Abstract
The present study determined mutations of phosphatase and tensin homolog (PTEN) gene in patients suffering from high-grade gliomas (WHO grades III and IV) and further investigated the mutations in correlation to patients' histopathological classification and short-term survival. Total RNA and genomic DNA were extracted from tumor tissues. Full-length PTEN cDNA sequences were amplified by polymerase chain reaction (PCR). The PCR products were directly sequenced, and the PTEN mutations were analyzed. It demonstrated that the incidence of PTEN mutations was 8/22 in these patients: one patient with WHO grade III glioma (1/11) and 7 patients with WHO grade IV glioma (7/11). Most patients had three or more mutations in the PTEN gene, with exons 2, 3, 4, 5, 6 and 7 as hot mutation regions, with mutation incidence from 62.5% to 75%. About 68.4% of mutations were missense, 26.3% same-sense and 5.3% nonsense mutations. The median survival times of the WHO grade III and IV groups were 250 and 53 weeks after surgery, respectively (p=0.016). The 36-week survival rate of patients with and without PTEN mutations was 62.5% and 92.9% (p=0.038, odds ratio=7.80), respectively. The present study suggests that PTEN mutations are late events in the malignant progression of glioma and the occurrence of PTEN mutations are significantly correlated to patients' short-term survival.
Glioma is the most common neoplasm of the central nervous system, accounting for about 40%-50% of brain tumors (1). It is suggested that WHO high-grade gliomas, including anaplastic astrocytoma (AA) and glioblastoma multiform (GBM), have poor prognosis and high mortality. Despite the advances in diagnostics and treatments, such as modern diagnostic procedures, novel surgical techniques, and effective radiation therapy as well as chemotherapy, the prognosis of patients suffered from high-grade gliomas remains dismal. The majority of patients with GBM die within one year of diagnosis (2).
It has been suggested that genetic alterations of certain genes are critical events behind the pathogenesis of gliomas. Several investigations demonstrated that the deletions and/or mutations of tumor suppressor genes might play an important role. Phosphatase and tensin homolog (PTEN), a tumor suppressor gene, located on chromosome 10q23.3, encodes a 403 amino acid, which is a dual-specificity phosphatase with homology both to the protein tyrosine phosphatase (PTP) family and to the cytoskeletal proteins, i.e. tensin and auxilin (3-5). In general, PTEN negatively regulates the anti-apoptotic action of akt phosphorylation (6). It has been reported that mutations of PTEN have been implicated in the malignant progression of astrocytic gliomas, as these alterations are most frequently observed in GBM, less commonly found in AA, and very rare in the lower grade astrocytoma (7-8).
We previously examined PTEN expression in glioma tissue, tumor-adjacent normal brain tissues and benign brain tumor tissues, and found that mRNA levels of PTEN were much lower in the glioma tissues. PTEN mRNA levels were significantly different between the highly differentiated gliomas (WHO grades I-II) and high-grade gliomas (WHO grades III-IV). As mutations of PTEN are very rare in highly differentiated gliomas (9), in the present study, we investigated the mutation types and frequencies of PTEN alterations in 22 patients suffering from high-grade glioma and investigated the correlation between these alterations and the patients' histopathological classification as well as their short-term survival.
Characteristics of the patients.
Patients and Methods
Patients and tissue samples. A total of 22 patients (17 males and 5 females, 25- to 72-years-old; median age 51 years) suffering from gliomas and hospitalized in our hospital from 2000 to 2002 were surgical treated and followed up in the present study. All tumor samples were quickly frozen in liquid nitrogen after resection and were kept at −80°C until nucleotide extraction. All samples were examined histopathologically and were classified according to the WHO classification of tumors in the central nervous system (10). There were 11 cases of grade III and 11 cases of grade IV that were all defined as high-grade gliomas. None of the patients received chemotherapy or radiotherapy before surgery. The characteristics of the patients are listed in Table I.
