Introduction
The recently updated World Health Organization (WHO) classification of central nervous system (CNS) neoplasms incorporated molecular information into the definition of some CNS tumors, thereby officially turning a page into the era of molecular diagnosis of CNS neoplasms. Among such neoplasms, oligodendroglioma was defined as IDH-mutant and 1p/19q-codeleted, making the 1p/19q-codeletion part of the definition of this tumor a quarter of a century after it was first noticed in oligodendroglioma [
33]. This genetic alteration is caused by unbalanced translocation of chromosome (chr.) 19p to 1q, leading to the whole arm loss of 1p and 19q. Recent research using next-generation sequencing analysis has revealed the mutational landscape of lower-grade gliomas including oligodendroglioma [
13,
35]. Interestingly, the 1p/19q-codeletion has tight positive association with
IDH mutations and
TERT promoter mutations, while it is mutually exclusive with
ATRX loss and
TP53 mutation, which are the hallmark of diffuse astrocytoma, IDH-mutant. Some (30-60%) of 1p/19q-codeleted tumors also have accompanying mutations of
CIC,
FUBP1 or
NOTCH1, but these mutations do not appear to be essential for establishment of the histological and clinical features of oligodendrogliomas [
4]. Although it is still unknown how 1p/19q-codeletion contributes to the oncogenesis of oligodendroglioma, this alteration is known to be clinically important because tumors with 1p/19q-codeletion have shown remarkable response to combined chemotherapy with procarbazine, lomustine, and vincristine (PCV therapy) [
11], which has been confirmed in multiple clinical trials [
8,
10,
37]. In contrast to diffuse astrocytoma, IDH-mutants that often undergo malignant progression [
3,
20], oligodendroglioma has longer progression free survival and a lower tendency to progress to very aggressive tumors [
22]. However, again, the molecular mechanisms that underlie such behaviors are not well known. To gain insight into the molecular mechanism underlying this behavior of oligodendroglioma, we investigated the genetic and epigenetic profile of 1p/19q-codeleted oligodendroglioma at recurrence and compared them to those of the original tumor.
Discussion
In the present study, similar to the results of previous studies of astrocytic tumors [
3,
20,
35], the mutation retention rate at recurrence in oligodendrogliomas was relatively low. In ten out of the twelve tumors, more than half of the mutations found in the primary tumor were not found in the recurrent tumor. These observations indicated that oligodendrogliomas show a complex branched evolutionary pattern at recurrence similar to other malignant gliomas. Indeed, in some recurrent tumors, mutations such as
CIC,
TP53, and
PIK3CA mutations that are generally regarded as potent drivers were not maintained at recurrence. On the other hand, mutations in
FUBP1, which is a transcriptional modulator of c-MYC [
19], were maintained or newly acquired at recurrence, suggesting that these
FUBP1 mutations may confer a survival advantage. Similarly, although less frequently, inactivating
TCF12 mutations were acquired in 2 recurrent tumors; these mutations were frameshift indels, p.97_97del in patient 1 and p.I162fs in patient 4. Mutations leading to truncation of a basic helix-loop-helix (bHLH) domain of the transcription factor TCF12 were previously detected in an aggressive type of 1p/19q-codeleted tumor [
26]. Those results, together with our data, mean that such truncation can be considered as one of the driving genetic alterations in recurrent oligodendroglioma. Regarding copy number alterations, apart from 1p/19q-codeletion, the 9p21 locus containing the
CDKN2A gene was the most frequently altered locus. Alteration of this locus was not so frequent in 1p/19q-codeleted tumors in previous large scale analyses [
13,
35]. However, alteration of the 9p21 locus was previously reported to be associated with histological malignancy such as microvascular proliferation and necrosis [
6,
15], as well as worse prognosis in 1p/19q-codeleted tumors [
1]. Even though the alterations described above might have been clonally selected at recurrence and were potentially associated with tumor growth, there was no increase in malignant histological characteristics in most of the recurrent tumors, and most of such tumors could still be controlled by the treatment, demonstrating that these events were not sufficient to enhance tumor malignancy.
