There has been an explosion in genetic “causes” for many
cancers and prostate cancer, PCa, is not the exception. We have completed a White Paper which covers the material herein in some detal.
One of the most
significant factors has been the ability by some to take metrics of multiple
gene expressions and allege that with the proper weightings these single
dimensional metrics are prognostic. The problem with the metrics is often that
they do not relate to actual genetic control mechanisms. We consider here an
example in PCa of several genes and miRNAs which taken together create a putative
malignant state.
Specifically we examine three elements:
1. p53, the classic oncogene which is a control element for
keeping cells in a homeostatic state and avoiding malignant changes.
2. miRNA 34, or miR-34 which is a micro RNA and is also
found to have a controlling effect upon a cell.
3. MET, a tyrosine kinase receptor which can be activated by
HGF, the hepatocellular growth factor ligand, and which can activate multiple
pathways and if activated and done so in an uncontrolled manner can result in
malignancies.
This examination is predicated on a recent paper by Cheng et
al (2014) which discusses the joint regulation effects of p53 and miR-34.
This section discusses the micro RNA process and its impact
on PCa. Micro RNAs, miRNA, are small single stranded RNAs which when in the
cytoplasm may often bind to other RNA on complement binding sites and thus
change or incapacitate the mRNA to which it binds from being translated into a
protein. Craig Mello was awarded the Nobel Prize in 2006 for the discovery and
his Nobel Lecture provides an excellent overview of the early stages of miRNA
investigation.
In a recent paper by Cheng et al (2014) they state:
The miR-34 family was originally found to be a direct
target of p53 and is a group of putative tumor suppressors. Surprisingly, mice
lacking all mir-34 genes show no increase in cancer formation by 18 months of
age, hence placing the physiological relevance of previous studies in doubt.
Here, we report that mice with prostate
epithelium-specific inactivation of mir-34 and p53 show expansion of the
prostate stem cell compartment and develop early invasive adenocarcinomas and
high-grade prostatic intraepithelial neoplasia, whereas no such lesions are
observed after inactivation of either the mir-34 or p53 genes alone by 15
months of age.
Consistently, combined deficiency of p53 and miR-34 leads
to acceleration of MET-dependent growth, self-renewal, and motility of prostate
stem/ progenitor cells.
Our study provides direct genetic evidence that mir-34
genes are bona fide tumor suppressors and identifies joint control of MET
expression by p53 and miR-34 as a key component of prostate stem cell
compartment regulation, aberrations in which may lead to cancer
This is a murine model which putatively demonstrates that a
blocking of both miR-34 and p53 leads to PCa. Specifically, this is MET pathway
dependent growth.
As noted in Bioscience Technology[1]:
Previous research at Cornell and elsewhere has shown that
another gene, called p53, acts to positively regulate miR-34. Mutations of p53
have been implicated in half of all cancers. Interestingly, miR-34 is also
frequently silenced by mechanisms other than p53 in many cancers, including
those with p53 mutations.
The researchers showed in mice how interplay between
genes p53 and miR-34 jointly inhibits another cancer-causing gene called MET.
In absence of p53 and miR-34, MET overexpresses a receptor protein and promotes
unregulated cell growth and metastasis.
This is the first time this mechanism has been proven in
a mouse model, said Alexander Nikitin, a professor of pathology in Cornell’s
Department of Biomedical Sciences and the paper’s senior author. Chieh-Yang
Cheng, a graduate student in Nikitin’s lab, is the paper’s first author.
In a 2011 Proceedings of the National Academy of Sciences
paper, Nikitin and colleagues showed that p53 and miR-34 jointly regulate MET
in cell culture but it remained unknown if the same mechanism works in a mouse
model of cancer (a special strain of
mice used to study human disease).
The findings suggest that drug therapies that target and
suppress MET could be especially successful in cancers where both p53 and
miR-34 are deficient.
Also, the number of stem cells in mice with both p53 and
miR-34 silenced increased substantially compared with control mice or mice with
only miR-34 or p53 independently silenced.
“These results indicated that together [miR-34 and p53]
regulate the prostate stem cell compartments,” said Nikitin.
