Primary and Metastatic Brain Tumors Dissipate – Understanding and Intervention

ABSTRACT

We researched and reviewed peer-reviewed articles to provide an overview of the primary and metastatic brain tumor growth,
dissemination mechanisms in their microenvironment. We studied the role of alternative pre-mRNA splicing events of
KCNMA1, which encodes the pore forming α-subunit of calcium-activated voltage-sensitive potassium (BKCa) channels, in
migration, invasion, proliferation, dispersal of brain tumor. It is conceivable that by targeting epigenetic events and gene
variants that contribute to brain tumor growth, we might attenuate tumor diffusion, distant metastasis and angiogenesis. We
reviewed literature on the alternative splicing events of KCNMA1, specific to brain tumor, its microenvironment and the
biological activity of known alternatively spliced isoforms. The blood-brain tumor barrier (BTB) prevents delivery of
anticancer drugs to micro-macro metastases requiring novel strategies to enhance drug delivery across the BTB. We also
revealed the interaction between the BKCa channel isoform expression and VEGF secretion in brain tumors that can be
exacerbated under hypoxia with significant implications on neoangiogenesis, vascular permeability and anticancer drug
delivery.

INTRODUCTION

Primary and metastatic cancers
Primary brain tumors start in the brain or brain’s contiguous structures. Most common types of primary brain tumors are
gliomas, which begin in the glial tissue. Major types of brain
tumors are gliomas that also include, astrocytoma arising
from astrocytes and oligodendrogliomas arising from
oligodendrocytes or from a glial precursor cell. In children,
medulloblastoma is common arising from cerebellar
primitive neuroectodermal tumor (PNET), originating from
the posterior fossa and can spread to other parts of the brain
and to the spinal cord. In fact, there are 16 types of brain
tumors according to WHO report. The 2016 CNS WHO
presents major restructuring of the diffuse gliomas,
medulloblastomas and other embryonal tumors and
incorporates new entities that are defined by both histology
and molecular features, including glioblastoma [1]. This
report addresses the challenges of primary brain tumor
diagnosis, prognosis and targeted treatment based on the
molecular basis of the tumor. Secondary brain tumors or
metastatic brain tumors are those that spread to the brain
from somewhere else in the body. For example, cancers of the lung, breast, kidney, stomach, colon and melanoma skin
cancer have the potential to travel through the bloodstream
and lodge themselves in the brain. Then they will begin to
grow into new tumors in the new microenvironment
supported by increased vascularization by avoiding host
immune response (Figure 1). With more-sensitive and
accurate detection of distant metastases by improved
imaging modalities should increase incidence of brain
metastases and prolong survival of patients.

Brain tumor dissipation

New generation sequencing (NGS) technologies have
contributed to the giant leap in understanding the genomic
and epigenomic analyses of both primary and metastatic
brain tumor tissues and tumor cells. Mutations identified at
the genome or transcriptome levels helped us understand the
differences in gene regulation of cellular functions in
identical brain tumors as related but functionally different
tumor entities [2-6]. Novel mutations detected in brain
metastases suggested potential drivers tumor progression [7],
strengthening the argument that brain tumors are highly
heterogeneous with complex microenvironments.
Epigenetics play an important role in cancer initiation,
growth and progression.
Understanding the precise mechanism helps us in developing
diagnosis, prognosis and treatment strategies for affected
cancer patients. For example, overexpression of Ezh2 plays
a role in many cancers, including breast cancer and brain
tumors. H3K27M serves as an oncohistone and, if mutated it
contributes to tumor development as Ezh2 is no longer able
to methylate the histone and gene expression is aberrantly
upregulated. We studied the methylation status of the
promoter region using low grade and high-grade glioma cell
lines and showed that there is a significant difference in the
expression of genes between low and high-grade glioma
cells, when treated with 5-aza plus TSA. Our study
suggested that studying the methylation status of ADFP, CDCP1 and ZFP42 in brain tumor biopsies may indicate the
potential aggressive nature of gliomas [8]. More functional
genomics and cell-based molecular analyses are required to
qualify mutated or amplified genes as clinical and
therapeutic markers. A case in point is the data available
from The Cancer Genome Atlas (TCGA) and The Human
Protein Atlas that refers to testing of KCNMA1 and BKCa
channels as a pathological marker in cancer [9].
The complication in understanding metastatic process and
metastatic spread in the brain calls for rationalized tumor
model systems. Researchers are using both in vitro and ex
vivo primary brain tumor model systems to better understand
the molecular mechanisms of brain tumors and neovascularization for designing targeted novel anti-
metastatic therapies [10]. Metastatic brain tumors dissipate and colonize in distant parts of brain metastases grow by
drafting existing blood vessels and/or by forming new blood
vessels. The unique brain microenvironment such as hypoxia
promotes tumor cell survival, tumor growth and resistance to
therapy. The ion channels are shown to be involved in
hypoxia-induced aggressiveness of glioblastomas [11]. We
also found that KCNMA1 and the alternate splicing of
KCNMA1, which encode for BKCa channels, under hypoxia
impact vascular endothelial growth factor (VEGF) secretion
in glioma cells (Figure 2). Hence, there is an opportunity to
control cancer growth by developing safe and effective
VEGF and BKCa channel inhibitors.

CONCLUSION

We know that primary and metastatic brain tumors are
distinct in their etiology, biology, response and resistance to
anticancer drugs. Hence innovative preclinical and clinical
study designs are required to develop effective diagnosis, prognosis and treatment. We should embrace the new DNA,
RNA, protein sequencing and imaging technologies to
understand each tumor type and customize treatment
strategies. Furthermore, studies are essential to understand
the host microenvironment including molecular aberrations
such as gene mutations and alternative splicing. Specifically,
in hypoxic microenvironment metastatic brain tumor cells
adapt very well and thrive by forming new blood vessels.
Targeting VEGF and VEGFR that are involved in
angiogenesis with Bevacizumab like molecules should take a
priority. In addition, BBB/ BTB pose hurdles to anti-cancer
drug and imaging agents’ delivery. These challenges are
different in primary and metastatic brain tumors. Molecules
involved in extravasation, metabolism, cell adhesion and
cellular signalling in brain-specific metastatic clones should
be identified for targeted therapies.
More effective anticancer drugs and specific biologics
similar to clinically used inhibitors of EGFR, HER2, PI3K
and BRAF, should be developed to get an upper hand on
unique challenges posed by both primary and metastatic
brain tumors. Finally, our work suggests that validating
KCNMA1/BKCa channel variants in clinically relevant
tumor samples will be useful in identifying biological
process that promote malignancy and affect prognosis and
survival of brain tumor patients.


ACKNOWLEDGEMENT:
The authors thank the Scintilla Group, Bangalore, India;
Anderson Cancer Institute and Mercer University Medical
Center, Savannah, GA, USA; Vanderbilt-Ingram Cancer
Center, Nashville, TN, USA; Cedars-Sinai Medical Center,
Los Angeles, CA, USA; American Cancer Society, USA;
Georgia Cancer Coalition, Atlanta, GA, USA; and NIH for
providing the opportunity and research grant support.