Triglyceride deposit cardiomyovasculopathy: a rare cardiovascular disorder

Triglyceride deposit cardiomyovasculopathy (TGCV) is a phenotype primarily reported in patients carrying genetic mutations in PNPLA2 encoding adipose triglyceride lipase (ATGL) which releases long chain fatty acid (LCFA) as a major energy source by the intracellular TG hydrolysis. These patients suffered from intractable heart failure requiring cardiac transplantation. Moreover, we identified TGCV patients without PNPLA2 mutations based on pathological and clinical studies. We provided the diagnostic criteria, in which TGCV with and without PNPLA2 mutations were designated as primary TGCV (P-TGCV) and idiopathic TGCV (I-TGCV), respectively. We hereby report clinical profiles of TGCV patients. Between 2014 and 2018, 7 P-TGCV and 18 I-TGCV Japanese patients have been registered in the International Registry. Patients with I-TGCV, of which etiologies and causes are not known yet, suffered from adult-onset severe heart disease, including heart failure and coronary artery disease, associated with a marked reduction in ATGL activity and myocardial washout rate of LCFA tracer, as similar to those with P-TGCV. The present first registry-based study showed that TGCV is an intractable, at least at the moment, and heterogeneous cardiovascular disorder.

. Another phenotype of NLSD involving the skin was reported as NLSD with ichthyosis (NLSD-I) by Chanarin and Dorfman in the 1970s [15][16][17]. The genetic cause of NLSD-I was found to be mutations in ABHD5 encoding CGI-58, a co-enzyme of ATGL [18]. Using skin fibroblasts and iPS cells from patients with NLSDs, unique intracellular metabolism of TG has been extensively analyzed. These cell-biological experiments showed that cytoplasmic lipid droplets are dynamic cellular organelles interacting with ATGL, CGI-58, and other proteins, and could be a therapeutic target [19][20][21][22][23].
Discovery of TG-deposit cardiomyovasculopathy (TGCV) with PNPLA2 (ATGL) mutation Since the early 1980s, patients with Jordans' anomaly and severe heart failure (HF), though very rare, had been reported in Japan [24]. In the early 2000s, our institution started to take care of two patients with severe HF and vacuolar formation in peripheral leukocytes. HF was progressive and intractable, and a couple of years later, they became candidates for cardiac transplantation (CTx). Preoperative examination of their hearts exhibited dilated cardiomyopathy-like morphology in chest X-ray and ultrasonography; however, endomyocardial biopsy specimens showed neutral lipid deposition in cardiomyocytes [25]. When they underwent CTxs, pathological and biochemical analyses of their explanted hearts were performed, demonstrating that their coronary arteries showed unusual coronary atherosclerosis with TG deposition in endothelial and smooth muscle cells (SMCs). We named this novel phenotype as TGCV [26][27][28]. These patients were identified as homozygous for genetic mutations in PNPLA2 encoding ATGL, which is also known to be responsible for NLSD-M as described above [2].

Postmortem analyses revealed undiagnosed individuals with TGCV
Retrospective postmortem analyses of autopsied cases identified individuals with TGCV phenotype who had TG deposit in both myocardium and coronary arteries, as presented in Fig. 1 irrespective of intensive treatment for coronary artery disease (CAD) and HF. His heart was heavy in weight and hypertrophied with multiple myocardial fibrous scars. Coronary arteries showed diffuse and concentric stenosis in multi-vessels. Biochemical analyses and imaging mass spectrometry showed TG deposition in both myocardium and coronary arteries [29,30]. TG-deposit SMCs were observed in his renal and mesenteric arteries as well (data not shown). These data mimic genetic ATGL deficiency; however, the immunoreactive mass of ATGL was detected, and the genetic test using genomic DNA extracted from stored specimens showed no mutation in all exons and exon/intron boundaries of PNPLA2 gene (data not shown). In addition, pathological records showed that he did not have skeletal myopathy.

Development of diagnostic methods for TGCV
The above postmortem studies suggested that it is difficult to diagnose TGCV, and many undiagnosed patients should have died, which motivated us to develop diagnostic tools and methods for TGCV. We reported that myocardial scintigraphy with iodine-123-β-methyl iodophenyl-pentadecanoic acid (BMIPP) [31,32], a radioactive analogue of LCFA, was useful in detecting abnormal LCFA metabolism in patients with TGCV [33,34]. In addition, we reported the use of automated hematology analyzers to detect Jordans' anomaly in patients with PNPLA2 mutation [35][36][37]. Recently, we developed CT-based TG imaging to detect myocardial and coronary TG deposition [34,38] and selective immunoinactivation assay to measure functional ATGL activities using peripheral leukocytes [39].

