Genetic evidence of the neoplastic nature of gemistocytes in astrocytomas.

Gemistocytic astrocytoma is characterized by a predominance of large astrocytes with plump processes and massive accumulation of glial fibrillary acidic protein (gemistocytes). This histological variant of low-grade diffuse astrocytoma (WHO grade II) is prone to more rapid progression to anaplastic astrocytoma and glioblastoma than the ordinary fibrillary astrocytoma. The biological basis of this unfavorable prognosis is unclear, since gemistocytes themselves have low proliferative activity, even if present in anaplastic astrocytomas or glioblastomas. This has raised the question of whether gemistocytes are neoplastic cells or dysplastic reactive astrocytes. In this study, gemistocytes and non-gemistocytic neoplastic cells were separated by laser-assisted microdissection from six gemistocytic astrocytomas carrying TP53 mutations. In all cases, identical TP53 mutations were identified in both cell types, indicating that gemistocytes are indeed neoplastic cells. Their lack of proliferative activity may indicate terminal differentiation.

Promoter methylation of the DNA repair gene MGMT in astrocytomas is frequently associated with G:C --> A:T mutations of the TP53 tumor suppressor gene.

O(6)-Methylguanine-DNA methyltransferase (MGMT) is a repair protein that specifically removes promutagenic alkyl groups from the O(6) position of guanine in DNA. Repair of O(6)-alkylguanine adducts by tumour cells has been implicated in drug resistance since it reduces the cytotoxicity of alkylating chemotherapeutic agents. We assessed promoter methylation of the MGMT gene in astrocytic brain tumours by methylation-specific PCR. MGMT promoter methylation was detected in 26 of 54 (48%) low-grade diffuse astrocytomas (WHO grade II) and in 12 of 16 (75%) of secondary glioblastomas (WHO grade IV) that had progressed from low-grade astrocytomas. The frequency of MGMT methylation was significantly lower in primary (de novo) glioblastomas (13 of 36, 36%, P = 0.0155). The majority of low-grade astrocytomas with MGMT methylation (24/26, 92%) contained a TP53 mutation, whereas only 11 out of 28 (39%) cases without MGMT methylation carried a TP53 mutation (P < 0.0001). Furthermore, G:C --> A:T transition mutations at CpG sites were significantly more frequent in low-grade astrocytomas with MGMT methylation (15/26, 58%) than in those without (3/28, 11%, P = 0.0004). These results suggest that loss of MGMT expression as a result of promoter methylation, which frequently occurs at an early stage in the pathway leading to secondary glioblastomas, appears to be associated with increased frequency of TP53 mutations, in particular G:C --> A:T transitions.

Smoking cessation in cancer prevention.

Tobacco smoking is the largest preventable risk factor for morbidity and mortality in industrialized countries. WHO estimates that tobacco will become the largest single health problem by 2020, causing an estimated 8.4 million deaths annually. Tobacco has central importance in the etiology of cancers of the lung, head and neck, urinary tract, and pancreas. Reducing the number of young people who take up smoking and helping those who have started to smoke to quit the habit are the key ways of preventing these cancers. Tobacco- or nicotine-dependence is a common, chronic, relapsing medical condition. Studies of twins have implicated genetic factors in most of the differences in vulnerability to tobacco smoke and in the persistence of the smoking phenotype. The available interventions for reducing tobacco use and treatment for nicotine dependence offer public health officials and clinicians the greatest single opportunity for disease prevention. Five medications -- nicotine chewing gum, nicotine patches, nicotine inhalers, nicotine nasal sprays and bupropion -- and behavioural therapy appear to be both effective and safe: they double the quitting rates and are associated with a dropout rate due to adverse events of less than 5%.

Reduced expression of the Aalpha subunit of protein phosphatase 2A in human gliomas in the absence of mutations in the Aalpha and Abeta subunit genes.

