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Ryan Gardner
ARG-3
12/6/05
Stem Cells: A New Cardiovascular Therapy?
A position paper for the non-scientific community
Ryan Gardner
Fall, 2005
A paper for a current events magazine (Time)
There is a good chance that mostly everyone in the United States has had a
personal experience with a heart attack, whether it was a family member, a friend or even
themselves. According to the center for disease control, heart disease is the number one
cause of death in the United States for both male and females (Figure 1), and the rates are
on the rise. Due to the high rate of heart attacks in the U.S. and throughout the rest of the
world, the medical community is in great need for a practical and effective therapy for a
damaged heart.
Until very recently the heart was thought to be a post-mitotic organ (Side Note 1)
(Orlic, D. 2005), the consequences of which are that if damage does occur in the heart, it
is unable to repair itself. However, recent evidence indicates that the heart does have
stem cells that can be used for cardiac repair (Side Note 2). Unfortunately, cardiac stem
cells are very difficult to isolate, making them impractical as a possible therapy after a
heart attack. At this point in time only mesenchymal (Side Note 3) stem cells have been
shown to differentiate into cardiac muscle cells in a damaged heart and improve heart
health (Smits, Vliet, Hassink, Goumans, Doevendans, 2005). However, as is the case
with cardiac stem cells, these are difficult to isolate. Bone-marrow stem cells are much
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more easily isolated, making implantation of them a possible alternative to the obstacle
created by isolation of more potent (Side Note 4) stem cells. Unfortunately, there is
mixed evidence on what the fate (Side Note 5) of bone-marrow stem cells is, and if there
is an increase in heart function after implantation. By determining a simple and efficient
means of cardiovascular therapy using stem cells, the scientific community will open
numerous doors for cardiovascular repair.
Before this review can proceed any further, it is important that I define some of
the terms that I will be using throughout the explanation of my position. I will frequently
be using the terms myocardial infarction, ischemia, cardiomyocytes, hematopoietic cells
and hematopoietic stem cells, and autologous cells. Myocardial Infarction is an event
where blood flow is blocked (ischemia) to a portion of the heart, and as a result, that area
cannot receive any oxygen or other nutrients. The lack of oxygen to the cardiac muscle
cells (cardiomyocytes) eventually kills them and scar tissue forms (infarction). This is
described as a myocardial infarction because the infarction occurs in the functional layer
of the heart called the myocardium; which is where cardiac muscle cells are found.
Another important term that is useful to understand is hematopoietic. Hematopoietic
stem cells are a synonym for bone marrow stem cells, which mature into hematopoietic
cells or blood cells. An important topic to understand is that of autologous transplants
(Side Note 6). If a bone marrow cell is taken from someone and implanted into that same
person (self to self), it is considered an autologous cell and an autologous transplant. In
addition to these terms, it is also important to understand the anatomy of the heart. The
heart is a four-chambered organ consisting of a right atrium and ventricle and a left
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atrium and ventricle, all four of which have a specific purpose. The chamber that will be
focused on to the greatest extent in this paper is the left ventricle.
Recently, several studies have investigated the possibility of stem cell
implantation after an ischemic event to improve heart function and determine the fate of
the implanted stem cells. Unfortunately, no study examines both the fate of the stem
cells and the overall function of the heart simultaneously; therefore there are essentially
two questions being asked, and under each question there are inconsistent findings.
Question 1: Does Bone-Marrow Stem Cell Implantation Increase Heart Function?
The studies that examined this question did not come to the same conclusion
(Table 1). First of all, Stamm et al. (2003) found that upon implantation of autologous
hematopoietic stem cells the ability of the left ventricle to function properly was
significantly higher, finding that Left Ventricular Ejection Fraction (LVEF) (Side Note 7)
increased significantly after implantation. This conclusion was also found by Wollert et
al. (2004), Strauer et al. (2002) and Chen et al. (2004). A second conclusion was found
by Deten et al. (2004). In this study it was found that there was no improvement in heart
function. Figure 2 shows a comparison of stroke volume (Side Note 7) values for these
studies.
