By analyzing the plasma metabolome of HCM patients before and after surgical myectomy, we have demonstrated, for the first time, that the plasma metabolomic profiles of HCM patients exhibit measurable, important changes in the postoperative state reflective of improvement in organ metabolic function. Although patient metabolic profiles sorted more by patient identity than by operative state, as is usually the case in patient studies of this nature, and we did not find dominant biomarkers that clearly separated the preoperative from the postoperative state in all patients, we were still able to identify broad shifts in metabolite patterns that inform potential metabolic changes in HCM patients after myectomy surgery. Specifically, we were able to identify trends in bilirubin, arginine derivatives, phospholipids and other metabolites that suggest important physiological changes as outlined below.
Bilirubin, biliverdin and urobilin are products of heme metabolism via HO and BR enzymes, which concludes with the conjugation of unconjugated bilirubin in hepatocytes. Increased serum levels of bilirubin are generally associated with decreased overall liver function, and bilirubin is a well-established clinical biomarker for liver function . In our study, plasma bilirubin showed a fold-change of 0.63 (p-value < 0.0001, q-value < 0.05) following surgical myectomy, which demonstrates that plasma levels of bilirubin are decreased in the postoperative state. These data therefore may suggest decreased HO or BR enzyme activities but may also indicate an increase in biliary and urinary excretion, potentially indicating improved liver and kidney function. Another indication of increased kidney function may be the decrease in a group of acetylated peptides, specifically phenylacetylglutamine, a biomarker of urea cycle disorders that is normally excreted via urine .
Homoarginine and dimethylarginine are components of the urea cycle, important for removal of nitrogenous waste product. Homoarginine can mediate inhibition of arginase activity and inflammation . In addition, homoarginine may support nitric oxide (NO) synthesis by serving as a substrate for nitric oxide synthase (NOS) and by inhibiting arginase activity . Reduced homoarginine would be consistent with a possible decrease in NO production and inflammation following surgery. Consistent with this finding, we note that none of our 18 patients had increased vasopressor requirements in the postoperative period. We did not do any further studies to assess the relationship between blood pressure and arginine metabolites in our patients. Dimethylarginine has been linked to increased risk for heart failure in human studies  and reduction of this metabolite is consistent with improved cardiac function. Taken together, these data are consistent with improvement in markers of cardiac health and inflammation.
The observed increases in circulating phospholipid metabolites may reflect improved secretion of lipoproteins from the liver. Importantly, previous studies suggest that circulating levels of PC are lower in people with heart damage compared to healthy adults , thus the increases in the current study may reflect improved heart function. Cardiomyopathy can also result in the heart relying increasingly more on glucose utilization than on fatty acid oxidation for energy, hence a decrease in plasma BHBA may reflect an improvement in fatty acid usage by the heart. The concurrent reduction in circulating sphingolipid metabolites is also interesting, as circulating sphingolipids, especially ceramides, have been postulated to promote type 2 diabetes via pathways involved in insulin resistance, pancreatic islet β-cell dysfunction and inflammation . Patients with HCM who undergo surgery thus may also demonstrate improved endocrine function.
PFOS and PFOA are man-made chemicals that function as fluoro-polymers and are used for industrial purposes. These chemicals have both been found to have potentially adverse effects on liver function, and have been associated with increased levels of hepatocellular injury biomarkers in humans  as well as increased risk of chronic kidney disease . Here, we show that plasma levels of PFOA and PFOS decrease following surgical myectomy in HCM patients, with fold-changes of 0.77 (p-value < 0.0000001, q-value < 0.00005) and 0.82 (p-value < 0.0000001, q-value < 0.00005), respectively. Decreased plasma levels of these chemicals are consistent with improved liver and kidney function resulting in more effective clearance of these chemicals from the systemic circulation. HCM is known to confer risk of end stage renal disease , and myectomy may reduce this risk. HCM may impact the liver and kidney function of patients by inducing congestive hepatopathy and reduced renal blood flow at a subclinical level, as has been described for other cardiovascular diseases such as left heart failure, cardiomyopathy, and constrictive pericardial disease , and this impact is likely mitigated after surgical myectomy.
3,5-dichloro-2,6-dihydroxybenzoic acid has been reported as a biomarker for red meat and dairy intake . Our study showed that 3,5-dichloro-2,6-dihydroxybenzoic acid was decreased in plasma post-myectomy with a fold-change of 0.79 (p-value < 0.000001, q-value < 0.0005). We speculate that reduction in plasma 3,5-dichloro-2,6-dihydroxybenzoic acid is in the postoperative state is indicative of improved liver and kidney function resulting in more rapid clearance, although a change in dietary intake of red meat and dairy products cannot be ruled out.
