Thus far, the risk of liver toxicity accompanied by loss or reduction of
transgene expression appears to be the most worrying toxicity associated
with liver targeted delivery of AAV as described before (Fig. 2). However,
this phenomenon can be readily controlled with short courses of
prednisolone and appears to be self-limiting with no evidence of persistent
hepatocellular damage. The precise pathophysiological basis for the
hepatocellular toxicity observed in this study remains unclear, in part
because it has not been possible to recapitulate this toxicity in animal
models. The vector preparation used in this study contained a large excess
(~80%) of empty capsids, which are fully assembled capsids that
lack a functional viral genome . These empty particles cannot mediate
FIX expression but can serve as antigenic targets for capsid-specific
cytotoxic T cells following transduction of hepatocytes (Fig. 2) .
Since the dose administered to humans is based on an estimate of the
proviral genome copy number and not the total viral capsid load, it
seems logical to assume that removal of these contaminating empty
particles, may allow administration of the high vector dose without provoking
hepatocellular toxicity or compromising the level of gene transfer.
To test this hypothesis further we have prepared another clinical lot
of self-complementary AAV2/8-LP1-hFIXco from which empty particles
have been removed by caesium chloride (CsCl) density centrifugation.
We plan to infuse this new vector preparation in subjects with severe
haemophilia B during the next year.
As expected, all subjects developed long lasting AAV8 capsid-specific
humoral immunity following peripheral vein delivery of scAAV8-LP1-
hFIXco. Whilst the rise in anti-AAV8 IgG did not have direct clinical consequences,
its persistence at high titres precludes subsequent successful
gene transfer with vector of the same serotype in the event that transgene
expression should fall below therapeutic levels. However, we
and others have established that it is possible to achieve successful
transduction in animals including non-human primates with preexisting
anti-AAV8 antibodies following administration of AAV vector
produced with an alternate serotype . Based on our current data in
subjects with haemophilia B this is likely to be required infrequently
and at periods that extend beyond 4 years.
Another potential problem of systemic administration of AAV is
spread of vector particles to non-hepatic tissues including the gonads.
Vector genomes were transiently detectable in the seminal fluid of all
subjects recruited to the AAV2 and AAV8 haemophilia B clinical trials
. The lack of persistence of the vector genome in semen of
haemophilia B patients is consistent with animal data that suggested
that AAV transduces adventitial cells present in semen and not germ
cells.
The risk of insertional mutagenesis following AAV-mediated gene
transfer has been judged to be low because it mediated transgene expression
from episomally-retained proviral DNA. This is consistent
with the fact that wild type AAV infection in humans though common
is not associated with oncogenesis. However, deep sequencing studies
show that integration of the AAV genome can occur in the liver
. Additionally, an increased incidence of hepatocellular carcinoma
(HCC) has been reported in the mucopolysaccharidoses type VII
(MPSVII) mouse model following perinatal gene transfer of AAV [59].
Remarkably, integration analysis studies suggested that the AAV genome
had integrated within a 5 kb segment of mouse chromosome
12, in an imprinted region rich in miRNAs and snoRNAs referred to as
“RIAN” . Subsequent studies in other murine models have failed to
recapitulate this finding and collectively the available data in mice as
well as larger animal models suggest that AAV has a relatively low risk
of tumourigenesis .