The Fontan immunophenotype and post-transplant outcomes in children: A multi-institutional study

BACKGROUND

Patients after Fontan palliation represent a growing pediatric population requiring heart transplant (HTx) and often have lymphopenia (L) and/or hypogammaglobinemia that may be exacerbated by protein-losing enteropathy (PLE, P). The post-HTx effects of this altered immune phenotype are not well studied.

METHODS

In this study of the Pediatric Heart Transplant Society Registry, 106 Fontan patients who underwent HTx between 2005 and 2018 were analyzed. The impact of lymphopenia and PLE on graft survival, infection, rejection, and malignancy was analyzed at 1 and 5 years post-HTx.

RESULTS

The following combinations of lymphopenia and PLE were noted: +L+P, n = 37; +L-P, n = 23; -L+P, n = 10; and -L-P, n = 36. Graft survival between the groups was similar within the first year after transplant (+L+P: 86%, +L-P: 86%, -L+P: 87%, -L-P: 89%, p = .9). Freedom from first infection post-HTx was greatest among -L-P patients compared to patients with either PLE, lymphopenia, or both; with a 22.1% infection incidence in the -L-P group and 41.4% in all others. These patients had a significantly lower infection rate in the first year after HTx (+L+P: 1.03, +L-P: 1, -L+P: 1.3, -L-P: 0.3 infections/year, p < .001) and were similar to a non-single ventricle CHD control group (0.4 infections/year). Neither freedom from rejection nor freedom from malignancy 1 and 5 years post-HTx, differed among the groups.

CONCLUSIONS

Fontan patients with altered immunophenotype, with lymphopenia and/or PLE, are at increased risk of infection post-HTx, although have similar early survival and freedom from rejection and malignancy. These data may encourage alternative immunosuppression strategies and enhanced monitoring for this growing subset of patients.

Pediatric heart transplant from an incompletely treated influenza A-positive donor

There is a shortage of pediatric donor hearts for waitlisted children, and yet nearly 50% of organs offered are not transplanted. Donor quality is often cited as a reason for declining organs offered from donors infected with influenza, presumably due to concern about disease transmission at transplant leading to severe disease. We previously described an excellent outcome after heart transplant from a donor infected with influenza B that had been treated with a complete course of oseltamivir. In this report, we describe a similar outcome after transplantation of an organ from an influenza A-positive donor with symptomatic disease incompletely treated with oseltamivir. Due to the availability of effective antiviral treatment, we suggest that influenza A is also a manageable donor infection that need not preclude heart placement.

Trajectory of right ventricular indices is an early predictor of outcomes in hypoplastic left heart syndrome

BACKGROUND

Children with hypoplastic left heart syndrome (HLHS) have risk for mortality and/or transplantation. Previous studies have associated right ventricular (RV) indices in a single echocardiogram with survival, but none have related serial measurements to outcomes. This study sought to determine whether the trajectory of RV indices in the first year of life was associated with transplant-free survival to stage 3 palliation (S3P).

METHODS

HLHS patients at a single center who underwent stage 1 palliation (S1P) between 2000 and 2015 were reviewed. Echocardiographic indices of RV size and function were obtained before and following S1P and stage 2 palliation (S2P). The association between these indices and transplant-free survival to S3P was examined.

RESULTS

There were 61 patients enrolled in the study with 51 undergoing S2P, 20 S3P, and 18 awaiting S3P. In the stage 1 perioperative period, indexed RV end-systolic area increased in patients who died or needed transplant following S2P, and changed little in those surviving to S3P (3.37 vs -0.04 cm² /m² , P = .017). Increased indexed RV end-systolic area was associated with worse transplant-free survival. (OR = 0.815, P = .042). In the interstage period, indexed RV end-diastolic area increased less in those surviving to S3P (3.6 vs 9.2, P = .03).

CONCLUSION

Change in indexed RV end-systolic area through the stage 1 perioperative period was associated with transplant-free survival to S3P. Neither the prestage nor poststage 1 indexed RV end-systolic area was associated with transplant-free survival to S3P. Patients with death or transplant before S3P had a greater increase in indexed RV end-diastolic area during the interstage period. This suggests earlier serial changes in RV size which may provide prognostic information beyond RV indices in a single study.