This is significant, as cancer frequently develops when
stem cells become unregulated and grow uncontrollably, he said.
Researchers further found that p53 and miR-34 affect stem
cell growth by regulating MET expression. In absence of p53 and miR-34, MET is
overexpressed, which leads to uncontrolled growth of prostate stem cells and
high levels of cancer in these mice.
From Tang’s Lab at MD Anderson we have[2] (see Liu et al):
Cancer stem cells (CSCs), or tumor-initiating cells, are
involved in tumor progression and metastasis1. MicroRNAs (miRNAs) regulate both
normal stem cells and CSCs and dysregulation of miRNAs has been implicated in
tumorigenesis6. CSCs in many tumors—including cancers of the breast, pancreas,
head and neck, colon, small intestine, liver, stomach, bladder and ovary—have
been identified using the adhesion molecule CD44, either individually or in
combination with other marker(s).
Prostate CSCs with enhanced clonogenic17 and
tumor-initiating and metastatic capacities are enriched in the CD44+ cell
population, but whether miRNAs regulate CD44+ prostate cancer cells and
prostate cancer metastasis remains unclear. Here we show, through expression
analysis, that miR-34a, a p53 target was underexpressed in CD44+ prostate
cancer cells purified from xenograft and primary tumors.
Enforced expression of miR-34a in bulk or purified CD44+
prostate cancer cells inhibited clonogenic expansion, tumor regeneration, and
metastasis. In contrast, expression of miR-34a antagomirs in CD44− prostate
cancer cells promoted tumor development and metastasis. Systemically delivered
miR-34a inhibited prostate cancer metastasis and extended survival of tumor-bearing
mice.
We identified and validated CD44 as a direct and
functional target of miR-34a and found that CD44 knockdown phenocopied miR-34a
overexpression in inhibiting prostate cancer regeneration and metastasis. Our
study shows that miR-34a is a key negative regulator of CD44+ prostate cancer
cells and establishes a strong rationale for developing miR-34a as a novel
therapeutic agent against prostate CSCs.
Overall we examine here a four part set of elements related
to PCa; receptors, pathway elements, mi RNAs and methylation. We outline this
graphically below:
Note that in the above each plays a role in the development
of PCa.
This has been known for a while. We see in Yamamura et al
(2012) that they observed:
MicroRNA-34a (miR-34a), a potent mediator of tumor
suppressor p53, has been reported to function as a tumor suppressor and miR-34a
was found to be downregulated in prostate cancer tissues. We studied the
functional effects of miR-34a on c- Myc transcriptional complexes in PC-3
prostate cancer cells. Transfection of miR-34a into PC-3 cells strongly
inhibited in vitro cell proliferation, cell invasion and promoted apoptosis.
Transfection of miR-34a into PC-3 cells also significantly inhibited in vivo
xenograft tumor growth in nude mice. miR-34a downregulated expression of c-Myc
oncogene by targeting its 39 UTR as shown by luciferase reporter assays.
miR-34a was found to repress RhoA, a regulator of cell migration and invasion,
by suppressing c-Myc–Skp2–Miz1 transcriptional complex that activates RhoA.
Overexpression of c-Myc reversed miR-34a suppression of
RhoA expression, suggesting that miR-34a inhibits invasion by suppressing RhoA
through c-Myc. miR-34a was also found to repress c-Myc-pTEFB transcription
elongation complex, indicating one of the mechanisms by which miR- 34a has
profound effects on cellular function. This is the first report to document
that miR-34a suppresses assembly and function of the c-Myc–Skp2–Miz1 complex
that activates RhoA and the c-Myc-pTEFB complex that elongates transcription of
various genes, suggesting a novel role of miR-34a in the regulation of
transcription by c-Myc complex.
It is interesting to see that we have a miRNA as a tumor
suppressor. It is a key change in the way we can understand the overall pathway
control paradigm. Thus the miRNA acts in a powerful manner to modulate cell
growth and proliferation.