2-hydroxylaurate is a lipid that is mainly important as chemical modification of lipopolysaccharide (LPS) and is specifically added to the endotoxic portion Lipid A used by all gram-negative bacteria as a virulence factor . LPS is cleared by the liver through portal circulation , suggesting a possible mechanism by which 2-hydroxylaurate appears in human plasma. Though studies of LPS clearance by the liver have not yet elucidated its metabolic mechanism, we propose that 2-hydroxylaurate may be a product of LPS breakdown. We demonstrate that 2-hydroxylaurate is decreased in plasma post-myectomy with a fold-change of 0.85 (p-value < 0.0001, q-value < 0.01). As with PFOS and PFOA, it is possible that 2-hydroxylaurate is decreased in the postoperative state because the liver is more efficiently clearing this metabolite.
Trigonelline (N-methylnicotinic acid) is a product of niacin metabolism and is found most prominently within coffee seeds, among other foods . Our study showed that trigonelline levels showed a fold change of 2.88 (p-value = 0.0004, q-value < 0.05) in the postoperative state, being the only identified metabolite to show a statistically significant increase in plasma level (Table 2). Trigonelline is also associated with lean soft tissue mass in cancer patients, which correlates with muscle mass . It is possible that the postoperative state is associated with improved lean soft tissue mass as a result of greater functional capacity, although a confounding factor such as recent coffee intake cannot be ruled out. Studies of trigonelline bioavailability have shown that maximum plasma levels of trigonelline occur around 2–3 h after coffee ingestion, with a half-life of approximately 5 h before being cleared in the urine [31, 32].
Curiously, five metabolites that were significantly downregulated in the postoperative state were unable to be structurally identified (Table 2). It is possible that, as with 2,3-dichloro-2,6-dihydroxybenzoic acid, there is little known about these metabolites because they may be byproducts of chemicals that have not yet been deeply investigated. Given that the other decreased metabolites that were successfully identified are cleared by the liver and/or kidney, we speculate that these metabolites could be other chemicals that are cleared by the liver and/or kidney. It will be necessary to further confirm the identity of these metabolites to validate this speculation and confirm their utility as biomarkers.
The overarching theme of our study is that metabolomic profiling suggests improvement in the metabolic function of various organs in HCM patients after myectomy to alleviate LVOT obstruction. These organs include the liver, kidney, heart, endocrine pancreas, cardiac and skeletal muscle. Although HCM has been associated with kidney and cardiac dysfunction [3, 24], associations with liver, pancreatic and skeletal muscle dysfunction in patients have not been described, although crosstalk between the liver and heart has been suggested in mouse models of HCM . A limitation of our study is that many confounding factors related to nutritional status, such as Body Mass Index, diet, herbal remedies, tobacco use, etc. can contribute to metabolomic profiles and were not assessed in our patients but were noted in the above discussion where relevant. It is also important to note that established biomarkers for kidney function (creatinine), liver function (AST and ALT), inflammation (C-reactive protein) and heart failure (Brain Natriuretic Peptide) were not different in HCM patients in the preoperative and postoperative states (Additional file 1: Table S1, Additional file 2: Table S2, line 210; ). These findings argue that the observed changes in metabolites are not simply related to improvement in heart failure, renal perfusion, liver perfusion or inflammatory state but may be unique to HCM patients. A corollary speculation could be that these metabolites may be more sensitive to low levels of heart failure, reduced organ perfusion and inflammation in HCM patients than established markers, but this hypothesis requires further testing and validation. Potential clinical relationships between HCM with LVOT obstruction and liver dysfunction, islet cell dysfunction or skeletal muscle dysfunction in patients remain to be explored in future studies.
Our study is a pilot study limited by small sample size, patient heterogeneity and the need for larger scale validation and mechanistic studies in animal and/or in vitro models. The patient heterogeneity results from selection solely based on need for a surgical myectomy and plan to follow up postoperatively at our institution, without further inclusion or exclusion criteria. Our study population is skewed towards female patients (67%), shows significant age heterogeneity and has a low prevalence of sarcomere mutations (17%), as a result. A future, larger study will try to balance sex, age and sarcomere mutation status to avoid bias and to allow stratification based on these characteristics. Some patients had concurrent coronary artery disease, atrial fibrillation and one even had aortic stenosis, necessitating additional surgical intervention beyond myectomy such as coronary artery bypass grafting, MAZE procedures and in one case an aortic valve replacement. These additional conditions and the additional surgical procedures may confound our analysis. A future study with narrowed selection criteria limiting the condition to uncomplicated HCM with LVOT obstruction may uncover additional metabolites and pathways. Another limitation of our study is that many confounding factors related to nutritional status, such as Body Mass Index, diet, herbal remedies, tobacco use, etc. can contribute to metabolomic profiles. Information about these confounding factors was not assessed in our patients but was noted in the above discussion where relevant.