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chromatin features, types of targeted genes,
CpG islands, and other genomic features.
Thus, using a standardized bioinformatic platform it is possible to visually compare the
AAV vector, lentiviral vector, and ␥RV integration profiles. The strength of the study is
that coupling the whole transcriptome analysis
to the integration site data obtained from
HCCs, it was possible to measure the impact
of AAV integration on the expression of genes
targeted in these tumors in vivo. Importantly,
genes in neighboring AAV vector integrations
did not display aberrant levels of expression.
Overall, the level of detail reached by this analysis sets a new standard for AAV vector safety
studies in mice.
It is also possible also to appreciate some
peculiar challenges in the study of AAV vector
safety that instead do not appear to be a problem with retroviruses. For example, vector
integration levels are difficult to estimate as
AAV vector genomes coexist both in episomal
(mostly) and integrated forms that cannot be
easily distinguished. Also, if the integration
frequency as determined by Li and colleagues
is ⬃ 1/1500, can we expect the HCCs to be
marked? It is difficult to answer this question
because in the complex tumor microenvironment, expanding tumor cells are mixed with
bystander cells marked by integrated and/or
episomal AVV genomes and unmarked cells,
rendering vector marking measurements
problematic. How is it possible, then, to distinguish the integrations in tumor cells (if any)
from the bystanders? In the HCCs in this
study, 1-2 integrations are represented by high
numbers of sequencing reads compared with
other integrations from the same sample.
Could these be the ones that form tumor cells?
Ultimately however, the relative contribution
of each integration site should be addressed
experimentally.
Why in some studies does AAV appear to
be genotoxic while in others it does not? The
AAV vector from Donsante et al4 contained a
viral-derived cytomegalovirus early enhancer/
chicken ␤ actin promoter driving the expression of the human ␤-glucuronidase, while
Li et al used a cellular promoter chimera composed of the apolipoprotein E enhancer/
alpha1-antitrypsin promoter driving Factor
IX expression. These differences may be an
important to considered in genotoxicity assays
to confirm that the use of cellular promoters
provides an added safety value to retroviral
3250
vectors.11 Other not mutually exclusive variables may be related to an enhanced susceptibility to insertional mutagenesis of the disease
model on the mouse strain or environmental
factors that may trigger chronic liver damage.12 All these factors play an important role
in HCC formation and could play a role in the
selection of hepatocytes with genotoxic AAV
integrations.
It is expected that in the quest for safer
AAV vectors, future studies will involve in
vivo testing and validation of optimized vector
constructs and dissection of the role of genetic
and environmental variables in genotoxicity.
Conflict-of-interest disclosure: The author
declares no competing financial interests. ■
3. Deyle DR, Russell DW. Adeno-associated virus vector
integration. Curr Opin Mol Ther. 2009;11(4):442-447.
4. Donsante A, Miller DG, Li Y, et al. AAV vector integration sites in mouse hepatocellular carcinoma. Science.
2007;317(5837):477.
5. Gariboldi M, Manenti G, Canzian F, et al. Chromosome mapping of murine susceptibility loci to liver carcinogenesis. Cancer Res. 1993;53(2):209-211.
6. Dupuy AJ, Rogers LM, Kim J, et al. A modified sleeping beauty transposon system that can be used to model a
wide variety of human cancers in mice. Cancer Res. 2009;
69(20):8150-8156.
7. Zhang L, Volinia S, Bonome T, et al. Genomic and epigenetic alterations deregulate microRNA expression in human epithelial ovarian cancer. Proc Natl Acad Sci U S A.
2008;105(19):7004-7009.
8. Kool J, Berns A. High-throughput insertional mutagenesis screens in mice to identify oncogenic networks. Nat Rev
Cancer. 2009;9(6):389-399.
9. Baum C. Insertional mutagenesis in gene therapy and
stem cell biology. Curr Opin Hematol. 2007;14(4):337-342.
REFERENCES
10. Manno CS, Pierce GF, Arruda VR, et al. Successful
transduction of liver in hemophilia by AAV-Factor IX and
limitations imposed by the host immune response. Nat
Med. 2006;12(3):342-347.
1. Li H, Malani N, Hamilton SR, et al. Assessing the
potential for AAV vector genotoxicity in a murine model.
Blood. 2011;117(12):3311-3319.