Micro RNA
The development of our understanding of micro RNAs has
evolved from that of elements just left over to key control factors in major
pathway expression. From Pekarik et al:
Among all previously described factors involved in the
initiation and development of prostate cancer another element interconnecting
several cellular processes may be traced. This element is represented by
microRNAs (miRNAs), short non-coding regulatory molecules involved in multitude
of processes in eukaryotic cells. They play a role in virtually each step of
tumour formation and progression. miRNAs networks affect apoptotic pathways,
cellular growth, responsiveness to growth factors and anticancer drugs, inhibit
expression of tumour suppressor genes or permit expression of oncogenes.
Classical textbooks refer to carcinogenesis as a harmonic
process caused by a loss of function of tumour suppressor genes and
simultaneous activation of oncogenic genes. Recent progress in miRNAs function
studying did not change this definition substantially; it only extended our
understanding of regulation of this intrinsic network by miRNAs which can be
likewise characterised as oncogenic miRNAs and antitumour miRNAs.
Indeed, we now see that tumor growth is a highly complex
amalgam of genetic elements and supra-genetic elements as well. We have also
argued that in many cases we see extracellular matrix interactions as well as
free radical excitation of cells as well.
Oncogenic miRNAs are those that directly or indirectly
suppress gene expression of tumour suppressors or proapoptotic genes and vice
versa anti-tumorigenic miRNAs are those that reduce expression of oncogenic
proteins. miRNAs are involved in nearly all types of cancer studied so far and
they target classical oncological pathways. However, certain miRNAs were
specifically associated with defined tumour types suggesting that they are
involved in specific processes related to a cancer type or a tissue of origin.
With regard to the number of genes regulated by miRNAs it is not surprising
that these small regulatory molecules play a role also in the resistance of
cancer cells to various anti-cancer drugs. In that respect, miRNAs become very
attractive target for potential therapeutic interventions.
Recent research has revealed existence of miRNAs
circulating in human blood serum. More surprisingly, it was found that levels
of various miRNAs are altered in response to various physiological changes and
some of these changes are well correlated with tumour existence. This makes
circulating miRNAs a very attractive non-invasive cancer biomarker.
miRNAs have come to the fore as one of the several
epigenetic factors which can precipitate various malignancies. The added factor
of methylation as a silencing mechanism also adds to but further complicates
the understanding of cancer progression. Thus, when we see loss of a miRNA, we
may actually be indirectly observing the effects of methylation of the CpG
region about that miRNA encoding region.
The relationship between miRNAs and pathway control elements
is now being better understood. From Yamamura et al:
MicroRNAs (miRNAs) are highly conserved, single stranded,
non-coding RNAs of approximately 22 nucleotides that regulate gene expression
by posttranscriptional silencing of specific target mRNAs, by repressing
translation or cleaving RNA transcripts. miRNAs regulate diverse cellular
processes such as cell-cycle progression, proliferation, apoptosis and
development. miRNAs have been shown to function as oncogenes or tumor
suppressor genes.
The p53 tumor suppressor is deleted or mutated in more
than 50% of human tumors and is a key molecule which suppresses malignancies.
p53 has been found to target the miR-34 family and the ectopic expression of
miR-34 genes has drastic effects on cell proliferation and survival. Ectopic
miR-34a causes cell-cycle arrest in the G1 phase and apoptosis. As p53 has been
found to target miR-34a and since, cell-cycle arrest and apoptosis are also end
points of p53 activation, the miR-34a gene may be a mediator of p53 function.
The proto-oncogene c-Myc regulates cell proliferation and transformation both
transcriptionally and non-transcriptionally and is frequently deregulated in
human cancers
miR-34 is one of now hundreds of micro RNAs, which are short,
generally 22 base pairs, and non-coding RNA segments. They are now well known
as control elements in the expression of genes and have significant control
mechanisms.
From NCBI[3] (1p36.22; 1p36.22):
microRNAs (miRNAs) are short (20-24 nt) non-coding RNAs
that are involved in post-transcriptional regulation of gene expression in
multicellular organisms by affecting both the stability and translation of
mRNAs. miRNAs are transcribed by RNA polymerase II as part of capped and
polyadenylated primary transcripts (pri-miRNAs) that can be either
protein-coding or non-coding.
