ANALYSIS OF GENETIC ALTERATIONS DURING HEPATOCARCINOGENESIS

推定的p53肿瘤抑制基因的突变和丢失

Mutations in the putative p53 tumor suppressor gene and loss of heterozygosity (LOH) at several chromosomal sites in a tumor indicates that inactivation of several tumor suppressor genes are necessary for development of the tumor. Aflatoxin B1 has been suggested as a causative agent for a G to T mutation at codon 249 in the p53 tumor suppressor gene in human hepatocellular carcinomas (HCCs) from southern Africa and Qidong in China. It seems to be important to examine whether the causative agent for the selective p53 mutation also is involved in other tumor suppressor genes. To clarify the involvement of aflatoxin B1 in HCCs in China, we examined the p53 gene for mutations and LOH on the p53, retinoblastoma (Rb) and adenomatis polyposis coli (APC) genes, and chromosomes 4, 13, and 16 using polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis, as well as restriction fragment length polymorphism (RFLP) analysis. HCCs were examined from two different areas of China: Qidong, where exposure to hepatitis B virus (HBV) and aflatoxin B1 is high; and Beijing, where exposure to HBV is high but aflatoxin B1 is low. Frequencies of mutation, loss, and aberration (mutation and loss) of the p53 gene in 25 HCCs from Qidong were 60, 58, and 80%, respectively. The frequencies in 9 HCCs from Beijing were almost the same, being 56, 57, and 78%, respectively. However, the frequencies of a G to T transversion at codon 249 in HCCs from Qidong were 52% versus 0% for Beijing. Loss of the Rb and APC genes were observed in 44% and 7%, respectively, of HCCs from Qidong. LOH on chromosome 4 and 16 was relatively frequent in HCCs from Qidong, but rare in HCCs from Beijing. LOH on chromosome 13 was frequent in both Qidong and Beijing. These data suggest that aflatoxin-B1 might be one of the causative agents for a G to T transversion at codon 249 of the p53 gene in Qidong, but other factors might be involved in the development of HCCs in Beijing. Therefore, although the p53 gene plays a critical role in the development of HCCs, distinct mechanisms appear to be responsible for human hepatocarcinogenesis in different geographic areas.