Mitochondria Matter: Systemic Aspects of Nonalcoholic Fatty Liver Disease (NAFLD) and Diagnostic Assessment of Liver Function by Stable Isotope Dynamic Breath Tests

The liver plays a key role in systemic metabolic processes, which include detoxification, synthesis, storage, and export of carbohydrates, lipids, and proteins. The raising trends of obesity and metabolic disorders worldwide is often associated with the nonalcoholic fatty liver disease (NAFLD), which has become the most frequent type of chronic liver disorder with risk of progression to cirrhosis and hepatocellular carcinoma. Liver mitochondria play a key role in degrading the pathways of carbohydrates, proteins, lipids, and xenobiotics, and to provide energy for the body cells. The morphological and functional integrity of mitochondria guarantee the proper functioning of β-oxidation of free fatty acids and of the tricarboxylic acid cycle. Evaluation of the liver in clinical medicine needs to be accurate in NAFLD patients and includes history, physical exam, imaging, and laboratory assays. Evaluation of mitochondrial function in chronic liver disease and NAFLD is now possible by novel diagnostic tools. "Dynamic" liver function tests include the breath test (BT) based on the use of substrates marked with the non-radioactive, naturally occurring stable isotope 13C. Hepatocellular metabolization of the substrate will generate 13CO2, which is excreted in breath and measured by mass spectrometry or infrared spectroscopy. Breath levels of 13CO2 are biomarkers of specific metabolic processes occurring in the hepatocyte cytosol, microsomes, and mitochondria. 13C-BTs explore distinct chronic liver diseases including simple liver steatosis, non-alcoholic steatohepatitis, liver fibrosis, cirrhosis, hepatocellular carcinoma, drug, and alcohol effects. In NAFLD, 13C-BT use substrates such as α-ketoisocaproic acid, methionine, and octanoic acid to assess mitochondrial oxidation capacity which can be impaired at an early stage of disease. 13C-BTs represent an indirect, cost-effective, and easy method to evaluate dynamic liver function. Further applications are expected in clinical medicine. In this review, we discuss the involvement of liver mitochondria in the progression of NAFLD, together with the role of 13C-BT in assessing mitochondrial function and its potential use in the prevention and management of NAFLD.

First intraoperative measurement of liver functional capacity during liver surgery using the 13 C-methacetin breath test: early results of a pilot study

Highlight Postoperative liver failure remains one of the most dreadful complications of modern liver surgery. In search of a "real-time" measurement of liver functional capacity, Makridis and colleagues explored the intraoperative application of the 13C-methacetin breath test during major liver resection and report the preliminary results of this pilot study.

Potential use of metabolic breath tests to assess liver disease and prognosis: has the time arrived for routine use in the clinic?

The progression of liver disease may be unique among organ system diseases in that progressive fibrosis compromises not only the sufficiency of hepatocyte mass but also impairs blood flow to the liver, resulting in porto-systemic shunting. Although liver biopsy as an assessment of fibrosis has become the key biomarker of and target for new therapies, it is invasive and subject to sampling error, and cannot quantify metabolic function or porto-systemic shunting. Measurement of the hepatic venous pressure gradient accommodates some of the deficiencies of biopsy but requires expertise not widely available and misses minor changes in hepatocellular mass and thereby information about metabolic function. Thus, an unmet need in clinical hepatology remains unfulfilled: a noninvasive biomarker which quantitates both the hepatocellular insufficiency and porto-systemic shunting inherent in progressive hepatic fibrosis. Ideally, such a biomarker should correlate with clinical endpoints including liver-related survival and cirrhotic complications, be performed at the point-of-care, and be affordable and easy to use. This review, an expert opinion, summarizes background and recent data suggesting that metabolic breath tests may now meet these requirements and have a valid place in clinical hepatology to supplant the time-honoured assessment of hepatic fibrosis.

Prognostic value of enzymatic liver function for the estimation of short-term survival of liver transplant candidates: a prospective study with the LiMAx test

LiMAx has been recently proposed as a new quantitative liver function test. Thus, we aimed to evaluate the diagnostic ability of LiMAx to assess short-term survival in liver transplant candidates and compare its performance to the model for end-stage liver disease (MELD) and indocyanine green plasma disappearance rate (ICG-PDR). Liver function of 167 chronic liver failure patients without hepatocellular carcinoma was prospectively investigated when they were evaluated for liver transplantation. Primary study endpoints were liver-related death within 6 months of follow-up. Within 6 months of follow-up, 18 patients died and 36 underwent liver transplantation. Median LiMAx results on evaluation day were significantly lower in patients who died (99 μg/kg/h vs. 55 μg/kg/h; P = 0.024), while median ICG-PDR results did not differ within both groups (4.4%/min vs. 3.5%/min; P = 0.159). LiMAx showed a higher negative predictive value (NPV: 0.93) as compared with ICG-PDR (NPV: 0.90) and the MELD (NPV: 0.91) in predicting risk of death within 6 months. In conclusion, LiMAx provides good prognostic information of liver transplant candidates. In particular, patients who are not at risk of death can be identified reliably by measuring actual enzymatic liver function capacity.