Nucleotide extraction and polymerase chain reaction (PCR). Total RNA and genomic DNA (for amplifying the first and last exon of PTEN) were extracted by using the total RNA isolation classic kit (SNBC, Shanghai, P. R. China) and UIIQ-10 column blood genomic DNA isolation kit (Sangon, Shanghai, P. R. China), respectively. The quality of the RNA samples was determined by absorbance measurements at 260/280 nm. Before RNA extraction, frozen sections of the specimens were examined histologically to ensure that they were representative of the tumor tissues. First-strand cDNAs were synthesized by using a RevertAid first-strand cDNA synthesis kit (MBI Fermentas Inc., Canada). According to the mRNA (HSU93051) and DNA (AC063965) sequences of PTEN in the GenBank, four PCR primer sets were designed respectively (as shown in Table II) to amplify four fragments (747 bp, 516 bp, 430 bp and 362 bp, respectively) which can be joined to form a full-length cDNA sequence (1212 bp). The PCR reaction for each fragment was performed in a 50 μl volume, containing 0.2 μl 100 mM primers and probes, 2 μl cDNA, 5 μl 10x buffer, 3 μl MgCl2 (25 mM), 1 μl dNTP (10 mmol/l) and 1 μl Taq DNA polymerase. The thermal cycling conditions included the following steps: 1 min at 95°C to activate Taq polymerase, then a 40-cycle 3-step PCR was performed consisting of 30 s at 95°C, 30 s at 60°C (56°C and 58°C when 747 bp and 516 bp fragments were amplified, respectively) and 60 s at 72°C. All PCRs were performed on a GeneAmp PCR system 9700 (Perkin-Elmer Applied Biosystems, USA)
Primers of human PTEN gene.
Sequencing of PCR products and analyses of PTEN mutations. The PCR products were directly sequenced on an automatic sequencer from Applied Biosystems, model 3730 (Invitrogen, Shanghai, P. R. China). After sequencing, four fragments were spliced and joined to obtain full length PTEN cDNA of 1212 bp. Vector NTI advance 10 software package (Invitrogen Corporation, CA, USA) was used to analyze the PTEN mutations.
Statistic analyses. All data were statistically analyzed by Prism version 4 (GraphPad Software, Inc. CA, USA). Comparisons between these with and without mutations of PTEN in the two groups (WHO grade III and WHO grade IV) were statistically evaluated by the Chi-square test. A p-value less than 0.05 was considered as significant. Correlation of patients' life span with tumor grades and mutations of PTEN were analyzed by the Kaplan-Meier curve.
Results
Frequency and genotype of PTEN mutations. Eight out of a total 22 cases were found to have mutations in their PTEN cDNA. Among these mutations, there was only one case in the WHO grade III patient group, which represents mutation incidence of 1/11 and 7 cases in the WHO grade IV patient group (7/11). The mutation distribution is shown in Table III. The frequency of mutation was similar for each exon, but there were only two cases with mutation of exon 8 and none for mutation of exon 1 and 9. Most patients had three or more mutations of their PTEN (Table III). Exons 2, 3, 4, 5, 6 and 7 are hot mutation regions, with a mutation incidence from 5/8 to 6/8 (Table III). In the present study, there were 19 different mutations found in these patients (Table III). There were 13/19 mutations which represented missense mutation, 5/19 as same-sense mutation and 1/19 as nonsense mutation.
PTEN mutations in the WHO high-grade gliomas.
PTEN mutations in correlation to tumor histopathological classification and to patients' short-term survival. The incidence of PTEN mutations was significantly different between WHO grade III and WHO grade IV (Table IV) (p=0.024). The risk rate of histopathological diagnosis of WHO grade IV was 3.06-fold for a patient with mutation of PTEN than for those without (95% confidence interval, CI: 1.284-7.302). The median survival time of WHO grade III and IV groups were 250 and 53 weeks, respectively (Figure 1A) (p=0.016). At the time of writing, there are four patients still alive in the WHO grade III group, however, all patients in the WHO grade IV group have died. As shown in Figure 1B, the median survival period of patients with and without PTEN mutations were 67 and 99 weeks (p=0.292), respectively. However, as shown in Table V, the 36-week survival rates of these two group patients were 62.5% and 92.9% respectively, which is statistically significant (p=0.038, odds ratio, OR=7.80, one-sided test).
