Distinct overlapping sequences at the carboxy-terminus of merlin regulate its tumour suppressor and morphogenic activity

Pathogenic noncoding variants in the neurofibromatosis and schwannomatosis predisposition genes

Neurofibromatosis type 1 (NF1), type 2 (NF2), and schwannomatosis are a group of autosomal dominant disorders that predispose to the development of nerve sheath tumors. Pathogenic variants (PVs) that cause NF1 and NF2 are located in the NF1 and NF2 loci, respectively. To date, most variants associated with schwannomatosis have been identified in the SMARCB1 and LZTR1 genes, and a missense variant in the DGCR8 gene was recently reported to predispose to schwannomas. In spite of the high detection rate for PVs in NF1 and NF2 (over 90% of non-mosaic germline variants can be identified by routine genetic screening) underlying PVs for a proportion of clinical cases remain undetected. A higher proportion of non-NF2 schwannomatosis cases have no detected PV, with PVs currently only identified in around 70%-86% of familial cases and 30%-40% of non-NF2 sporadic schwannomatosis cases. A number of variants of uncertain significance have been observed for each disorder, many of them located in noncoding, regulatory, or intergenic regions. Here we summarize noncoding variants in this group of genes and discuss their established or potential role in the pathogenesis of NF1, NF2, and schwannomatosis.

Phosphorylation of Merlin by Aurora A kinase appears necessary for mitotic progression

Although Merlin's function as a tumor suppressor and regulator of mitogenic signaling networks such as the Ras/rac, Akt, and Hippo pathways is well-documented, in mammals as well as in insects, its role during cell cycle progression remains unclear. In this study, using a combination of approaches, including FACS analysis, time-lapse imaging, immunofluorescence microscopy, and co-immunoprecipitation, we show that Ser-518 of Merlin is a substrate of the Aurora protein kinase A during mitosis and that its phosphorylation facilitates the phosphorylation of a newly discovered site, Thr-581. We found that the expression in HeLa cells of a Merlin variant that is phosphorylation-defective on both sites leads to a defect in centrosomes and mitotic spindles positioning during metaphase and delays the transition from metaphase to anaphase. We also show that the dual mitotic phosphorylation not only reduces Merlin binding to microtubules but also timely modulates ezrin interaction with the cytoskeleton. Finally, we identify several point mutants of Merlin associated with neurofibromatosis type 2 that display an aberrant phosphorylation profile along with defective α-tubulin-binding properties. Altogether, our findings of an Aurora A-mediated interaction of Merlin with α-tubulin and ezrin suggest a potential role for Merlin in cell cycle progression.

Personalized comprehensive molecular profiling of high risk osteosarcoma: Implications and limitations for precision medicine

Background

Despite advances in molecular medicine over recent decades, there has been little advancement in the treatment of osteosarcoma. We performed comprehensive molecular profiling in two cases of metastatic and chemotherapy-refractory osteosarcoma to guide molecularly targeted therapy.

Patients and Methods

Hybridization capture of >300 cancer-related genes plus introns from 28 genes often rearranged or altered in cancer was applied to >50 ng of DNA extracted from tumor samples from two patients with recurrent, metastatic osteosarcoma. The DNA from each sample was sequenced to high, uniform coverage. Immunohistochemical probes and morphoproteomics analysis were performed, in addition to fluorescence in situ hybridization. All analyses were performed in CLIA-certified laboratories. Molecularly targeted therapy based on the resulting profiles was offered to the patients. Biomedical analytics were performed using QIAGEN's Ingenuity^® Pathway Analysis.

Results

In Patient #1, comprehensive next-generation exome sequencing showed MET amplification, PIK3CA mutation, CCNE1 amplification, and PTPRD mutation. Immunohistochemistry-based morphoproteomic analysis revealed c-Met expression [(p)-c-Met (Tyr1234/1235)] and activation of mTOR/AKT pathway [IGF-1R (Tyr1165/1166), p-mTOR [Ser2448], p-Akt (Ser473)] and expression of SPARC and COX2. Targeted therapy was administered to match the P1K3CA, c-MET, and SPARC and COX2 aberrations with sirolimus+ crizotinib and abraxane+ celecoxib. In Patient #2, aberrations included NF2 loss in exons 2–16, PDGFRα amplification, and TP53 mutation. This patient was enrolled on a clinical trial combining targeted agents temsirolimus, sorafenib and bevacizumab, to match NF2, PDGFRα and TP53 aberrations. Both the patients did not benefit from matched therapy.

Conclusions

Relapsed osteosarcoma is characterized by complex signaling and drug resistance pathways. Comprehensive molecular profiling holds great promise for tailoring personalized therapies for cancer. Methods for such profiling are evolving and need to be refined to better assist clinicians in making treatment decisions based on the large amount of data that results from this type of testing. Further research in this area is warranted.

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.

Clinical and molecular predictors of mortality in neurofibromatosis 2: a UK national analysis of 1192 patients

BACKGROUND

Neurofibromatosis 2 (NF2) is an autosomal-dominant tumour predisposition syndrome characterised by bilateral vestibular schwannomas, considerable morbidity and reduced life expectancy. Although genotype-phenotype correlations are well established in NF2, little is known about effects of mutation type or location within the gene on mortality. Improvements in NF2 diagnosis and management have occurred, but their effect on patient survival is unknown.

