MAPPING OF THE HEPATITIS-B VIRUS GENOME IN HEPATOCELLULAR-CARCINOMA USING PCR AND DEMONSTRATION OF A POTENTIAL TRANSACTIVATOR ENCODED BY THE FREQUENTLY DETECTED FRAGMENT

The association of hepatitis B virus (HBV) infection with hepatocellul ar carcinoma (HCC) is well established. Insertional mutagenesis, trans -activation by truncated X or preS2/S regions and activation of growth regulatory genes or oncogenes have all been suggested as possible mec hanisms for this carcinogenesis. However, no consensus regarding the m echanism or region of the HBV genome involved has been established. Of the 36 HCC tissues analysed for the presence and extent of the HBV ge nome, using multiple overlapping PCR, 22 (61%) were found to be positi ve. Twenty of these showed the presence of a fragment (nucleotides 636 to 746) that covered part of the surface antigen gene. The recognized trans-activators, X and preS2/S, were present in only seven (31.8 %) and 12 (54.5 %) cases, respectively. In two cases the entire viral gen ome was detected. The trans-activation potential of the cloned S fragm ent(nucleotides 426 to 851) covering the frequently detected fragment (nucleotides 636 to 746) was investigated in cotransfection experiment s. This fragment was able to trans-activate the HBV enhancer-X promote r target. To define the specificity of the trans-activation and the se quences involved, frameshift and deletion mutants of this fragment wer e constructed and analysed. The transactivation activity was lost in t he frameshift mutants. The deletion mutants that retained nucleotide s equences 436 to 679 showed trans-activation activity whereas the other ones (nucleotide sequences 436 to 611) did not show any activity. It is suggested that the frequently detected HBV genome fragment belongin g to the S gene frame has a trans-activation potential. This may expla in the mechanism for pathogenicity of HBV-associated HCC.