Pediatric heart transplantation from an influenza B-positive donor

As heart transplantation demand is increasing without subsequent growth of the donor pool, need for expansion of acceptance criteria is paramount, particularly when considering critically ill, highly sensitized patients. We present a case report of a pediatric heart transplant recipient of an organ refused by 197 prior potential recipients due to the donor being infected with influenza virus. We perform a literature review of recent solid organ transplant cases from influenza-positive donors and conclude that the donor pool may be expandable by allowing donors with treatable infections to be included.

Gain-of-function mutations in SMAD4 cause a distinctive repertoire of cardiovascular phenotypes in patients with Myhre syndrome

Myhre syndrome is a rare, distinctive syndrome due to specific gain-of-function mutations in SMAD4. The characteristic phenotype includes short stature, dysmorphic facial features, hearing loss, laryngotracheal anomalies, arthropathy, radiographic defects, intellectual disability, and a more recently appreciated spectrum of cardiovascular defects with a striking fibroproliferative response to surgical intervention. We report four newly described patients with typical features of Myhre syndrome who had (i) a mildly narrow descending aorta and restrictive cardiomyopathy; (ii) recurrent pericardial and pleural effusions; (iii) a large persistent ductus arteriosus with juxtaductal aortic coarctation; and (iv) restrictive pericardial disease requiring pericardiectomy. Additional information is provided about a fifth previously reported patient with fatal pericardial disease. A literature review of the cardiovascular features of Myhre syndrome was performed on 54 total patients, all with a SMAD4 mutation. Seventy percent had a cardiovascular abnormality including congenital heart defects (63%), pericardial disease (17%), restrictive cardiomyopathy (9%), and systemic hypertension (15%). Pericarditis and restrictive cardiomyopathy are associated with high mortality (three patients each among 10 deaths); one patient with restrictive cardiomyopathy also had epicarditis. Cardiomyopathy and pericardial abnormalities distinguish Myhre syndrome from other disorders caused by mutations in the TGF-β signaling cascade (Marfan, Loeys-Dietz, or Shprintzen-Goldberg syndromes). We hypothesize that the expanded spectrum of cardiovascular abnormalities relates to the ability of the SMAD4 protein to integrate diverse signaling pathways, including canonical TGF-β, BMP, and Activin signaling. The co-occurrence of congenital and acquired phenotypes demonstrates that the gene product of SMAD4 is required for both developmental and postnatal cardiovascular homeostasis. © 2016 Wiley Periodicals, Inc.

Bortezomib use in a pediatric cardiac transplant center

Data are limited on the efficacy and safety of bortezomib for the treatment of AMR following OHT for pediatric acquired or CHD. Retrospective chart review identified patients who received bortezomib for acute (n = 3, within two wk of diagnosis) and chronic (n = 1, three months after diagnosis) AMR or as part of a desensitization regimen (n = 1). Bortezomib was associated with a 3-66% reduction in class I DSA and a 7-82% reduction in class II DSA. Two of the three acute AMR cases resolved by the first follow-up biopsy. Two patients with AMR resolution are currently well. One patient developed a second episode of AMR, which was unresponsive to bortezomib therapy and required retransplantation for progressive coronary allograft vasculopathy. One patient died shortly after the third cycle from multi-organ failure. The desensitization patient showed transient HLA reduction with two cycles, but died five months after transplant from sepsis. Complications included infection (3/5), peripheral neuropathy (2/5), AKI (2/5), and thrombocytopenia (3/5). Adverse events appear more common in critically ill patients. Bortezomib therapy resulted in variable DSA reduction and AMR resolution in AMR in OHT secondary to pediatric acquired or CHD.

Orthotopic heart transplant: a therapeutic option for unresectable cardiac fibroma in infants

Primary cardiac tumors are rare lesions in childhood, with the two most common being rhabdomyoma and fibroma. We report two infants who successfully underwent orthotopic heart transplant for massive interventricular septal cardiac fibromas. For unresectable infantile cardiac fibroma, orthotopic heart transplant may be considered a therapeutic option.

