p190RhoGAP can act to inhibit PDGF-induced gliomas in mice: a putative tumor suppressor encoded on human chromosome 19q13.3

Identification of novel oligodendroglioma-associated candidate tumor suppressor genes in 1p36 and 19q13 using microarray-based expression profiling

Loss of heterozygosity (LOH) on chromosomal arms 1p and 19q is the most common genetic alteration in oligodendroglial tumors and associated with response to radio- and chemotherapy as well as favorable prognosis. Using microsatellite analysis, we previously identified the chromosomal regions 1p36.22-p36.31 and 19q13.3, as candidate tumor suppressor gene regions being commonly deleted in these tumors. To identify genes within these regions that are downregulated in oligodendroglial tumors with LOH 1p/19q, we performed cDNA microarray-based RNA expression profiling of 35 gliomas with known allelic status on 1p and 19q, including 7 oligodendrogliomas and 8 diffuse astrocytomas of World Health Organization (WHO) grade II, as well as 14 anaplastic oligodendrogliomas and 6 anaplastic oligoastrocytomas of WHO grade III. The microarrays used for expression profiling carried approximately 7,000 gene-specific cDNAs, with complete coverage of the genes located in 1p36.13-p36.31 and 19q13.2-q13.33. Microarray analysis identified 8 genes from these regions (MGC4399, SRM, ICMT, RPL18, FTL, ZIN, FLJ10781 and DBP), which all showed significantly lower expression in 1p/19q-deleted gliomas when compared to gliomas without 1p/19q losses. Quantitative real-time reverse transcription-PCR analyses were performed for the MGC4399, ICMT and RPL18 genes and confirmed the microarray findings. In addition, we found that the cytosolic phospholipase A2 (PLA2G4C) gene at 19q13.3 demonstrated significantly lower expression in anaplastic oligodendrogliomas (WHO grade III) when compared to well-differentiated oligodendrogliomas (WHO grade II). Taken together, our study provides a set of interesting novel candidate genes that may play important roles in the pathogenesis of oligodendroglial tumors.

Transforming growth factor beta regulates the expression of the M2 muscarinic receptor in atrial myocytes via an effect on RhoA and p190RhoGAP

Transforming growth factor beta (TGFbeta) signaling is involved in the development and regulation of multiple organ systems and cellular signaling pathways. We recently demonstrated that TGFbeta regulates the response of atrial myocytes to parasympathetic stimulation. Here, TGFbeta(1) is shown to inhibit expression of the M(2) muscarinic receptor (M(2)), which plays a critical role in the parasympathetic response of the heart. This effect is mimicked by overexpression of a dominant negative mutant of RhoA and by the RhoA kinase inhibitor Y27632, whereas adenoviral expression of a dominant activating-RhoA reverses TGFbeta inhibition of M(2) expression. TGFbeta(1) also mediates a decrease in GTP-bound RhoA and a reciprocal increase in the expression of the RhoA GTPase-activating protein, p190RhoGAP, whereas total RhoA is unchanged. Inhibition of M(2) promoter activity by TGFbeta(1) is mimicked by overexpression of p190RhoGAP, whereas a dominant negative mutant of p190RhoGAP reverses this effect of TGFbeta(1). In contrast to atrial myocytes, in mink lung epithelial cells, in which TGFbeta signaling through activation of RhoA has been previously identified, TGFbeta(1) stimulated an increase in GTP-bound RhoA in association with a reciprocal decrease in the expression of p190RhoGAP. Both effects demonstrated a similar dose dependence on TGFbeta(1). Thus TGFbeta regulation of M(2) muscarinic receptor expression is dependent on RhoA, and TGFbeta regulation of p190RhoGAP expression may be a cell type-specific mechanism for TGFbeta signaling through RhoA.

