Molecular and cell biology of liver carcinogenesis and hepatitis

Overview of molecular etiology

Recent advances in molecular genetics have emphasized the multistep process of tumorigenesis. It is evident that cancer is a genetic disease involving aberrant chromosome rearrangements, genetic mutations, and epigenetic silencing of tumor suppressor genes. Independent of the etiology, hepatocellular carcinoma (HCC) generally develops where sustained hepatocyte turnover occurs in the setting of injury-inflammation-regeneration, which leads to the accumulation of chromosomal aberrations (see Chapters 68 and 89 ). The monoclonal populations of hepatocytes become preneoplastic and, after additional genomic alterations, change into dysplastic cells and eventually HCC. Accumulated genetic alterations in preneoplastic lesions and HCC result in the activation, as well as inactivation, of many growth factor signal transduction pathways involved in hepatic transformation. It is believed that increased hepatocyte turnover associated with chronic liver injury may be a major feature of hepatic oncogenesis. However, another central question is whether hepatitis viruses, the leading cause of HCC worldwide, directly contribute to the development of this disease. Accumulating evidence suggests that chronic hepatitis B virus (HBV) and hepatitis C virus (HCV) infection play a direct role in the molecular pathogenesis of HCC through specific viral–cellular protein interactions (see Chapter 68 ). In this chapter, we discuss the molecular mechanisms of liver carcinogenesis and focus on the role of HBV and HCV.

Epidemiology

Primary liver cancer was the fourth leading cause of cancer death in 2015 after lung, colorectal, and stomach cancer. It is estimated that 782,000 new patients with the disease were diagnosed in 2012. The 5-year survival rate is less than 15% in developed countries, and the United States has a survival rate of 20.3%, making liver cancer the second most fatal tumor after pancreatic cancer. Presumably because of its poor prognosis, liver cancer is the second leading cause of cancer death in men and the sixth among women in the world. It is estimated that about 745,000 individuals worldwide died from this disease in 2012. It is one of the few neoplasms with a steadily increasing incidence and mortality in the United States.

Primary liver cancer comprises a heterogeneous group of malignant tumors with different histologic features. The unfavorable prognosis varies from hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma (iCCA) to mixed hepatocellular cholangiocarcinoma (HCC-CCA), fibrolamellar HCC, and the pediatric neoplasm hepatoblastoma (see Chapters 50 , 87 , and 89 ). The most common is HCC, which accounts for 70% to 85% of all hepatic tumors. Approximately 80% of HCC is caused by chronic infection with HBV or HCV (see Chapter 68 ). The HCC burden is unevenly distributed worldwide; areas where tumors are most prevalent include West and Central Africa and East and Southeast Asia, with China alone accounting for more than 50% of the world cases. The incidence of HCC varies with both geographic location and ethnicity. For example, HCV is the leading etiology of HCC in the United States, Europe, Japan, and South America, whereas HBV is the major cause in the majority of Asian and African countries.

Trends in HCC incidence are likely to be different in regions of high and low persistence of HBV and HCV infection. Comparative studies performed between 1977 and 1982 and between 1993 and 1997 show that the incidence of HCC in Hong Kong, Shanghai, Singapore, and Japan has begun to decrease. The fall in incidence is apparently because of vaccination against HBV, which has been accomplished in greater than 80% of newborns, because chronic HBV infection in those countries is usually acquired through mother-to-newborn or sibling-to-sibling transmission at a young age. Although HBV vaccination reduces the incidence of HCC, many unvaccinated persons are still infected with HBV (257 million in 2015), mostly in Asia and sub-Saharan Africa.

The incidence of HCC has increased in some countries, such as Australia, the United States, and the United Kingdom, probably as the result of chronic HCV infection and nonalcoholic steatohepatitis (NASH; see Chapter 69 ). The annual incidence of liver cancer increased from 2.6 per 100,000 population for the years 1978 to 1980 to 8 per 100,000 in 2010, of which at least 3 of 4 cases are because of HCC. Reasons for this increased incidence are not entirely clear but may reflect a greater prevalence of NASH and role of persistent HCV infection. Recent advances in direct-acting antiviral (DAA) agents for HCV will cure most individuals with chronic HCV infection, and it has been estimated that reducing the frequency of chronic HCV infection by 90% would eliminate 15% of HCC in the United States. However, there is debate over the effects of DAA agents on tumor progression.

The age of onset of HCC varies in different parts of the world. HCC tends to occur later in life in Japan, North America, and European countries, where the median age of onset is above 60 years. In contrast, in parts of Asia and most African countries, HCC is commonly diagnosed in the age range of 30 to 60 years. In the United States, however, recent trends have revealed a peak incidence shifting toward a relatively younger age group.

Significant gender and ethnic variation in incidence, as well as mortality from HCC, has also been found; male rates are nearly triple that of females. The most likely explanation for gender variation is that men have more risk factors, such as exposure to hepatitis virus infection, excessive alcohol intake, smoking, and increased iron stores in the liver. In addition, androgen levels may accelerate the progression of HCC through interaction with the HBV genome. ^, The incidence of HCC also varies with race and ethnicity in the same area. In the United States, the incidence and subsequent mortality rates are two times greater in Asians than African Americans, which are two times greater than those found in Caucasians. These variations are explained, in part, by the accumulation of major risk factors in each ethnic group.

