Chronic hepatitis Viruses Flashcards
What is hepatitis B virus?
Small circular partially duplex 3.2kb genome
Compact genome every nt is coding and over 50% codes for more than 1 protein
Genome replication involves RNA copy and reverse transcriptase activity
HBV epidemiology?
One of most prevalent infections in world
Ab vs surface Ag can be used to determine prevalence in globe
8% (high) in northern Canada/Alaska central/Southern Africa central/Southeast Asia
2% in Northern Europe/USA and Central America
WHO estimates 2 billion have serological evidence of HBV infection
360 million have chronic infections
500,000-700,000 deaths a year
Responsible for 33% of cirrhosis
50% of hepatocellular carcinoma
Virus very stable in blood, secretions and excretions. All fluids capable
of HBV transmission
109 virions/ml blood from chronic carriers and can withstand up to 1 week drying and while maintaining infectivity
Vertical transmission possible 90% of mothers transmit and 85% babies become chronic carriers
Also by sexual transmission and percutaneous(shared razor/toothbrushes)
Clinicl progression of HBV?
95% cases of adult acute HBV infection results in recovery
5% establish chronic infection
Infants is 10% v 90% much worsei
1% can cause Fulminant hepatitis- very rapid, aggressive and often leads to death via acute liver failure
75%-80% will stabilise showing no obvious signs of infection
0.5% develop hepatocellular carcinoma
2-3% develop cirrhosis from stabile chronic infection
20-25% develop cirrhosis which liver can compensate or can’t be in chronic infection
2-6% of those with compensated or decompensated develop Hepatocellular carcinoma
Steps of HBV replication
- Membrane fuses with cell membrane, releasing the core virus into the cytoplasm, where DNA translocates to the nucleus
- Viral DNA is repaired by cellular enzymes to form covalently closes circular DNA (cccDNA)
- cccDNA acts as a template for transcription and the transcripts are then exported + translated
- One transcript is longer than the viral genome (pre-genomic RNA) which is packaged into viral particles
- Reverse transcriptase synthesises a DNA copy of the RNA genome (minus then plus strand)
- Travels to the ER to be exocytosed or transported back to nucleus
Basically opposite of retrovirus replication
dsDNA->ssRNA->dsDNA
Retrovirus-> ssRNA-dsRNA-ssRNA
Some anti-HIV drugs also target HBV replication
Coding capacity of HBV?
Every nt is coding for at least 1 protein. 50% encode for more than 1
4 ORFs in total
- S fragment encodes for the envelope proteins(within region)
- Region P encodes the polymerase
- ORF X
- ORF C
Due to extensive overlapping theee is little variation between strains
HBV genome ?
Has partially dsDNA genome that is not covalently linked
Both + and - ve strand have exposed 3’ and 5’ ends
Describe -ve strand of HBV genome?
5’en d is linked to terminal protein (polymerase) whole strand is circular 3.2kb
Describe +ve strand of HBV genome?
+ve strand starts with small piece of RNA at 5’end goes for about two thirds of genome
DR1 and DR2 are direct repeats that link the +be and -be strand via proteins
What forms HBV capsid?
ORF C/ core protein forms the cost translated form the 3.5kb mRNA
HBV core antigen
Can produce 2 proteins
HBeAg or HBcAg
Initiate from different start codons. 1st start coding is inefficient and sometimes misses and ribosome starts from a
Second.
1st=HBeAg 2nd=HBcAg 29amino acids missed out are a hydrophobic signal sequence that takes the protein into the ER
Form monomer and dimer spikes on core particle surface
Loop on monomers that enables Ag presentation
Are nucleic acid binding proteins
HBcAg vs HBeAg
HBcAg can form core particles in e.g. E. coli expression systems
Accumulates in cytoplasm and dimerises via a disulphides bridge
HBeAg has different functions is transported to ER for secretion
- high levels of HBeAg in sera implies it acts to suppress immune system via mopping up of Abs
- Hbe -ve woodchuck hepatitis fails to establish persistent infection
HBV surface Ag
3 surface Ag presented on envelope derivedflr
2 mRNAs (2.1/2.4kb)
-Large,medium and small HBsAg which are all related proteins
Alternative promoters drive production of alternate mRNA
Large Ag is made from translation from 1st start codon of 2.4kb mRNA has 3 domains: preS1, preS2 and S with a myristic acid
Medium Ag is made from the 2nd start codon of the 2.1kb mRNA containing preS2 domain and S domain
Small Ag is made from the 3rd start codon of the 2.1kb mRNA contains just S domain
Function of large HBsAg
Key protein O surface that interacts with the receptor
–4 TM domain helices within the S domain
Has 2 topologies it can adopt
1– PreS1 and PreS2 presented external to the cell with myr anchoring allowing interaction with cellular receptor
—–sodium taurochlorate co-transporting polypep(NTCP), a 10 TM protein located on the sinusoidal membrane of membrane of hepatocytes(facing blood)
—–naturally transports Na and bile acids
2– 2nd conformation has 3 TMs, N-terminus in internal where PreS1/PreS2 can stabilise viral membrane-core particle interactions(important for stability)
Membrane topology of medium and small HBsAg
Small Ag forms a 4 TM protein with C+N external
Medium Ag forms a 4 TM protein with C+N external, PreS2 is presented externally
Look at diagrams and memorise
What are the multiple forms of HBV?
As seen in serum
- Dane(mature, infectious) particles contain nucleocapsid in the middle and surface Ag/lipids- enriched in L protein
-20nm diameter spheres/filaments which are lipid membranes containing mainly S ag, very little M and L + lipid- possibly immune decoy
Very high conc of HBsAg in serum (<300ug/ml)
