Edited by: Andrea Carai, Bambino Gesù Ospedale Pediatrico (IRCCS), Italy
Reviewed by: Akane Kawamura, University of Oxford, United Kingdom; Maria Berdasco, Biomedical Research Institute of Bellvitge, Spain
This article was submitted to Pediatric Oncology, a section of the journal Frontiers in Pediatrics
†These authors have contributed equally to this work
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Medulloblastoma is one of the most frequent among pediatric brain tumors, and it has been classified in various subgroups. Some of them already benefit from quite good therapeutic options, whereas others urgently need novel therapeutic approaches. Epigenetic modulators have long been studied in various types of cancer. Within this review, we summarize the main preclinical studies regarding epigenetic targets (such as HDAC, SIRT, BET, EZH2, G9a, LSD1, and DNMT) inhibitors in medulloblastoma. Furthermore, we shed light on the increasing number of applications of drug combinations as well as hybrid compounds involving epigenetic mechanisms. Nevertheless, in the studies published so far, mainly un-specific or old modulators have been used, and the PKs (brain permeability) have not been well-evaluated. Thus, these findings should be considered as a starting point for further improvement and not as a final result.
Medulloblastoma (MB) is one of the most frequent and extensively studied pediatric brain tumors. According to the WHO-classification of central nervous system tumors, four main genetically defined subgroups have been described: WNT, SHH, group 3, and group 4. Each of these groups has its unique expression signature and clinical outcome (
In this review we would like to highlight the latest preclinical and clinical efforts regarding the application of epigenetic modulators in MB. An overview of the presented compounds, their targets and effects in MB can be found in Table
Summary of the epigenetic modulators and combinations active in MB.
Suberoylanilide-hydroxamic acid, vorinostat | HDACs | Active in DAOY and D283med MB cancer stem cells ( |
Synergistic effect in D283med cells, but not in DAOY with decitabine ( |
|
Romidepsin | HDAC1/2 | Active in DAOY and D283med MB cancer stem cells ( |
– | |
Panobinostat | HDACs | Suppressed leptomenigeal seeing in a MB mouse model ( |
– | |
Valproic acid, VPA | Class I/IIa HDACs | Potential treatment in various MB cell lines ( |
Synergistic effects of VPA and vorinostat with irradiation in MB ( |
|
Parthenolide | HDAC1 | Active in DAOY and D283med MB cancer stem cells ( |
– | |
MS-275, entinostat | Class I HDACs | Active in DAOY and D283med MB cancer stem cells ( |
– | |
HDiA | HDAC1/2 | Block GLI1/2 activities and SHH MB growth ( |
– | |
HDiB | HDAC1/2 | Block GLI1/2 activities and SHH MB growth ( |
– | |
Curcurmin | HDACs | Increased survival in the Smo/Smo transgenic MB mouse model ( |
– | |
MAZ1863 | Class IIa HDACs | Only very weak effects on MYC-MB cells ( |
– | |
MAZ1866 | Class IIa HDACs | Only very weak effects on MYC-MB cells ( |
– | |
TH34 | HDAC 6/8/10 | Induced caspase-dependent programmed cell death in various MB cell lines ( |
– | |
Nicotinamide | SIRTs | SIRT1 inhibition might be a double edge sword in MB treatment ( |
– | |
JQ-1 | BETs | Decreased proliferation and tumor growth in SHH MB via reducing the expression of. GLI1 and GLI2 ( |
Effective combination with the CDK inhibitor milciclib, as both regulate the MYC function in MB via different actions, prolonging survival in a MB animal model ( |
|
I-BET151 | BETs | Inhibition of the SHH pathway in SHH-MB cells as well as in a MB mouse model ( |
– | |
Decitabine | DNMTs | Found to be quite inactive in DAOY ( |
Synergistic effect in D283med cells, but not in DAOY with decitabine ( |
|
Zebularine | DNMTs | Inhibits the expression of SHH pathway components, such as SMO and GLI1, in DAOY and ONS-76 MB ( |
DNMTi zebularine has been tested in combination with vincristine in SHH MB cells, displaying a synergistic effect ( |
|
MC2840 (compound 2) | DNMTs | Impaired MB-SC growth led to high MB-SC differentiation rates ( |
– | |
MC3343 (compound 5) | DNMTs | Significantly impaired the MB-SC growth rate ( |
– | |
3-Deazaneplanocin A, DZNep | EZH2 | Indirect and rather unspecific EZH2i in MB ( |
– | |
MC3629 | EZH2 | Reduces in a MB xenografted mice the tumor volume, stemness and cell proliferation and lastly induces apoptosis ( |
– | |
UNC0638 | G9a | Reduces DAOY proliferation via controlling the USP37 expression mediated by G9a ( |
– | |
SP2509 | LSD1 | Disruptor of the CoREST–LSD1 complex active in various MB cell lines ( |
– | |
4SC-202 | HDAC1/2/3 and LSD1 | – | Active in various MB cell lines ( |
|
Sodium Phenylbutyrate | HDACs | – | Phenylbutyrate in combination with decitabine and the tyrosine kinase inhibitor imatinib induced apoptosis in DAOY and UW228 3 MB cell lines ( |
|
NL-103 | HDAC/HH | – | NL-103 is a dual inhibitor of the HDACs and HH pathway with potential activity in MB ( |
HDACi are among the oldest and deeply studied class of epigenetic modulators. In time, the most widely studied HDACi have not been isoform selective, but they were targeting more than one HDAC, specially class I and/or IIa/b HDACs. Nevertheless, selective isoform-specific modulators are more and more developed (
