Development of Kinetic Biomarkers of Liver Fibrosis Measuring NAFLD

Management of NASH and NAFLD remain a significant unmet medical challenge that is growing in importance as part of the obesity epidemic. Minimally invasive tools for monitoring disease progression and evaluating therapeutic interventions in NASH would be extremely valuable. Utilizing in vivo heavy water labeling, multiple pathways related to protein metabolism (fibrogenesis) and lipid metabolism can be quantified in human subjects. We have recently discovered that plasma lumicam synthesis represents a non-invasive kinetic biomarker of tissue fibrogenesis in patients with viral hepatitis. In addition, synthesis of fatty acids in plasma VLDL-triglycerides provide a window into hepatic lipid metabolism.

Stable isotopes have a long history as a safe, effective tracer for measuring synthesis of molecules in humans (1). Recently, new developments in stable isotope labeling techniques and advances in mass spectrometry have made in vivo kinetic measurement of slow metabolic processes possible. Through the use of 2H2O as the source of labeling, we and others have measured T-cell proliferation (2), mammary epithelial cell proliferation (3), prostate epithelial cell proliferation (4), triglyceride synthesis (5) and protein synthesis (6) in humans. We have recently evaluated this approach for the measurement of fibrogenesis patients with fibrotic liver disease.

Excess accumulation of collagen in the liver is termed fibrosis. Fibrosis is a common pathological feature of several chronic liver diseases (e.g. Hepatitis C, alcoholic liver disease, primary biliary sclerosis, drug/toxin induced liver disease, etc.). Currently, the standard method for detection of fibrosis is liver biopsy and histochemical analyses of tissue collagen content (8, 9). Although effective in diagnosing existing, advanced fibrosis, a single biopsy cannot measure current disease activity or predict rate of progression. To determine whether disease is progressing using current methods, a second biopsy is required. If significant additional collagen has accumulated since the first biopsy, this suggests that the disease is progressing. However, this measurement represents the history of the disease, not the current disease activity at the time of the second biopsy. There are also significant limitations in current methods, since changes in collagen pool size measurable by histochemistry cannot measure small changes in collagen content and intra-laboratory variability inherent in histochemical assays reduce their sensitivity (10, 11).

This stable isotope / mass spectrometry based method will be applied here for the quantification of fibrogenesis in vivo (from a bone marrow biopsy) and the identification of novel biomarkers of fibrogenesis in plasma in patients receiving investigational therapies.

If successful, this research will identify plasma proteins which can be easily measured by tandem mass spectrometry (LC/MS/MS) methods and whose synthesis rate reflects disease activity in the heart. Ideally, a set of markers related to NASH/ NAFLD will be developed that can detect and differentiate among multiple disease phenotypes, based on the kinetic signature measured in a single blood draw from a patient labeled with deuterated water.

The role of de novo lipogenesis (DNL) has been suggested by several clinical studies (Donnelly JCI 2005, Puri Hepatology 2009). DNL contributes significantly to the accumulation of lipid in NASH (Donnelly JCI 2005). Moreover DNL is elevated in many other inflammatory states and may be a useful marker of hepatic inflammation. DNL as well as hepatic TG assembly and cholesterogenesis are easily measured in plasma or dried blood spot samples from patients consuming 2H2O, after several days of labeling the plasma DNL reaches a steady state and reflects hepatic DNL rates.