Dear Editor,
Dilated cardiomyopathy (DCM) is a common form of inherited cardiomyopathy. In the
past decades, single mutations in various genes encoding sarcomeric, cytoskeletal,
and channel proteins etc. have been found to be associated with DCM (Hershberger et
al., 2013; McNally and Mestroni, 2017). However, the mechanisms how single mutations
in sarcomeric or structural genes lead to the disease remain elusive. An interesting
phenomenon often seen in familial cardiomyopathy is that different single mutations
on the same gene can cause either DCM or hypertrophic cardiomyopathy (HCM) (Kathiresan
and Srivastava, 2012), which exhibit almost opposite disease phenotypes. DCM is characterized
by thinned myocardium and septum, ventricular chamber dilation, and systolic dysfunction
(Jefferies and Towbin, 2010; McNally and Mestroni, 2017), while HCM exhibits thickened
myocardium and septum, reduced ventricular chamber, and diastolic dysfunction (Richard
et al., 2003). At the cellular level, HCM cardiomyocytes exhibit concentric hypertrophy
characterized by assembly of myofilaments in parallel and widening of the myocytes.
In contrast, DCM cardiomyocytes show eccentric hypertrophy, with assembly of the myofilaments
in series and myocyte elongation (Kehat and Molkentin, 2010).
The gene TNNT2, which encodes cardiac troponin T (cTnT), is one of such gene showing
ramification of cardiomyopathy phenotypes. Deletion of lysine 210 (ΔK210) in one allele
of TNNT2 was found to cause familial DCM. Patients carrying this heterozygous mutation
exhibit dilated ventricular chamber and systolic dysfunction leading to progressive
heart failure with high mortality (Kamisago et al., 2000). In contrast, deletion of
glutamic acid 160 (ΔE160) in one allele of TNNT2 causes HCM (Watkins et al., 1995).
Previous studies showed that the ΔK210 mutation and ΔE160 mutation causes an opposite
myofilament Ca2+ sensitivity and force generation (Morimoto et al., 2002), possibly
leading to opposite macroscopic disease phenotypes. The contrast cardiomyopathy phenotypes
arisen from these two different cTnT mutations provide us a nice model to explore
detailed molecular basis triggering DCM disease development.
To investigate the key factors involved in this disease divergence, we generated isogenic
human embryonic stem cell (hESC) lines carrying the cTnT-ΔK210 or cTnT-ΔE160 mutation
by TALEN-mediated genomic engineering (Fig. S1A and S1B). This eliminated the genetic
variations of induced pluripotent stem cells (iPSCs) even from individuals from the
same family. Southern blotting using a probe targeting the PGK-Puro cassette confirmed
that no non-specific integration within the genome (Fig. S1C–E). Sanger’s sequencing
analyses of each of these cell lines further confirmed their heterozygous and homozygous
nature (Fig. 1A). We next differentiated the heterozygous and homozygous cTnT-∆K210
and -∆E160 hESCs toward the cardiac lineage. cTnT-∆K210 and -∆E160 hESC-cardiomyocytes
showed abnormal beating activities (Fig. 1B), which suggested a correlation with cardiac
arrhythmia. To examine whether ∆K210 and ∆E160 mutations have an impact on myofilament
organization, we further analyzed cardiomyocytes derived from these mutant hESCs by
immunostaining the sarcomeric proteins cTnT and α-actinin. Compared to WT hESC-cardiomyocytes,
cTnT-∆K210 and -∆E160 cardiomyocytes showed distinct phenotypes in sarcomeric organization
(Fig. 1C). A much higher percentage of heterozygous TNNT2
WT/∆K210 cardiomyocytes showed DCM hallmarks, such as fewer myofibrils, more irregular
disrupted sarcomere organization, and punctate cellular distribution of cTnT, which
was even more pronounced in homozygous TNNT2
∆K210/∆K210 cardiomyocytes (Fig. 1D). Ultra-structures analyzed by transmission electron
microscopy (TEM) showed ∆K210 cardiomyocytes exhibited relative irregular Z-lines
and expanded endoplasmic reticulum (Fig. 1E). In contrast, heterozygous TNNT2
WT/∆E160 cardiomyocytes showed relative denser myofibrils with thickened Z lines.
Homozygous TNNT2
∆E160/∆E160 cardiomyocytes showed markedly disrupted myofibrils and Z-lines (Figs. 1E
and S2).
Figure 1
Generation of ∆K210 and ∆E160 mutant hESC lines, phenotypic characterizations of mutant
cardiomyocytes, and identification of actin binding Rho activating protein (ABRA)
as a candidate gene involved in the earliest disease divergence.