Protein phosphatase 2A (PP2A) consists of 3 subunits: the catalytic subunit, C, and the regulatory subunits, A and B. The A and C subunits both exist as 2 isoforms (alpha and beta) and the B subunit as multiple forms subdivided into 3 families, B, B' and B". It has been reported that the genes encoding the Aalpha and Abeta subunits are mutated in various human cancers, suggesting that they may function as tumor suppressors. We investigated whether Aalpha and Abeta mutations occur in human gliomas. Using single strand conformational polymorphism analysis and DNA sequencing, 58 brain tumors were investigated, including 23 glioblastomas, 19 oligodendrogliomas and 16 anaplastic oligodendrogliomas. Only silent mutations were detected in the Aalpha gene and no mutations in the Abeta gene. However, in 43% of the tumors, the level of Aalpha was reduced at least 10-fold. By comparison, the levels of the Balpha and Calpha subunits were mostly normal. Our data indicate that these tumors contain very low levels of core and holoenzyme and high amounts of unregulated catalytic C subunit.

Germline SDHD mutation in paraganglioma of the spinal cord.

Hereditary paraganglioma of the head and neck is associated with germline mutations in the SDHD gene, which encodes a mitochondrial respiratory chain protein. Paragangliomas of the central nervous system are very rare, occur almost exclusively in the cauda equina of the spinal cord and are considered non-familial. In the present study, we screened 22 apparently sporadic paragangliomas of the cauda equina for SDHD mutations. One spinal paraganglioma and similar cerebellar tumours that developed 22 years later in the same patient contained a missense mutation at codon 12 (GGT-->AGT, Gly-->Ser) and a silent mutation at codon 68 (AGC-->AGT, Ser-->Ser). There was no family history of paragangliomas but DNA from white blood cells of this patient showed the same sequence alterations, indicating the presence of a germline mutation. All other cases of spinal paraganglioma had the wild-type SDHD sequence, except one case with a silent mutation at codon 68 (AGC-->AGT, Ser-->Ser). This is the first observation indicating that inherited SDHD mutations may occasionally cause the development of paragangliomas in the central nervous system.

Mutation analysis of hBUB1, hBUBR1 and hBUB3 genes in glioblastomas.

Glioblastomas, the most malignant human brain tumors, are characterized by marked aneuploidy, suggesting chromosomal instability which may be caused by a defective mitotic spindle checkpoint. We screened 22 glioblastomas for mutations in the mitotic spindle check-point genes hBUB1, hBUBR1 and hBUB3. DNA sequencing revealed a silent mutation at codon 144 of hBUB1 (CAG-->CAA, Gln-->Gln) in one glioblastoma, a silent mutation at codon 952 of hBUBR1 (GAC-->GAT, Asp-->Asp) in another glioblastoma, and a silent mutation at codon 388 of the hBUBR1 gene (GCG-->GCA, Ala-->Ala) in 8 glioblastomas. We also observed a known polymorphism at hBUBR1 codon 349 (CAA/CGA, Gln/Arg), with an allelic frequency of 0.75 for Gln and 0.25 for Arg, which is similar to that among healthy Caucasian individuals (0.73 vs 0.27). The coding sequence of the hBUB3 gene did not contain any mutation, but in 4 glioblastomas (18%), a C-->T point mutation was detected at position -6 (6 nucleotides upstream of the ATG initiator codon). Analysis of blood DNA of these patients showed identical sequence alterations, indicating that this is a polymorphism. Again, the frequency in glioblastomas was similar to that in healthy Caucasians (15%). We further screened hBUB1 in 18 cases of giant cell glioblastoma, a variant characterized by a predominance of bizarre, multinucleated giant cells. There were no changes, except for a silent mutation at codon 144 in two cases. These results suggest that mutations in these mitotic spindle checkpoint genes do not play a significant role in the causation of chromosomal instability in glioblastomas.

p14ARF deletion and methylation in genetic pathways to glioblastomas.

