Hepatitis B virus is a precursor to several other illness such as liver cirrhosis and hepatocellular carcinoma and from their names you notice that they all have to do with the liver.
The virus which is one of the smallest DNA virus has a protein coat which encloses a double stranded deoxyribose nucleic acid. When the virus infects the hepatocytes which are the main cells of the parenchymal tissue of the liver, its DNA particle changes to a form which enables transcription of the viruses mRNAs with the aid of the hosts RNA polymerase. This eventually leads to Virus replication and the production of its protein coat.
The transformed form of the nucleic acid which is the covalently closed circular DNA form has several characteristics such as its ability to have every of its nucleotide code for something and this makes its structure both compact and efficient. The protein coat of the new viral particles are produced during translation of AUG codons which are the start codons. The virus possess 7 viral proteins which needs several transcripts to enable their expression.
As mentioned earlier the HBV is compact and its compactness can lead to it having multiple effects on naturally occurring mutation from a single gene and this leads to difficulty in our efforts to figure out its pathogenic pathway.
The HBV genome is able to generate 4 polyadenylated RNAs through the aid of 4 promoters found on different position on the genome These 4 RNAs are transcribed by 2 enhancer elements and also by a transactivator known as HBx which is capable of causing multiple effects from a single gene. This transactivator is needed for HBV infection to occur and can also result in cancer of the liver.
The previous RNAs are transported to the cytoplasm of the host cell so that it can lead to protein translation and also replication of DNA.During this process, hepatitis B antigen is secreted and this protein is required for infection to be established. The pregenomic RNA which is the shorter version of the 3.5kb polyadenylated RNA produced and responsible for expression of core proteins is the only HBV transcript that is needed for genome replication.
Infection of a HBV does not directly lead to liver cirrhosis and HCC but however repeated actions of destruction and regeneration of the liver cells as a result of trying to flush out HBV may result to these diseases.
There are 8 genotypes of the HBV which are further dived into several subgenotypes with the exception of 3 genotypes, with each having little variation from the next. These genotypes of HBV are also restricted to certain regions of the world. Some genotypes such as genotype g which is popular among homosexual men are known to interact with several other diseases which can also result in more diseases originating. Although most of the genotypes of HBV are geographically specific, 1 host may possess more than 1 type of genotype of HBV (although 1 dominates over the other) as a result of increased migration. In cases of more than 1 HBV genotype infecting the same host, there may be formation of recombinant virus as a result of interaction between both viral genome.
In recent times, an additional 2 genotypes were discovered, although they had similarities with other genotypes, they possessed variations that could not be left unnoticed. The new genotype I is geographically specific to Southern China and also has a relationship with the development of HCC. Genotype J was isolated from an 88 year old man believed to have been infected during his stay at a south eastern Asian island. Genotype J is believed to be a recombinant of two HBV from different species.
To be continue…