The worldwide obesity epidemic has increased the risk of accumulating fat in the liver, a preamble to liver inflammation, liver disease, and ultimately hepatocellular carcinoma (HCC) or liver cancer.
The burden of HCC, considered the most common form of liver cancer, is steadily growing because obesity, type 2 diabetes (T2DM), metabolic syndrome, and nonalcoholic fatty liver disease (NAFLD) are replacing viral- and alcohol-related liver disease as major pathogenic promoters. This change worries scientists since the “new” risk factors are largely spread in the general population.
Today, the systemic and hepatic molecular mechanisms involved in obesity- and NAFLD-induced hepatocarcinogenesis as well as potential early markers of HCC are being extensively investigated. However, in addition to an increase in the observed risk factors, a still very intriguing paradox is the development of fatty liver in lean and normal-weight individuals and in people following a healthy diet.
Although researchers have shown that individuals diagnosed with NAFLD who lack classical risk factors also have the ability to develop nonalcoholic steatohepatitis (NASH, inflamed liver) and can progress to more advanced liver diseases, the pathophysiology and risk factors for the development of NAFLD in non-obese persons are not fully understood.
A genetic link
Scientists know that two genes, RNF43 and ZNRF3, are mutated in liver cancer patients. However, their role in the development of liver cancer was unknown so far.
Researchers at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany, for the first time describe now that a loss or mutation of these genes causes an accumulation of lipids and inflammation in the liver in non-obese mice fed a normal diet.
These genetic alterations not only increase the accumulation of fat but also the number of liver cells in proliferation. In human patients, these alterations also increase the risk of developing NASH and fatty liver and reduce the patient’s survival time. These findings might facilitate the discovery of people at risk and could promote novel therapeutic interventions and better management of the disease
Central metabolic organ
The liver is a key player in metabolizing food and eliminating waste. It is our central metabolic organ, which is vital for detoxification and digestion.
NAFLD results from a buildup of fat in the liver of someone who drinks little or no alcohol. And while NAFLD may cause the liver to be enlarged, most people with a fatty liver probably won’t notice any symptoms.
About 25% of patients with NAFLD will go on to develop NASH. NASH is much more serious and it reflects underlying cell damage, liver fibrosis or cirrhosis, and can lead to liver cancer.
Understanding the cause
Unfortunately, chronic liver diseases, NAFLD and NASH as well as such diseases as cirrhosis and liver cancer, are on the rise worldwide, with combined mortality of two million individuals dying each year.
It is, therefore, more important than ever to understand their causes and the underlying molecular mechanisms of liver diseases in order to prevent, manage, and treat these increasing patient population subgroups. Previous cancer genomic studies identified RNF43 and ZNRF3, as genes mutated in colon and liver cancer patients. However, their role in liver disease has been unexplored.
The research lab of Meritxell Huch at the Max Planck Institute of Molecular Cell Biology and Genetics, together with colleagues at the Gurdon Institute (Cambridge, UK) and at the University of Cambridge, has now investigated the mechanisms by which alterations in these two genes can affect the emergence of liver diseases.
To pursue this goal, the researchers worked with mice as an animal model, data from human individuals, human tissues, and liver organoid cultures, which are 3D cellular microstructures made out of hepatocytes that resemble the liver in a dish.
“With the organoid, we were able to grow hepatocytes mutated only in these genes, and we saw that the loss of these activates a signal that regulates the metabolism of lipids,” explained Germán Belenguer, the first author of the study and postdoctoral researcher in the group of Meritxell Huch.
“As a result, the fat metabolism is no longer under control and lipids accumulate in the liver, which leads in turn to a fatty liver. Another result of the activated signal is that hepatocytes multiply uncontrollably. Both mechanisms combined facilitate the progression towards fatty liver disease and cancer,” Belenguer added.
A worse prognosis
The scientists then compared the results from the experiments with patient data in a publicly available dataset from the International Cancer Genome Consortium. They evaluated the prognosis of survival when the two genes are mutated in liver cancer patients and found that patients with these mutated genes show fatty liver disease and have a worse prognosis than liver cancer patients with the two genes unmutated.
“Our findings can help identify individuals with a RNF43/ZNRF3 mutation and therefore at risk of developing a fatty liver or liver cancer,” says Meritxell Huch.
“With the alarming increase in the consumption of fat and sugar worldwide, recognizing those individuals already predisposed because of bearing those genetic mutations might be important for the therapeutic intervention and management of the disease, especially at very early stages or even before the disease is initiated,” she continued.
“We will need more studies to further characterize the roles of the two genes in human fatty liver disease, NASH, and human liver cancer and to identify therapeutics that could help those patients that are already intrinsically predisposed to develop the disease,” Huch concluded.
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 Phipps M, Wattacheril J. Non-alcoholic fatty liver disease (NAFLD) in non-obese individuals. Frontline Gastroenterol. 2019 Dec 13;11(6):478-483. doi: 10.1136/flgastro-2018-101119. PMID: 33101626; PMCID: PMC7569516.
 Belenguer G, Mastrogiovanni G, Pacini C, Hall Z, Dowbaj AM, Arnes-Benito R, Sljukic A, Prior N, Kakava S, Bradshaw CR, Davies S, Vacca M, Saeb-Parsy K, Koo BK, Huch M. RNF43/ZNRF3 loss predisposes to hepatocellular-carcinoma by impairing liver regeneration and altering the liver lipid metabolic ground-state. Nat Commun. 2022 Jan 17;13(1):334. doi: 10.1038/s41467-021-27923-z. PMID: 35039505; PMCID: PMC8764073.
Featured image: Liver liver cell organoids derived from normal (left) or mutant (right) mice showing intrinsic accumulation of lipids (in green). When the two genes (RNF43 and ZNRF3) are mutated, liver cells autonomously produce higher levels of lipids that also accumulate as big droplets inside each cell (revealed by the presence of their nuclei in blue and the surrounding cell membrane in white). Photo courtesy: © 2022 Belenguer et al./ Max-Planck-Gesellschaft/Max Planck Institute of Molecular Cell Biology and Genetics. Usee with permission.