Download Diapositiva 1 - Laboratorio de Genómica Viral y Humana

Retrotransposons &
Retrovirus
CA García Sepúlveda MD PhD
Laboratorio de Genómica Viral y Humana
Facultad de Medicina, Universidad Autónoma de San Luis
Potosí
1
Session #27 Retroposons
Introduction
• Retroposon or Retro-transposons are transposons that mobilize via an RNA
form.
• The DNA element is transcribed into RNA and then reverse-transcribed into
DNA, which is inserted at a new site in the genome.
• Transposition that involves an obligatory intermediate of RNA is unique to
eukaryotes.
• Some eukaryotic transposons are related to retroviral proviruses in their
general organization.
• Retroviruses insert DNA copies (proviruses) of an RNA viral genome into the
chromosomes of a host cell.
2
Session #27 Retroposons
Introduction
• The life cycle of retroviruses and retroposons
involve alternation of reverse transcription
from RNA to DNA with transcription from DNA
to RNA.
• Only retroviruses can generate infectious
particles.
• Retroposons are confined to an intracellular
cycle.
• Retroposons & retrovirus share features with other transposons
– short direct repeats of target DNA at the site of an insertion.
3
Session #27 Retroposons
Retroviruses
dsRNA
ssRNA+
dsDNA
ssRNARNA
Virus
RT
DNA
Virus
ssDNA
Virus
4
Session #27 Retroposons
Retrovirus
– Belong to the Retroviridae family of single-stranded RNA viruses that
Reverse Transcribe (ssRNA-RT).
– Enveloped viruses possessing an RNA genome.
AKA Pararetrovirus
dsDNA-RT
ssRNA-RT
5
Session #27 Retroposons
Retrovirus
– Belong to the Retroviridae family of single-stranded RNA viruses that
Reverse Transcribe (ssRNA-RT).
– Enveloped viruses possessing an RNA genome.
– Its members include:
ssRNA-RT
6
Session #27 Retroposons
Retrovirus
– Belong to the Retroviridae family of single-stranded RNA viruses that
Reverse Transcribe (ssRNA-RT).
– Enveloped viruses possessing an RNA genome.
– Its members include:
ssRNA-RT
• Avian leukosis virus
7
Session #27 Retroposons
Retrovirus
– Belong to the Retroviridae family of single-stranded RNA viruses that
Reverse Transcribe (ssRNA-RT).
– Enveloped viruses possessing an RNA genome.
– Its members include:
ssRNA-RT
• Mouse mammary tumour virus
8
Session #27 Retroposons
Retrovirus
– Belong to the Retroviridae family of single-stranded RNA viruses that
Reverse Transcribe (ssRNA-RT).
– Enveloped viruses possessing an RNA genome.
– Its members include:
ssRNA-RT
• Murine & feline leukemia virus
9
Session #27 Retroposons
Retrovirus
– Belong to the Retroviridae family of single-stranded RNA viruses that
Reverse Transcribe (ssRNA-RT).
– Enveloped viruses possessing an RNA genome.
– Its members include:
ssRNA-RT
• Human T-lymphotropic virus
10
Session #27 Retroposons
Retrovirus
– Belong to the Retroviridae family of single-stranded RNA viruses that
Reverse Transcribe (ssRNA-RT).
– Enveloped viruses possessing an RNA genome.
– Its members include:
ssRNA-RT
• Walleye dermal sarcoma virus
11
Session #27 Retroposons
Retrovirus
– Belong to the Retroviridae family of single-stranded RNA viruses that
Reverse Transcribe (ssRNA-RT).
– Enveloped viruses possessing an RNA genome.
– Its members include:
ssRNA-RT
• Human immunodeficiency virus 1
12
Session #27 Retroposons
Retrovirus
– Belong to the Retroviridae family of single-stranded RNA viruses that
Reverse Transcribe (ssRNA-RT).
– Enveloped viruses possessing an RNA genome.
– Its members include:
ssRNA-RT
• Chimpanzee foamy virus
13
Session #27 Retroposons
Retrovirus
– Rely on reverse transcriptase to
perform the reverse transcription
of its genome from RNA into DNA.