The primary transcript is cleaved by the Drosha
ribonuclease III enzyme to produce an approximately 70-nt stem-loop precursor
miRNA (pre-miRNA), which is further cleaved by the cytoplasmic Dicer
ribonuclease to generate the mature miRNA and antisense miRNA star (miRNA*)
products. The mature miRNA is incorporated into a RNA-induced silencing complex
(RISC), which recognizes target mRNAs through imperfect base pairing with the
miRNA and most commonly results in translational inhibition or destabilization
of the target mRNA.
There has been a great amount of research regarding the
impact of miRNA on cancer and especially on PCa. miRNAs may downregulate tumor
suppressor genes such as PTEN. This has been seen in miRNA 21. Colin and Croce
have provided several review article regarding miRNA and their influence on
cancers. They argue that miRNA alterations are heavily involved in the
initiation of many cancers. Their focus had been on CLL, chronic lymphocytic
leukemia, and its initiating miRNAs, miR 15 and miR 16. Coppola et al (2010)
provide a detailed summary of miRNAs and PCa.
For example miR34 can cause the activation and recapitulate
p53 which in turn induces cell cycle arrest and apoptosis. Loss of the miR34
can result in the impairment of the p53 control of apoptosis and permit the
cells to proliferate. Coppola et al perform a detailed analysis of all of the
above related miRNAs and their resultant impact on PCa. miR-21 up-regulation
leads to PTEN loss and thus is an oncogene.
Recent work by Poliseno et al has shown that PTEN can be
down regulated via miR-106b. It had already been known that PTEN could be
down-regulated by miR-22, miR-25 and miR-302. Their work demonstrated that
miR-22 and miR-106b are overexpressed in
PCa miR-106b is an intronic miRNA. The work of Poliseno thus has demonstrated a
proto-oncogenic miRNA dependent network that regulates PTEN and thus can have a
significant role in initiating PCa.
Micro RNAs are regulators of mRNA, the post transcriptional
result which is then used to generate via translation the operative protein.
Currently there are nearly 1,000 identified miRNAs. They are generally 22
nucleotides long, short segments, and they usually target specific mRNA and
silence it. Each one of the miRNA may act upon many mRNAs.
As He and Hannon state:
Non-coding RNAs
participate in a surprisingly diverse collection of regulatory events, ranging
from copynumber control in bacteria1 to X-chromosome inactivation in mammals2.MicroRNAs
(miRNAs) are a family of 21–25-nucleotide small RNAs that, at least for those
few that have characterized targets, negatively regulate gene expression at the
post-transcriptional leve.
Members of the miRNA family
were initially discovered as small temporal RNAs (stRNAs) that regulate
developmental transitions in Caenorhabditis elegans6. Over
the past few years, it has become clear that stRNAs were the prototypes of a
large family of small RNAs, miRNAs, that now claim hundreds of members in
worms, flies, plants and mammals.
The functions of
miRNAs are not limited to the regulation of developmentally timed events.
Instead, they have diverse expression patterns and probably regulate many
aspects of development and physiology. Although the mechanisms through which
miRNAs regulate their target genes are largely unknown, the finding that at
least some miRNAs feed into the RNA INTERFERENCE (RNAi) pathway has
provided a starting point in our journey to understand the biological roles of
miRNAs.
miRNAs are simple yet complex entities and key players in
the epigenetics which control gene expression.
It is clear from the above that miRNAs can positively and
negatively impact many elements in the pathways we have considered in HGPIN and
PCa. Coppola et al review several of the key ones. For example:
·
miR-146: Down regulates the
AR.
·
miR-34: Can recapitulate
p53 resulting in apoptosis and arrest.
·
miR-23: can result in c-myc
overexpression and cell proliferation.
In a recent paper by Poliseno et al they have identified
several others:
·
miR-106b: Down-regulates
PTEN and triggers PIN in murine models.
·
miR-22, miR-25, miR-302:
Down-regulating of PTEN.
Similarly the papers by Petrocca et al and that by Calin and
Croce detail many of the miRNAs and their impacts on many cancers. As seen in
the above graphic these are but a few in the overall targeting of PCa control
genes. As Coppola et al state:
The hypothesis that
miRs can be regarded as new broad-spectrum oncogenes or tumor suppressor genes
has opened a revolutionary field of research with exciting diagnostic and
therapeutic perspectives.