Oligodendroglial-specific transcriptional factor SOX10 is ubiquitously expressed in human gliomas

The two most common types of gliomas: astrocytoma and oligodendroglioma are distinguished based on their morphologic similarities to mature astrocytes and oligodendroglia. Whereas prototypical examples of the tumors have distinct pathogenetic and prognostic differences, the majority of the gliomas falls in the intermediate category and their distinction is problematic. The transcriptional factor SOX10 is one of the key determinants of oligodendroglial differentiation. We applied immunohistochemistry to analyze whether the expression of SOX10 can differentiate astrocytoma and oligodendroglioma. The majority of oligodendrogliomas, but also a large fraction of astrocytomas, including the least differentiated glioblastomas, expressed SOX10, albeit at lower levels. Comparison with 1p and 19q deletion status by FISH analysis also revealed no obvious associations. High levels of expression were also found in pilocytic astrocytoma, consistent with recent studies suggesting that pilocytic astrocytomas have greater overlap with oligodendroglial than astrocytic tumors. Our data raise a possibility that histogenesis of gliomas have more common features than previously anticipated.

An update on oligodendroglial neoplasms

PURPOSE OF REVIEW

The most widely accepted brain tumor classification system remains morphology-based but the increasing knowledge of the molecular pathogenesis of oligodendroglial tumors has spurred translational research yielding new diagnostic and therapeutic paradigms. These data have accumulated rapidly and, in combination with exciting new insights in the cellular origin of these tumors, necessitate a review.

RECENT FINDINGS

'Cancer stem cells' have been identified in gliomas. Further study of these cells will not only provide information on the cellular origin and pathogenesis of these tumors but may also give rise to new treatments that target a cell pool not amenable to current therapeutic strategies. Molecular tumor characteristics have been correlated with imaging findings, treatment response and prognosis. This has enabled neuro-oncologists to take a risk-stratified approach to patients with oligodendrogliomas that optimizes treatment efficacy and minimizes toxicity. Furthermore, more accurate epidemiological data have become available from population-based studies.

SUMMARY

In spite of remarkable progress over the last 15 years, these tumors remain incurable. The search for a cure has to go on, while currently available multidisciplinary treatments are refined.

Quantitation of chromosome 1p and 19q deletions in glial tumours by interphase FISH on formalin-fixed paraffin-embedded tissue

Deletions on chromosomes 1p and 19q identify oligodendroglial tumours that are likely to have a complete response to some chemotherapy regimens and are associated with prolonged patient survival. Detection of these chromosomal alterations is becoming increasingly important in the evaluation of glial tumours. Interphase fluorescence in situ hybridisation (FISH) is one of a number of techniques for detecting deletions, and is an efficient method for screening large numbers of tumours. We used FISH to detect 1p and 19q deletions in formalin-fixed paraffin sections of 11 oligodendrogliomas, 11 oligoastrocytomas, two astrocytomas and four glioblastomas multiforme. 1p and 19q deletion and non-deletion ratios were tabulated in 200 nuclei in each tumour. We found considerable variation between tumours in the fraction of cells with deletions. This variation has not been reported previously and its clinical significance will be clarified with patient follow-up. There was little variation between regions within the same tumour.

Applications of mouse glioma models in preclinical trials

Gliomas are the most common primary tumors that arise from glial cells and their precursors in the central nervous system. Most of the genetic alterations identified in human gliomas result in signal transduction abnormalities or disruption of cell cycle arrest pathways. Over the past years, several mouse glioma models have been generated based on human genetic abnormalities and the induced gliomas exhibit histological similarities to their human counterparts. There is emerging evidence suggesting that an oncogenic signaling initiating tumorigenesis is also required for tumor maintenance, these glioma models can be used to further characterize the mechanisms of oncogenic signaling in tumor formation, as well as identify molecular targets in preclinical trials.

Polymorphisms in GLTSCR1 and ERCC2 are associated with the development of oligodendrogliomas

BACKGROUND

Deletions of 19q have been associated with gliomas, especially oligodendrogliomas. In addition, cases with oligodendrogliomas with the 19q deletion have been observed to have a better survival compared with cases without the 19q deletion. The authors have previously described a 150-kilobase minimal deletion region in gliomas that maps to 19q13.33 and contains 3 novel candidate genes (GLTSCR1, EHD2, and GLTSCR2).

METHODS

The authors performed an association study using 141 cases with gliomas (61 cases with astrocytomas, 40 cases with oligodendrogliomas, 40 cases with mixed oligoastrocytomas) and 108 general controls. They evaluated 7 single nucleotide polymorphisms (SNPs) in 6 genes within and nearby the minimal 19q deletion region (ERCC2, RAI, ASE-1, ERCC1, GLTSCR1, and LIG1).