Conclusion 1:
There is an improvement
in function.
Question 1:
Does Bone-Marrow
Stem Cell
Implantation
Increase Heart
Function?
Conclusion 2:
There is no improvement
in function.
Table 1: Conclusions to
Question 1.
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Question 2: What is the Fate of Bone-Marrow Stem Cells After Implantation into an
Infarct Heart?
There are three studies that examined this question, and as is the case with the
first question, there are two conclusions to this question (Table 2). One study found that
when bone-marrow stem cells were implanted, they fused with surrounding cells; in this
case, cardiac muscle cells (Terada et al. 2002). When this fusion occurred the stem cells
would take on the characteristics of cardiac muscle cells. The other two studies
concluded that bone-marrow stem cells, when implanted into the heart, will have
traditional blood cell fates (Murry et al. 2004 and Balsam et al. 2004). This means that
Question 2:
What is the
fate of
implanted
blood cells
into a heart
after a heart
attack?
Table 2: Conclusions to
Question 2.
Conclusion 1:
Implanted cells fuse
with existing cardiac
muscle cells.
Conclusion 2: Blood
stem cells become
traditional blood cells.
even in the medium of an infarct heart, bone-marrow stem cells will become what they
would traditionally become in their original environment. The following table is an
overview of the conclusion found by each study (Table 3).
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Studies that Measure Heart Function
Study
Stamm et al. (2003)
Wollert et al. (2004)
Chen et al. (2004)
Strauer et al. (2002)
Deten et al. (2004)
Outcome
Improvement
Improvement
Improvement
Improvement
No Improvement
Studies that Determine Stem Cell Fate
Study
Outcome
Terada et al. (2002)
spontaneous cell fusion and phenotype* adoption
Murry et al. (2004)
traditional blood cell fate
Balsam et al. (2004)
traditional blood cell fate
Table 3: Comparison of Outcomes; * phenotype is defined as morphological characteristics.
At this point it is clear that there is no uniform conclusion on the topic of bone-marrow
stem cell implantation into an infarct heart.
The conclusion that I support is the combination that bone-marrow stem cells
spontaneously fuse with existing cardiac muscle cells, and through this mechanism,
improve heart function. By coming to this conclusion, one must ask what process is
responsible for improved ventricular function if in fact spontaneous cell fusion does
occur. Intuitively, one would guess that the only mechanisms by which an injured
myocardium would improve are by regenerating the population of cardiac muscle cells,
or by increasing the strength of the surviving cells to counter balance the sustained injury.
The latter is what I propose is occurring. As I explained previously, there is evidence that
suggests that when stem cells are in a medium of different cell types, the stem cells will
fuse with the other cells and take on their characteristics (Figure 3) (Terada et al., 2004).
In other words, a new cell is not formed; however, an existing cell is supplemented with
increased volume. In our case, a stem cell would fuse with a cardiomyocyte rather than
creating a new one. This fusion would supplement the existing cardiomyocyte with the
addition of extra volume and increased room for the protein units responsible for
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contraction. With the addition of these proteins, the contractile strength of the cell will
increase, explaining the increased function and contractile power that was seen by some
of the studies.
There is robust evidence in support of both parts of my conclusion. For example,
Strauer et al. (2002) found that left ventricular end-systolic volume (LVESV) (Side Note
7) decreased significantly, from 82 mL before stem cell therapy, to 67 mL after therapy.