(A) Sanger’s sequencing of PCR-amplified genomic DNA in wildtype (WT), heterozygous,
and homozygous ∆K210 and ∆E160 hESCs confirmed deletion of AAG (K210) and GAG (E160)
in exon 14 and exon 12, respectively, in the TNNT2 gene. (B) The beating rate (times/min)
of hESC-cardiomyocytes of different groups at day 10, 20, and 30 post differentiation.
(C) Representative cellular myofilament organization of day 35 single WT, heterozygous
and homozygous ∆K210 or ∆E160 cardiomyocytes immunostained with cTnT (red) and α-actinin
(green). Scale bars, 50 μm. (D) The percentage of cells with disorganized sarcomeric
pattern in each group at day 35 post cardiac differentiation. (E) Representative TEM
images of myofibrillar organization of day 35 cardiomyocytes in each group. Z, Z-line.
Red arrows indicate disorganized or thickened myofibrils. Scale bars, 500 nm. (F)
Quantification of the relative spontaneous contraction forces for day 35 single hESC-cardiomyocytes
in each group. (G and H) Representative Ca2+ line scan images of spontaneous Ca2+
transients and statistics of Ca2+ handling parameters of WT, ∆K210, and ∆E160 cardiomyocytes
at day 35 post differentiation. (I and J) Whole transcriptomic RNA-seq profiles and
principal component analysis (PCA) showed significant separation between WT, DCM ∆K210,
and HCM ∆E160 cardiomyocytes at day 35 post differentiation. (K) Heatmap of the differentially
expressed genes in day 35 WT, WT/∆K210 and WT/∆E160 hESC-cardiomyocytes. Compared
with WT, genes showed opposite expression changes are listed in the box. (L) Quantitative
PCR verification of ABRA, AZGP1, and HRASLS5 expression in day 35 WT, WT/∆K210 and
WT/∆E160 hESC-cardiomyocytes. *P < 0.05, **P < 0.01 and ***P < 0.001
For the cardiomyocyte function, both heterozygous and homozygous cTnT-∆K210 cardiomyocytes
exhibited a reduced relative contraction force compared with WT cardiomyocytes, while
cTnT-∆E160 cardiomyocytes showed increased contractility (Fig. 1F). Moreover, cTnT-∆K210
and -∆E160 cardiomyocytes exhibit a divergent Ca2+ handling properties and more irregular
Ca2+ transients at day 35 post differentiation (Fig. 1G and 1H). Overall, these results
showed that heterozygous ∆K210 and ∆E160 cardiomyocytes largely recapitulated cellular
phenotypes of DCM and HCM respectively, while homozygous ∆K210 and ∆E160 cardiomyocytes
showed more severe but abnormal cellular phenotypes.
Since DCM or HCM is progressively developed in patients’ lifetime, we hypothesized
that genes involved in the earliest disease development and divergence will be valuable
targets for interfering the disease. In this study, we used ∆E160 cardiomyocytes as
a contrast to look for the key genes involved in the earliest disease progress during
DCM development. Whole transcriptomic analyses of day 14 and day 35 ∆K210 and ∆E160
cardiomyocytes was performed using RNA-sequencing. Day 14 and day 35 cardiomyocytes
exhibited distinct gene expression patterns and automatically grouped separately into
two large clusters (Fig. 1I and 1J), suggesting they are at different maturation stages.
Within the cluster of day 35 cardiomyocytes, it began to show differential gene expression
patterns and almost automatically sub-clustered into WT, ∆K210 DCM, and ∆E160 HCM
groups (Fig. 1I and 1J). To further narrow down the candidate genes in early disease
development and divergence of DCM and HCM, we next compared those differentially expressed
genes in day 35 ∆K210 and ∆E160 cardiomyocytes (the time that showed disease divergence)
and searched for those genes exhibiting an opposite expression. As shown in Fig. 1K,
there were 74 overlapped genes and among which only 3 genes showed opposite direction
in expression changes. Of the 3 genes, ABRA (actin binding Rho activating protein,
also known as striated muscle activator of Rho signaling (STARS)) were down-regulated
in TNNT2
WT/∆K210 DCM cardiomyocytes and up-regulated in TNNT2
WT/∆E160 HCM cardiomyocytes. The other two genes HRASLS5 and AZGP1, which are strongly
linked to metabolism, were up-regulated in TNNT2
WT/∆K210 cardiomyocytes and down-regulated in TNNT2
WT/∆E160 cardiomyocytes (Fig. 1K). Expression of ABRA, AZGP1, and HRASLS5 in day 35
cardiomyocytes were further validated by quantitative PCR (Fig. 1L). Since ABRA was
shown up-regulated in skeletal muscle hypertrophy and down-regulated in atrophy in
human patients (Lamon et al., 2009), which is closely corresponding to heart muscle
hypertrophy and dilation, we speculated that ABRA play an important role in the earliest
disease divergence.