The CDKN2A locus on chromosome 9p21 contains the p14ARF and p16INK4a genes, and is frequently deleted in human neoplasms, including brain tumors. In this study, we screened 34 primary (de novo) glioblastomas and 16 secondary glioblastomas that had progressed from low-grade diffuse astrocytomas for alterations of the p14ARF and p16INK4a genes, including homozygous deletion by differential PCR, promoter hypermethylation by methylation-specific PCR, and protein expression by immunohistochemistry. A total of 29 glioblastomas (58%) had a p14ARF homozygous deletion or methylation, and 17 (34%) showed p16INK4a homozygous deletion or methylation. Thirteen glioblastomas showed both p14ARF and p16INK4a homozygous deletion, while nine showed only a p14ARF deletion. Immunohistochemistry revealed loss of p14ARF expression in the majority of glioblastomas (38/50, 76%), and this correlated with the gene status, i.e. homozygous deletion or promoter hypermethylation. There was no significant difference in the overall frequency of p14ARF and p16INK4a alterations between primary and secondary glioblastomas. The analysis of multiple biopsies from the same patients revealed hypermethylation of p14ARF (5/15 cases) and p16INK4a (1/15 cases) already at the stage of low-grade diffuse astrocytoma but consistent absence of homozygous deletions. These results suggest that aberrant p14ARF expression due to homozygous deletion or promoter hypermethylation is associated with the evolution of both primary and secondary glioblastomas, and that p14ARF promoter methylation is an early event in subset of astrocytomas that undergo malignant progression to secondary glioblastoma.

Promoter hypermethylation and homozygous deletion of the p14ARF and p16INK4a genes in oligodendrogliomas.

The INK4a/ARF locus on chromosome 9p21 encodes two gene products that are involved in cell cycle regulation through inhibition of CDK4-mediated RB phosphorylation (p16INK4a) and binding to MDM2 leading to p53 stabilization (p14ARF). The locus is deleted in up to 25% of oligodendrogliomas and 50% of anaplastic oligodendrogliomas, but little is known on the frequency of gene silencing by DNA methylation. We assessed promoter hypermethylation of p14ARF and p16INK4a using methylation-specific PCR, and homozygous deletion of the p14ARF and p16INK4a genes by differential PCR in 29 oligodendrogliomas (WHO grade II) and 20 anaplastic oligodendrogliomas (WHO grade III). Promoter hypermethylation of the p14ARF gene was detected in 6/29 (21%) oligodendrogliomas and 3/20 (15%) anaplastic oligodendrogliomas. None of the oligodendrogliomas and only 1 out of 20 anaplastic oligodendrogliomas showed hypermethylation of p16INK4a. Homozygous deletion was not detected in any of the WHO grade II oligodendrogliomas but was present in 5/20 (25%) anaplastic oligodendrogliomas and always affected both genes. In one tumor containing distinct areas with and without anaplasia, p14ARF hypermethylation was detected in the WHO grade II area, while homozygous co-deletion of p14ARF and p16INK4a was found in the region with anaplastic features (grade III). These data suggest that aberrant p14ARF expression due to hypermethylation is the earliest INK4a/ARF change in the evolution of oligodendrogliomas, while the presence of p14ARF and p16INK4a deletions indicates progression to anaplastic oligodendroglioma.

Soluble decoy receptor 3 is expressed by malignant gliomas and suppresses CD95 ligand-induced apoptosis and chemotaxis.

Decoy receptor 3 (DcR3) is a newly identified soluble protein that binds to CD95 ligand (CD95L) and inhibits its proapoptotic activity. Here we report that DcR3 is expressed by the majority of long-term and ex vivo malignant glioma cell lines as well as in human glioblastoma in vivo. Expression of DcR3 correlates with the grade of malignancy: 15 of 18 (83%) glioblastomas (WHO grade IV) but none of 11 diffuse astrocytomas (WHO grade II) exhibited DcR3 immunoreactivity. We also demonstrate that human malignant glioma cells engineered to release high amounts of DcR3 into the cell culture supernatant are protected from CD95L-induced apoptotic cell death. In contrast, DcR3 does not confer protection from the death ligand Apo2 ligand (TRAIL). Importantly, ectopic expression of DcR3 resulted in substantial differences in immune cell infiltration in the 9L rat gliosarcoma model. Thus, the infiltration of CD4+ and CD8+ T cells as well as microglia/macrophages into glioma was substantially decreased in DcR3-producing tumors compared with control tumors. Chemotaxis assays revealed that DcR3 counteracts the chemotactic activity of CD95L against microglial cells in vitro. These findings suggest that DcR3 may be involved in the progression and immune evasion of malignant gliomas.

Second primary glioblastoma.