The rise of a hypermutator phenotype after TMZ chemotherapy against low-grade gliomas has been reported, raising some concern regarding the management strategy for this tumor [
20]. In our series of 12 pairs of primary and recurrent tumors, in which PAV chemotherapy was used in the majority (7/12), neither a significant increase in mutation number in recurrent tumors nor a hypermutator phenotype was observed. One possible reason for the absence of hypermutation in our series is that astrocytic tumors with
IDH mutation may be more prone to a hypermutator phenotype compared to oligodendrogliomas, since a previous report demonstrated that a hypermutator phenotype is frequently found in astrocytic tumors harboring
IDH mutation that were treated with TMZ [
20,
24]. It has also been reported that a hypermutator phenotype might be infrequent in glioblastoma without
IDH mutation, suggesting that the incidence rate of the hypermutator phenotype is different among glioma subtypes [
24]. Another possible explanation is that only three cases were treated with TMZ, which is thought to be a major driver for the hypermutator phenotype, and that PAV therapy, which is an analogous regimen to PCV chemotherapy in Japan, is less likely to cause a hypermutator phenotype. The emergence of a hypermutator phenotype is thought to be related to the mechanism of action of TMZ, and to chemical reactions of alkylating agents belonging to the triazene group such as TMZ, procarbazine, and dacarbazine, which differ from those of nitrosoureas such as ACNU, BCNU, and CCNU. Briefly, TMZ adds a methyl group to the O
6 position of a guanine residue to make O
6-methylguanine, which leads to the addition of a thymine residue instead of a cytosine into the paired DNA strand when DNA replicates. These mismatch residues are recognized by the mismatch repair system. An attempt to repair this mismatch is then initiated, which cannot be completed in the presence of O
6-methylguanine and therefore the process ends up with thymine reinsertion, leading to a futile mismatch repair cycle and eventually apoptosis [
16,
32]. A defect in the mismatch repair system confers resistance to TMZ but leads to a large amount of C > T/G > A mutations [
12,
20]. On the other hand, nitrosoureas such as ACNU add a chloroethyl group to the O
6 position of a guanine residue, making O
6-chloroethylguanine, and subsequent cross-linking prevents DNA replication and induces apoptosis [
34]. Thus, the mechanism of action of ACNU is not related to the mismatch repair system, and therefore these drugs will not cause a hypermutator phenotype. Although procarbazine that is used in PAV chemotherapy has a similar pharmacological action to TMZ, the dosage and duration of procarbazine treatment are different from those of TMZ and such differences might affect the incidence rate of a hypermutator phenotype. Indeed, there did not seem to be a frequent rise in a hypermutator phenotype after chemotherapy that consisted mainly of nitrosourea in our oligodendroglioma cases. A recent phase III study showed that radiation plus PCV chemotherapy elongates progression-free survival and overall survival of high-risk low-grade glioma, especially of oligodendroglioma [
8]. Although TMZ is a candidate substitute for PCV therapy, which often results in relatively severe side effects, it may be necessary to consider such possible different consequences of these regimens and to investigate genomic status in a larger series of recurrent gliomas in the future. Understanding those molecular dynamic features might be essential for planning of the future treatment strategy, including molecular targeting therapy, for sufficient control of this tumor.
Recent studies using whole-exome sequencing have revealed that lower-grade gliomas also harbor intratumoral heterogeneity, which is widely observed in malignant tumors such as glioblastomas [
17,
25,
35]. We observed here that oligodendroglioma, which generally has a better prognosis compared with astrocytic tumors, also demonstrates marked intratumoral heterogeneity. Mutant allele frequencies were generally low; < 0.75, < 0.5, and < 0.25 in 96%, 87%, and 42% of the number of identified gene mutations, respectively, which most likely reflect heterogeneity within the tumor. Among genetic and chromosomal alterations,
IDH1 mutation and 1p/19q-codeletion were prevalent in almost all tumor cells and are considered to be the trunk alterations. However, in two analyzed cases, analysis of regions that showed different histological and imaging features within a tumor demonstrated that
TERT promoter mutation was not identical in these regions. Previous reports also demonstrated slightly less than 100% incidence of
TERT promoter mutation in 1p/19q-codeleted oligodendrogliomas [
13,
35]. In addition, the presence of other mutations that are frequently observed in oligodendrogliomas can vary temporally and spatially within a tumor. Therefore, mutations other than
IDH1 and 1p/19q-codeletion including
TERT promoter mutation appear to be later events in oligodendroglioma oncogenesis.
The combined data presented here suggest interesting genetic features of oligodendrogliomas. The number of mutations harbored by oligodendroglioma is not necessarily smaller than that reported for astrocytic tumors, and there appears to be marked intratumoral heterogeneity. Although tumor heterogeneity is often related to the malignant nature of tumors, in recurrent oligodendrogliomas there was no tendency for mutation increase at recurrence, and, in some cases, there were fewer mutations and copy number aberrations at recurrence than in the primary tumor. In addition to such observations of genetic and genomic changes, minimal epigenetic changes were observed in recurrent oligodendroglioma compared to the primary tumor. Such epigenetic stability is in contrast to that of astrocytic tumors that harbor
IDH mutation, which demonstrate a dynamic methylation profile change during malignant progression [
14,
28]. In our series,
MGMT promoter methylation was also stable among tissues from the same patient, although a previous study suggested that there is clonal heterogeneity of
MGMT promoter methylation even in oligodendroglioma [
31].
Acknowledgements
We are grateful to the patients for donating their tissue for research and we acknowledge the excellent technical assistance of Reiko Matsuura and Kayoko Iwata, Yuko Matsushita and Saki Shimizu in extracting DNA from blood, Hiroko Meguro for performing the Infinium Methylation Assays, Kaori Shiina and Saori Kawanabe for gene sequencing, and Ruriko Miyahara for assisting with the analysis of patient information.