METHODS

We evaluated clinical and molecular predictors of mortality in 1192 patients (771 with known causal mutations) identified through the UK National NF2 Registry. Kaplan-Meier survival and Cox regression analyses were used to evaluate predictors of mortality, with jackknife adjustment of parameter SEs to account for the strong intrafamilial phenotypic correlations that occur in NF2.

RESULTS

The study included 241 deaths during 10 995 patient-years of follow-up since diagnosis. Early age at diagnosis and the presence of intracranial meningiomas were associated with increased mortality, and having a mosaic, rather than non-mosaic, NF2 mutation was associated with reduced mortality. Patients with splice-site or missense mutations had lower mortality than patients with truncating mutations (OR 0.459, 95% CI 0.213 to 0.990, and OR 0.196, 95% CI 0.213 to 0.990, respectively). Patients with splice-site mutations in exons 6-15 had lower mortality than patients with splice-site mutations in exons 1-5 (OR 0.333, 95% CI 0.129 to 0.858). The mortality of patients with NF2 diagnosed in more recent decades was lower than that of patients diagnosed earlier.

CONCLUSIONS

Continuing advances in molecular diagnosis, imaging and treatment of NF2-associated tumours offer hope for even better survival in the future.

Merlin Isoforms 1 and 2 Both Act as Tumour Suppressors and Are Required for Optimal Sperm Maturation

The tumour suppressor Merlin, encoded by the gene NF2, is frequently mutated in the autosomal dominant disorder neurofibromatosis type II, characterised primarily by the development of schwannoma and other glial cell tumours. However, NF2 is expressed in virtually all analysed human and rodent organs, and its deletion in mice causes early embryonic lethality. Additionally, NF2 encodes for two major isoforms of Merlin of unknown functionality. Specifically, the tumour suppressor potential of isoform 2 remains controversial. In this study, we used Nf2 isoform-specific knockout mouse models to analyse the function of each isoform during development and organ homeostasis. We found that both isoforms carry full tumour suppressor functionality and can completely compensate the loss of the other isoform during development and in most adult organs. Surprisingly, we discovered that spermatogenesis is strictly dependent on the presence of both isoforms. While the testis primarily expresses isoform 1, we noticed an enrichment of isoform 2 in spermatogonial stem cells. Deletion of either isoform was found to cause decreased sperm quality as observed by maturation defects and head/midpiece abnormalities. These defects led to impaired sperm functionality as assessed by decreased sperm capacitation. Thus, we describe spermatogenesis as a new Nf2-dependent process. Additionally, we provide for the first time in vivo evidence for equal tumour suppressor potentials of Merlin isoform 1 and isoform 2.

NF2/merlin in hereditary neurofibromatosis 2 versus cancer: biologic mechanisms and clinical associations

Inactivating germline mutations in the tumor suppressor gene NF2 cause the hereditary syndrome neurofibromatosis 2, which is characterized by the development of neoplasms of the nervous system, most notably bilateral vestibular schwannoma. Somatic NF2 mutations have also been reported in a variety of cancers, but interestingly these mutations do not cause the same tumors that are common in hereditary neurofibromatosis 2, even though the same gene is involved and there is overlap in the site of mutations. This review highlights cancers in which somatic NF2 mutations have been found, the cell signaling pathways involving NF2/merlin, current clinical trials treating neurofibromatosis 2 patients, and preclinical findings that promise to lead to new targeted therapies for both cancers harboring NF2 mutations and neurofibromatosis 2 patients.

A neuronal function of the tumor suppressor protein merlin

Mutagenic loss of the NF2 tumor suppressor gene encoded protein merlin is known to provoke the hereditary neoplasia syndrome, Neurofibromatosis type 2 (NF2). In addition to glial cell-derived tumors in the PNS and CNS, disease-related lesions also affect the skin and the eyes. Furthermore, 60% of NF2 patients suffer from peripheral nerve damage, clinically referred to as peripheral neuropathy. Strikingly, NF2-associated neuropathy often occurs in the absence of nerve damaging tumors, suggesting tumor-independent events. Recent findings indicate an important role of merlin in neuronal cell types concerning neuromorphogenesis, axon structure maintenance and communication between axons and Schwann cells. In this review, we compile clinical and experimental evidences for the underestimated role of the tumor suppressor merlin in the neuronal compartment.

Merlin, the NF2 gene product

Merlin, the protein product of NF2 gene, is one of the most versatile tumor suppressors capable of integrating different mechanisms that regulate cell proliferation, motility, survival and signaling pathways underlying and governing those mechanisms. Merlin is considered a member of the band 4.1 families of cytoskeleton-associated proteins also called ERM family and acts as tumor suppressor. The main cause for transformation of Schwann cells into schwannomas is credited to the inactivation of the neurofibromin 2 (NF2) gene and the consecutive loss of its protein merlin. Recent scientific advances improved our understanding of pathogenic mechanisms involving NF2 gene. The present review brings genetic properties of NF2 gene, molecular characteristics of merlin, summarizes mutational spectra and explains merlin's multifunctional roles regarding its involvement in neurofibromatosis associated tumorigenesis.