Gliomas are the most common primary brain tumors and are classified on the basis of histopathological characteristics. Glioblastoma (GBM) is the most common and malignant glioma subtype characterized by rapid growth and poor prognosis.1 Like cancer in general, GBM develop as a consequence of genetic alterations that accumulate with tumor progression. However, pathogenesis of GBM recurrence is still poorly understood. Certain molecular pathways and novel biomarkers have been established as diagnostic, prognostic and predictive markers, such as is epidermal growth factor receptor (EGFR) amplification, O6-methylguanine-DNA methyltransferase (MGMT) methylation and others.1, 2, 3, Among novel biomarkers are non-coding RNAs, of which the most studied are microRNAs (miRNAs). The miRNAs are small RNAs that act as endogenous regulators of gene expressions and function in different physiological processes, contributing to pathogenesis of different types of cancer, including gliomas. miRNA expression might be a potential biomarker for GBM used as diagnostic support or for prognostic and therapeutic application.4, 5, 6, However, differential expression of certain miRNAs might be also involved in pathophysiological mechanisms of GBM recurrence, but there are limited data regarding miRNA expression as GBM progresses (at its recurrence).7, 8, Comprehensive knowledge and understanding of the molecular pathways underlying disease progression, tumor recurrence and response to therapy might be of great importance in future development of more efficient therapies in GBM.
Local infiltration and progressive growth of GBM tumor cells into the adjacent brain tissue invalidate any possibility for a radical surgical tumor resection.9 Consequently, recurrence is observed in almost all tumors.7 Additionally, the difficulty in treatment of this lethal disease is in tumor acquisition of several mechanisms of radio and chemoresistance. In addition to methylation of MGMT10, a change in miRNA’s expression is also a cause for drug resistance in GBM.11 Several miRNAs have been related to radio- and chemo-sensitivity as well as radio- and chemo-resistance of glioma cells, and recognized as crucial regulators during glioma pathogenesis and progression.12 However, a certain degree of this aberrant regulation may also be the consequence of treatment.
In aim to characterize at least some miRNAs that might be involved in GBM formation/progression and recurrence, and may also contribute to therapy resistance, we selected 83 consecutive patients with operable recurrent GBM, where samples from first and from second surgery were available. We selected ten miRNAs, which expression was up-regulated (
List of the analysed miRNAs with known functions in glioma pathogenesis
MicroRNAs | Invasion | Proliferation | Tu growth | Migration | Cell cycle | cell Survival/death | Other functions |
---|---|---|---|---|---|---|---|
+ | + | + | nd | + | + | Differentiation, Vasculogenic mimicry | |
nd | + | nd | + | nd | nd | Self-renewal, Vasculogenic mimicry | |
+ | + | + | nd | + | + | Angiogenesis | |
+ | + | + | + | + | + | / | |
+ | + | + | + | nd | + | Vasculogenic mimicry | |
+ | + | + | + | + | + | Differentiation, Angiogenesis | |
nd | + | nd | nd | nd | nd | / | |
nd | + | + | + | nd | nd | / | |
+ | nd | nd | + | nd | nd | / | |
nd | nd | nd | nd | nd | nd | / | |
+ | + | nd | + | nd | nd | / |
Our retrospective study included eighty-three patients with recurrent GBM. The data were obtained from the patients charts from hospital registry of the Institute of Oncology and from the Cancer Registry of Slovenia. Each patient underwent the first surgery and the second one after the first recurrence of GBM between January 1997 and November 2011 at the University Clinical Centre Ljubljana, Slovenia. The selected patients were consecutive; of the 1117 that underwent the first surgery for GBM, 83 underwent also the second surgeryat tumor recurrence since these patients were capable of re-operation according to clinical standards (Karnofsky performance status over 80). Patients received radiotherapy (RT) after the surgery, and RT with concomitant and adjuvant chemotherapy (ChT) after 2005.
Tissue samples were fixed in 10% buffered formalin and embedded in paraffin (FFPE) for routine diagnostics. The grading criteria based on WHO classification was used.14 Paired tissue samples were retrieved from the archive of the Institute of Pathology, Faculty of Medicine, University of Ljubljana. Samples were classified in two groups,
The patients’ data, including age, gender and therapy are summarized in Table 2.
