Hepatitis B virus (HBV) is one of the most widespread viruses in the world. HBV has high variability, which is due to the fact that HBV replicates through RNA intermediates (pregenomic RNA) that are reverse transcribed into negative-stranded DNA using the virus’ own DNA polymerase (polymerase) – DNA-P. During reverse transcription, DNA-P lacks proofreading enzyme activity and cannot correct nucleotide mismatches, resulting in HBVDNA sequence variation. HBVDNA contains at least four open-reading frames (ORF), namely S region (including S region, pre-S1 region and pre-S2 region), C region (including C region and pre-C region), P region and X region. In this paper, we review the latest research progress on HBV drug resistance and prevention strategies. HBV is a hepatophilic DNA virus with a gene length of only 3.2 kb. Within the HBV envelope, the HBV gene exists as an incomplete double-stranded circular DNA. The host RNA polymerase uses cccDNA as a template to transcribe mRNA. mRNA is also pregenomic RNA. HBV polymutase uses pregenomic RNA as a template to reverse transcribe and synthesize negative-stranded HBVDNA, and then uses negative-stranded DNA as a template to synthesize positive-stranded HBV DNA. The HBV multimerase plays a crucial role in HBV replication. The definition and nomenclature of HBV drug-resistant mutations are divided into phenotypic resistance and genotypic resistance to antiviral drugs. Phenotypic resistance refers to an increase in viral levels during treatment, generally measured by the concentration of antiviral drugs (IC50), and an increase in IC50 indicates a decrease in drug sensitivity or an increase in drug resistance, requiring a higher drug dose to suppress the mutated virus. Genotypic drug resistance refers to mutations in the viral polymerase gene, forming a new viral gene sequence, which is generally determined by DNA sequencing and gene chips. The mutated virus can often change its biological properties, which brings a series of problems to the clinical control. For HBV drug resistance can be divided into phenotypic resistance, genotypic resistance, clinical resistance, clinical resistance refers to the clinical appearance of viral replication can not be inhibited, or HBV replication was once inhibited and then HBV DNA rebound, accompanied by an increase in ALT. HBV is divided into 8 genotypes (A to H). The length of HBV multimerase varies among genotypes. The HBV polypeptide can be divided into four functional regions: terminal protein, spacer, reverse transcriptase and RNase H. HBV drug-resistant variants are based on the internationally accepted amino acid single letter plus variant. The HBV resistance variants were labeled with the internationally accepted amino acid single letters plus variant sites. For example, YMDD represents four amino acids (tyrosine-methionine-aspartate-aspartate) in the reverse transcriptase region, where a change from methionine (M) to leucine (V) or isoleucine (I) would cause lamivudine resistance. Initially, the location of the HBV resistance mutation was from the first amino acid number of the HBV polymorpha. Since the length of the HBV multimerase varies among the eight HBV genotypes, the YMDD is located at different sites with different HBV genotypes as reference. The same YMDD variant was reported as M552V, M550V, M539V and other different designations. To avoid confusion, Stuyverg et al. proposed to standardize HBV drug resistance mutations from the first amino acid number of the functional region of reverse transcriptase (344 amino acids for each genotype) with the prefix rt (e.g., the YMDD variant was named rtM204V). The HBV polycomb reverse transcriptase region contains seven gene retention subregions (A to G). The three-dimensional structure of HBV polymutase reverse transcriptase is like a half-open right hand. rtM204V or rtM204I mutations are a direct cause of lamivudine resistance. In terms of molecular structure, both V and I have one more β-methyl group than M. This extra methyl group causes lamivudine resistance. This extra methyl group causes space congestion in the lamivudine binding site, thus making lamivudine ineffective in binding to HBV polymorphic enzymes. The mechanism of drug-resistant mutations in HBV As HBV replication in vivo requires a reverse transcription step, the lack of DNA-P proofreading enzyme activity during reverse transcription does not correct nucleotide mismatches, resulting in HBVDNA sequence variation, and the natural replication error rate in HBV replication is about 10 times higher than that of other DNA viruses. The natural variation of HBV genes during replication results in the formation of quasispecies, a group of genetically similar but not identical viral strains in HBV-infected patients. In patients not treated with nucleoside analogues, HBV wild-type strains account for the majority of HBV quasispecies. Drug-resistant variants may be present before or during the course of drug administration. The drug-sensitive wild-type strains of HBV are suppressed after the patient is treated with the drug. The strains containing drug-resistant