Although characterized by a highly variable phenotype and multiple genetic alterations, glioblastomas are considered monoclonal in origin. We here report on a 64-yr-old patient who developed a second glioblastoma in the left frontal lobe 10 yr after surgical resection of a glioblastoma of right frontal lobe. The first tumor contained 2 p53 mutations, in codon 213 (CGA-->TGA, Arg-->stop) and codon 306 (CGA-->TGA, Arg-->stop), further, 1 missense PTEN mutation (codon 257, TTC-->TTA, Phe-->Leu) and a silent PTEN mutation (codon 154, TTC-->TTT, Phe-->Phe). The second glioblastoma also contained multiple, but different mutations: p53 mutations in codons 158 (CGC-->CAC, Arg-->His) and 273 (CGT-->TGT, Arg-->Cys), and a PTEN mutation in codon 233 (CGA-->TGA, Arg-->Stop). Both neoplasms had a homozygous p16 deletion. The discordant pattern of mutations indicates that the second glioblastoma was not a recurrence but an independent second glioblastoma. The presence in these neoplasms of multiple mutations in tumor suppressor genes suggests the involvement of a novel disease mechanism but there was no indication of a DNA mismatch repair deficiency or of an inherited tumor syndrome.

Promoter hypermethylation of the RB1 gene in glioblastomas.

Loss of expression of the retinoblastoma gene (RB1) has been shown to occur in up to 25% of glioblastomas (WHO Grade IV). To elucidate the underlying mechanism, we assessed RB1 promoter hypermethylation using methylation-specific polymerase chain reaction and RB1 expression by immunohistochemistry in 35 primary (de novo) glioblastomas and in 21 secondary glioblastomas that had progressed from low-grade diffuse astrocytoma (WHO Grade II) or anaplastic astrocytoma (WHO Grade III). Promoter hypermethylation was significantly more frequent in secondary (9 of 21, 43%) than in primary glioblastomas (5 of 35, 14%; p = 0.0258). There was a clear correlation between loss of RB1 expression and promoter hypermethylation. In the majority of glioblastomas with loss of RB1 expression, there was promoter hypermethylation (11 of 13, 85%), whereas 93% of tumors with RB1 expression had a normal RB1 gene status (p < 0.0001). In three glioblastomas, areas with and without RB1 expression were microdissected; promoter hypermethylation was detected only in areas lacking RB1 expression. In patients with multiple biopsies, methylation of the RB1 promoter was not detectable in the less malignant precursor lesions, ie, low-grade diffuse and anaplastic astrocytoma. These results indicate that promoter hypermethylation is a late event during astrocytoma progression and is the major mechanism underlying loss of RB1 expression in glioblastomas.

Invasiveness in vitro and biological markers in human primary glioblastomas.

Invasion of spheroids from 20 human primary glioblastomas into precultured fetal rat brain tissue in culture has been studied and quantified. Between 30 and 98 percent of the normal brain tissue was destroyed by invading glioma cells within 4 days. The degree of invasion did not correlate with patient survival. A slightly higher invasiveness and shorter survival was seen in tumors with EGF receptor overexpression, and the opposite pattern was found for tumors with a TP53 mutation. The degree of invasiveness in vitro was far higher than would be expected from the dynamics of clinically observed tumor spread. This suggests that mechanisms suppressing invasion may be operative in the normal brain; alternatively the differences may be due to a higher permissiveness of the fetal brain tissue for invasion in vitro.

Concurrent inactivation of RB1 and TP53 pathways in anaplastic oligodendrogliomas.