Perioperative renal failure in pediatric heart transplant recipients: outcome and risk factors

PRF is encountered in 10-13% of adult heart transplants. Only one study of a single center's experience with PRF has been reported in pediatric patients. This study examines the effect of PRF on pediatric heart transplant outcome using the UNOS database. A total of 3598 patients met inclusion criteria, of whom 254 (7%) had PRF. The PRF group comprised 31 recipients requiring PRE and 223 recipients requiring POST. Compared with No-PRF patients, PRE patients had similar survival rate and POST patients had decreased survival rate at 30 days, one, five, and 10-yr post-transplant (p < 0.001). PRF patients also had significantly lower graft survival at one, five, and 10 yr (p < 0.001). Risk factors for developing PRF included ECMO, ventilator, and inotropic support at listing and CHD as the listing diagnosis. PRF increased the duration of hospital stay and the incidence of chronic severe renal dysfunction. PRF that requires POST (whether or not it began pretransplant) has a significant negative impact on pediatric heart transplant outcome. Specific characteristics identify patients at particular high risk of developing PRF.

Effect of hypothermia on doxorubicin-induced cardiac myoblast signaling and cell death

BACKGROUND

Anthracyclines (AC) are useful chemotherapeutic agents whose principal limitation is cardiac toxicity, which may progress to heart failure, transplantation or even death. We have shown that this toxicity involves oxidative stress-induced activation of the DNA damage pathway. Hypothermia has been shown to be protective against other diseases involving oxidative stress but has not been studied in models of AC toxicity.

METHODS

In the current experiments, H9C2 cardiac myoblasts were treated with varying concentrations of the AC doxorubicin (DOX) during normothermia (37°C) or mild hypothermia (35°C). Total cell death was assayed using trypan blue exclusion and apoptosis by terminal deoxynucleotidyl transferase-mediated deoxyuridine-biotin nick end labeling (TUNEL) staining. Oxidative stress was assayed using the fluorescent indicator 2'7'-dichlorofluorescein diacetate. DNA damage pathway activation was assayed by immunostaining for H2AX and p53. Mitochondrial membrane potential was assayed by JC-1 staining.

RESULTS

At all concentrations of DOX examined (1, 2.5 and 5 μM), hypothermia reduced oxidative stress, activation of H2AX and p53, loss of mitochondrial membrane potential and total and apoptotic cell death (P=.001-.03 for each observation).

CONCLUSIONS

The reduction of oxidative stress-induced activation of the DNA damage pathway and consequent cell death by mild hypothermia supports a possible protective role to reduce the clinical impact of DOX-induced cardiac toxicity. Such an approach may allow expanded use of these effective chemotherapeutic agents to increase cancer cure rates.

Low donor-to-recipient weight ratio does not negatively impact survival of pediatric heart transplant patients

A major limitation to success in pediatric heart transplantation is donor organ shortage. While the use of allografts from donors larger than the recipient is accepted, the use of undersized donor grafts is generally discouraged. Using the UNOS database, we wanted to evaluate whether using smaller donor hearts affects the short- and long-term survival of pediatric heart transplant patients. A retrospective analysis of data entered into the UNOS database from April 1994 to May 2008 was performed. Pediatric heart transplant recipients (ages 0-18 yr) with DRWR <2.0 were identified and divided into two groups: Low-DRWR (<0.8) and Ideal-DRWR (0.8-2.0). Patients' demographics, pretransplant diagnoses, age at transplantation, severity of pretransplant condition, and rate of complications prior to hospital discharge after transplantation were noted. Fisher's exact, chi-square, and Wilcoxon rank sum tests were used to compare patients' baseline characteristics. Kaplan-Meier curves and Cox proportional hazard regression were used to compare patients' survival and to identify independent risk factors for outcomes. There were 3048 patients (204 with Low- and 2844 with Ideal-DRWR). The Low-ratio group patients were older (8.3 vs. 6.9 yr; p = 0.001), there was a slight male predominance in the Low-DRWR group (p = 0.055). The Low-DRWR group had longer transplant wait time than the Ideal-DRWR group (97 vs. 85 days; p = 0.04). The groups did not differ in race, primary diagnoses, severity of pretransplant condition (medical urgency status, need for ventilation, inotropic support, ECMO, nitric oxide, or dialysis, the PVR for those with bi-ventricular anatomy), or post-transplant complications (length of stay, need for inotropic support, dialysis, and rate of infections). The Low-DRWR patients had less episodes of acute rejection during the first-post-transplant month. Infants with DRWR 0.5-0.59 had lower 30-day survival rate (p = 0.045). There was no difference in short- and long-term survival between the patients with DRWR 0.6-0.79 and DRWR 0.8-2.0. Use of smaller allografts (DRWR 0.6-0.8) has no negative impact on the short- and long-term survival of pediatric heart transplant patients.