The compelling hint of
a widespread miR deregulation in cancer pathogenesis came from the analysis of
the genomic distribution of 186 miR. In this study, it was demonstrated that
more than half of them mapped in cancer-associated genomic regions, namely in
chromosomal sites prone to deletions, amplifications or recombinations. These
aberrations can result in miR down- or up-regulation, conferring selective
advantages to mutated cells.
Additional mechanisms
of miR deregulation include altered expression of miRs as a consequence of
excessive or deficient processing; aberrant transcription of the precursors by
epigenetic silencing of miR promoters or as a result of the activity of
oncogenic transcription factors; and more rarely, point mutations in mature
miRs or in target sequences that can interfere with normal target recruitment
The problem that we will have in any modeling of HGPIN and
PCa is not only do we have issues regarding the somewhat well-known genes but
the impact of the epigenetic factors is unknown, complex, and possibly random.
Furthermore miRNAs can act in a positive or negative manner
depending upon the cell and the activated networks in the cell. From Croce
(2009) we have:
Importantly, miRNAs
should not be described as oncogenes or tumor suppressor genes, unless the
tissue or cell type involved in their action is specified. For example, miR-221
and miR-222 target an oncogene, KIT, and inhibit the growth of erythroblastic leukaemia30, and
therefore function as tumor suppressors in erythroblastic cells. but they also
target at least four important tumor suppressors — phosphatase
and tensin homologue (PTEN), p27, p57 and tissue inhibitor of
metalloproteinases 3 (TIMP3) — and function as oncogenic
miRNAs by suppressing these tumor suppressors in various human solid tumours31 (TABLE 1).
Therefore, before describing an miRNA as a tumor suppressor or an oncogene, it
is necessary to specify in which cell or tissue, as cellular context is crucial
for the function of miRNAs….
Recent work on miR-34 has demonstrated its impact on p53
(Rokhlin et al) and the fact that miR-34 significantly mediates the role of p53
in apoptosis in AR dependent PCa.
As Sevli et al state:
The miRNAs have critical functions in gene expression and
their dysregulation may cause tumor formation and progression. Today, it is
known that tumors possess widespread deregulated miRNA levels. Over-expression
or down-regulation of specific miRNAs in different tumor types make them
potential therapeutic targets and diagnostic markers. Up-regulated miRNAs
inhibiting tumor suppressor genes in tumor cells are commonly termed as
oncogenic miRNAs or oncomirs. The miRNAs whose down-regulation promotes tumor
progression are tumor suppressor miRNAs. One type of mRNA may possibly be
targeted by multiple different miRNAs with variable efficiencies. Conversely, a
single miRNA may target more than one mRNA. Thus, to be able to observe a
tumorigenic phenotype, some significant changes should occur in microRNome
content of the cells.
As we have indicated elsewhere, the concept of the cancer
stem cell has received significant attention. There has also been a great deal
of work on the area of linking miRNAs and the stem cell model for PCa. In a
recent work by Liu et al (2011) the authors demonstrate the nexus between
miR-34a and its ability to inhibit PCa stem cells by directly repressing CD44.
They observe that cancer stem cells have been observed in many solid cancers by
using the fact that CD44 adheres to the cell surface. PCa stem cells with
enhance clonogenic and tumor initiating and metastatic capacities are often
enriched with CD44+ cell population. The work of Liu et al demonstrated that
the administration of miR-34a to PCa cells inhibited PCa metastasis and
inhibited PCa regeneration. This is one of the first uses of miRNA as a tumor
suppressor.
In a recent paper by Xia (2008) the author states:
The key
characteristics of stem cells are that they are capable of self-renewal and
differentiation. The mechanisms by which stem cells maintain self-renewal and
differentiation are complicated. In the past years, protein-coding genes had
been broadly investigated in stem cell self-renewal and differentiation.
Recent studies
indicate miRNAs as one of the most abundant classes of post-transcriptional
regulators proved to be crucial in a wide range of biological processes, which
suggest that miRNAs may also play essential roles in stem cell self-renewal and
differentiation. Disruption of Dicer function in murine ESs influences miRNA
processing and greatly impairs their ability to differentiate …
Cancer stem cells
(CSCs) are the cells within a tumor that possess the capacity to self-renew and
to produce the heterogeneous lineages of cancer cells that comprise the tumor.