RESULTS

The prevalence of a germline GLTSCR1-exon-1 T allele (SNP rs1035938) was 40% in cases with oligodendrogliomas compared with 27% in controls (P = 0.029), and the prevalence of an ERCC2-exon-22 T allele (SNP rs1052555) was 35% in cases with oligodendrogliomas compared with 18% in controls (P = 0.043). One high-risk and 1 low-risk haplotype were associated with oligodendroglioma development (P = 0.003 and 0.026, respectively). Cases with oligodendrogliomas with the 19q deletion had a significantly higher frequency of the GLTSCR1-exon-1 T allele compared with cases without the 19q deletion (P = 0.01). It was noteworthy that cases with gliomas who were homozygous for the GLTSCR1-exon-1 T allele had a significantly better survival: 77% and 68% survival at 2 and 5 years compared with 56% and 34% for other genotypes (P = 0.02, log-rank test). Multivariable analysis identified grade, age, and the GLTSCR1-exon-1 and ERCC2-exon-22 genotypes as independent predictors for survival.

CONCLUSIONS

These results suggested that alterations in GLTSCR1 (or a closely linked gene) were associated with the development and progression of oligodendroglioma.

Advances in the biology of astrocytomas

PURPOSE OF REVIEW

Conventional surgery, radio- and chemotherapy have failed to significantly improve the prognosis of patients with malignant astrocytomas--hence the need for understanding their molecular biology. Harvesting this understanding to yield novel biological targeted therapies has approached the clinical doorstep. Therapeutic efficacy will likely require combinatorial therapy involving biologicals and conventional therapies, with small incremental efficacy in selected sub-groups. This review highlights some of the findings over the past year (June 2003-2004) that have contributed to this slow but essential journey towards our understanding of the biology of astrocytomas.

RECENT FINDINGS

The accumulation of loss and/or gain of function molecular alterations underlying astrocytoma formation, progression and key growth parameters including proliferation, angiogenesis, apoptosis, invasion and resistance are emerging. These alterations involve those regulating the growth factor/receptor and downstream signaling networks, cell cycle, immune modulators and other key biological processes. The advances are facilitated by interactions amongst clinician and basic scientists, in both academia and industry. They have incorporated high-throughput bioinformatics analysis of genomic and expression array data, the emerging field of proteomics and development of various genetically engineered models of astrocytomas.

SUMMARY

Astrocytomas, like other cancers, are a result of several molecular alterations, some of which strongly correlate to their pathological grade. However, molecular heterogeneity exists between astrocytomas of similar grades and likely between varying micro-environmental regions of a single tumor. Characterization of the molecular signature of an astrocytoma and linking with the appropriate 'tailored' therapie(s) is the hope of the future.

An FF domain-dependent protein interaction mediates a signaling pathway for growth factor-induced gene expression

FF domains are poorly understood protein motifs found in all eukaryotes but in a very small number of proteins. They typically occur in tandem arrays and appear predominantly in splicing and transcription factors. Curiously, they are also present in the p190 family of cytoplasmic Rho GTPase activating proteins (GAPs). We identified the serum-responsive transcriptional regulator TFII-I as a specific interactor with the p190 RhoGAP FF domains. p190 sequesters TFII-I in the cytoplasm via the FF domains, but upon PDGF receptor-mediated phosphorylation of an FF domain, TFII-I is released from p190 and translocates to the nucleus where it can activate transcription of serum-inducible genes including c-fos. These findings reveal a pathway by which mitogens promote gene transcription and indicate a role for FF domains in phosphorylation-mediated signal transduction.

Allelic Losses at 1p36 and 19q13 in Gliomas: Correlation with Histologic Classification, Definition of a 150-kb Minimal Deleted Region on 1p36, and Evaluation of CAMTA1 as a Candidate Tumor Suppressor Gene

Purpose: Allelic loss at 1p is seen in 70% to 85% of oligodendrogliomas (typically in association with 19q allelic loss) and 20-30% of astrocytomas. Because most 1p deletions in gliomas involve almost the entire chromosome arm, narrowing the region of the putative tumor suppressor gene has been difficult. To better define the histologic correlates of different patterns of 1p and 19q loss, we evaluated 1p/19q status in a large group of gliomas. This also allowed us to define a very small minimal deleted region (MDR) on 1p36.

