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So hello, everybody.
This is Luigi Naldini from
the San Raffaele-Telethon
Institute of Gene
Therapy in Milan, Italy.
Today we will be discussing
properties of retroviral vectors
and, in particular,
lentiviral vectors.
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The first slide, I'm
listing key properties
of retroviral vectors in general.
First of all, these vectors
are capable of integrating
the transgene into the genome.
So they're very well
suitable for stable gene
transfer in target cell.
Transgene expression can
also be stable in long term,
but this is not
necessarily the case.
At least in some situation, you
may have the vector gene stable
inserted in a cell, but
lack of its expression.
There is a number or
reason why this may happen.
In some cases, most of the vector
may be silenced, because the cell
can recognize viral sequence, most
often in the viral long terminal
repeat-- so-called LTRs--
and specifically silence
that by epigenetic changes.
This usually occurs
in some cell type--
most often embryonic stem cell.
And this usually happens with the
gamma retroviral vector, which
contains long terminal repeat,
which carries sequence which are
recognized by this
surveillance system.
So if you are
transducing ES cell, you
may want to use a vector which is
devoided of those long terminal
repeat sequence targeted by
the surveillance mechanism.
You may use a lentiviral
vector, or a vector
which has a deletion of those LTR.
In other cases,
silencing may only affect
a fraction of the integration site.
This could be a small
subset, or a sizable subset.
And these are those integration
which randomly occur into chromatin
being condensed, and so not
permissive to transcription;
or chromatin, which
becomes condensed
after that initial transduction.
In this case it will be
extinction of the O expression.
But most of the insertion site
could still be open and expressed.
So this statement about
general stability of expression
remains true, although not
for every insertion site,
but for a majority of
them in most of the cases.
In term of fidelity of
replication retroviral vector
are dependent on
reverse transcription
for the transfer of the
genome to target cell.
Reverse transcription
is highly error-prone
so there will be a significant
rate of mutation-- up
to one in thousand basis
In the transduced cell.
