1Department of Clinical Research and 2Ransome Research Laboratory, Singapore General Hospital, Singapore 169608, Republic of Singapore
Sir,
Lamivudine, the negative enantiomer of 2'-dideoxy-3'-thiacytidine, is a reverse transcriptase inhibitor for both human immunodeficiency virus (HIV) and human hepatitis B virus (HBV). Its application has so far resulted in a reduction of serum HBV DNA in chronic carriers and also improved liver function.1 However, prolonged lamivudine therapy has resulted in mutations in the Tyr-Met-Asp-Asp (YMDD) motif of the HBV DNA polymerase.2 Although the HBV DNA polymerase and viral surface antigen (HBsAg) are translated from the same coding region, albeit different reading frames, the YMDD motif is located in domain C of the polymerase, whereas the antigenic a determinant of HBsAg (amino acids 124147) is in the variable linker region between the two conserved domains A and B of the polymerase.3 Mutations in the YMDD motif associated with lamivudine therapy should therefore not result in changes in the antigenicities of the particular mutants.3 However, recent evidence has suggested that concomitant mutations may occur in the a determinant following lamivudine therapy.4 These HBsAg mutants (G130D and G145R) displayed altered viral antigenicity,5 leading eventually to their escaping detection by current immuno-based methods.
The significance of these HBsAg mutants associated with lamivudine was further analysed in nine additional patients undergoing lamivudine therapy (Table 1): the earlier reported patient4 and eight others, selected because of rebound of their HBV DNA during lamivudine therapy. The coding region covering both HBV DNA polymerase and HBsAg was amplified by PCR as described previously.4 Sequence analysis of the amplified DNA fragment revealed the presence of three types of mutants (Table 1). These included those carrying mutations in both the HBV DNA polymerase and HBsAg (patients 1, 2, 4, 7 and 8), those with mutations only in the HBV DNA polymerase (patient 3) and those with mutations only in HBsAg (patients 5, 6 and 9).
|
Among the nine patients analysed in this study, only patient 3 carried the YIDD mutation in HBV DNA polymerase with no mutation in HBsAg. Unlike most other mutations in the YMDD motif, the YIDD mutation was not seen at the rebound of serum HBV DNA (January 2000) but became detectable 7 months later (July 2000).
The third category of mutant included those found only in HBsAg but not in HBV DNA polymerase despite rebound of serum HBV DNA, with Y100C in patient 5, W182stop codon in patient 6 and T126I/S in patient 9. These HBsAg mutants appeared to persist despite lamivudine therapy in a similar pattern to those found in patients 1 (G145R and G130D), 2 (P120S) and 8 (L175S/F179Y).
In summary, our data support our earlier finding that mutations associated with lamivudine therapy may be found in HBV DNA polymerase and concomitantly in the non-overlapping HBsAg. Some of these HBsAg mutants were detected before the onset of lamivudine therapy and persisted thereafter, possibly as a result of the concomitant lamivudine-associated mutations in the YMDD motif of HBV DNA polymerase. Future monitoring will be required for this new class of lamivudine-associated mutants, as they are less sensitive to lamivudine and may also escape current immuno-based detection due to changes in their viral antigenicities.
Acknowledgements
We thank the National Medical Research Council of Singapore and Singapore General Hospital Liver Cancer Endowment Fund for support.
Footnotes
* Corresponding author. Tel: +65-63214664;Fax: +65-62259865; E-mail: wnchen{at}ntu.edu.sg
References
1 . Liaw, Y. F., Leung, N. W., Chang, T. T., Guan, R., Tai, D. I., Ng, K. Y. et al. (2000). Effects of extended lamivudine therapy in Asian patients with chronic hepatitis B. Gastroenterology 119, 17280.[ISI][Medline]
2 . Doo, E. & Liang, T. J. (2001). Molecular anatomy and pathophysiologic implications of drug resistance in hepatitis B virus infection. Gastroenterology 120, 10008.[ISI][Medline]
3 . Chen, W. N. & Oon, C. J. (1999). Human hepatitis B virus mutants: significance of molecular changes. FEBS Letters 453, 23742.[ISI][Medline]
4 . Oon, C. J., Chen, W. N., Lim, N., Koh, S., Lim, G. K., Leong, A. L. et al. (1999). Hepatitis B virus variants with lamivudine-related mutations in the DNA polymerase and the a epitope of the surface antigen are sensitive to ganciclovir. Antiviral Research 41, 1138.[ISI][Medline]
5
.
Chen, W. N. & Oon, C. J. (2000). Changes in the antigenicity of a hepatitis B virus mutant stemming from lamivudine therapy. Antimicrobial Agents and Chemotherapy 44, 1765.
6 . Chen, W. N., Oon, C. J. & Toh, I. (2000). Altered antigenicities of hepatitis B virus surface antigen carrying mutations outside the common a determinant. American Journal of Gastroenterology 95, 10989.[ISI][Medline]
7 . Gutfreund, K. S., Williams, M., George, R., Bain, V. G., Ma, M. M., Yoshida, E. M. et al. (2000). Genotypic succession of mutations of the hepatitis B virus polymerase associated with lamivudine resistance. Journal of Hepatology 33, 46975.[ISI][Medline]