinal, and some low-grade central nervous system malignancies (ependymomas). Ophthalmic features are also prominent and include reduced visual acuity and cataract. About 70% of NF2 patients have skin tumours (intracutaneous plaque-like lesions or more deep-seated subcutaneous nodular tumours). Neurofibromatosis type 2 is a dominantly inherited tumour predisposition syndrome caused by mutations in the NF2 gene on chromosome 22. More than 50% of patients represent new mutations and as many as one-third are mosaic for the underlying disease-causing mutation. Although truncating mutations (nonsense and frameshifts) are the most frequent germline event and cause the most severe disease, single and multiple exon deletions are common. A strategy for detection of the latter is vital for a sensitive analysis. Diagnosis is based on clinical and neuroimaging studies. Presymptomatic genetic testing is an integral part of the management of NF2 families. Prenatal diagnosis and pre-implantation genetic diagnosis is possible. The main differential diagnosis of NF2 is schwannomatosis. NF2 represents a difficult management problem with most patients facing substantial morbidity and reduced life expectancy. Surgery remains the focus of current management although watchful waiting with careful surveillance and occasionally radiation treatment have a role. Prognosis is adversely affected by early age at onset, a higher number of meningiomas and having a truncating mutation. In the future, the development of tailored drug therapies aimed at the genetic level are likely to provide huge improvements for this devastating condition.

Neurofibromatosis type 2 (NF2): a clinical and molecular review

Neurofibromatosis type 2 (NF2) is a tumour-prone disorder characterised by the development of multiple schwannomas and meningiomas. Prevalence (initially estimated at 1: 200,000) is around 1 in 60,000. Affected individuals inevitably develop schwannomas, typically affecting both vestibular nerves and leading to hearing loss and deafness. The majority of patients present with hearing loss, which is usually unilateral at onset and may be accompanied or preceded by tinnitus. Vestibular schwannomas may also cause dizziness or imbalance as a first symptom. Nausea, vomiting or true vertigo are rare symptoms, except in late-stage disease. The other main tumours are schwannomas of the other cranial, spinal and peripheral nerves; meningiomas both intracranial (including optic nerve meningiomas) and intraspinal, and some low-grade central nervous system malignancies (ependymomas). Ophthalmic features are also prominent and include reduced visual acuity and cataract. About 70% of NF2 patients have skin tumours (intracutaneous plaque-like lesions or more deep-seated subcutaneous nodular tumours). Neurofibromatosis type 2 is a dominantly inherited tumour predisposition syndrome caused by mutations in the NF2 gene on chromosome 22. More than 50% of patients represent new mutations and as many as one-third are mosaic for the underlying disease-causing mutation. Although truncating mutations (nonsense and frameshifts) are the most frequent germline event and cause the most severe disease, single and multiple exon deletions are common. A strategy for detection of the latter is vital for a sensitive analysis. Diagnosis is based on clinical and neuroimaging studies. Presymptomatic genetic testing is an integral part of the management of NF2 families. Prenatal diagnosis and pre-implantation genetic diagnosis is possible. The main differential diagnosis of NF2 is schwannomatosis. NF2 represents a difficult management problem with most patients facing substantial morbidity and reduced life expectancy. Surgery remains the focus of current management although watchful waiting with careful surveillance and occasionally radiation treatment have a role. Prognosis is adversely affected by early age at onset, a higher number of meningiomas and having a truncating mutation. In the future, the development of tailored drug therapies aimed at the genetic level are likely to provide huge improvements for this devastating condition.