Nomenclature, definition, and classification of TGCV
It is well known that disease nomenclature is made not only by their genotypes, but also by their phenotypes in many diseases and by discoverer's names in some diseases. The nomenclature of TGCV was made by its phenotype that TG accumulated in both myocardium and coronary arteries, resulting from abnormal intracellular metabolism of TG and LCFA ( Fig. 2) [26][27][28]. ATGL is a known enzyme involved in the phenotypic expression of TGCV. The Japan TGCV study group provided the diagnostic criteria for TGCV, in which TGCV with and without PNPLA2 mutations was designated as primary TGCV (P-TGCV) and idiopathic TGCV (I-TGCV), respectively [40][41][42].

Pathophysiology of TGCV
The pathophysiological schema of TGCV is shown in Fig. 3. In normal condition (left panel, Fig. 3), LCFAs are taken up through transporters and receptors such as CD36. Some are transported to the mitochondria for β-oxidation, and the remaining LCFAs are utilized as a source of TG and rapidly hydrolyzed by intracellular lipases such as ATGL. In TGCV (right panel, Fig. 3), LCFAs are taken up and used to synthesize TG that cannot be hydrolyzed due to ATGL insufficiency, leading to energy failure and lipotoxicity with massive TG accumulation [28,43]. It is emphasized that TG-deposit atherosclerosis is an important characteristic of TGCV [44] and distinct from usual cholesterol-deposit atherosclerosis, because the former showed diffuse and concentric narrowing formed by TG-deposit SMCs, whereas the latter showed discrete and eccentric stenosis initiated by the response to injury in the endothelium and  4). We reported that TG-deposit SMCs and endothelial cells had pro-inflammatory and vulnerable phenotype in vitro [46,47].

A clinical case presentation of I-TGCV
A 58-year-old woman was referred to our hospital due to sudden chest tightness with ST-segment elevation in the electrocardiogram, followed by cardiopulmonary arrest. Under the diagnosis of acute myocardial infarction, she underwent coronary artery bypass grafting (CABG). Past history included type 2 diabetes mellitus requiring insulin treatment and hemodialysis. Cytoplasmic vacuoles in her peripheral polymorphonuclear leukocytes were observed less frequently (< 10% of neutrophils), compared with that in genetic ATGL deficiency (panel A in Fig. 5). ATGL activity in peripheral leukocytes was very low, comparable to that of genetic ATGL deficiency, as shown in Table 1. Myocardial washout rate (WOR) of BMIPP was defective in scintigraphy (panel B in Fig. 5). Pathological analyses of endomyocardial biopsy specimens demonstrated numerous vacuoles filled with stained lipid but positive reactivity for ATGL in cardiomyocytes and adipocytes (right, panel C in Fig. 5). Coronary CT angiogram showed diffuse narrowing coronary arteries, and in TG imaging [25], outside-in involvement of diffuse and abundant lipid components expressed as low CT numbers was seen within the wall in a peninsular pattern (arrows in panel D in Fig. 5). Her laboratory data and imaging tests were similar to those observed in TGCV with genetic ATGL deficiency, except for the conserved expression of ATGL protein in the myocardium. However, it is noted that the case was clinically distinct from genetic ATGL deficiency because there was no skeletal myopathy and no elevation of MM type creatine kinase. Genetic tests showed no mutations or substitutions in any of the exons or intron/exon boundaries of genes encoding ATGL, 1-acylglycerol-3-phosphate O-acyltransferase, hormone-sensitive lipase, or GOS2 (data not shown).
Clinical characteristics of P-and I-TGCV Table 1 shows the clinical characteristics of 7 and 18 patients with P-and I-TGCV, respectively, registered to the international registry for NLSD and TGCV between February 2014 and March 2018 in Japan. Both TGCV types were adult onset with chest pain at rest or dyspnea and palpitation. Most patients with either types of TGCV developed severe HF or CAD with diffuse narrowing multivessel lesions or both. Myocardial metabolism of LCFA, detected by WOR of BMIPP and ATGL activities in peripheral leukocytes, was reduced in both TGCV types. Most patients with P-TGCV developed intractable and critical HF, as reported recently [26,48,49]. Two of them underwent CTx [26,48]. Many patients with I-TGCV required percutaneous coronary intervention and CABG. As comorbidity, neither type of TGCV had skin lesions, which suggests that TGCV is not associated with NLSD-I. All patients with P-TGCV had skeletal myopathy, whereas none of those with I-TGCV did. Five of 7 and 3 of 18 registered patients with P-and I-TGCV, respectively, died.