CSCs can thus only be defined experimentally by their ability of self-renewal
and tumor propagation.
The implementation of
this approach explains the use of alternative terms in the literature, such as
“tumor-initiating cells” to describe putative CSCs. …
The identification of
growth and differentiation pathways responsible for CSC proliferation and
survival will help in the discovery of novel therapeutic targets. Previous
studies have shown that many signal pathways may participate in regulating CSC
functions, including Wnt/β-catenin, Notch, and
Sonic hedgehog homolog (SHH). The canonical Wnt cascade has emerged as a
critical regulator of stem cells and activation of Wnt signalling has also been
associated with various cancers …
CSC maintenance is
dependent on β catenin signaling. Moreover, because Wnt/β-catenin
signalling is not essential for normal epidermal homeostasis, such a
mechanistic difference may thus be targeted to eliminate CSCs and consequently
eradicate squamous cell carcinomas. It is therefore hypothesized that
inhibition of Wnt signaling may provide an effective way to reduce the unwanted
stem cell renewal which results in cancers.
Inhibition of Wnt
signalling may prove to be an effective road to inhibit the uncontrolled cell
renewal that drives cancer. Acting as novel and pivotal regulators of
protein-encoding genes, miRNAs will have great potential in regulating CSCs’
biological functions by targeting CSCs-related signal pathway molecules.
We have performed various analyses of CSCs especially for
PCa. This is a critical area for ongoing research and most likely will prove
quite useful.
MET is a tyrosine kinase receptor. It is activated by HGF
the hepatic growth factor and it in turn activates a multiplicity of pathways.
It is considered a proto-oncogene and thus is of general concern. From NCBI[4]:
The proto-oncogene MET product is the hepatocyte growth
factor receptor and encodes tyrosine-kinase activity. The primary single chain
precursor protein is post-translationally cleaved to produce the alpha and beta
subunits, which are disulfide linked to form the mature receptor. Various mutations
in the MET gene are associated with papillary renal carcinoma.
MET is located on 7q31. We now examine the MET structure and
then examine its control over several pathways.
From Benvenuti and Comoglio we have:
Both MET and RON are tyrosine kinases crucially involved
in the control of the ‘‘invasive growth’’ (Giordano et al., 2002). Under
physiological conditions such as embryonic development and organ regeneration,
they contribute to establishing the normal tissue patterning by orchestrating
cellular proliferation, disruption of intercellular junctions, migration
through the EMC and protection from apoptosis. In transformed tissues, receptor
deregulation is responsible for cancer progression and metastasis formation and
dissemination. Either upon ligand stimulation or receptor constitutive
activation, cancerous cells are induced to leave the primary tumor, degrade the
basal membrane, move towards different organs and there give rise to metastasis
MET controls many pathway elements in a cell. We show some
of them in the Figure below from Dickinson and Duncan. Note the HGF binds to
MET and thus it activates a set of pathways facilitating invasion and stopping
cell cycle arrest.
The above demonstrates the MET pathway and its relationship
to the many other key pathways.
MET can be over expressed and over activated and the result
is a malignant growth. Thus MET has the potential for becoming a significant
factor in cancer development. From Benvenuti and Comoglio:
It has been extensively demonstrated that when used in a
deviant cellular environment and without spatial and temporal regulation, MET
exerts a major role in tumor formation and progression. Cells which
over-express either MET or HGF are tumorigenic when implanted into nude mice
and become extremely metastatic, moreover transgenic mice for either MET or HGF
develop metastatic tumors while, on the
contrary, endogenously expressing cancer cells become less aggressive when MET
is switched off.