Demographic and clinicopathological characteristics of patients included in the study
Histopathological diagnosis | GBM, grade WHO IV | |
---|---|---|
83 | ||
50 (min ± 12.8 8, max 71) | ||
50 (60%) | ||
33 (40%) | ||
83 | ||
83/83 (100%) | ||
54/83 (65%) | ||
15/83 (18%) | ||
47/83 (56%) | ||
38/47 (81%) | ||
46/47 (98%) | ||
0.84–67.30 | ||
0.03–25.75 | ||
0.84–70.48 |
MGMT = methyl guanine methyl transferase; TMZ = temozolamide
Tissue samples were cut at 10 μm from FFPE tissue blocks and for the isolation procedure six to eight 10 μm sections were used. Total RNA isolation was performed using miRNeasy FFPE Kit (Qiagen, Germany) according to the manufacturer’s protocol. The RNA was eluted in 30 μl of nuclease-free water. The yield was measured spectrophotometrically using the NanoDrop-1000 (Thermo Scientific, USA) and the quality was evaluated on the Bioanalyzer 2100 (Agilent Technologies, USA).
All the reagents were from Qiagen, except where otherwise indicated. Quantitative PCR (qPCR) was carried out using the Rotor Gene-Q Real-Time PCR System and all the qPCR reactions were performed in duplicates or triplicates. Prior to qPCR analysis, two pools of RNA samples were created from FFPE tissue samples (primary, recurrence) and, along with reference RNA, tested for qPCR efficiency. Reverse transcription using miScript reverse transcription kit was performed in a 10 μl reaction master mix with 50 ng of total RNA according to manufacturer instruction. The resulting cDNA was diluted 100-fold and qPCR reaction was carried out in 10 μl PCR master mix, according to manufacturer instruction. As the reference genes,
In a subset of patients that were treated with temozolamide (TMZ) (n = 47), MGMT methylation status was determined. Tissue samples from the first surgery and the surgery at first recurrence were cut at 10 μm from FFPE tissue blocks and for the isolation procedure, six to eight 10 μm sections were used. Total DNA isolation was performed using QIAamp DNA FFPE Tissue Kit (Qiagen) according to the manufacturer’s protocol. The DNA was eluted in 60 μl of nuclease-free water. The yield was measured fluorescently using the Quant-It (Life Technologies) according to manufacturer instruction and Rotor Gene Q (Qiagen).
For determining the MGMT methylation status, methyl-specific polymerase chain reaction (MSP) was used in a two-step approach with primers previously described.16 Briefly, prior to MSP, 500 ng of DNA was used for bisulfite conversion using innuCONVERT Bisulfite Basic Kit according to manufacturer instruction (Analytik Jena) and stored at -20°C for subsequent MSP. For MSP, 15 ng of bisulfite converted DNA was used with 0.2 μM of each primer for methylated form and 0.3 μM of each primer for unmethylated form, 2 mM of dNTP and 0.25 U of Hot Master Polymerase (5 Prime), all in 10 μl reaction. Amplification was performed according to manufacturer instruction using 59°C for primer annealing. In each run, fully methylated (EpiTect Control DNA, methylated, Qiagen) as well as fully unmethylated controls (EpiTect Control DNA, unmethlyated, Qiagen) were used as assay controls. Results were analyzed using 2% agarose gel. The investigator who analyzed the GBM samples was blinded to all clinical information.
To present a relative gene expression the 2-ΔΔCt method was used.17 For paired samples (primary, recurrent), the calculated ΔCt was tested for statistical significance using Wilcoxon Signed Rank test. For independent group of samples, ΔΔCt was calculated relatively to Human brain Reference RNA and used for Mann-Whitney test (treatment, MGMT status,
Schematic representation of the workflow and statistical comparisons.
GBM = glioblastoma; Reference RNA = Human Brain Reference RNA; 1 = Willcoxon Signed Rank Test between primary glioblastoma and corresponding recurrent glioblastoma using ΔCt; 2 = normalization to Human Brain reference RNA resulting in ΔΔCt; 3 and 4 = Mann-Whitney test between independent groups of samples using ΔΔCt (normalization of glioblastoma samples to Human Brain Reference RNA)
Study has been approved by Slovene National Medical Ethics Committee (Number 113/05/13).