ABRA is a cardiac and skeletal muscle-specific actin-binding protein, which specifically
localizes to the Z disc and M-line and directly binds actin (Arai et al., 2002). Previous
studies demonstrated that ABRA activates the Rho signaling and serum response factor
(SRF) mediated transcription by allowing nuclear translocation of MRTF-A and -B. ABRA
also promotes actin polymerization by increasing the binding of G-actin to F-actin,
strengthens myofilaments of striated muscle cells and modulates the heart response
to stress signaling, which may function as a cytoskeletal intermediary of stress sensing
and intracellular signaling regulation (Arai et al., 2002; Kuwahara et al., 2007;
Wallace et al., 2012). Compared to WT cardiomyocytes, ABRA showed a striated localization
pattern but tended to localize to the center of TNNT2
WT/∆K210 cells and more disrupted in TNNT2
∆K210/∆K210 cells (Fig. S3A). ABRA and cTnT protein levels were also decreased in
cTnT-∆K210 cardiomyocytes (Fig. S3B and S3C). Overexpression of ABRA using lentivirus
in cTnT-∆K210 cardiomyocytes also rescued the DCM-relevant cellular phenotypes, such
as sarcomere disorganization, punctate distribution of cTnT and decreased contractile
force (Fig. S3D–H). We reasoned that down-regulation of ABRA in cTnT-∆K210 cardiomyocytes
reduced F-actin filament formation and may directly weaken myofilament structures,
thereby leading to decreased cell contractility and DCM-relevant phenotypes.
We next investigated whether heart-specific expression of ABRA could reverse DCM phenotypes
of the cTnT-∆K210 DCM mice in vivo. The results showed reduced ABRA protein in cTnT-∆K210
mice (Fig. S4A), staining the Z-lines with α-actinin and A-bands with tropmodulin
(TMOD1) in cTnT-∆K210 mice further indicated the Z-lines were relative intact but
A-bands were disrupted (Fig. S4B). Recombinant adeno-associated virus 9 (AAV9) with
cardiac-specific cTnT promoter driving ABRA expression (AAV9-ABRA) was constructed
and AAV9 with cTnT promoter driving luciferase (AAV9-Luci) served as a control. AAVs
were injected at 2–3 days after birth through intraperitoneal injection. The luciferase
protein was detected only in the hearts, which validated the virus performance (Fig.
S5A).
Mice heart function was measured every month after AAV injection by echocardiography.
It showed that impaired heart functions were effectively rescued in AAV9-ABRA treated
cTnT-∆K210 mice at 3 months (Fig. 2A). Left ventricular ejection fraction (EF) and
fractional shortening (FS) were reversed to a level close to WT values (Fig. 2B).
The left ventricular end-systolic internal diameter (LVIDs) and LV posterior wall
(LVPW) thickness of cTnT-ΔK210 mice treated with AAV9-ABRA were significantly improved
(Figs. 2B and S5B). Heart size and heart weight/body weight value of AAV9-ABRA treated
cTnT-ΔK210 mice were also much smaller and close to those of the WT (Fig. 2C and 2D).
Histological analyses showed that AAV9-Luci treated cTnT-ΔK210 mice developed cardiac
dilation with enlarged cardiomyocytes and increased myocardial fibrosis, while AAV9-ABRA
treated mice reversed these typical DCM phenotypes and pathological cardiac remodeling
(Fig. 2E). Moreover, AAV9-ABRA treatment restored the disrupted sarcomeric A-bands
and M lines in cTnT-ΔK210 mice, although the sarcomere length still seemed compact
(Fig. 2F). AAV9-ABRA treatment also improved overall survival rate of cTnT-ΔK210 DCM
mice (Fig. 2G). These results indicated that the DCM phenotypes in cTnT-ΔK210 mice
were markedly reversed by cardiac-specific ABRA expression. We next used the classical
cTnT-R141W knock-in DCM mice (Juan et al., 2008) to examine whether ABRA heart-specific
expression is effective in other cTnT mutation-induced familial DCM. As shown in Fig.
S6, impaired heart functions in cTnT-R141W mice were rescued after AAV9-ABRA injection.
Figure 2
Cardiac-specific expression of ABRA rescued DCM phenotypes.