– cDNA is integrated into the host's
genome with the help of an
integrase.
The virus then replicates as part of the cell's DNA, which means:
mRNA
Positive sense
5'-Capped
3'-Poly-A
14
Session #27 Retroposons
Retrovirus
– These retrovirus are exogenous.
ssRNA-RT
– A retroviral sequence that is integrated in the germline remains in the
cellular genome as an endogenous PROVIRUS.
– Like a lysogenic bacteriophage, a provirus behaves as part of the genetic
material of the organism.
– They are inherited to the progeny.
15
Session #27 Retroposons
Retrovirus
– Retrovirus that have been incorporated to the genome are called
ENDOGENOUS (ERV).
– They are derived from ancient infections of germ cells in humans,
mammals & other vertebrates.
– ERVs make up 5-8% of the human genome (98,000 elements).
– Most insertions have no known function (junk DNA) but some play
important roles in host biology:
• Control of gene transcription.
• Control of cell fusion during placental development.
• Resistance to exogenous retroviral infection.
• Immunosuppresion
16
Session #27 Retroposons
Retrovirus
– ERVs are activated during pregnancy in viviparous mammals (not
Monotremes) and particularly during the implantation of the embryo.
They act as immunosupresors
protecting the embryo from its mother's
immune system.
Viral fusion proteins involved in the
formation of placental syncytium limits
cell migration (something an epithelium
will not do well, as certain blood cells
are able to diapedize).
17
Session #27 Retroposons
Retrovirus
– ERV's are similar to HIV (which causes AIDS in humans).
– The immunodepressive action was the initial normal behavior of the virus,
as in HIV.
– The fusion proteins were a way to spread the infection to other cells by
merging them with the infected one, as in HIV.
– It is believed that the ancestors of modern vivipary mammals evolved after
an infection by this virus, enabling the fetus to survive the immune system
of the mother.
18
Session #27 Retroposons
Retrovirus
– 24 ERV families identified by Human Genome Project (HGP).
– Broadly classified into 3 classes,
on the basis of relatedness to
exogenous genera:



Class I are similar to the
gammaretroviruses
Class II are similar to the
betaretroviruses &
alpharetroviruses
Class III are similar to the
spumaviruses
19
Session #27 Retroposons
Retrovirus
– Most ERVs appear to be defective, containing nonsense mutations or
major deletions which prevent them from producing infectious virus
particles.
– However, there is one family of viruses that have been active since the
divergence of humans and chimpanzees [ hERV-K(HML2)].
– Make up less than 1% of hERV elements but is one of the most studied.
– There are indications it has been active in the past few hundred thousand
years, as some humans carry more copies of the virus than others.
20
Session #27 Retroposons
Retrovirus
– Human ERVs (hERVs) are suspected of
involvement in some autoimmune diseases
(multiple sclerosis).
– Especially human endogenous retrovirus W
known (MSRV).
– Also a possible hERV involvement in the
HELLP (Hemolytic anemia, Elevated Liver
enzymes & Low Platelet count) syndrome &
pre-eclampsia.
– hERVs very likely associated with some
types of schizophrenia.
21
Session #27 Retroposons
Retrovirus
– Retroviruses have ssRNA
genomes.
22
Session #27 Retroposons
Retrovirus
– Retroviruses have ssRNA
genomes.
– Genomes are replicated
through a dsDNA
intermediate.
23
Session #27 Retroposons
Retrovirus
– Retroviruses have ssRNA
genomes.
– Genomes are replicated
through a dsDNA
intermediate.
– The life cycle involves an
obligatory stage of
integration in which the
proviral dsDNA is inserted
into the host genome by a
transposition-like event that
generates short direct
repeats of target DNA.
24
Session #27 Retroposons
Retrovirus
– The enzyme responsible
for generating the initial
DNA copy of the RNA is
reverse transcriptase.
– Converts the RNA into a
linear (or circular) duplex
of DNA in the cytoplasm
of the infected cell.