Oligodendrogliomas are characterized by frequent loss of heterozygosity (LOH) on chromosomes 1p and 19q, but additional genetic alterations are likely to be involved. In this study, we screened 28 oligodendrogliomas (WHO grade II) and 20 anaplastic oligodendrogliomas (WHO grade III) for alterations in the RB1/CDK4/p16INK4a/p15INK4b and TP53/p14ARF/MDM2 pathways. In oligodendrogliomas, hypermethylation of RB1 (1 case) and p14ARF (6 cases) were the only detectable genetic changes (7/28, 25%). In anaplastic oligodendrogliomas, the RB1/CDK4/p16INK4a/p15INK4b signaling pathway regulating the G1 -->3 S transition of the cell cycle was altered in 13/20 (65%) cases, by either RBI alteration, CDK4 amplification, or p16IN4a/p15INK4b homozygous deletion or promoter hypermethylation. Further, 50% (10/20) of anaplastic oligodendrogliomas showed alterations in the TP53 pathway through promoter hypermethylation or homozygous deletion of the p14ARF gene and, less frequently, through TP53 mutation or MDM2 amplification. Of 13 anaplastic astrocytomas with an altered RB1 pathway, 9 (69%) also showed a dysregulated TP53 pathway. Thus, simultaneous disruption of the RB1/CDK4/p16INK4a/p15INK4b and the TP53/p14ARF/MDM2 pathways occurs in 45% (9/20) of anaplastic oligodendrogliomas, suggesting that these phenomena contribute to their malignant phenotype.

Loss of heterozygosity on chromosome 19 in secondary glioblastomas.

Glioblastomas develop rapidly de novo (primary glioblastomas) or slowly through progression from low-grade or anaplastic astrocytoma (secondary glioblastomas). Recent studies have shown that these glioblastoma subtypes develop through different genetic pathways. Primary glioblastomas are characterized by EGFR amplification/overexpression, PTEN mutation, homozygous p16 deletion, and loss of heterozygosity (LOH) on entire chromosome 10, whereas secondary glioblastomas frequently contain p53 mutations and show LOH on chromosome 10q. In this study, we analyzed LOH on chromosomes 19q, 1p, and 13q, using polymorphic microsatellite markers in 17 primary glioblastomas and in 13 secondary glioblastomas that progressed from low-grade astrocytomas. LOH on chromosome 19q was frequently found in secondary glioblastomas (7 of 13, 54%) but rarely detected in primary glioblastomas (1 of 17, 6%, p = 0.0094). The common deletion was 19q13.3 (between D19S219 and D19S902). These results suggest that tumor suppressor gene(s) located on chromosome 19q are frequently involved in the progression from low-grade astrocytoma to secondary glioblastoma, but do not play a major role in the evolution of primary glioblastomas. LOH on chromosome 1p was detected in 12% of primary and 15% of secondary glioblastomas. LOH on 13q was detected in 12% of primary and in 38% of secondary glioblastomas and typically included the RB locus. Except for 1 case, LOH 13q and 19q were mutually exclusive.

Genetic profile of gliosarcomas.

There are distinct genetic pathways leading to the glioblastoma, the most malignant astrocytic brain tumor. Primary (de novo) glioblastomas develop in older patients and are characterized by epidermal growth factor (EGF) receptor amplification/overexpression, p16 deletion, and PTEN mutations, whereas secondary glioblastomas that progressed from low-grade or anaplastic astrocytoma develop in younger patients and frequently contain p53 mutations. In this study, we assessed the genetic profile of gliosarcoma, a rare glioblastoma variant characterized by a biphasic tissue pattern with alternating areas displaying glial and mesenchymal differentiation. Single-strand conformation polymorphism followed by direct DNA sequencing revealed p53 mutations in five of 19 gliosarcomas (26%) and PTEN mutations in seven cases (37%). Homozygous p16 deletion was detected by differential polymerase chain reaction in seven (37%) gliosarcomas. The overall incidence of alterations in the Rb pathway (p16 deletion, CDK4 amplification, or loss of pRb immunoreactivity) was 53%, and these changes were mutually exclusive. Coamplification of CDK4 and MDM2 was detected in one gliosarcoma. None of the gliosarcomas showed amplification or overexpression of the EGF receptor. Thus gliosarcomas exhibit a genetic profile similar to that of primary (de novo) glioblastomas, except for the absence of EGFR amplification/overexpression. Identical PTEN mutations in the gliomatous and sarcomatous tumor components were found in two cases. Other biopsies contained p16 deletions, an identical p53 mutation, or coamplification of MDM2 and CDK4 in both tumor areas. This strongly supports the concept of a monoclonal origin of gliosarcomas and an evolution of the sarcomatous component due to aberrant mesenchymal differentiation in a highly malignant astrocytic neoplasm.

