Prions, emerging viral infections and HIV Flashcards
a) Overview of prions
b) TSE
c) Prion structure
a) They are infectious proteins that cause diseases called transmissible spongiform encephalopathies (TSEs)
b) These are chronic, neurodegenerative diseases, forming vacuoles in the brain. They can infect both man and animals and are commonly fatal, although may take decades to develop. eg i) Scrapie in sheep ii) Bovine spongiform encephalopathy (BSE) in cattle iii) Chronic wasting disease (CWD) in deer (cervids) iv) Creutzfeldt-Jakob disease (CJD) and variant CJD (vCJD) in humans v) Kuru in tribes in Papua New Guinea where cannibalism is traditional
c) PrPᶜ - a 240 aa membrane protein, mainly α-helical (found in neural tissue and lymphoid). PrPˢᶜ - the abnormal protein, mainly β-sheet (making it resistant to proteases, so it accumulates in neural tissue, forming amyloid plaques). PrPᶜ can be converted to PrPˢᶜ, this can be spontaneous, but can be promoted by specific aa changes in PrPᶜ, or be catalysed by PrPˢᶜ (so the more abnormal protein their is, the more conversion will occur). PrPˢᶜ join with eachother to form oligomers, which go on further to form amyloid fibrils
a) How are prions transmitted
b) CJD and vCJD
c) Detection and screening for prions
d) Scrapie in sheep
a) Eating infected animals (BSE in cattle transmitted by feeding cattle with infected material, vCJD in humans transmitted by eating contaminated beef). Iatrogenic from medical or surgical procedures, or blood transfusions. (Also eating of infected humans, tradition of cannibalism transmits Kuru)
b) TSEs in humans are rare, but CJD is the most common. Most cases are sporadic, a few are familial, and rarely iatrogenic. There was an increased number of vCJD cases following the BSE epidemic.
c) There is no adaptive immune response against abnormal PrPᶜ. Monoclonal antibodies are able to distinguish between PrPᶜ and PrPˢᶜ. Additionally, PrPᶜ and PrPˢᶜ can be distinguished be protease digestion/SDS-PAGE/immunoblotting and by analysis of glycoforms. PrPˢᶜ may eb identified by bioassay
d) In the preclinical phase, infectious material is first associated with gut lymphoid tissue. Then, the PrPˢᶜ is detected in the peripheral lymph nodes, suggesting haematogenic dissemination. Finally PrPˢᶜ accumulates in the CNS. There is a breeding scheme to select flocks that are resistant to TSEs
SARS-CoV-2
a) Describe the virion
b) Describe the genome
a) Enveloped with surface spikes, 80-140nm. The spike (S) mediates binding to cells via angiotensin converting enzyme II (ACE-2). S is cleaved into S1 and S2 subunits. S1 contains the receptor binding domain (RBD) and is the most variable protein.
b) +ve ssRNA, 22.9kb. From 5’ end, first 21kb encodes ORF 1a and 1b, which are polyproteins cleaved into mature proteins for genome replication and immune evasion. 1b polyprotein is expressed by ribosomal frameshifting. The next 3.9kb encodes S, the spike protein. There are several non-structural proteins (nsps) needed for genome replication and immune evasion (preventing the induction and action of interferon). Nsp14/nsp10 complex detects and repairs 3’ nucleotide mismatches produced during replication (allowing virus genome to be large)
Human immunodeficiency virus (HIV)
a) Structure
b) Genome
a) A retovirus, subgroup lentivirus. 110nm. Has surface glycoproteins gp120 and gp41
b) +ve ssRNA, diploid, 10kb. Genome is copied into DNA and then integrated in the host as a provirus. Genes are transcribed by host DNA-dependent RNA polymerase II. There are extra regulatory genes expressed by differential splicing
a) HIV transmission and entry to cells
b) Polymorphism in CCR5
a) Transmission - sexual contact, intravenous drug abuse, mother to baby, contaminated blood products.
Entry - virus gp120 binds to cells expressing both CD4 and a co-receptor CCR5 (macrophage tropic)/CXCR4 (T cell tropic) - chemokine receptors
b) There is a mutant allele of CCR5 with a 32bp deletion. This causes termination of the protein after transmembrane domain 4. The mutant protein doesn’t function as a HIV-1 co-receptor. This mutant allele is found in Caucasians (16% heterozygous, 1% homozygous), with homozygotes resistant to infection, but not immune to the disease itself - transmission is largely by CCR5 tropic viruses
Outcomes of HIV infection
a) Slow progressors
b) Long term non-progressors
c) Rapid progressors
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a) Anti-HIV drugs (3)
b) HIV drug resistance
a) i) AZT (zidovudine, retrovir) - nucleoside analogue, after incorporation into DNA it blocks further elongation (functions as a chain terminator). Has specificity as AZT is a better substrate for the HIV reverse transcriptase than host cell DNA polymerase
ii) Dideoxycytidine (ddC) and dideoxyinosine (ddI) 3TC (lamivudine) - other chain terminating nucleoside analogues, has some toxicity
iii) Ritonavir, saquinovir, indinavir - protease inhibitors, target the HIV aspartate protease and prevents cleavage of HIV polyprotein precursors into mature proteins
b) Mutations arise due to error prone nature of reverse transcriptase and RNA pol II, so with time the virus becomes resistant to a single drug. Can change the drug, but mutations arise again. This is solved by using several drugs simultaneously - HAART (highly active anti-retrovirus therapy) is in widespread use in developed countries, it is harder for the virus to develop multidrug resistance simultaneously