Critical appraisal of 13C breath tests for microsomal liver function: aminopyrine revisited

As liver diseases are a major health problem and especially the incidence of metabolic liver diseases like non-alcoholic fatty liver disease (NAFLD) is rising, the demand for non-invasive tests is growing to replace liver biopsy. Non-invasive tests such as carbon-labelled breath tests can provide a valuable contribution to the evaluation of metabolic liver function. This review aims to critically appraise the value of the (13) C-labelled microsomal breath tests for the evaluation of metabolic liver function, and to discuss the role of cytochrome P450 enzymes in the metabolism of the different probe drugs, especially of aminopyrine. Although a number of different probe drugs have been used in breath tests, the perfect drug to assess the functional metabolic capacity of the liver has not been found. Data suggest that both the (13) C(2) -aminopyrine and the (13) C-methacetin breath test can play a role in assessing the capacity of the microsomal liver function and may be useful in the follow-up of patients with chronic liver diseases. Furthermore, CYP2C19 seems to be an important enzyme in the N-demethylation of aminopyrine, and polymorphisms in this gene may influence breath test values, which should be kept in mind when performing the (13) C(2) -aminopyrine breath test in clinical practice.

The LiMAx test: a new liver function test for predicting postoperative outcome in liver surgery

BACKGROUND

Liver failure has remained a major cause of mortality after hepatectomy, but it is difficult to predict preoperatively. This study describes the introduction into clinical practice of the new LiMAx test and provides an algorithm for its use in the clinical management of hepatic tumours.

METHODS

Patients with hepatic tumours and indications for hepatectomy were investigated perioperatively with the LiMAx test. In one patient, analysis of liver volume was carried out with preoperative three-dimensional virtual resection.

RESULTS

A total of 329 patients with hepatic tumours were evaluated for hepatectomy. Blinded preoperative LiMAx values were significantly higher before resection (n= 139; mean 351 microg/kg/h, range 285-451 microg/kg/h) than before refusal (n= 29; mean 299 microg/kg/h, range 223-376 microg/kg/h; P= 0.009). In-hospital mortality rates were 38.1% (8/21 patients), 10.5% (2/19 patients) and 1.0% (1/99 patients) for postoperative LiMAx of <80 microg/kg/h, 80-100 microg/kg/h and >100 microg/kg/h, respectively (P < 0.0001). A decision tree was developed to avoid critical values and its prospective preoperative application revealed a reduction in mortality from 9.4% to 3.4% (P= 0.019).

DISCUSSION

The LiMAx test can validly determine liver function capacity and is feasible in every clinical situation. Combination with virtual resection could enable the calculation of residual liver function. The LiMAx decision tree algorithm for hepatectomy might significantly improve preoperative evaluation and postoperative outcome in liver surgery.

A continuous 13C methacetin breath test for noninvasive assessment of intrahepatic inflammation and fibrosis in patients with chronic HCV infection and normal ALT

Up to 30% of patients with hepatitis C virus (HCV) infection and normal serum alanine aminotransferase (NALT) have significant liver disease. Currently, many of these patients undergo a liver biopsy to guide therapeutic decisions. The BreathID continuous online (13)C-methacetin breath test (MBT) reflects hepatic microsomal function and correlates with hepatic fibrosis. To assess its role in identifying intrahepatic inflammation and fibrosis in NALT patients, we tested 100 patients with untreated chronic HCV infection, and 100 age- and sex-matched healthy volunteers using (13)C MBT following ingestion of 75 mg methacetin. All HCV patients had undergone a liver biopsy within 12 months of performing the MBT. Patients with a necroinflammatory grade <or=4 or >4, based on Ishak (modified HAI) score, HAIa + HAIb + HAIc + HAId, were defined as having low or high inflammation, respectively. Patients with a histological activity fibrosis stage <or=2 or >2, were defined as having nonsignificant or significant fibrosis, respectively. A proprietary algorithm to differentiate intrahepatic inflammation within chronic HCV patients with NALT achieved an area under the curve (AUC) of 0.90. Setting a threshold on the point of best agreement (at 83%) results in 82% sensitivity and 84% specificity. With application of another proprietary algorithm to differentiate patients with nonsignificant or significant fibrosis, 67% of liver biopsies performed in the patient group could have been avoided. This algorithm achieved an AUC of 0.92, with a sensitivity of 91% and a specificity of 88%. There was no correlation between body mass index (BMI) and MBT scores for patients with the same histological score. The continuous BreathID(13)C MBT is an accurate tool for measuring the degree of inflammation and fibrosis in patients with chronic HCV infection and NALT. As such, it may prove to be a powerful, noninvasive alternative to liver biopsy in the management of this patient population.