A
new Johns Hopkins study has unraveled the changes in a key cardiac
protein that can lead to heart muscle malfunction and precipitate heart
failure.
Troponin
I, found exclusively in heart muscle, is already used as the
gold-standard marker in blood tests to diagnose heart attacks, but the
new findings reveal why and how the same protein is also altered in
heart failure. Scientists have known for a while that several heart
proteins—troponin I is one of them—get “out of tune” in patients with
heart failure, but up until now, the precise origin of the “bad notes”
remained unclear.
The
discovery, published online ahead of print on Sept. 12 in the journal
Circulation, can pave the way to new—and badly needed—diagnostic tools
and therapies for heart failure, a condition marked by heart muscle
enlargement and inefficient pumping, and believed to affect more than 6
million adults in the United States, the researchers say.
Troponin
I acts as an on-off switch in regulating heart relaxation and
contraction and, in response to, adrenaline—the “flight-fight” response.
But when altered, troponin I can start acting as a dimmer switch
instead, one that ever so subtly modulates cardiac muscle function and
reduces the heart’s ability to pump efficiently and fill with blood, the
researchers found.
The
Hopkins team used a novel method to pinpoint the exact sites, or
epicenters, along the protein’s molecule where disease-triggering
changes occur. They found 14 such sites, six of them previously unknown.
In revealing new details about the molecular sequence of events leading
up to heart failure, the researchers said their work may spark the
development of tests that better predict disease risk and monitor
progression once the heart begins to fail.
“Our
findings pinpoint the exact sites on troponin I’s molecule where
disease-causing activity occurs, and in doing so they give us new
targets for treatment,” says researcher Jennifer Van Eyk, Ph.D.,
director of the Johns Hopkins Proteomics Innovation Center in Heart
Failure.
In
the current study, the team analyzed tissue from the hearts of patients
with end-stage heart failure and from deceased healthy heart donors.
The 14 sites the researchers identified are sites where troponin I binds
with phosphate, a process known as phosphorylation. Phosphate can
activate or deactivate many enzymes, thus altering the function of a
protein and, in the case of heart failure, ignite disease. The six newly
identified sites represent new “hot spots” involved in heart
contraction, the researchers say, and could be used as diagnostic
markers or a target for treatment to restore function. The Hopkins
researchers found that in some sections of the molecule, phosphorylation
ratcheted up the dimmer switch, while ratcheting it down in other
sections, but it invariably led to muscle dysfunction.
“Our
goal would be to zero in on these new sites, gauge risk of heart
failure and, hopefully, restore heart muscle function,” Van Eyk says.
Heart
failure is a complex progressive disorder, and while cardiac pacemakers
can restore or “resynchronize” heart function in many people, about
one-third of patients do not improve even with pacemaker therapy in
addition to standard medication treatments.
“This
is a devastating disorder for which we desperately need new and less
invasive therapies,” says senior investigator Anne Murphy, M.D., a
cardiologist at Johns Hopkins Children’s Center.
In
their analysis, the researchers used a novel technique, called
multiple-reaction monitoring (MRM), which pinpoints the exact locations
along the protein’s molecule where faulty signaling occurs and disrupts
heart muscle function. MRM is an ultra-sensitive type of mass
spectrometry that measures the exact size and chemical composition of
protein fragments. Phosphorylated protein fragments have different
molecular weights than non-phosphorylated ones. In this way, MRM
accurately homes in on sites where phosphate is bound to troponin I to
modulate heart muscle function.
The
researchers found that patients with heart failure had markedly
different levels of phosphorylation in certain protein segments compared
with healthy heart muscle.
The
advantage of MRM analysis—one of the first non-antibody based troponin I
tests—is that it can measure phosphorylation levels without the need
for antibodies, the traditional method currently used to monitor heart
muscle function. The researchers believe that MRM can be developed as a
clinical diagnostic test, and the Hopkins team is already working to
develop a test that would measure phosphorlyation levels of proteins in
the blood and would allow physicians to monitor the progression of the
disease as well as predict which heart attack patients will progress to
heart failure. About one-third of them do so.
“Right
now, we don’t really know which heart attacks patients will develop
heart failure and which ones will maintain normal heart muscle
function,” Murphy says. “Monitoring specific phosphorylation sites might
be one way to help us foresee and forestall this complication on an
individual patient basis.”
Other
Johns Hopkins investigators on the study included Pingbo Zhang, Ph.D.,
Weihua Ji, M.S., Cristobal G. dos Remedios, D.Sc., Jonathan Kirk, Ph.D.,
and David Kass, M.D.
Source: Johns Hopkins University