11. Zychlinski D, Schambach A, Modlich U, et al. Physiological promoters reduce the genotoxic risk of integrating
gene vectors. Mol Ther. 2008;16(4):718-725.
2. Mueller C, Flotte TR. Clinical gene therapy using
recombinant adeno-associated virus vectors. Gene Ther.
2008;15(11):858-863.
12. Farazi PA, DePinho RA. Hepatocellular carcinoma
pathogenesis: from genes to environment. Nat Rev Cancer.
2006;6(9):674-687.
● ● ● IMMUNOBIOLOGY
Comment on Liu et al, page 3257
Lineage-specific pleiotropy in immune
aging
---------------------------------------------------------------------------------------------------------------Parisa Eshraghi and K. Lenhard Rudolph
UNIVERSITY OF ULM
In this issue of Blood, Liu et al provide the first genetic evidence that p16 defines a
lymphoid lineage intrinsic gatekeeper, which prevents B-lymphocyte transformation but impairs T-lymphocyte function in aging mice.1
magine if we could influence our immune
system to produce youthful numbers of
highly functional lymphocytes throughout life
up to a very advanced age, without increasing the
risk of tumors derived from the lymphoid lineage. It is conceivable that such an intervention
would reduce the risk of deleterious infections
and cancer.2 To therapeutically target the immune system to prevent aging-associated declines in immune function, it is of utmost importance to identify molecular causes of this process.
Molecular mechanisms that limit cell proliferation (eg, telomere shortening) have been
implemented in cancer protection, but were
also shown to contribute to the decline in tissue maintenance and regeneration during ag-
I
ing.3 The cycling-dependent kinase inhibitor
p16 represents one example of a cell-cycle inhibitor, which has not only been implemented
to prevent cancer formation at early age, but
also to impair stem cell function and tissue
maintenance during aging.4 The concept that
biologic processes, regulating organismal
function (such as the expression of specific
genes), can have pleiotropic effects during
lifetime (tissue protective during early life but
tissue destructive in late life) has led to the
theory of “antagonistic pleiotropy” in aging.5,6
It remains an open debate whether genes that
exhibit a pleiotropic function during lifetime
could represent targets for novel therapies,
aiming to prevent the evolution or progression
24 MARCH 2011 I VOLUME 117, NUMBER 12
blood
From www.bloodjournal.org by guest on June 18, 2017. For personal use only.
It has been shown that tumor suppressor genes (eg, p16) have pleiotropic effects during lifetime preventing the
formation of cancer during early life but contributing to impairments in stem-cell function and organ
maintenance during aging. Liu et al provide the first experimental evidence for lineage-specific pleiotropic
effects of p16 in the aging immune system. B lineage–specific deletion of p16 leads to B cell–derived
neoplasms in middle-aged mice without improving B lymphopoiesis at this age. In contrast, T lineage–specific
deletion of p16 does not lead to tumorigenesis but significantly improves T lymphopoiesis and immune
functions in aging mice. The data indicate that p16 does not contribute to tumor suppression in T-lymphoid
cells but has an important tumor-suppressive role in B-lymphoid cells. In contrast, p16 contributes to
impairments in T lymphopoiesis during aging but has no significant effects on B lymphopoiesis in middle-aged
mice. Inhibitory effects of p16 on B lymphopoiesis at advanced age cannot be excluded. The data support a new
concept indicating that lineage-specific gene-targeting (targeting of p16 in T-lymphoid lineage) could represent
a therapeutic option to improve tissue maintenance during aging (T lymphopoiesis) without affecting gene
expression and transformation in other compartments (B-lymphoid lineage).
of age-associated organ dysfunction. In this
regard, it is an important question whether
lifecycle-dependent pleiotropic effects of
genes occur within a single cell lineage or in
separate lineages. If the latter holds true, a
pleiotropic tumor suppressor gene may prevent tumor formation in one cell lineage and
induce the evolution of age-associated dysfunction in a different lineage. In the case of
lineage-specific pleiotropy, cell type–specific
gene targeting could improve age-associated
impairments in maintenance of one tissue
without affecting gene expression and cancer
risk in other tissues.