mutations are given more space to replicate. When a drug-resistant strain becomes the dominant strain in a quasi-species, the drug loses its efficacy. The nucleoside analogs for the treatment of hepatitis B mainly inhibit HBV replication by competing with the natural substrate (dNTP) of HBV polycombase to achieve inhibition of HBV polycombase activity. The P region is the largest ORF in the HBV genome, with a total length of 2496 bp, encoding a polypeptide containing 832 amino acids, called HBVDNA polypeptidase (DNAP), located between nt2357 and 0-1621. mutations in drug-resistant strains of HBV occur in the rt region of the DNAP gene. Clinical studies have identified H B V resistant variants of various drugs, and the available drugs can be divided into three groups according to the different forms of nucleoside (acid) analogue resistant variants, (1) L-nucleoside (acid) analogue group including Lamivudine (LMV), Emtricitabine (FTC), Tebivudine ( (2) acyclic phosphate group, including adefovir dipivoxil (ADV) and tenofovir (TFV); (3) cyclopentene group, including entecavir (ETV), etc. 1, L- nucleoside (acid) analog group DNA polymerase C region sub-structural domain (motif) is the active site of HBV for reverse transcription, consisting of tyrosine (Y), methionine (M) – aspartate (D) – aspartate (D), that is, YMDD, this site is the binding site of LAM interference with HBV replication This site is the binding site for LAM to interfere with HBV replication. It has been demonstrated that the methionine at rt204 in YMDD can be replaced by valine (V) or isoleucine (I) to produce YVDD or YIDD, resulting in a change in the spatial configuration of this substructural domain, which prevents binding to LAM. the YIDD variant can exist alone, while YVDD is often accompanied by a variant in which the leucine (L) at rt180 in the B region of the polymorpha is replaced by a methionine (M) . The sensitivity of mutated HBV to LAM is significantly reduced. The main resistance mutation site associated with lamivudine is rtM204I/V. Different types of mutant forms associated with resistance have been identified in current studies including: (1 ) r t M 2 0 4 I / V + rtL180M; (2) rtM204I; (3) rtV173L + rtL180M + rtM204V; (4) r1L80I + rtM204I; ( (5) rtQ215S + rtM204I/V + rtL180M; (6) rtIl69T + rtV173L + rt 180M + rtM204V; (7) rtA181T; (8) rtT184S + rtM204I/V ± rtL180M; (9) rtM204S + rtL180M. Some of these mutations are as compensatory mutations can enhance viral replication activity or drug resistance, and a portion of the variants affect subsequent treatment selection. The resistance characteristics of other L-nucleoside (acid) analogues are similar to those of lamivudine, with the main variant site focused on rtM204I/V accompanied by some other site variants. Unlike lamivudine resistance, which is mainly concentrated at rtM204, the adefovir resistance variants are more scattered, and in addition to the above two variants, rtP237H, rtN238T/D, rtV84M, rtS85A, rtQ85A, rtV84M, and rtS85A were also found. rtP237H, rtN238T/D, rtV84M, rtS85A, rtQ215S and rtV214A, etc. Since the form of adefovir resistance mutation is different from that of lamivudine, patients with adefovir resistance can be treated with lamivudine or other L-nucleoside (acid) analogues, and lamivudine resistance can also be treated with adefovir. t F V is effective against lamivudine-resistant strains, but mutations resistant to both T F V and lamivudine were found in patients co-infected with H I V and H B V. The mutation site is located in the B region and the C region binding site rtA194T, with the rtL180M+rtM204V mutation. This combined mutant strain showed a more than 10-fold decrease in sensitivity to TFV drugs. 3, cyclopentene group There are two main forms of mutations in entecavir resistance, both occurring in lamivudine-resistant patients, (1) rtM250V ± rtI169T + rtM204V + rtL180M, (2) rtT184G + rtS202G/I + rtM204V + rtL180M. rtV173L variant was recently discovered. V. Prevention and treatment of HBV drug resistance 1. Predictors of hepatitis B virus drug resistance variants: A variety of factors can predict the chance of HBV resistance to nucleoside (acid) analogues. For example, a high HBV DNA load at the start of treatment, a foundation of liver fibrosis/cirrhosis, previous antiviral treatment with nucleoside (acid) analogs, and a high adaptive capacity of resistant viral strains all suggest a high risk of drug resistance. An increasing number of studies suggest that early virological response is also an important predictor of the incidence of drug resistance. 