APC mutations in sporadic medulloblastomas.

The cerebellar medulloblastoma (WHO Grade IV) is a highly malignant, invasive embryonal tumor with preferential manifestation in children. Several molecular alterations appear to be involved, including isochromosome 17q and the p53, PTCH, and beta-catenin gene mutations. In this study, 46 sporadic medulloblastomas were screened for the presence of mutations in genes of the Wnt signaling pathway (APC and beta-catenin). Single-strand conformational polymorphism (SSCP) analysis followed by direct DNA sequencing revealed 3 miscoding APC mutations in 2 (4.3%) medulloblastomas. One case contained a GCA-->GTA mutation at codon 1296 (Ala-->Val), and another case had double point mutations at codons 1472 (GTA-->ATA, Val-->Ile) and 1495 (AGT-->GGT, Ser-->Gly). Miscoding beta-catenin mutations were detected in 4 tumors (8.7%). Three of these were located at codon 33 (TCT -->TTT, Ser-->Phe) and another at codon 37 (TCT-->GCT, Ser-->Ala). Adenomatous polyposis coli (APC) gene and beta-catenin mutations were mutually exclusive and occurred in a total of 6 of 46 cases (13%). Although germline APC mutations are a well established cause of familial colon and brain tumors (Turcot syndrome), this study provides the first evidence that APC mutations are also operative in a subset of sporadic medulloblastomas.

Loss of heterozygosity on chromosome 10 is more extensive in primary (de novo) than in secondary glioblastomas.

Glioblastomas develop de novo (primary glioblastomas) or through progression from low-grade or anaplastic astrocytoma (secondary glioblastomas). There is increasing evidence that these glioblastoma subtypes develop through different genetic pathways. Primary glioblastomas are characterized by EGFR and MDM2 amplification/overexpression, PTEN mutations, and p16 deletions, whereas secondary glioblastomas frequently contain p53 mutations. Loss of heterozygosity (LOH) on chromosome 10 (LOH#10) is the most frequent genetic alteration in glioblastomas; the involvement of tumor suppressor genes, other than PTEN, has been suggested. We carried out deletion mappings on chromosome 10, using PCR-based microsatellite analysis. LOH#10 was detected at similar frequencies in primary (8/17; 47%) and secondary glioblastomas (7/13; 54%). The majority (88%) of primary glioblastomas with LOH#10 showed LOH at all informative markers, suggesting loss of the entire chromosome 10. In contrast, secondary glioblastomas with LOH#10 showed partial or complete loss of chromosome 10q but no loss of 10p. These results are in accordance with the view that LOH on 10q is a major factor in the evolution of glioblastoma multiform as the common phenotypic end point of both genetic pathways, whereas LOH on 10p is largely restricted to the primary (de novo) glioblastoma.

Phenotype vs genotype in the evolution of astrocytic brain tumors.

Astrocytic brain tumors are the most frequent human gliomas and they include a wide range of neoplasms with distinct clinical, histopathologic, and genetic features. Diffuse astrocytomas are predominantly located in the cerebral hemispheres of adults and have an inherent tendency to progress to anaplastic astrocytoma and (secondary) glioblastoma. The majority of glioblastomas develop de novo (primary glioblastomas), without an identifiable less-malignant precursor lesion. These subtypes of glioblastoma evolve through different genetic pathways, affect patients at different ages, and are likely to differ in their responses to therapy. Primary glioblastomas occur in older patients and typically show epidermal growth factor receptor (EGFR) overexpression, PTEN mutations, p16 deletions, and, less frequently, MDM2 amplification. Secondary glioblastomas develop in younger patients and often contain TP53 mutations as their earliest detectable alteration. Morphologic variants of glioblastoma were shown to have intermediate clinical and genetic profiles. The giant cell glioblastoma clinically and genetically occupies a hybrid position between primary (de novo) and secondary glioblastomas. Gliosarcomas show identical gene mutations in the gliomatous and sarcomatous tumor components, which strongly supports the concept that there is a monoclonal origin for gliosarcomas and an evolution of the sarcomatous component due to aberrant mesenchymal differentiation in a highly malignant astrocytic neoplasm.