25
Session #27 Retroposons
Retrovirus
– The enzyme responsible
for generating the initial
DNA copy of the RNA is
reverse transcriptase.
– Converts the RNA into a
linear (or circular) duplex
of DNA in the cytoplasm
of the infected cell.
– The enzyme responsible
for integrating the DNA
copy is integrase.
26
Session #27 Retroposons
Retrovirus
– Implications of proviral DNA integration:
• A retroviral sequence that is integrated in the germline remains in the
cellular genome as an endogenous provirus (behaves as part of the
genetic material of the organism).
• Cellular sequences may be transposed with proviral DNA and
inserted into the genome as duplex sequences in new locations.
• Retrovirally transposed cellular sequences may change the properties
of the cell that becomes infected with the virus.
27
Session #27 Retroposons
HIV as an example
1) Tropism
2) Fusion
3) RT
4) Integration
5) Transcription
6) Assembly
7) Budding
8) Release
9) Maturation
28
Session #27 Retroposons
HIV as an example
1) Tropism
2) Fusion
3) RT
4) Integration
5) Transcription
6) Assembly
7) Budding
8) Release
9) Maturation
Integration mediated by
integrase.
Can occur at one or more sites.
29
Session #27 Retroposons
HIV as an example
1) Tropism
2) Fusion
3) RT
4) Integration
5) Transcription
6) Assembly
7) Budding
8) Release
9) Maturation
Proviral DNA is transcribed by
the host machinery to produce
viral RNAs:
- mRNAs
- genomic RNA.
30
Session #27 Retroposons
HIV as an example
1) Tropism
2) Fusion
3) RT
4) Integration
5) Transcription
6) Assembly
7) Budding
8) Release
9) Maturation
The enzymes reverse
transcriptase and integrase are
carried with the genome in the
viral particle.
31
Session #27 Retroposons
HIV as an example
Two copies of the RNA
genome are packaged into
each virion (diploid virus).
When a cell is infected by
two different viruses,
it is possible to generate
heterozygous virus particles
carrying one genome of each
type.
32
Session #27 Retroposons
Retroviral translation & processing
A typical retroviral sequence
contains 3 or 4 "genes".
Gag-Pol-Env
Coding regions give rise to multiple
proteins by processing reactions.
Retroviral mRNA has a
conventional structure; it is capped
at the 5′ end and polyadenylated at
the 3′ end.
33
Session #27 Retroposons
Retroviral translation & processing
Retroviral mRNAis represented
by two subgenomic species.
1. Gag-Pol-Env (Full length)
2. Env
34
Session #27 Retroposons
Retroviral translation & processing (Gag)
The Gag product is translated
by reading from the initiation
codon to the first termination
codon.
Gag derived products p6, p7,
p17 & p24 are generated by
post-translational processing.
Gag derived products have
structural functions.
35
Session #27 Retroposons
Retroviral translation & processing (Pol)
To translate the Pol product,
the first termination codon
must be bypassed.
Depends on the “reading frame
correspondence” between Gag
and Pol.
If Gag and Pol are in the same
reading frame...
Termination codon usage is
suppressed by a glutamyl-tRNA,
generating a Gag-Pol peptide.
36
Session #27 Retroposons
Retroviral translation & processing (Pol)
Gag-Pol peptide is then
processed to yield:
1- Gag derived products (p6,
p7, p17 & p24).
2- Pol derived products (p10,
p32, p51, p66).
Pol derived products have
enzymatic functions.
37
Session #27 Retroposons
Retroviral translation & processing (Pol)
When Gag & Pol are in different
reading frames, a ribosomal
frameshift occurs to generate a
single protein.
Readthrough is ~5% efficient,
so Gag protein outnumbers
Gag-Pol protein by 20x.
←20x
←1x
38
Session #27 Retroposons
Retroviral translation & processing (env)
The Env polyprotein is expressed
by post-transcriptional splicing
generating a shorter subgenomic
messenger that is translated into
the Env product.
Env products gp120 & gp41 are
envelope proteins.
39
Session #27 Retroposons
Retroviral translation & processing
40