Differential diagnosis of TGCV
Myocardial disorders such as dilated cardiomyopathy, hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, mitochondrial cardiomyopathy, alcoholic heart disease, and metabolic myocardial disorders (e.g., Fabry disease, Pompe disease, cholesteryl ester storage disease) need to be differentiated from TGCV [41,42]. Furthermore, known diabetic and metabolic heart diseases need to be differentiated from TGCV. One is diabetic cardiomyopathy, which was originally defined as cardiomyopathy without significant stenosis in epicardial coronary arteries [50]. Another concept is epicardial fat accumulation, which is the oeverdeposition of TG in physiological tissues. TGCV is distinct from these two entities because TGCV is characterized by the ectopic deposition of TG in the cardiomyocytes and SMCs with apparent involvement of epicardial coronary arteries, as shown in Figs. 1 and 5.

Academia-initiated development of specific treatment for TGCV
We found that the chow with tricaprin, TG form of capric acid, improved LCFA metabolism, lipid deposition, cardiac function, and life span in ATGL-targeted mice [4], raising a therapeutic hypothesis that capric acid may be an alternative energy source and reduce TG deposition and lipotoxicity in TGCV [51]. Based upon Fig. 4 Schemes for cholesterol-(Left) and TG-deposit atherosclerosis (Right). In cholesterol-deposit atherosclerosis, cholesterol (green) accumulates in macrophages, leading to eccentric stenosis. In TGdeposit atherosclerosis, TG (red) accumulates in SMCs, leading to concentric stenosis, which is a major feature of TGCV these data, the Osaka University Hospital manufactured GMP-graded capsules containing the active gradients called CNT-01. We developed the assay to measure plasma capric acid levels [52,53]. After finishing toxicity tests using rats and dogs required, we are finally conducting investigator-initiated clinical trials.

Comparison between NLSD-I, NLSD-M, and TGCV
As mentioned above, the nomenclature of TGCV was made by its phenotype that TG accumulated in both myocardium and coronary arteries, resulting from abnormal intracellular metabolism of TG and LCFA (Figs. 2 and 3). As described in the first paragraph in this letter, there have been known related disorders; NLSD-M and NLSD-I. Figure 6 shows the comparison of phenotype and genotype between TGCV and NLSDs. NLSD-M and NLSD-I are caused by mutations in PNPLA2 and ABHD5, mainly involved in the skeletal muscle and skin, respectively. Genotype of P-TGCV is known to be PNPLA2 mutation which is responsible for NLSD-M as well.  (before, after registration) (3, 2) (2, 1) *Three patients with P-TGCV and fourteen with I-TGCV were enrolled We did not have opportunity for the measurement in the remaing four patients with P-TGCV and four with I-TGCV **Two patients were dismissed before the genetic analysis The Japan TGCV study group certified I-TGCV according to the diagnostic guideline Abbreviations: CT Computed tomography, CVD Cardiovascular disease, TGCV Triglyceride deposit cardiomyovasculopathy Issues to be resolved The following points are important focus for future researches: 1. Possible clinical continuum between P-TGCV and NLSD-M As mentioned above, both P-TGCV and NLSD-M is caused by genetic ATGL deficiency. It would be of interest to know whether patients with NLSD-M have TG-deposit atherosclerosis, which is the important feature for P-TGCV. 2. Etiologies of I-TGCV and its prevalence in countries other than Japan As shown in Table 1, 13 out of 18 patients with I-TGCV had family history of cardiovascular disease, suggesting that any genetic factors might be involved in the pathogenesis of I-TGCV. The mechanism underlying downregulation of ATGL activities of I-TGCV and possible involvement of other lipases and related enzymes is of significance to elucidate. In order to elucidate these issues, the development of screening methods for the diagnosis of I-TGCV is under way in our laboratory.

Conclusions
TGCV is a severe cardiovascular disorder named by its phenotype of cardiomyovascular TG deposition, of which etiologies seem heterogeneous.

Availability of data and materials
The datasets generated and/or analyzed in the current study are available from the corresponding author on reasonable request.
Authors' contributions ML wrote the manuscript and contributed to data analyses and discussion. KH designated the research concept and contributed to the discussion and writing of the manuscript. YoI performed autopsy for the undiagnosed case ( Fig. 1), collected data, and contributed to the discussion and writing of the manuscript. KH and JK took care of the clinically identified I-TGCV case (Fig. 5). CH, BZ, JK, KSu, HM, ASu, YH, AT, YaI, KKo, MH, YF, NZ, SYamag, RS, and SYamad collected data and contributed to the discussion. HNak, KaK, E S, S-PH, YNak, TIn, YS, Y Yasui, YNag, ASa, SK, KSh, HH, DN, TId, TA, HNai, and HNag interpreted and discussed the data. KuK designated the registry, collected data, and contributed to the discussion. All the authors have read and revised the manuscript and approved the final manuscript.

Ethics approval and consent to participate
The protocol of the international registry was approved by the Osaka University Hospital Ethical Committee (Approved No. 13204).

Consent for publication
Not applicable.

Competing interests
The authors declare that they have no competing interests.

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