Risk factors

Unlike most malignancies, HCC has well-established extrinsic risk factors that account for at least 80% of tumors (namely chronic infection with HBV or HCV; see Chapter 68 ). Key epidemiologic aspects of HBV- and HCV-induced HCC are summarized in Table 9B.1 . Chronic HBV infection is the leading cause of HCC, and it has been estimated that there are 350 to 400 million HBV carriers, which account for 5% of the global population. About 59% of HCC patients in developing countries and 23% of HCC patients in developed countries are chronically infected with HBV. Risk factors for HBV-related HCC include demographic features such as male sex, older age, Asian or African ancestry, family history of HCC, viral properties (higher levels of HBV replication; HBV genotype; longer duration of infection; co-infection with HCV, human immunodeficiency virus [HIV], or hepatitis delta virus), clinical factors (cirrhosis), and environmental factors (exposure to aflatoxin, heavy intake of alcohol or tobacco). The 5-year cumulative incidence rates of HCC from HBV-related cirrhosis are 17% in highly endemic areas and 10% in Europe and the United States. HBV can cause HCC in the absence of cirrhosis, although approximately 70% to 90% of HBV-related HCC cases develop in patients with this disease. Nucleoside/nucleotide analogues that suppress viral replication are associated with risk reduction of HCC in patients with chronic hepatitis B. Aflatoxin B1 (AFB1) is produced by Aspergillus flavus and related fungi that contaminate corn, rice, and peanuts in China and sub-Saharan Africa. High rates of dietary exposure to AFB1 increase the risk for HCC 4-fold. When people with chronic HBV infections are exposed to AFB1, the relative risk for HCC dramatically increases to about 60-fold. This synergistic effect between AFB1 exposure and chronic HBV infection is an important observation because in some regions of the world, AFB1 exposure and chronic HBV infection rates are high.

Chronic HCV infection is the second leading cause of HCC. The estimated number of HCV carriers worldwide is 180 million, which accounts for 2% of the global population. Approximately 33% of HCC tumors in developing countries and 20% of HCC in developed countries are attributable to persistent HCV infection. According to cross-sectional and case-control studies, HCC risk is increased 15- to 20-fold in HCV-infected people compared with the HCV-negative population. Patients with HCV-induced cirrhosis are at particularly high risk for the development of HCC, with an annual incidence of HCC ranging from 0.5% to 10%. Sustained virologic response (SVR) with DAA agents has emerged as the most dominant modifier of HCC in patients with HCV. Although DAA is likely to change the epidemiology of HCV-related HCC in those who are treated, most HCV-infected populations remain untreated. The DAAs offer a chance for cure of patients with advanced cirrhosis, older age, and alcohol use. These patients had a poor response to interferon (IFN)-based therapies. However, there was no evidence for differential occurrence of HCC or recurrence risk after SVR attributable to DAA or IFN-based therapy.

Excessive ethanol consumption (>50–70 g/day) is another well-defined risk factor for HCC. Alcoholic cirrhosis is the second most common risk factor for HCC in the United States and Europe. ^, There was a linear increase of a relative risk for HCC by 5-fold when 60 to 140 g/day alcohol was consumed. The 5-year cumulative HCC incidence in alcoholic cirrhosis without HBV and HCV infection is 8% ; however, it is unlikely that ethanol itself has a direct carcinogenic effect. Rather, excessive ethanol ingestion indirectly affects hepatocarcinogenesis through the promotion of cirrhosis. Indeed, greater than 80% of HCC tumors found in alcoholics develop in the background of a cirrhotic liver. A synergistic effect between heavy alcohol consumption and hepatitis virus infection has been observed in several studies. The relative risk for HCC attributable to heavy alcohol consumption alone was only 2.4-fold, whereas in combination with chronic HCV infection, it increased to 50-fold. Others have reported that concomitant HCV infection in alcoholics increases the risk for HCC 2-fold, whereas HBV infection moderately increases this risk 1.2- to 1.5-fold. ^,

Growing evidence now suggests that metabolic dysfunction, including obesity, diabetes, and nonalcoholic fatty liver disease (NAFLD), are important risk factors for HCC, especially in developed countries (see Chapter 69 ). Several large cohort studies revealed that obesity is a definitive risk factor for HCC, with the 1.5- to 4-fold increased risk. Men are more susceptible to obesity-associated HCC than women. NAFLD-associated HCC also occurs frequently in the absence of cirrhosis. Diabetes mellitus has also been established as a moderately strong risk factor for HCC, with a two- to four-times higher risk. ^, The use of metformin is associated with decreased risk, and the use of insulin or sulfonylureas may increase HCC risk. Longer duration of diabetes may be associated with an incremental increase in risk. More recently, NASH, the more aggressive form of NAFLD, is considered to be a cause of a large proportion of cryptogenic cirrhosis, which is risk for HCC development. However, the overall incidence of HCC in patients with NAFLD is lower than in patients with other well-established etiologies, such as chronic viral infection.

Cigarette smoking is one risk factor for HCC. The HCC risk of current and former smoking was 1.55 and 1.39 times, respectively. Cirrhosis of any cause increases the risk for HCC, with an annual incidence between 2% and 4%.