This means that after a contraction of the heart muscles, there was less blood remaining
in the left ventricle during the post-implant phase of rehabilitation; therefore, the left
ventricle was ejecting a larger amount of blood into circulation. This study also found an
increased stroke volume (Side Note 7), starting with 49 mL/m2 before treatment and
finishing with 56 mL/m2. This means that for a given area of left ventricle, there is an
increased amount of blood that is ejected from the heart. Similarly, Chen et al. (2004)
found a LVESV decrease from 76 mL before transplant to 58 mL after transplant, and a
stroke volume index increase from 40% to 58%. The data that supports this part of my
conclusion is very consistent. The other component of my conclusion is that when in a
medium of different cell types bone-marrow stem cells will fuse with the other cells and
take on their characteristics. Terada et al. (2002) originally found that bone marrow stem
cells did in fact differentiate into cardiomyocytes; however, upon genetic analysis of the
new cardiomyocytes they found that the cells had higher than normal counts of
chromosomes (Figure 3). This led them to the conclusion that the bone marrow stem
cells actually fuse with surrounding cells and act as if they differentiated, taking on the
characteristics of the specialized cells, in this case cardiac muscle cells.
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The main reason that I propose this outcome is due to the overwhelming evidence
that suggests that heart function improves after bone-marrow stem cell implantation. For
the five studies that measured cardiac function only one suggested that there was no
improvement (Deten et al., 2004). The body of evidence that proposes that benefits are
seen simply overwhelms the conclusion that there is no benefit. Another reason that I
believe this conclusion is stronger is that the studies that found heart function increased
were all performed on human populations, whereas the other studies used mice for
subjects. While animal studies are useful in the sense that they allow us to take
measurements that would not be able to be taken on humans, it is important for this type
of study to be performed on humans. The Reason it is important is because the only real
beneficiaries at this point are humans that have had a myocardial infarction, and by
performing these studies on humans, they are more representative of actual clinical
effects.
As is the case with any position there are strengths and limitations on both sides
of the argument. Within this argument I have sided with the conclusion that through
implantation of bone-marrow stem cells into an infarct myocardium the implanted cells
will fuse with existing cardiomyocytes and through this mechanism will improve heart
function. The main strength of this position is that in general, the studies had highly
controlled research methods. With the exception of the Stamm et al. (2003) study, all of
the studies included a non-treatment control group (Side Note 8) to compare to the
treatment group. This is an important feature because it allows the researchers to
determine if any benefit was caused by the non-stem cell treatment of the myocardial
infarction. Unfortunately, there are also limitations to this position. First of all, there is
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only one study (Terada et al. 2002) that suggests bone-marrow stem cells will fuse
surrounding cells. It is an important aspect of research to be able to reproduce data, and
because only one study comes to the conclusion that fusion occurs, it is difficult to
determine if the results are valid and reproducible. A second limitation is the lack of data
given by the Terada et al. (2002) study. This study only concluded that they found cells
with higher than normal levels of chromosomes and gave no data in support of this
conclusion. This makes it very difficult to tell if the fusion process is common, or to
determine any other characteristics that are important for assessing the study.
In opposition to the evidence that suggests that heart function improves by cell
fusion, there is also a body of evidence that suggests implanted bone-marrow stem cells
take on traditional blood cell fates and have no affect on heart function. For example,
Balsam et al. (2004) found that upon implantation of the stem cells up to 48% stayed in
the myocardium and the rest went in to circulation. For those cells that stayed in the
infarct region of the heart, there were no cardiomyocytes that exhibited the genetic
marker that denoted the cells as implanted in any of the fourteen subjects. This means
that none of the implanted cells differentiated into cardiac muscle cells. Similarly, Murry
et al. (2004) found that there were no cardiomyocytes exhibiting the same genetic marker
in any of the 145 subjects. When it comes to the question of heart function, Deten et al.
(2004) found no significant improvement. For all measures of cardiac function this study
consistent found no significant differences between the non-treatment group versus the
treatment population. For example, Deten et al. (2005) found that stroke volume stayed
constant with 51 ml/m2 in the non-treatment group and a value of 50 ml/m2 for the
population that received a bone marrow stem cell implantation.