To investigate the role of p16 in lymphocyte aging and transformation, Liu et al analyzed aging mouse cohorts carrying B or
T lymphocyte–specific deletions of the
INK4A gene locus encoding for p16 protein
compared with a wild-type cohort. The authors demonstrate that p16 deletion in Tlymphocyte progenitor cells impairs agedependent involution of the thymus and improves the production of naive and memory
blood 2 4 M A R C H 2 0 1 1 I V O L U M E 1 1 7 , N U M B E R 1 2
T lymphocytes in aging mice. Importantly,
this enhancement in age-dependent T-cell
production was associated with improved immune responses in aging mice and did not increase the cancer risk. These results provide a
first example of a T cell–specific gene knockout impairing age-dependent declines in thymopoiesis. The findings do not argue against
inhibitory effects of the aging environment
(thymic niche or systemic blood circulation)
on T lymphogenesis. It is conceivable that
inhibitory effects of the aging environment on
T lymphogenesis could lead to a cell-intrinsic
up-regulation of p16 in T-lymphocytic progenitor cells. An important question in future
studies is to delineate the molecular pathways
that control the up-regulation of p16 in Tlymphocyte progenitor cells during aging.
In sharp contrast to the beneficial effects of
p16 deletion in the T-lymphocyte lineage, Liu
et al demonstrate that the deletion of p16 in
B-lymphocyte progenitor cells led to an early
formation of B cell– derived neo plasms in
middle-aged mice. Positive effects of p16 dele-
tion on the prevention of B-lymphocyte aging
could not be observed in this time window.
The contrasting effects of p16 deletion on aging and transformation in the B-lymphopoietic versus T-lymphopoietic cells represent
the most important finding of the current
study. To our knowledge, these data provide
the first experimental evidence that lineagespecific deletion of a single gene (p16) can have
age-dependent, pleiotropic effects, improving
maintenance and function in one compartment (T lmphocytes), but promoting tumorigenesis in another compartment (B lymphocytes). The authors conclude from their
findings that “this work serves as a cautionary
tale for those who would seek to ameliorate
aging by globally attenuating tumor suppressor function.”1 This conclusion is important,
but to see it more positively we can also conclude from these data that cell type–specific
targeting of pleiotropic tumor suppressor
genes could represent a novel therapeutic concept for the improvement of tissue maintenance and function during aging (see figure).
In addition, it should be noted that the deletion of tumor suppressor genes (p21 or Exonuclease-1) at the level of the whole organism
(germline deletion) has been shown to elongate
the lifespan of prematurely aging mice with
dysfunctional telomeres.7,8 These data indicate that a systemic inactivation of tumor suppressor genes could have beneficial effects at
advanced age, when defects in organ maintenance become life limiting.
Conflict-of-interest disclosure: The authors
declare no competing financial interests. ■
REFERENCES
1. Liu Y, Johnson SM, Fedoriw Y, et al. Expression of
p16INK4a prevents cancer and promotes aging in lymphocytes. Blood. 2011;117(12):3257-3267.
2. Dorshkind K, Montecino-Rodriguez E, Signer RA. The
ageing immune system: is it ever too old to become young
again? Nat Rev Immunol. 2009;9(1):57-62.
3. Artandi SE, DePinho RA. Telomeres and telomerase in
cancer. Carcinogenesis. 2010;31(1):9-18.
4. Sharpless NE, DePinho RA. How stem cells age and
why this makes us grow old. Nat Rev Mol Cell Biol. 2007;
8(9):703-713.
5. Williams GC. Pleiotropy, natural selection, and the evolution of senescence. Evolution. 1957;11:398-411.
6. Kirkwood TB. Understanding the odd science of aging.
Cell. 2005;120(4):437-447.
7. Choudhury AR, Ju Z, Djojosubroto MW, et al. Cdkn1a
deletion improves stem cell function and lifespan of mice
with dysfunctional telomeres without accelerating cancer
formation. Nat Genet. 2007;39(1):99-105.
8. Schaetzlein S, Kodandaramireddy NR, Ju Z, et al. Exonuclease-1 deletion impairs DNA damage signaling and
prolongs lifespan of telomere-dysfunctional mice. Cell.
2007;130(5):863-867.
3251
From www.bloodjournal.org by guest on June 18, 2017. For personal use only.
2011 117: 3250-3251
doi:10.1182/blood-2011-02-332650
Lineage-specific pleiotropy in immune aging
Parisa Eshraghi and K. Lenhard Rudolph
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