Association between mutation of PTEN and histopathological classification.
Comparison of 36-week survival rate.
Discussion
In the present study, we analyzed PTEN mutations in WHO high-grade gliomas by direct sequencing of full-length PTEN cDNA. Only one case had PTEN mutation in WHO grade III patients (1/11), whereas there were seven cases with PTEN mutations in the WHO grade IV patients, which revealed that mutation of PTEN appears to be a late event in the malignant progression of gliomas. Although the PTEN gene contains 9 exons and encodes a 403-amino acid protein, we found PTEN mutations mainly occurred with high frequency in the region encoding the first 240 amino acids, which suggests that the latter part of the PTEN gene is a relative conserved region.
Comparison of survival curves. A: Histopathological classification; B: PTEN gene alteration.
The present study demonstrated that the median survival time of patients with WHO grade III was significantly longer than these with WHO grade IV, which suggests that there is a strong correlation between tumor histopathological classifications and patients' prognoses (11, 12). According to the literature, it is still controversial whether PTEN mutation is correlated to prognosis in patients with WHO high-grade glioma. Several studies have addressed whether PTEN gene aberrations could be considered as a potential prognostic index for patients suffered from malignant glioma. Sasaki et al. (13) investigated 47 patients with anaplastic oligodendroglioma and demonstrated that the presence of PTEN aberrations was significantly associated with shorter survival time in these patients. The median survival time was only 14.8 months in patients with PTEN aberration, however, in patients without PTEN aberration, the median survival time was extended to 123.4 months. This suggested that PTEN gene alterations might be regarded as one of the key roles in the pathogenesis of certain anaplastic oligodendrogliomas and may be associated with an aggressive tumor phenotype regardless of chemosensitivity. In another study of 62 anaplastic astrocytomas, Smith et al. (14) reported that PTEN mutations were detected in 11 out of 62 cases, with median survival times of 4.4 months and 34.4 months in patients with and without PTEN mutations, respectively. This indicated that PTEN mutation may be a powerful independent index for reduced survival time in patients suffering from anaplastic astrocytomas (WHO grade III). In the case of glioblastoma, however, Zhou et al. (15) analyzed the correlation between clinical data and PTEN mutations, and showed there was no significant correlation between PTEN mutation and patients' survival time; the median survival time was 11 and 12 months in patients with and without PTEN-mutated tumors, respectively. A similar study revealed that mutations in the PTEN gene lacked prognostic significance for overall survival time in GBM patients (16). In addition, another two independent studies suggested that loss of heterozygosity at microsatellite markers flanking the PTEN gene locus on 10q23.3 might be a significant predictor of the shorter survival time among glioblastoma patients (17-18).
In order to elucidate whether PTEN mutation is correlated to patient prognosis in WHO high-grade gliomas, we therefore assessed PTEN mutations in corresponding cDNA obtained from patients with WHO grade III and IV gliomas. Our data demonstrated that the median survival time of patients with PTEN mutations was only 67 weeks, while it was 99 weeks in the patients without PTEN mutation, but the survival analysis indicated that the difference of the median survival time between these two groups did not reach the significance level. However, the 36-week survival rate was statistically different between patients with and without PTEN mutations, which may suggest that PTEN mutation may only be correlated to patients' short-term survival.
Conclusion
PTEN mutations mainly occurred with high frequency in the first part of the PTEN gene. These mutations appear to be late events in the malignant progression of glioma, as well as being significantly correlated to patients' short-term survival.
Footnotes
- Received September 23, 2009.
- Revision received February 22, 2010.
- Accepted February 23, 2010.
- Copyright© 2010 International Institute of Anticancer Research (Dr. John G. Delinassios), All rights reserved