Experimental Design: Among 205 consecutive cases of glioma studied for 1p loss of heterozygosity (LOH), 112 tumors were evaluated for both 1p and 19q LOH using at least three polymorphic markers on 1p and 19q each. The latter group included both low-grade tumors (oligodendroglioma, diffuse astrocytoma, and “oligoastrocytoma”) and high-grade tumors (anaplastic oligodendrogliomas, anaplastic astrocytomas, anaplastic oligoastrocytomas). Tumors with small segmental 1p losses (defined as LOH at some loci with retention of heterozygosity at other loci) were studied using a more extensive panel of markers to define the 1p MDR. The candidate gene was screened for mutations and its expression was studied by qualitative and quantitative reverse transcriptase-PCR and Northern blotting.

Results: Allelic losses on 1p and 19q, either separately or combined, were more common in classic oligodendrogliomas than in either astrocytomas or oligoastrocytomas (P < 0.0001). Classic oligodendrogliomas showed 1p loss in 35 of 42 (83%) cases, 19q loss in 28 of 39 (72%), and these were combined in 27 of 39 (69%) cases. There was no significant difference in 1p/19q LOH status between low-grade and anaplastic oligodendrogliomas. In contrast, no astrocytomas and only 6 of 30 (20%) oligoastrocytic tumors had combined 1p/19q loss. Although rare, 1p deletions were more often segmental in astrocytomas (5 of 6, 83%) than in oligodendrogliomas (3 of 35, 9%; P = 0.006). Eleven tumors (6 oligodendrogliomas or having oligodendroglial components, 5 purely astrocytic) with small segmental 1p losses underwent further detailed LOH mapping. All informative tumors in the oligodendroglial group and 2 of 3 informative astrocytomas showed LOH at 1p36.23, with a 150-kb MDR located between D1S2694 and D1S2666, entirely within the CAMTA1 transcription factor gene. Mutation analysis of the exons encoding conserved regions of CAMTA1 showed no somatic mutations in 10 gliomas, including 6 cases with and 4 cases without 1p LOH. CAMTA1 is normally expressed predominantly in non-neoplastic adult brain tissue. Relative to the latter, the expression level of CAMTA1 was low in oligodendroglial tumors and was further halved in cases with 1p deletion compared with those without 1p deletion (Mann-Whitney, P = 0.03).

Conclusions: Our data confirm the strong association of combined 1p/19q loss with classic oligodendroglioma histology and identify a very small segment of 1p36 located within CAMTA1 that was deleted in all oligodendroglial tumors with 1p LOH. This MDR also overlaps the neuroblastoma 1p36 MDR. CAMTA1 shows no evidence of inactivation by somatic mutations but its expression is reduced by half in cases with 1p LOH, suggesting that the functional effects of CAMTA1 haploinsufficiency warrant further investigation.

Molecular changes in gliomas

PURPOSE OF REVIEW

Despite optimal clinical treatment, the prognosis for gliomas remains poor, and little progress has been observed during the last few years. Meanwhile, understanding of glioma oncogenesis has improved greatly. This review focuses on recent advances in molecular biology of glial tumors, with particular emphasis on lineage markers, genetic mechanisms underlying tumor progression, new diagnostic and prognostic markers, and potential therapeutic targets.

RECENT FINDINGS

The question of the cell of origin, illustrated by the evidence of tumor-derived multipotent progenitors, by the animal models of gliomas, and by lineage markers such as Olig1/2 markers, remains unsolved. Genotype/phenotype correlation studies have identified early and late genetic alterations related either to astrocytic or oligodendroglial phenotype. They complement the existing World Health Organization morphologic classification and provide additional prognostic markers such as 1p/19q deletion in oligodendrogliomas. Most of these genetic alterations result in the disruption of three main cellular systems: RB1, P53, and tyrosine kinase receptor pathways. New gene alterations have also been identified in glioma, promoting mitotic signal transduction, cell cycle regulation, apoptosis, angiogenesis, or invasion. Gene and protein profiling has been correlated with outcome.