Identification and cloning of a new protein that binds the 3(') untranslated region of alpha-striated tropomyosin

The 3' untranslated region of muscle tropomyosin (TM UTR) induces muscle differentiation when transcribed in primary fibroblasts. This sequence binds protein in extracts from cell types that differentiate upon TM UTR transcription. To identify the protein(s) bound by the TM UTR, an avian embryo fibroblast library was induced to express protein in solution and extracts from these pools were screened with electromobility shift assays using a TM UTR RNA probe. Positive pools were progressively fractionated until a pool containing a single positive clone was obtained. The TM UTR-binding protein (UBP) clone thus isolated contains 751 nt, 618 of which represent a single open reading frame. UBP is related to a human autoantigen, Sjogren's syndrome antigen B (SSB) beginning with the start of the UBP open reading frame. This homology is to the 5' end of SSB in a region containing an RNA-binding motif of 70 amino acids. The deduced amino acid sequence of UBP predicts phosphorylation sites for protein kinase C, casein kinase 2, and cAMP-dependent protein kinase and asparginine glycosylation sites. The observed size of UBP by UV cross-linking with a TM UTR probe is of the same size as the protein bound in fibroblast extract. UBP is expressed in primary fibroblasts, but not in fibroblast or myogenic cell lines, suggesting that its expression is restricted. The full-length UBP mRNA is approximately 3 kB, suggesting a long 5' untranslated region. Transient transfection of cultured cells with UBP directs production of a protein that binds the TM UTR, confirming that these sequences interact in vivo. These observations suggest that we have identified a novel protein that binds to the TM UTR in vitro and in vivo. Determining the function of this protein will facilitate determining the mechanism by which the TM UTR induces differentiation.

Myocardial infarction due to coronary abnormalities in pulmonary atresia with intact ventricular septum

We describe the clinical course, angiography, and histopathology of a newborn male with pulmonary atresia and intact ventricular septum who succumbed to a myocardial infarction. Angiography demonstrated right ventricular-dependent coronary circulation and focal areas of coronary narrowing. His clinical course was characterized by attacks of sudden irritability, consistent with ischemia. Histology demonstrated significant coronary artery narrowing secondary to fibromuscular dysplasia as well as evidence of new and old infarction. This case illustrates the severity of coronary lesions in pulmonary atresia and the potential for progression of coronary obstruction and insufficiency, and it provides correlation between angiography, ischemic symptoms, and pathology.

Molecular basis of anthracycline-induced cardiotoxicity and its prevention

Anthracyclines are a class of highly potent antitumor antibiotics utilized against hematologic and solid tumors in children and in adults. Their use has been limited primarily by their cardiotoxic side effects, which may lead to congestive heart failure. Although there is a linear relationship between the cumulative dose received and the incidence of cardiotoxicity, in some patients cardiotoxicity may develop at doses below the generally accepted threshold level. Anthracycline-induced cardiotoxicity is believed to be related to the generation of highly reactive oxygen species, which, by means of membrane lipid peroxidation, cause direct damage to cardiac myocyte membranes. Another important factor may be the relatively poor antioxidant defense system of the heart. In an attempt to circumvent these toxic effects, a wide variety of antioxidants have been used in cell culture, animal, and human studies without consistent beneficial effects. Moreover, none of the agents used to date are designed to act selectively upon the heart. If the cardiac complications resulting from anthracyclines could be reduced and/or prevented, higher doses could potentially be used, thereby increasing cancer cure rates. Furthermore, the incidence of cardiac toxicity resulting in congestive heart failure or even heart transplantation would be reduced, therefore increasing the quality and extent of life for cancer survivors. This article will review the basic science of free radical biology, the biology of oxygen-derived free radicals and antioxidant proteins, and explore some new and innovative approaches to limiting and/or preventing anthracycline-induced cardiotoxicity.