2. Prevention strategies for drug-resistant mutations of hepatitis B virus: ( 1) Rational selection of nucleoside (acid) analogues for antiviral therapy: nucleoside (acid) analogues are not recommended for immune-tolerant or inactive HBV-infected patients, especially those of younger age, who do not need to receive immunosuppressive or chemotherapeutic drugs. (2) Rational selection of antiviral treatment regimen: Refer to the Chinese “Guidelines for the Prevention and Treatment of Chronic Hepatitis B” for the treatment regimen. The virological response of the patient during treatment should be closely monitored. In addition, single-drug sequential therapy should be avoided as much as possible to avoid the occurrence of multi-drug resistance. (3) Improve patient compliance: During antiviral therapy with nucleoside (acid) analogs, it should be repeatedly emphasized that the medication should be taken on time and in sufficient quantity as prescribed by the doctor. (3) Regular monitoring of response and timely adjustment of treatment regimen: HBVDNA levels should be tested every 3 months during treatment, and genotypic resistance testing should be performed promptly for primary treatment failure or virological breakthroughs if not due to poor compliance, and to identify mutation patterns to guide switching to other treatment regimens. dynamic changes in HBV DNA levels are important indicators for early detection of drug resistance mutations. However, when analyzing the results, it should be noted that the sensitivity of different laboratories and different testing methods vary. The clinical management of drug-resistant mutations is recommended: for a small number of patients with normal ALT and mild inflammatory or fibrotic lesions (< G1S1) on histological examination of the liver before treatment, antiviral therapy can be stopped, but close monitoring is required, and once there is a sudden outbreak of hepatitis, antiviral therapy should be administered again in a timely manner; for most nucleoside (acid) analogue-resistant patients, especially those with decompensated cirrhosis, early rescue therapy is required. The majority of patients with nucleoside (acid) analogue resistance, especially in decompensated cirrhosis, require early rescue therapy. Usually, virological breakthrough precedes biochemical breakthrough, and rescue therapy before biochemical breakthrough can save patients from hepatitis flare-up and liver disease deterioration. Rescue therapy requires the addition or substitution of non-cross-resistant nucleoside analogs according to the resistance of the virus to different nucleoside analogs; if not contraindicated, IFN2α or pegylated interferon may also be used. If there is no contraindication, IFN2α or pegylated interferon can be used. PCR amplification - restriction enzyme slicing polymorphism analysis (PCR-RFLP) is currently commonly used in domestic laboratories for the detection of YMDD variants, rtA181V adefovir resistance, BCP area variant loci. Real-time fluorescence PCR amplification is the more recent domestic application of HBV drug resistance detection methods. 2, gene chip analysis, including fluid chip technology, probe hybridization technology, mainly through reverse speckle hybridization technology to achieve the detection of variant sites. 3.Pyrophosphate sequencing, reproducibility and reliability testing of serum specimens showed that the mutation detection rate and reproducibility of plasmid standards were 100%, while the serum specimens were 98.8%. 4. Gene cloning and sequencing, using specific primers to clone and sequence the target fragment, and identifying the mutation site by comparison with the standard strain. This is the most objective detection method among all the detection methods, but the operation is complicated, high cost and time consuming are the main defects of this method. VII. Conclusion Variation (mutation) exists in any species in nature, and it is an important way for organisms to adapt to their environment and maintain survival. mismatches, so that 1010 to 11 mismatches occur per day, and most mutations are silent mutations with no biological significance. the variability of HBV is so high that no two strains of the virus have identical nucleotide sequences. HBV undergoes superior selection for mutation under the influence of factors such as its infection chronicity process, immune response, vaccination, and viral drug therapy to achieve species survival. Recent studies have shown that host immune pressure plays an important role in the generation and selection of HBV gene mutations. In conclusion, HBV mutations have a significant impact on hepatitis B diagnosis, treatment, and prevention, and mutations occurring in each ORF in HBV persistent infection are not isolated and can occur as multi-locus mutations. Researchers have now successfully deciphered new genes encoding the HBV genome - the pre-pre-S gene and pre-X gene. This has increased the number of ORFs in the HBV genome to six. In addition, the HBV genome includes several other regulatory elements involved in viral replication, including four promoters, two enhancers, packaging signal ε, etc. Therefore, HBV variants must be treated from a genome-wide perspective, and HBV variants must be viewed from a developmental perspective and analyzed from all angles in order to correctly analyze the disease and avoid clinical omissions, blind drug use and delayed treatment, which may aggravate the patient's condition. Only by correctly treating the variation of HBV dots can we judge the choice of antiviral drugs and disease prognosis, thus providing a basis for personalized patient treatment.