SUMMARY

Management of gliomas, especially oligodendrogliomas with 1p19q deletion, benefits from advances in molecular genetics. A better understanding of the molecular pathogenesis and cellular lineage of gliomas will improve tumor classification and define more reliable prognostic markers. There is a hope that it will also lead to novel targets for therapy.

Cell adhesion receptors, tyrosine kinases and actin modulators: a complex three-way circuitry

The interaction of cells with surrounding matrix and neighbouring cells governs many aspects of cell behaviour. Aside from transmitting signals from the external environment, adhesion receptors also receive signals from the cell interior. Here we review the interrelationship between adhesion receptors, tyrosine kinases (both growth factor receptor and non-receptor) and modulators of the actin cytoskeletal network. Deregulation of many aspects of these signalling pathways in cancer highlights the need for a better understanding of the complexities involved.

Mutation analysis of the Ras pathway genes NRAS, HRAS, KRAS and BRAF in glioblastomas

Aberrant activation of Ras signaling is a common finding in human glioblastomas. To determine the contribution of Ras gene mutations to this aberration, we screened 94 glioblastomas for mutations in the three Ras family genes NRAS, KRAS and HRAS. All tumors were additionally analyzed for mutations in BRAF, which encodes a Ras-regulated serine/threonine kinase with oncogenic properties. Mutation analysis of the entire coding regions of NRAS and KRAS, as well as the known mutation hot-spot sites in HRAS, identified somatic point mutations in two glioblastomas, both affecting codon 12 of NRAS (c.35G>A, p.G12D). Three additional tumors carried BRAF mutations altering the known hot-spot codon 599 (c.1796T>A, p.V599E). None of these five glioblastomas showed amplification of the EGFR or PDGFRA genes, while three of the tumors, including two with NRAS and one with BRAF mutation, demonstrated PTEN missense mutations or loss of PTEN mRNA expression. Taken together, our data suggest activating mutations in NRAS or BRAF as a molecular alteration that contributes to aberrant Ras signaling in a small fraction of glioblastomas.

Pathology and molecular genetics of oligodendroglial tumors

Oligodendroglial gliomas are second only to astrocytic gliomas in frequency. The lack of stringent diagnostic criteria cause high interobserver variation in regard to classification and grading of these tumors. Previous studies have described oligodendrogliomas with features that overlap with those of neurocytic tumors, thus further complicating diagnostic decisions. The increasing need for standardized diagnostic criteria in this subset of gliomas is emphasized by the benefit of adjuvant therapies in patients with anaplastic oligodendrogliomas. Characteristic chromosomal aberrations have been successfully determined for oligodendroglial tumors in recent years. In contrast to astrocytomas, however, no genes in the affected regions have been clearly linked to their pathogenesis. However, the molecular findings promise to be helpful for diagnostic and therapeutic decisions. This review compiles clinical, pathological, and molecular genetic findings on WHO grades II and III oligodendrogliomas and oligoastrocytomas.

A novel testicular RhoGAP-domain protein induces apoptosis

The GTPase-activating proteins (GAPs) accelerate the hydrolysis of GTP to GDP by small GTPases. The GTPases play diverse roles in many cellular processes, including proliferation, cell motility, endocytosis, nuclear import/export, and nuclear membrane formation. Little is known about GAP-domain proteins in spermatogenesis. We isolated a novel RhoGAP domain-containing tGAP1 protein from male germ cells that exhibits unusual properties. The tGAP1 is expressed at low levels in early spermatogonia. Robust transcription initiates in midpachytene spermatocytes and continues after meiosis. The 175-kDa tGAP1 protein localizes to the cytoplasm of spermatocytes and to the cytoplasm and nucleus in spermatids. The protein contains four GAP domain-related sequences, in contrast to all other GAP proteins that harbor one such domain. No activity toward RhoA, Rac1, or Cdc42 could be detected. Results of transfection studies in various somatic cells indicated that low-level tGAP1 expression significantly slows down the cell cycle. Expression of higher levels of tGAP1 by infection of somatic cells with recombinant adenoviruses demonstrated that tGAP1 efficiently induces apoptosis, which to our knowledge is the first such demonstration for a RhoGAP protein. Based on its subcellular location in spermatids and its activity, tGAP1 may play a role in nuclear import/export.

