Clinicopathologic significance of genetic alterations in hepatocellular carcinoma
Introduction
Hepatocellular carcinoma (HCC) is one of the most common cancers in the world [1], with a marked geographic variation in incidence, being very prevalent in sub-Saharan Africa and Asia, but uncommon in the West [1]. This is due to different risk factors. Chronic hepatitis B virus (HBV) infection [2] and exposure to the carcinogen aflatoxin [3] are important risk factors for African and Asian populations. Chronic HBV carriers have a relative risk of nearly 200:1 for developing HCC [2], which may be due to cirrhosis and the oncogenic potential of the HBV genome [4]. The carcinogenic potential of aflatoxin relates to its capacity to induce G:C to T:A transversion in the TP53 gene, with specific clustering at the third base of codon 249 [5]. It also acts synergistically with HBV infection to increase the risk of HCC [6]. Chronic hepatitis C virus (HCV) is a risk factor in the West [7], although the underlying pathogenetic mechanisms remain undefined. Hepatocellular carcinoma has a poor prognosis, with a 5-year survival of less than 3% in inoperable cases.
Genetic mutations also play significant roles in hepatocarcinogenesis. The TP53 gene is pivotal in maintaining genomic stability, and mutations of the gene have been implicated in HCC formation. In addition to the clustering of mutations to codon 249 in aflatoxin prevalent areas [3], [5], mutations also occur in other exons in sporadic cases in most parts of the world [8], [9], [10].
CDKN2A (alias p16, MTS1) is another candidate gene involved in hepatocarcinogenesis [10]. It is a potent cyclin-dependent kinase inhibitor [11] that halts cell-cycle progression at the G1āS phase boundary. Loss of CDKN2A function may lead to unregulated cellular proliferation [11]. The localization of the CDKN2A gene to 9p21, which is often deleted in HCC [12], suggests that it may be a tumor-suppressor gene. Interestingly, aberrations of CDKN2A appear to show some heterogeneity in different localities. Several studies in Japan, Australia, and Taiwan failed to show any homozygous deletion, mutation, or methylation inactivation of the CDKN2A gene in HCC [12], [13], [14]. On the other hand, studies from Hong Kong showed that CDKN2A was often inactivated in HCC, predominantly through methylation of the 5ā² CpG island in the promoter region [15]. The reason for this difference is unclear. CDKN2B (alias p15, MTS2) is another inhibitor of the cyclin-dependent kinases CDK4 and CDK6 [16]. Methylation inactivation of CDKN2B occurred predominantly in hematologic malignancies [16]. It also is localized to 9p21 and so may also be a candidate gene in HCC.
Hepatocarcinogenesis is likely to involve multiple steps of genetic mutations. To test the hypotheses that the different genetic alterations involved in this multistep process may have distinctive pathogenetic roles; that these aberrations may be interrelated; and that they may be of clinicopathologic importance, we conducted a prospective study of HCC patients for TP53 gene mutation, CDKN2B and CDKN2A gene methylation, and genomic aberrations with comparative genomic hybridization (CGH). We further examined if these genetic lesions were interrelated, and defined their significance with a comprehensive panel of clinicopathologic parameters.
Section snippets
Patients
Sixty-eight consecutive patients with HCC undergoing surgical resection at a single institute from 1998ā99 were included in the study. Surgically resected tumor specimens were snap-frozen until analysis.
Pathologic evaluation
Pathologic evaluation was performed as previously described [17] by one of the investigators (I.O.N.) prospectively and without prior knowledge of the results of the genetic analyses. Ten pathologic characteristics were noted, including gross assessment of tumor size and number of tumor nodules,
Patients
There were 52 male and 16 female patients (M:FĀ =Ā 3.3:1), with a median age of 51 (13ā79) years. HBV was positive in 49 cases (76.5%) and HCV in 2 cases (3%).
Histologic evaluation
The mean size of the tumors was 7.8 cm (range: 1.5ā25.5). In the surrounding liver parenchyma, cirrhosis was found in 30 cases (44.1%), chronic hepatitis in 33 cases (48.5%), and apparently normal histology in 5 cases (7.4%).
TP53 mutation
Seventeen (25%) of the cases were found to have TP53 gene mutations, located at exons 5 to 9 (Table 1). Nine of these
Discussion
The study of genetic alterations in HCC may give important information on hepatocarcinogenesis. Karyotyping of HCC has limited success, however, owing to the low mitotic index and poor metaphases. Genetic studies have therefore focused on alterations of single genes, including TP53 and CDKN2A [8], [9], [10], [11], [12], [14], [15], [16]. Comparative genomic hybridization, a technique that assesses the overall genetic lesions in a tumor, has recently been used in the study of HCC [22], [23], [24]
Acknowledgements
The authors thank C.M. Lo, R.T.P. Poon for clinical care of the patients; Chris Tam and Eunice Chan for technical help; and M Ng for assistance in statistical analysis. This study was supported by grant no. HKU 7289/97M from the Research Grant Council.
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