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This conclusion is severely limited in several ways. First of all, these studies used
a specific protein called GFP to determine if a cell was implanted or was a previously
existing cell. When using genetic markers such as GFP as measurements, it is very
difficult to obtain accurate numeric data. The data that is collected is something of an
estimate. For example, when the stem cells are implanted into the heart, the researchers
are not implanting just a few cells; on average they are implanting anywhere from 50,000
cells to 1,000,000 cells. This makes it very difficult to collect data. In fact, neither
Murry et al. (2004) nor Balsam et al. (2004) gave any data in support of their conclusion.
For this reason, the only conclusions that the researchers can come to is that something
happened or it did not, and no actual data is given. This is exactly what occurred. In
addition to this, when examining the fate of the implanted cells it is common practice to
only look in the implanted area. The mechanism by which I propose cardiac
improvement occurs, would assume that the GFP cells would be in a non-infarct area
around the infarct area rather than in the infarct region itself. By limiting themselves to
only examining the infarct area the researchers leave out other possible fates of the
implanted cells that could improve cardiac function.
Although there are tremendous limitations to this conclusion, there all also some
strengths. Most importantly, these studies utilized a larger population than the other
studies, allowing a more representative study to take place. By including more subjects,
the researchers ensure that the data is representative of a larger population. Additionally,
the conclusions were very consistent for each study. For example, according to Murry et
al. (2004) and Balsam et al. (2004) there was not a single cardiomyogenic event out of
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millions of implanted stem cells. This means that two different studies found the exact
same thing, adding strength to the studies.
Thorough examination of the current evidence on the topic of bone-marrow stem
cell implantation clearly indicates that this issue is unresolved. It is important to
determine the actual affects of this treatment. Through utilization of a well developed
research methodology, the use of bone-marrow stem cell implantation could bring forth a
new generation of procedures in cardiovascular medicine. By determining the exact
mechanism and degree to which this procedure can be utilized, scientists may develop a
common and highly efficient therapeutic tool for myocardial infarctions.
Side Note 1: Beginning with an embryo and continuing all the way
through human development, cells will divide to create a larger
number of cells or to replenish damaged cells. The divisional
process is called mitosis. The term post-mitotic describes a tissue
that, after maturity, does not divide.
Side Note 2: The human body begins development by fusion of a
sperm and an egg. This fusion creates one cell that eventually,
through mitosis, will become a mass of hundreds of different cell
types that will make up the human body. The way in which we get
from one type of cell to multiple types of cells is through stem cells.
They differentiate into a certain type of cell depending on the
chemical environment around them, and therefore can be very
useful in regenerating various tissues.
Side Note 3: Mesenchyme is a type tissue in the body that is
derived from the mesoderm, the middle layer of cells created during
embryonic development.
Side Note 4: The ability of a stem cell to transform or differentiate
into a specialized cell is the potency of that cell. Cardiac stem cells
and mesenchymal stem cells have a greater potential for changing
into cardiac muscle cells, and are therefore more potent cells.
Side Note 5: The fate of a stem cell is what type of cell it will
change into. For bone-marrow stem cells there fate is usually a
traditional red blood cell, but we are assessing the possibility of
other fates.
Side Note 6: One of the major difficulties that come along with
transplantation of cells or organs is the immune system. When
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donor cells are implanted into a different person’s body, their
immune system will notice this and destroy the implanted cells.
This is why autologous implantation is very important; the donor of
the cells is also the acceptor, so the immune system will not see the
implanted cells as invading.
Side Note 7: Some of the most common measurements used by
these studies are LVEF, LVESV, and Stroke Volume. All three of
these measure the amount of volume that leaves the left ventricle or
how much volume stays in the left ventricle. LVEF is a
measurement of the percentage of blood that fills the volume of the
left ventricle that is ejected upon contraction. LVESV is the
amount of blood that remains in the left ventricle after a
contraction. Lastly, Stroke Volume is the amount of blood that is
ejected, divided by the area of the heart so that the value is standard
for each person. It is important to understand that the more blood
that can be ejected from the left ventricle the better.