Neurofibromatosis type 2

Neurofibromatosis type 2 is an autosomal-dominant multiple neoplasia syndrome that results from mutations in the NF2 tumour suppressor gene located on chromosome 22q. It has a frequency of one in 25,000 livebirths and nearly 100% penetrance by 60 years of age. Half of patients inherit a germline mutation from an affected parent and the remainder acquire a de novo mutation for neurofibromatosis type 2. Patients develop nervous system tumours (schwannomas, meningiomas, ependymomas, astrocytomas, and neurofibromas), peripheral neuropathy, ophthalmological lesions (cataracts, epiretinal membranes, and retinal hamartomas), and cutaneous lesions (skin tumours). Optimum treatment is multidisciplinary because of the complexities associated with management of the multiple, progressive, and protean lesions associated with the disorder. We review the molecular pathogenesis, genetics, clinical findings, and management strategies for neurofibromatosis type 2.

Molecular studies of vestibular schwannomas: a review

PURPOSE OF REVIEW

To summarize advances in understanding the molecular biology of vestibular schwannomas over the past year.

RECENT FINDINGS

The role of the neurofibromatosis type 2 protein, denoted as merlin or schwannomin, in embryonic development, cellular adherence, and in cell proliferation has become better elucidated in the past year. Likewise, the role of merlin in Schwann cell-axon interaction has been studied. Additionally, two comprehensive analyses of the spectrum of human neurofibromatosis type 2 mutations have been compiled which make up a valuable resource in understanding critical regions of the neurofibromatosis type 2 gene. Neurofibromatosis type 2 screening guidelines for young patients with solitary vestibular schwannomas have been published. The role of electromagnetic radiation via cellular and portable telephones as a predisposing factor to vestibular schwannoma formation has also been the topic of several studies. Based on increased knowledge of the pathways in which merlin functions and the available transgenic and xenograft mouse models, preliminary data regarding directed pharmacotherapy are also summarized.

SUMMARY

With increased knowledge of the pathologic mechanisms and interacting proteins associated with merlin, the research community is poised to begin trials of targeted interventions in vitro and in the current mouse models.

Mosaicism in neurofibromatosis type 2: an update of risk based on uni/bilaterality of vestibular schwannoma at presentation and sensitive mutation analysis including multiple ligation-dependent probe amplification

BACKGROUND

Neurofibromatosis type 2 (NF2) is almost unique among inherited disorders in the frequency of mosaicism in the first affected generation. However, the implications of this on transmission risks have not been fully elucidated.

METHODS

The expanded database of 460 families with NF2 and 704 affected individuals was analysed for mosaicism and transmission risks to offspring.

RESULTS

64 mosaic patients, with a projected mosaicism rate of 33% for sporadic classical NF2 with bilateral vestibular schwannoma at presentation and 60% for those presenting unilaterally, were identified. Offspring risks can be radically reduced on the basis of a sensitive mutation analysis of blood DNA including multiple ligation-dependent probe amplification (MLPA, which detects 15% of all mutations), but even MLPA cannot detect high levels of mosaicism.

CONCLUSION

The chances of mosaicism in NF2 and the resultant risks of transmission of the mutation to offspring in a number of different clinical situations have been further delineated. The use of MLPA in this large NF2 series is also reported for the first time.

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.

Self-masking in an intact ERM-merlin protein: an active role for the central alpha-helical domain

Ezrin/radixin/moesin (ERM) family members provide a regulated link between the cortical actin cytoskeleton and the plasma membrane to govern membrane structure and organization. Here, we report the crystal structure of intact insect moesin, revealing that its essential yet previously uncharacterized alpha-helical domain forms extensive interactions with conserved surfaces of the band four-point-one/ezrin/radixin/moesin (FERM) domain. These interdomain contacts provide a functional explanation for how PIP(2) binding and tyrosine phosphorylation of ezrin lead to activation, and provide an understanding of previously enigmatic loss-of-function missense mutations in the tumor suppressor merlin. Sequence conservation and biochemical results indicate that this structure represents a complete model for the closed state of all ERM-merlin proteins, wherein the central alpha-helical domain is an active participant in an extensive set of inhibitory interactions that can be unmasked, in a rheostat-like manner, by coincident regulatory factors that help determine cell polarity and membrane structure.