Identification of candidate cancer-causing genes in mouse brain tumors by retroviral tagging

Murine retroviruses may cause malignant tumors in mice by insertional mutagenesis of host genes. The use of retroviral tagging as a means of identifying cancer-causing genes has, however, almost entirely been restricted to hematopoietic tumors. The aim of this study was to develop a system allowing for the retroviral tagging of candidate genes in malignant brain tumors. Mouse gliomas were induced by a recombinant Moloney murine leukemia virus encoding platelet-derived growth factor (PDGF) B-chain. The underlying idea was that tumors evolve through a combination of PDGF-mediated autocrine growth stimulation and insertional mutagenesis of genes that cooperate with PDGF in gliomagenesis. Common insertion sites (loci that were tagged in more than one tumor) were identified by cloning and sequencing retroviral flanking segments, followed by blast searches of mouse genome databases. A number of candidate brain tumor loci (Btls) were identified. Several of these Btls correspond to known tumor-causing genes; these findings strongly support the underlying idea of our experimental approach. Other Btls harbor genes with a hitherto unproven role in transformation or oncogenesis. Our findings indicate that retroviral tagging with a growth factor-encoding virus may be a powerful means of identifying candidate tumor-causing genes in nonhematopoietic tumors.

Rho Proteins and Cancer

The Rho family of GTPases has been intensively studied for their roles in signal transduction processes leading to cytoskeletal-dependent responses, including cell migration and phagocytosis. In addition, they are important regulators of cell cycle progression and affect the expression of a number of genes, including those for matrix-degrading proteases implicated in cancer invasion. So far, the expression of some Rho family members has been found to be increased in some human cancers, and some cancer-associated mutations in Rho family regulators have been characterized. This makes Rho protein signalling pathways attractive targets for cancer therapy. However, there is little evidence so far from animal studies to define if and how Rho proteins contribute to cancer cell proliferation, survival, invasion and metastasis.

p190RhoGAP is cell cycle regulated and affects cytokinesis

p190RhoGAP (p190), a Rho family GTPase-activating protein, regulates actin stress fiber dynamics via hydrolysis of Rho-GTP. Recent data suggest that p190 also regulates cell proliferation. To gain insights into the cellular process(es) affected by p190, we altered its levels by conditional or transient overexpression. Overexpression of p190 resulted in a multinucleated phenotype that was dependent on the GTPase-activating protein domain. Confocal immunofluorescence microscopy revealed that both endogenous and exogenous p190 localized to the newly forming and contracting cleavage furrow of dividing cells. However, overexpression of p190 resulted in abnormal positioning of the furrow specification site and unequal daughter cell partitioning, as well as faulty furrow contraction and multinucleation. Furthermore, levels of endogenous p190 protein were transiently decreased in late mitosis via an ubiquitin-mediated degradation process that required the NH₂-terminal GTP-binding region of p190. These results suggest that a cell cycle–regulated reduction in endogenous p190 levels is linked to completion of cytokinesis and generation of viable cell progeny.

Angiogenesis in transgenic models of multistep angiogenesis

The histopathology and the epidemiology of human cancers, as well as studies of animal models of tumorigenesis, have led to a widely accepted notion that multiple genetic and epigenetic changes have to accumulate for progression to malignancy. Formation of new blood vessels (tumor angiogenesis) has been recognized, in addition to proliferative capabilities and ability to down-modulate cell death (apoptosis), as essential for the progressive growth and expansion of solid tumors. Mice overexpressing activated forms of oncogenes or carrying targeted mutations in tumor suppressor genes have proven extremely useful for linking the function of these genes with specific tumor features such as continuous proliferation, escape from apoptosis, invasion and neo-angiogenesis. The interbreeding of these mice allows for studying the extent of cooperativity between different genetic lesions in disease progression, leading to a greater understanding of multi-stage nature of tumorigenesis.