Misdirected vimentin messenger RNA alters cell morphology and motility

Localized messenger RNAs were first observed as embryonic determinants that altered development when mislocalized. In recent years localized mRNAs have been found for several cytoskeletal proteins, including actin, vimentin and several microtubule associated proteins. We sought to determine whether redirecting mRNA for a cytoskeletal protein to an inappropriate address would alter cellular phenotypes. To do so we generated vimentin mRNAs with a myc epitope tag and the (beta)-actin 3′ untranslated region (3′ UTR) as a localization signal. When misdirected vimentin mRNAs are expressed in either fibroblasts or SW13 cells, cells develop numerous, extremely long processes; these cells also move more slowly to enter a wound of the monolayer. In situ hybridization revealed that the misdirected mRNA was often localized in the processes, in contrast to endogenous vimentin mRNA. The processes usually contained actin distal to the transgenic vimentin and microtubules proximal to it. SW13 cells lacking vimentin produced fewer and shorter processes, suggesting a dominant negative effect that involves recruitment of endogenous vimentin. Control experiments that transfected in constructs expressing tagged, correctly localized vimentin, or (beta)-galactosidase that localized through the (beta)-actin 3′ UTR, indicate that neither the shape nor the motility changes are solely due to the level of vimentin expression in the cell. This is direct evidence that the site of expression for at least one cytoskeletal mRNA alters the phenotype of the cell in which it is expressed. Messenger RNA localization is proving to be as essential for the normal maintenance of somatic cell phenotypes as embryonic determinants are for embryogenesis.

Protein binding by the 3' untranslated region of alpha-striated tropomyosin

Tropomyosin is a component protein of the thin filament system in striated muscle, regulating the interaction between actin and myosin. The 3' untranslated region of the alpha-striated tropomyosin gene (TM UTR) induces muscle differentiation when expressed in primary fibroblasts, but the mechanism has not been defined. We hypothesize that fibroblasts utilize resident proteins to effect this response, perhaps by TM UTR binding to protein(s). In order to facilitate identification of protein(s) involved in mediating this differentiation response, we investigated the potential for this sequence to bind to cellular protein utilizing electrophoretic mobility gel shifting analysis (EMSA) with and without UV cross-linking. Under very specific conditions (including pH, KCl, and Mg concentration and extent of phosphorylation of protein), the TM UTR is able to bind protein in cells that differentiate upon TM UTR expression. Protein binding is significantly more extensive in cytoplasmic than nuclear protein preparations. Secondary structure of the RNA probe facilitates protein binding. The molecular masses of bound proteins are approximately 42 and 115 kDa under basal conditions. EMSA analysis of extract from cultured skeletal muscle confirms that protein binding by the TM UTR occurs in this cell type, and is more extensive in less differentiated cells. The demonstration of highly regulated protein binding by the TM UTR raises the possibility that this sequence may cause differentiation by binding to endogenous proteins, and further that this sequence may play a role in normal differentiation. Identification of proteins bound by the TM UTR will be necessary to completely define the mechanism by which it causes differentiation.

Percutaneous pulmonary artery gene transfer in pigs using naked plasmid DNA delivered during angioplasty

Gene transfer techniques are increasingly being used to study blood vessel biology and develop models for gene therapy. To date, there are no reports of pulmonary vascular gene transfer performed either without adjunctive agents or during angioplasty. We sought to demonstrate the feasibility of recombinant gene transfer to the pulmonary artery of juvenile pigs using naked plasmid DNA delivered via percutaneous angioplasty techniques. Plasmid DNA directing the expression of beta-galactosidase was used to transfect one pulmonary artery while the contralateral vessel served as an untreated control. One delivery technique used a standard angioplasty balloon coated with a DNA-heparin mixture. The second involved infusion of DNA between an angioplasty balloon and a surrounding, microporous balloon. Vessels were harvested 3 or 4 days following gene delivery. Protein expression was demonstrated by immunohistochemical staining in transfected but not control vessels in 9/9 pigs. Vascular wall expression was limited to endothelial cells. Pulmonary artery gene transfer using naked plasmid DNA delivered via percutaneous angioplasty techniques is feasible. Using naked plasmid DNA removes the potential for toxicity associated with adjunctive agents. The described techniques provide novel methods for studying pulmonary vascular biology in vivo and for developing future gene therapies.