Older studies described valproic acid (VPA) as a potential treatment in various MB cell lines (
Recently, another well-known pan-HDACi, panobinostat, was reported to suppress leptomeningeal seeding, a rare complication in MB spreading, causing brain and spinal cord inflammation in a mouse model (
Milde et al. developed a Group 3 MB HD-MB03 cell line and xenograft model with high HDAC expression levels and sensitivity to HDACi, such as vorinostat and panobinostat (
A meaningful example of the involvement of HDACi in the SHH signaling pathway has been given by Canettieri et al. They showed that the HDAC1/2 selective inhibitors HDiA and HDiB blocked GLI1 and GLI2 activity through their acetylation, and SHH MB cell growth in several SHH MB cell lines (
The natural compound curcumin, through HDAC inhibition and HDAC4 level depletion, reduced tumor growth and significantly increased survival in the Smo/Smo transgenic MB mouse model displaying HDAC4 overexpression. However, due to the pleiotropism displayed by curcumin, these positive results might not only be ascribed to HDAC4 inhibition but also to other off-target effects (
In 2015, Ecker et al. used the class IIa-selective HDACi MAZ1863 and MAZ1866 in Group 3 MB cancer cells and compared them to vorinostat (pan-HDACi) and to the class I specific inhibitor MS-275 (entinostat). MAZ1863 and MAZ1866 had only very weak effects on MYC-MB cells, whereas vorinostat and entinostat efficiently reduced the metabolic activity in MYC-MB cells. These results give precious hints on the development of novel therapies with selective HDACi in MYC- dependent MB (
Interestingly, when tested in the MED8A MB cell line, the novel non-toxic HDAC6/8/10 inhibitor TH34 modestly impaired colony growth and specifically induced caspase-dependent programmed cell death in a dose-dependent manner (
To sum up, the well-established and approved HDACi have so far failed to demonstrate a significant antitumoral effect in solid malignancies in preclinical and clinical settings (
Sirtuins (SIRTs), also known as class III HDACs, are NAD+ dependent deacetylases considered as a separate family of enzymes including seven different isoforms (hSIRT1-7). So far, there is very little literature evidence about the use of SIRT modulators in MB (
In contrast, Tiberi and coworkers found that the downregulation of the BLC6/BCOR/SIRT1 complex, a potent repressor of the SHH pathway, led to MB growth in human cells and in a mouse model. They demonstrated that SIRT1 is necessary for the BCL6 function (
The BET (Bromodomain and Extra-Terminal domain) proteins BRD2, BRD3, and BRD4, have been extensively studied in brain tumors including MB (
The BETi JQ-1 is one of the most studied in the literature. Tang et al. demonstrated that reduced expression of BRD4 via RNAi or its pharmacologic inhibition by JQ-1 resulted in decreased proliferation and tumor growth in SHH MB, reducing the expression of the glioma-associated oncogenes GLI1 and GLI2 (
Another BETi, namely I-BET151, has been shown to provide biological effects similar to JQ-1 in SHH MB. More precisely, this compound reduced the BRD4 binding to the GLI1 gene locus, thus resulting in the inhibition of the SHH pathway in SHH MB cells as well as in a MB mouse model (
Currently, JQ-1 is not in clinical trials for MB treatment due to its poor pharmacokinetic and pharmacodynamic properties (
Enhancer of zeste homolog 2 (EZH2) is a histone lysine
The deubiquitylase USP37 was identified as a target of REST, one of the main regulatory complexes in brain development and neurogenesis with aberrant overexpression in MB (
Lysine-specific demethylase 1 (LSD1), also known as KDM1A, has been the first of several protein lysine demethylases to be discovered. The modulation of this enzyme has also been studied in MB. Recently, it has been shown that SP2509 inhibited the enzymatic activity of LSD1 rather than acting as a protein-protein disruptor of the CoREST–LSD1 complex. SP2509 was able to block the growth of various human MB cell lines (DAOY, D283med, and ONS-76) through direct LSD1 inhibition (
DNA methyltransferases (DNMTs) are a family of enzymes that catalyze the transfer of a methyl group to the C5-cytosine residue of DNA. Aberration of DNA methylation leads to a wide variety of diseases, including cancer. DNMTi are one of the most studied epigenetic modulators after HDACi in cancer. The nucleoside analogs azacytidine and decitabine have been approved by FDA mainly in hematological malignancies (
Chemoresistance is one of the key reasons why drug combinations are applied in therapy. Targeting a disease by just one active principle often results in drug resistance. This problem might be overcome by using two different drugs that target two different molecular pathways involved in the same disease. In MB, this strategy has also been used to various epigenetic modulators in combination with other molecules either targeting epigenetic pathways or non-epigenetic ones. Furthermore, we shed light on novel, innovative hybrid compounds targeting at least one epigenetic molecule as following.
