Heart-like cells made in the laboratory from
the skin of patients with a common cardiac condition contract less strongly
than similarly created cells from unaffected family members, according to
researchers at the Stanford University School of Medicine. The cells also
exhibit abnormal structure and respond only dully to the wave of calcium
signals that initiate each heartbeat.
The
finding used induced pluripotent stem, or iPS, cell technology to create
heart-muscle-like cells from the skin of patients with dilated cardiomyopathy,
which is one of the leading causes of heart failure and heart
transplantation in the United States. It adds to a growing body of
evidence indicating that iPS cells can faithfully reflect the disease status of
the patients from whom they are derived.
Using
the newly created diseased and normal cells, the researchers were able to
directly observe for the first time the effect of a common beta blocker drug,
as well as validate the potential usefulness of a gene therapy approach
currently in clinical trials.
"Primary
human cardiac cells are
difficult to obtain and don't live long under laboratory conditions," said
Joseph Wu, MD, PhD, associate professor ofcardiovascular
medicine. Instead, researchers have relied on studies of cells from rodent
hearts, which beat much more quickly, to understand more about human heart
disease. "Now we've created heart cells from iPS cells derived from skin
that allow us to study in detail the mechanisms of a common cardiac disease and
how these cells respond to clinical interventions."
Wu is
the senior author of the research, which will be published April 18 inScience
Translational Medicine. Postdoctoral scholar Ning Sun, MD, PhD, is the
first author. The work is the latest in a type of research that's sometimes
referred to as "disease-in-a-dish" studies. Using iPS technology,
other researchers have created stem cells from patients with Parkinson's
disease, Marfan syndrome and amyotrophic lateral sclerosis, among others.
The
implications of such research are huge. According to Wu, one of the major
reasons cardiac drugs are pulled from the market is unexpected cardiac toxicity
— that is, they are damaging the very hearts they're meant to help. Currently,
such drugs are pre-screened for toxic effects on common laboratory cell lines
derived from either hamster ovaries or human embryonic kidney cells.
Even
though these ovarian and kidney cells have been artificially induced to mimic
the electrophysiology of human heart cells, they are still very different from
the real thing. A reliable source of diseased and normalhuman heart cells on
which to test the drugs' effect prior to clinical use could improve drug
screening, save billions of dollars and improve the lives of countless
patients.
Dilated
cardiomyopathy occurs when a portion of the heart muscle enlarges
and begins to lose the ability to pump blood efficiently. Eventually, the
enlarged muscle begins to weaken and fail, requiring either medication or even
transplant.
Although
many cases occur sporadically and without an apparent cause, dilated
cardiomyopathy can also be inherited via a variety of genetic mutations.
Wu and
Sun performed skin biopsies on seven members of three generations of a family
with the inherited form of the condition (called familial dilated
cardiomyopathy). Four of the family members had inherited a specific genetic
mutation — in a gene called TNNT2 — that causes the disease; the other three
had not.
The
researchers used iPS technology to convert skin cells from the affected and
unaffected family members into stem cells, which they
then coaxed to become heart muscle cells for further study. They then compared
cells from unaffected family members with those who had the disease.
"We
didn't know exactly how the mutation carried in this family would impact the
contractility of the cells," said Sun. "Other studies had indicated
that this mutation decreased calcium sensitivity in rodent cells, but we had no
direct biochemical data on human cells. We were able to show that the force of
contraction was lower in cells from patients with the mutation. We also saw
that, as predicted in the rodent model, they were less responsive to calcium signaling."
(In a normal heart, rapid, periodic increases in calcium levels inside heart cells trigger
each contraction.)
Wu and
Sun also saw that the diseased cells exhibit structural differences and are
more susceptible to mechanical stress than unaffected cells.
When
the researchers treated the diseased cells with metoprolol, a beta blocker
commonly used to treat cardiomyopathy, they found that it decreased the
frequency of contractions as expected. It also increased the responsiveness of
the cells to calcium and, over time, helped resolve some of the structural
differences between affected and unaffected cells.
Finally,
they showed that adding a protein called Serca2a, which may inhibit the
deleterious effect of the mutated TNNT2 gene, significantly improved the
contraction forcefulness of the diseased cells. Serca2a is currently in
clinical trials as a possible gene therapy for dilated cardiomyopathy.
"Next,
we'd like to continue looking at cells from patients with other mutations
associated with this disorder," said Wu. "How do they behave in
culture? Do they respond in the same way? What is the mechanism for their
response? What changes if we selectively introduce different mutations into
these cells? And how do
we scale up drug screening using cardiac specific iPS cell lines?"
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