Assembly of tropomyosin isoforms into the cytoskeleton of avian muscle cells

Tropomyosin (TM) is a component of microfilaments of most eukaryotic cells. In striated muscle, TM helps confer calcium sensitivity to the actin-myosin interaction. TM is a fibrillar, self-associating protein that binds to the extended actin filament system. We hypothesized that these structural features would permit TM to undergo assembly into the cytoskeleton during translation, or cotranslational assembly. Pulse-chase experiments with [35S]methionine and pulse experiments with [3H]puromycin followed by extraction and immunoprecipitation of TM were performed to examine the mechanism of assembly of TM into the cytoskeleton in cultured avian muscle cells. Pulse-chase experiments provide kinetic evidence for cotranslational assembly of TM in skeletal and cardiac muscle. Demonstration of a large majority of completed TM on purified skeletal muscle microfilaments after a short labeling period confirms that these kinetic data are not related to trapping of TM within the actin network of the cytoskeleton. Nascent TM peptides are demonstrated on the cytoskeleton of muscle cells after a short metabolic pulse followed by puromycin treatment to release nascent peptides from ribosomes or after labeling with [3H]puromycin. Nascent chain localization to the cytoskeleton independent of ribosomal attachment further confirms the high degree of cotranslational assembly of this protein. The extent of cotranslational assembly is similar before and after the formation of significant myofibril in myotubes, suggesting that cotranslational assembly of TM is active during contractile apparatus assembly in muscle differentiation. This is the first report where assembly mechanism has been predicted to be cotranslational based upon structural features of a cytoskeletal protein.

Transdifferentiation of chicken embryonic cells into muscle cells by the 3' untranslated region of muscle tropomyosin

Transfection with a plasmid encoding the 3' untranslated region (3' UTR) of skeletal muscle tropomyosin induces chicken embryonic fibroblasts to express skeletal tropomyosin. Such cells become spindle shaped, fuse, and express titin, a marker of striated muscle differentiation. Skeletal muscle tropomyosin and titin organize in sarcomeric arrays. When the tropomyosin 3' UTR is expressed in osteoblasts, less skeletal muscle tropomyosin is expressed, and titin expression is delayed. Some transfected osteoblasts become spindle shaped but do not fuse nor organize these proteins into sarcomeres. Transfected cells expressing muscle tropomyosin organize muscle and nonmuscle isoforms into the same structures. Thus, the skeletal muscle tropomyosin 3' UTR induces transdifferentiation into a striated muscle phenotype in a cell-type-specific context.

Specific and quantitative immunoprecipitation of tropomyosin and other cytoskeletal proteins by magnetic separation

Immunoprecipitation is a powerful technique for purifying many proteins for which specific antibodies exist. Magnetic separation has recently been demonstrated to be effective in the immunoprecipitation of cell-surface proteins. We have used magnetic separation with anti-immunoglobulin or protein A bound to magnetic particles to immunoprecipitate labeled muscle tropomyosin and several other cytoskeletal proteins for which specific antibodies exist. We have not found it necessary to bind antigen-specific antibody to the magnetic particles, increasing the versatility of the technique. The quantitative recovery of tropomyosin from muscle cultures using magnetic separation is superior to Staph A (protein A-positive Staphylococcus aureus cells). The specificity of magnetic separation also compares favorably with Staph A for immunoprecipitation of muscle tropomyosin. Fibroblast tropomyosin, vimentin (from muscle and osteoblast) and myosin heavy chain are other cytoskeletal proteins that are easily recovered with magnetic separation. Magnetic separation, therefore, appears to be a valuable technique for the immunoprecipitation of cytoskeletal proteins from various cell types.

Thin filament changes during in vivo rat heart development

Developmental differences in myocardial performance are known to exist. It is likely that the profile of protein isoforms present on the developing thin filament contributes to these observed differences. We have prepared thin filaments from developing and mature rat hearts by using an immunoprecipitation procedure developed in our laboratory. Analysis of these isolated thin filaments by Western immunoblots and two-dimensional gel electrophoresis demonstrates troponin I and troponin T isoform switching on the developing thin filament. Troponin I isoform switching begins by embryonic d 18 and is complete before the 3rd postnatal wk. Troponin T isoform switching begins between embryonic d 18 and birth and is complete between the 2nd and 3rd postnatal wk. The degree of phosphorylation of tropomyosin in thin filaments appears to be developmentally regulated, decreasing with advancing age. Nonmuscle isoforms of tropomyosin are also detectable in thin filaments from developing and mature rat hearts. These phenomena (troponin isoform switching, the degree of phosphorylation of tropomyosin, and the presence of nonmuscle isoforms of tropomyosin on cardiac thin filaments) likely play a role in the function of immature thin filaments and in the assembly of mature thin filaments.