In the above the issue is over expression. The question is;
what is driving that over expression? Is it truly an excess production or a
loss of control or modification? Is this on a cell by cell basis or is it
pandemic? They continue:
Accordingly, it was demonstrated that short hairpin RNA
(shRNA) mediated MET knockdown in rabdomyosarcomas (RMS)-derived cell lines
greatly affects cell proliferation, survival and invasion. Furthermore in
xenograft models of RMS MET silencing produced a dramatic reduction of tumor
mass. Similar results were obtained silencing MET in lung cancer cell lines
harboring MET amplification. In those cell lines receptor silencing (once more
achieved by shRNA technology) induced a significant growth inhibition; notably
the silencing sorted no effects on cell lines that did not display receptor
gene amplification.
This seems to answer the question regarding complete cell
line activation.
It has been extensively described, both in animal models
and in normally occurring human cancers, that constitutive activation of MET
can be achieved in three different ways:
(i) with establishment of ligand-receptor autocrine
loops;
(ii) via receptor over-expression, and
(iii) in presence of activating point mutations in the
receptor coding sequence.
Ligand-receptor autocrine circuits make cells independent
from the need of growth factors; receptor over-expression triggers receptor
oligomerization and reciprocal activation even in absence of ligands; point
mutations generate constitutively active receptors.
This last event is extremely uncommon; however, some
missense point mutations have been described in MET coding sequence in certain
human cancers.
The above discussion describes the ways in some detail. The
causes of over expression could then be addressed as a therapeutic methodology.
They continue:
Particularly missense mutations located in the tyrosine
kinase domain of MET were described in patients who suffer from hereditary and
sporadic papillary renal-cell carcinomas and head and neck squamous-cell
carcinomas, whereas alterations in the juxta-membrane region were mainly found
in human gastric and lung cancers
The above does also present the issue of mis-sense
mutations, changes that may not change anything but may cause a cessation of
genetic progression.
Observations
The paper which we have used to initialize the focus on this
report is one which combines: mir-34, MET, p53, and methylation. It is an
amalgam of receptors of the kinase inhibitor variety, key pathway oncogenes,
miRNAs and methylation. It is an interplay between all of the complex elements
which are now known in cancer genetics.
The Cheng et al results are simply as follows:
1.
miR-34 Cooperates with p53
in Suppression of Prostate Carcinogenesis
2.
p53 and miR-34 Cooperate in
the Control of Prostate Stem/Progenitor Cell Activity
3.
p53 and miR-34 Regulation
of Stem/Progenitor Cells Depends on MET
However in their conclusions we have also introduced the
methylation effects as well. They conclude:
Our study provides direct genetic proof that miRNAs of
the miR-34 family may act as tumor suppressors in concert with other genes,
such as p53. These findings offer a solid physiological basis for the rational
design of diagnostic and therapeutic approaches. Because the lack of mir-34
genes alone is insufficient for cancer initiation, their downregulation is
likely to occur at some point during tumor progression.
However, the preexistence of mir-34 methylation in some
normal cells cannot be excluded. Further genomic studies in conjunction with
animal modeling should be able to address this question. Although our current
studies have been focused on prostate cancer, tissue-specific inactivation of
mir-34 and p53 in other tissues will address likely interactions of these genes
in other cell lineages.
Thus we have exhibited here a complex interplay between
types of cell control mechanisms. The challenge will be how best to model this
complex interplay. In our prior analyses we have let epigenetic factors be
secondary and considered almost as noise. Here, however, they are pari passu
with all other elements and must be considered expressly.
Also Liu et al from Tang’s Lab state:
We have shown that miR-34a is underexpressed in
tumorigenic CD44+ prostate cancer cells and that it has potent antitumor and
antimetastasis effects. Our results establish miR-34a as a key negative
regulator of CD44+ prostate cancer cells and CD44 as an important target of
miR-34a. Our findings suggest that reduced expression of miR-34a in prostate
CSCs contributes to prostate cancer development and metastasis by regulating
expression of CD44 and the migratory, invasive and metastatic properties of
CSCs
Tang’s Lab has done extensive work on PCa CSC and the
implications of reduced miR-34 are significant. The issue here is several fold.
First, the measure of miR-34 activity can be prognostic. Second, the reasons
for reduced miR-34 is of prime concern. As we shall note later, the cause may
be methylation of CpG clusters. Thus if one were to try anti-methylation drugs,
would that assist? There is always a concern here since anti-methylation
therapeutics are non-selective.
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