(A) Representative M-mode echocardiography recordings of 1-month-old and 3-months-old
WT, AAV9-Luci injected cTnT-∆K210, and AAV9-ABRA injected cTnT-∆K210 mice. (B) Serial
echocardiographic measurements of ejection fraction (EF), fractional shortening (FS),
left ventricular end systolic internal diameter (LVIDs) at different time points post
virus injection. (C and D) Overall heart morphology and heart weight /body weight
(HW/BW) ratio of the whole hearts 3 months after AAV9 virus injection. (E and F) Representative
H&E, Masson’s trichrome, and WGA staining (E), was well as TEM images (F) of heart
sections 3 months after AAV9 virus injection. Scale bars: 100 μm (Masson), 50 μm (WGA),
1 μm (TEM). (G) survival curves for WT, AAV9-Luci injected cTnT-∆K210, and AAV9-ABRA
injected cTnT-∆K210 mice. (H and I) Western blot assessment and quantification of
calmodulin and Abra protein levels in heart tissues of 3-months-old WT and cTnT-∆K210
mice. A pan-calmodulin antibody was used. (J) Co-immunoprecipitation of calmodulin
and Abra from heart tissue extraction of WT mice. (K) Immunofluorescence staining
of SRF in mice heart tissues of different groups 3 months after AAV9 virus injection.
Scale bars, 100 μm. (L) Quantification of percentage of cardiomyocytes exhibiting
positive nuclear SRF staining. (M) RNA-seq heatmap profiling of SRF-regulated muscle
genes 3 months after AA9 virus injection. *P < 0.05. **P < 0.01.
In addition, our Ca2+ imaging results indicated that the cTnT-∆K210 mutation induced
abnormal Ca2+ handling (Fig. 1G) and significantly upregulated calmodulin in the heart
of cTnT-∆K210 mice (Fig. 2H and 2I). Previous study showed that the Ca2+-dependent
protein calmodulin directly binds to the N-terminus of ABRA and negatively regulates
muscle gene expression induced by the ABRA-serum response factor (SRF) pathway (Furuya
et al., 2016), thereby linking Ca2+-signaling to ABRA-mediated gene expression. Immunoprecipitation
assays showed that calmodulin binds to ABRA in cardiomyocytes (Fig. 2J). We then examined
whether ABRA mediated SRF activity was changed in cTnT-∆K210 mice. Compared to WT
mice, nucleus translocation of SRF was significantly reduced in heart tissues of cTnT-∆K210
mice (Fig. 2K and 2L). SRF-regulated muscle and contractile genes were significantly
downregulated in cTnT-∆K210 mice and increased after AAV9-ABRA treatment as shown
by whole transcriptome RNA-sequencing and further confirmed by QPCR (Figs. 2M and
S7). These results suggest that the cTnT-ΔK210 mutation causes irregular Ca2+ handling
in myofilaments and a resulted up-regulation of calmodulin, which negatively regulates
ABRA-SRF activity in cardiomyocytes and compromised downstream muscle related gene
expression.
Our current study showed that ABRA deficiency and compromised downstream SRF-regulated
muscle gene expression play a role in cTnT mutation induced-familial DCM. ABRA could
be a therapeutic gene for DCM patients carrying cTnT mutations. Our findings in this
study may also serve as a new strategy in discovering early disease-associated genes
for other mutation-caused familial DCM.
FOOTNOTES
This work was supported by the National Natural Science Foundation of China (NSFC
No.82070391, N.S.), the Postdoctoral Science Foundation (No.KLH1322109, B.L.), the
Young Elite Scientist Sponsorship Program by CAST (2018QNRC001), the Haiju program
of National Children’s Medical Center EK1125180102, and the National Key R&D Program
of China 2018YFC2000202 (N.S.).
We thank Dr. Lianfeng Zhang at Peking Union Medical College for his kindly provision
of the cTnT-R14W mice. We apologize to people whose work was relevant to but not cited
in this study due to limited space.
Bin Li, Yongkun Zhan, Qianqian Liang, Chen Xu, Xinyan Zhou, Huanhuan Cai, Yufan Zheng,
Yifan Guo, Lei Wang, Wenqing Qiu, Baiping Cui, Chao Lu, Ruizhe Qian, Ping Zhou, Haiyan
Chen, Yun Liu, Sifeng Chen, Xiaobo Li, Ning Sun declare that there is no conflict
of interests.
All institutional and national guidelines for the care and use of laboratory animals
were followed.
The data, analytic methods, and study materials will be made available to other researchers
for purposes of reproducing the results or replicating the procedure. The RNA-seq
data have been deposited in GEO (Gene Expression Omnibus) with accession code GSE154096
and GSE154097.
B.L., Y.K.Z., and Q.Q.L performed and interpreted the experiments and wrote the manuscript.
X.Y.Z. and H.H.C. performed H&E staining and MEA recording. C.X, Y.F.Z., Y.F.G, L.W.,
and W.Q.Q performed in vitro experiments. X.B.L., C.L., R.Z.Q and P.Z provided experimental
advice. H.Y.C. performed echocardiography. Y.L. performed RNA sequencing and bioinformatics
analyses. X.W. provided experimental advice. N.S. conceived the idea and experiments,
provided experimental assistance, manuscript writing and funding support.
Electronic supplementary material
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