Triglyceride deposit cardiomyovasculopathy: a rare cardiovascular disorder

TG is a major energy source for mammals. In normal condition, TG is either received via the diet, or synthesized endogenously and stored in adipose tissues. When required, TG is hydrolyzed by various enzymes called lipases and releases long-chain fatty acid (LCFA), which is delivered to non-adipose tissues for the production of ATP. It has been known that the ectopic TG deposition in non-adipose tissues causes some orphan diseases. In 1953, Jordans reported two brothers with phenotype of skeletal myopathy and vacuolar formation of peripheral leukocytes, called Jordans’ anomaly [1]. Fifty years later, Fischer et al. found that this phenotype is associated with mutations in PNPLA2 [2] encoding adipose TG lipase (ATGL) [3, 4], an essential molecule located in cytoplasmic lipid droplets for the intracellular TG hydrolysis [5, 6], and designated this phenotype as neutral lipid storage disease with myopathy (NLSD-M). Clinical manifestations of NLSD-M appeared variable from mild to severe symptoms [7‐13], which could be at least partially explained by function of mutated ATGL proteins [14]. Another phenotype of NLSD involving the skin was reported as NLSD with ichthyosis (NLSD-I) by Chanarin and Dorfman in the 1970s [15‐17]. 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 [19‐23].

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 [26‐28]. 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].

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. A 38-year-old man suddenly died 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.

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 [35‐37]. 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].

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) [26‐28]. 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 [40‐42].

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 accumulation of cholesterol-laden macrophages [45] (Fig. 4). We reported that TG-deposit SMCs and endothelial cells had pro-inflammatory and vulnerable phenotype in vitro [46, 47].

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).

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.

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.

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 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.

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.

The following points are important focus for future researches:

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