Side Note 8: When examining inputs and outcomes it is important
to include a group that does not receive the input. The nontreatment group is the control group. If a researcher finds a specific
outcome in a treatment group but not in a control group, it is
generally safe to assume that the outcome was due to the input.
Percentage
Increase in Stroke
Volume (mL/m2)
Perecntage Increase in Stroke
Volume after Stem Cell
Implantation
50
40
30
20
10
0
Series1
1
2
3
4
5
Study
Figure 1: Comparison of stroke volume values
1: Stamm et al. (2003)
2: Wollert et al. (2004)
3. Strauer et al. (2002)
4. Chen et al. (2004)
5. Deten et al. (2005)
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Causes of Death for People in U.S. in 2002 (In the Thousands)
500
494
434
400
300
289
269
Males
200
100
Females
69 61
34
64 42
39
0
A B C D E A B D
F E
A Heart Disease
B Cancer
C Accidents
D Chronic Lower Respiratory Diseases
E Diabetes Mellitus
F Alzheimer’s disease
Figure 2: Causes of Mortality in the U.S. (Source: Center for Disease Control)
Figure 3: Hyper-Diploid Chromosome Counts (Terada et al., 2002). This figure shows the
chromosome count of a cell that was found in the Terada et al. (2002) study. The normal cell should
only have two chromosomes per number. This cell has approximately four, evidence that two cells
fused together.
References
Balsam, L.B., Wagers, Christensen, Kofidis, Weissman, Robbins (2004). Haematopoietic stem cells adopt
mature haematopoietic fates in ischemic myocardium. Nature, 428, 668-673.
Chen, Fang, Ye, Liu, Qian, Shan, Zhang, Chunhua, Liao, Lin, Sun (2004). Effect on left ventricular
Function of intracoronary transplantation of autologous bone marrow mesenchymal stem cell in
patients with acute myocardial infarction. Am J Cardiol, 94, 92-95.
Deten, Volz, Clamors, Leiblein, Briest, Marx, Zimmer (2005). Hematopoietic stem cells do not repair the
infarcted mouse heart. Cardio Res, 65, 52-63.
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Murry, Soonpaa, Reinecke, Nakajima, Nakajima, Rubart, Pasmumarthi, Virag, Bartelmez, Poppa,
Bradford, Dowell, Williams, Field (2004). Haematopoietic stem cells do not transdifferentiate
into cardiacmyocytes in myocardial infarcts. Nature, 428, 664-668.
Orlic, D. (2005). BM stem cells and cardiac repair: where do we stand in 2004? Cytotherapy, 7(1), 3-15.
Smits, Vliet, Hassink, Goumans, Doevendans (2005). The role of stem cells in cardiac regeneration. J Cell
Mol Med, 9(1), 25-36.
Stamm, Westphal, Kleine, Petzsch, Kittner, Klinge, Schumichen, Nienber, Freund, Steinhoff (2003).
Autologous bone marrow stem cell transplantation for myocardial regeneration. Lancet, 361, 4546.
Strauer, Brehm, Zeus, Kostering, Hernandez, Sorg, Kogler, Wernet (2002). Repair of infarcted
myocardium by autologous intracoronary mononuclear bone marrow cell transplantation in
humans. Circulation, 106, 1913-1918.
Terada, Hamazaki, Oka, Hoki, Mastalrez, Nakano, Meyer, Morel, Petersen, Scott (2002). Bone marrow
adopts the phenotype of other cells by spontaneous cell fusion. Nature, 416, 542-545.
Wollert, Meyer, Lotz, Ringes, Lippolt, Breidenbach, Fichtner, Korte, Horning, Messinger, Arseniev,
Hertenstein, Gasner, Drexler. (2004). Intracoronary autologous bone-marrow cell transfer after
myocardial infarction: TheBoost randomized controlled clinical trial. Lancet, 364, 141-148.
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