Exploring risk factors of hepatitis B virus-associated hepatocellular carcinoma: prospective verse retrospective studies

Abstract

Major risk factors for hepatocellular carcinoma (HCC) vary by region and population. Chronic infection with hepatitis B virus (HBV) and/or hepatitis C virus (HCV), heavy alcohol intake, aflatoxin exposure, haemochromatosis, nonalcoholic fatty liver disease, obesity, diabetes mellitus and tobacco smoking have been suggested to be risk factors of HCC. However, chronic infection with HBV remains the major cause of HCC in the world. More than 50% of HCC cases worldwide and 80% of HCC cases in most high-risk areas, such as the Asia‐Pacific region and Africa, are attributable to HBV infection. The relative risk of HCC among individuals infected with HBV ranges from 5 to 49 in case‐control studies and from 7 to 98 in cohort studies [1]. In HBV endemic regions, HBV infection is usually acquired perinatally or in childhood, whereas in HBV nonendemic areas, it is mostly transmitted during adolescence or adulthood. Newborns become chronic HBV carriers at a very high rate (about 90%), while immune-competent adults are generally described as developing chronic infection at a rate of 5‐10%. HBV subgenotype C2 is more prone to causing chronic infection than HBV subgenotype B2 following acute hepatitis B [2]. Up to 40% of patients with chronic HBV infection will develop the life-threatening complications of HCC or decompensated liver cirrhosis (LC). Chronic hepatitis B (CHB), LC and HCC are progressive liver diseases and consecutive stages of HBVassociated hepatocarcinogenesis, although CHB may precede the asymptomatic hepatitis B surface antigen (HBsAg) carrier (ASC) state and HCC does not have to pass through the CHB or LC stage. Hepatitis B e antigen (HBeAg) expression indicates active viral replication. HBeAg expression, high viral load and alanine aminotransferase (ALT) levels are associated with an increased risk of HCC. HBV subgenotypes B2 and C2 are endemic in most parts of Asia, while subgenotype B1 is endemic in Japan. Chronic infection with HBV C2 is frequently associated with an increased risk of LC and HCC in patients older than 50 years, whereas chronic infection with HBV B2 is associated with HCC or HCC recurrence in young, mostly non-cirrhotic, patients. In comparison with infection with HBV B2 or C2, infection with HBV B1 is associated with fulminant hepatitis B, a lower incidence of HCC and the development of HCC when older. Different HBV genotypes display their own distinct mutation pattern in the preS region and in the enhancer II (EnhII)/basic core promoter (BCP)/precore region of the HBV genome. Takahashi and colleagues identified HCC-associated HBV mutations through the comparative analysis of full-length HBV isolates (95% genotype C) from sera of 40 Japanese patients with HCC. They found that deletions and missense mutations in the preS2 region, A1762T and G1764A (often appearing simultaneously, termed A1762T/G1764A) and T1753C/A mutations in the BCP region and G1613A and C1653T mutations in the EnhII region were more frequent in HCC patients [3]. This pioneering work has aroused great interest in the field. Many studies pertaining to the association of HBV mutations with the risk of HCC have been published during the past decade. It has been found that the HBV preS deletion, A1762T/G1764A, T1753V, C1653T and T31C are each associated with a significantly increased risk of HCC [4]. Of the two major HBV