NASH: Fomenting fat and forecasting fibrosis

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Fomenting fat and forecasting fibrosis
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Fomenting fat and forecasting fibrosis

With the revolution in direct-acting antivirals for hepatitis C upon us, it’s important not to lose track of another of our specialty’s great challenges, the fatty liver disease epidemic. The payoff of direct-acting HCV antivirals required 22years from the discovery of the virus in 1989 [4], yet 31years after the description of NASH [1], [10] we are still struggling to understand its pathogenesis, management, and impact. Two articles in this month’s issue offer incremental progress toward that goal.

A concise yet informative study by Tandra and colleagues derived from the NIH NASH Clinical Research Network defines a subset of 10% of 1022 patients with the disease whose biopsies demonstrate microvesicular steatosis rather than macrovesicular steatosis. This finding was strongly correlated with several histologic features of more aggressive disease, including ballooning cell injury, Mallory-Denk bodies, NASH activity scores, and advanced fibrosis, among others. Megamitochondria, a hallmark of mitochondrial disruption, were also prominent. Microvesicular steatosis, defined as “non-zonal, contiguous patches of foamy hepatocytes with centrally placed nuclei”, and illustrated by a figure in the Tandra paper, is typically associated with impairment in the mitochondrial β-oxidation of fatty acids [8], [21], either due to congenital metabolic disorders or drugs, including Reye’s syndrome. As fatty acids accumulate in the absence of oxidation they may be esterified into triglycerides and thereby accumulate in droplets. The development of small rather than large vacuoles may reflect the acuity of the insult, such that there is insufficient time for small vacuoles to coalesce into larger ones. Alternatively, impaired clearance of hepatic fat may also lead to lipid droplet accumulation. Either reduced VLDL secretion or defects in microsomal transfer protein (MTP), which normally transfers lipids to apolipoprotein B for packaging into VLDL, can contribute to reduced clearance.
These precipitants of microvesicular steatosis may provide some clues into the significance of this finding in patients with fatty liver.

An appealing hypothesis is that patients with microvesicular steatosis have a specific genetic polymorphism or defect in fatty acid metabolism that is unearthed in the setting of concurrent metabolic syndrome and fatty liver, an example of the ‘two-hit’ concept. Perhaps the fatty liver-associated SNP in the PNPLA3 gene [18] is contributing? Alternatively, applying an unbiased genome wide association study of the type that yielded the PNPLA3 might expose new genetic variants that influence the expression of fatty liver disease in patients with risk factors, which in turn could reveal new pathogenetic clues to NASH. Because the patients with or without microvesicular steatosis were matched in terms of NASH risk factors (e.g., BMI, HOMA and AST/ALT), it would seem unlikely that a genetic risk factor leading to microvesicular steatosis occurs first and then leads to features of fatty liver disease, but this remains a possibility.

Since this is a cross-sectional rather than a longitudinal study, one cannot determine whether microvesicular steatosis and associated abnormalities are present throughout the course of disease in these specific patients, or instead that the findings are evanescent and occur transiently in many more patients. Moreover, these more aggressive histologic features would be predicted to accelerate fibrosis, but long-term follow-up is required to validate this prediction.

Finally, the identification of a subset of patients with unique histologic features among those with fatty liver disease required the large and highly integrated approach of the NASH Clinical Research Network. The findings underscore the importance of large numbers of patients who are meticulously phenotyped by experts in order to generate robust, novel clinical, and genetic data, especially in those with the metabolic syndrome [11].

While the access to liver biopsies in a large number of patients proved critical to identifying a subset with microvesicular steatosis among those with fatty liver disease, the reality is that biopsy is not a routine management tool in clinical practice when fatty liver disease is suspected, and its role is unsettled.

Concurrently, non-invasive measures of liver injury, inflammation, fat, and fibrosis are steadily improving. For example, measurement of liver stiffness using transient elastography is increasingly accepted as a surrogate for inflammation, edema, and/or fibrosis in viral liver disease [3], [19], and helps discriminate fibrosis stage in fatty liver disease as well [5], although not as clearly as in viral hepatitis [9].

Even more exciting has been the evidence that liver stiffness values predict the likelihood of portal hypertensive complications in patients with chronic liver disease, with an accuracy comparable to hepatic venous pressure gradient [17]. These findings parallel recent studies demonstrating the prognostic value of serum tests under similar conditions [13]. Prognostic data of this type add momentum toward the use of non-invasive markers as endpoints in clinical trials, especially since liver stiffness may improve in response to effective therapies [12], [22].

While transient elastography has been widely accepted in Europe and Asia, its use in the United States has been limited, owing in part to the high cost of the devices and lack of approval by the Food and Drug Administration, preventing providers from recovering the costs of its purchase through reimbursement from patients or insurers. Moreover, in the case of fatty liver disease, severe adiposity limits its utility [2], although newer probes are under development to circumvent this problem [6].

Acoustic radiation force impulse (ARFI) is a newer ultrasound-based alternative to transient elastography that also measures stiffness, but can be incorporated into a conventional ultrasound device. A handful of papers have been reported using this technology in liver disease, which measures a region of interest of 5×4mm [7], [14], [15], [16], [20]. The technique reportedly has accuracy comparable to transient elastography in patients with chronic HCV [7], but experience is far more limited.

In the article by Palmeri and colleagues in this issue of the Journal of Hepatology, ARFI has been used in patients with NALFD, correlating the findings with specific histologic features on liver biopsies in 172 patients. The test was generally useful (90% sensitivity and specificity) in distinguishing between low fibrosis stage (0–2) and high fibrosis stage (3–4). There was no correlation between inflammation or hepatocyte ballooning, however. More encouraging was its utility even in obese patients, where transient elastography still poses challenges, although only a small number of obese patients were tested. While the data are preliminary and not thoroughly convincing, they are another example of the accelerating pace of technology development for non-invasive assessment of fibrosis and cirrhosis.

Along with other approaches including MRI techniques, serum markers, and metabolic breath tests, transient elastography and ARFI are expanding the prospects for using non-invasive markers alone or combination to assess disease stage and prognosis. To derive the most information from testing of these markers, prospective studies should be designed comparing as many techniques as possible head-to-head in the same patients, and correlating the results with clinical features, short- and long-term outcomes, and response to disease therapies. Minimizing the cost of new technologies and improving the quality of the data will enhance their appeal, and help usher in a new era of liver disease diagnosis.

Conflict of interest
The author declared that he does not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.

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Division of Liver Diseases, Mount Sinai School of Medicine, New York, NY, USA

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