Comparing expression of analyzed miRNAs to Human Brain Reference RNA (ΔΔCt), we found altered expression for all 11 analyzed miRNAs both in primary as well as in recurrent GBM. Whereas
Expression of miRNAs in primary and recurrent GBM (compared to the Human Brain Reference RNA, ΔΔCt) with Table presenting Average Fold Changes.
ChT = chemotherapy; RT = radiotherapy; A = primary GBM; B = recurrent GBM
Using calculated ΔCt and Wilcoxon Signed Rank test, we have observed significant alteration in expression for 7 miRNAs between primary and recurrent GBM. Five miRNAs,
Expression of miRNAs in recurrent glioblastoma (GBM) after treatment compared to expression in primary GBM. Results are represented separately for patients treated with RT and those treated with RT and ChT.
ChT = chemotherapy; RT = radiotherapy; * = significant differences in expression of miRNAs, p < 0.05; ** = p < 0.01
Using Mann-Whitney test for comparing subgroup of patients treated with RT alone to patients treated with RT and ChT (analyzed miRNAs normalized to Human Brain Reference RNA, ΔΔCt), we observed that expression of certain miRNAs was different before treatment (at first surgery) as well as after treatment (at second surgery). Therefore, only those miRNAs that showed significantly differential expression only after treatment (at second surgery) were considered truly as a consequence of treatment. Both
Distinctive expression of miRNAs after different treatment options in recurrent glioblastoma
ChT = chemotherapy; RT = radiotherapy; * = p < 0.05; ** = p < 0.01
There was weak, but statistically significant change in progression free survival between patients who received RT alone and those who received both, RT and ChT after first surgery (7 months and 11 months, respectively; p = 0.045). Results are summarized in Figure 5A
Survival curve analyses;
ChT = chemotherapy; RT = radiotherapy
We have also calculated the survival time from second surgery. The number of patients who received RT alone or RT and ChT was too small for reliable analysis of survival time. However, there was statistically significant longer overall survival for patients who received ChT compared to those that were not treated after second surgery (8 months and 5 months, respectively; p = 0.004). Results are summarized in Figure 5B.
We further calculate Spearman’s and Pearson’s correlation coefficient between miRNA expression (analyzed miRNAs normalized to Human Brain Reference RNA, ΔΔCt) in primary GBM and progression free survival and overall survival, and found weak inverse correlation for
Among patients treated with ChT, there were 47 treated with TMZ. Among them, there were 9 patients with unmethylated and 34 with methylated MGMT. All, except one, became methylated in recurrent tumor. Comparing miRNA expression normalized to Human Brain Reference RNA between MGMT methylated and MGMT unmethylated tumors revealed no correlation of investigated miRNA expression to MGMT methylation status.
In our 83 consecutive patients operated for primary GBM and re-operated at the first recurrence of GBM, we have analyzed the expression of 11 selected miRNAs (
We observed differential expression of the majority of analyzed miRNAs in both samples. In primary as well as in recurrent GBM we have observed overall
Our data also showed statistically significant alteration of expression of seven miRNAs (
Decrease expression of
In recurrent GBM, but not in primary GBM, we found significant change in expression between samples of patients who were treated with RT and ChT and those that were treated with RT alone for only
Summary of known functions in radio- and chemoresistance and sensitivity of miRNAs analyzed in our study.
Finally, in our patients we have observed longer disease free survival in patients treated with RT and ChT after first surgery compared to those treated with RT alone. The standard care treatment of GBM altered importantly after year 2005 when RT alone was replaced by RT associated with chemotherapeutic TMZ in great majority of our patients. Similarly to our results, it has been shown that for patients treated with surgery and RT alone median survival was shorter (12–12.1 months) compared to patients treated additionally with TMZ (14.2–14.6 months).35
Investigating miRNAs expression and survival in patients with GBM revealed that, both,
Although we did not found any differential expression of investigated miRNAs between methylated and unmethylated MGMT, we did observed change in MGMT methylation status from primary to recurrent GBM. Eight of nine unmethylated samples in primary GBM were methylated in recurrent GBM. This change in methylation status of MGMT is not surprising, since there are several publications describing similar percentage of change; however all changes were from more to less favorable state for tumor progress ion.36, 37, 38, 39
In conclusion, our results suggest that in recurrent GBM patients, miRNAs might play important role not only in pathogenesis of GBM, but also in tumor progression and its recurrence, as well as in response to therapy, in overall survival and progression free survival.