Patties et al. published in a first study the effects of combination of several epigenetic modulators, such as the DNMTi decitabine, VPA and vorinostat as HDACi, in MB and later they extended the previous study with the use of irradiation, a common physical therapy approach to fight various cancers (
They discovered that the treatment of D283med cells with vorinostat and decitabine produced a synergistic effect in reducing tumor cell viability, whereas the exposure of DAOY cells to the same compounds did not have a synergistic effect (
Interestingly, also the combination of VPA or vorinostat with irradiation showed similar effects compared to decitabine/irradiation treatments on the mentioned cell lines, even though to a lower extent (
Patties et al. did not only evaluate the combination of several epigenetic modulators, but also combined them with abacavir, a nucleoside analog HIV reverse transcriptase inhibitor, with or without irradiation (
Marino et al. evaluated the pan-HDAC inhibitor 4-phenylbutyrate in combination with the DNMTi decitabine and the tyrosine kinase inhibitor imatinib. The co-treatment reduced global methylation and induced apoptosis in DAOY and UW228 3 MB cell lines (
The DNMTi zebularine has been tested in combination with the well-known anti-cancer agent vincristine, able to interact with microtubules and tubulin, in SHH MB cells, displaying a synergistic effect (
However, also newer chemical entity combinations have been tested in MB. As reported earlier, MYC is an important player in Group 3 and Group 4 MB. JQ-1 is influencing this pathway via BET inhibition and has been combined with the CDK inhibitor milciclib, because CDKs regulate events in MYC function as well. This combination was well-tolerated, reduced tumor cell growth, and significantly prolonged survival in MB animal model (
However, for the treatment of MB all the aforementioned combinations are still in their early stage and need to be carefully evaluated before proceeding to the clinical area.
Hybrid compounds are single chemical entities hitting more than one target. Some of these innovative compounds have been also tested in MB. Inui et al. evaluated the dual HDAC1/2/3 and LSD1 inhibitor 4SC-202 in various MB cell lines (DAOY, D283med, and ONS-76). This compound proved to be active targeting both enzymes in the CoREST-HDAC-LSD1 complex. This study is one of the first examples using dual epigenetic inhibitors in MB (
NL-103, a dual inhibitor of the HDACs and SHH pathway, shows a hybrid structure merging those of vismodegib, a smoothened receptor (SMO) inhibitor approved by FDA for other solid cancers, and vorinostat, which is known to target the SHH pathway by influencing the acetylation status of GLI1 and GLI2 (
In this review, we have summarized and highlighted epigenetic modulators as promising drug targets in MB. However, there is still a long way to go: mainly not very specific, or older modulators have been used and often the brain permeability has not been well-evaluated. The molecular genetics and detailed epigenetic modulation of the various MB subgroups need to be further studied. As the MB subgroup is a key factor in choosing the right treatment, the development of personalized medicine with highly specific modulators could be a key in improving the poor survival rates of some MB subgroups. Therefore, research should not only focus on the design of more specific and selective epigenetic modulators, but also should study deeper the more biologically oriented factors, such as the tumor molecular genetics, the functional analysis of epigenetic factors and their potential modulation. In both cases, epigenetic modulators can be useful not only as tools to better understand the molecular mechanisms in MB, but also as novel potential drugs for innovative personalized treatments.
The literature research was conducted by all authors and the manuscript was written thereafter by the contribution of all authors. AM read, corrected, and supervised and coordinated all the work. All contributors read and approved the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The handling editor declared a past co-authorship with the authors GC and ZB.