Intrinsic to the ipsc generation process may pose

intrinsic to the IPSC generation process may pose risk of
enhancing tumorigenesis through both the introduced genes
themselves and in theory via the potential changes at specific
integration sites.
The IPSC field is evolving rapidly and moving away from
methods of induction that rely on genetic changes. This
approach is in its early days with some very promising initial
results [30, 31], but predictions are that such a move gener-
ally should reduce tumorigenicity and improve safety. How-
ever, important questions remain. Will it ever be possible to
make IPSC with absolutely no genetic changes? Can IPSC
ever totally escape from dependence (whether via genetic or
nongenetic approaches) on Myc, KLF4, and other possible
oncogenes? Although it may appear that the IPSC field has al-
ready answered this affirmatively for Myc in that IPSC can be
generated without added Myc, the omission of Myc reduces
the efficiency of IPSC generation and yet these IPSC can still
produce tumors in the form of teratoma [20, 21, 32]. These
studies also do not address the role of endogenous Myc. In
IPSC generated without genetic addition of Myc, the cells of
origin may well be characterized by unusually high levels of
endogenous Myc proteins required for the reprogramming and
could make the cells prone to tumorigenesis.
P
ROPOSING AN
A
LTERNATE
A
SSAY OF
T
UMORIGENICITY FOR
IPSC
AND
O
THER
S
TEM
C
ELLS
To gauge the tumorigenic nature of IPSC most often research-
ers have used what will be termed the derived mouse assay.
In this assay, through mouse genetics IPSC are used to con-
tribute to the formation of chimeric mice in which some tis-
sue lineages are derived from IPSC. The mice are then stud-
ied for tumor development. Using this assay, IPSC have been
reported to cause malignant tumors in up to 20% of such
derived mice [5], whereas IPSC generated without exogenous
Myc have been reported to not form tumors [20, 32]. The
Figure 1. The relationship between pluripotency and tumorigenicity.
Biological (top) and molecular (bottom) links between pluripotency
and tumorigenicity are described. Abbreviations: EC, embryonic
carcinoma cells; hESC, human embryonic stem cells; IPSC, induced
pluripotent stem cells.
1052
Confronting the Stem Cell Tumorigenicity Problem

cause of the tumors has been thought to be most often due to
reactivation of Myc from previously silenced viral insertions.
The problems with the derived mouse assay are twofold. First,
the IPSC must go through early embryogenesis and are sub-
ject to the powerful embryonic reprogramming forces that are
predicted to dramatically reduce the apparent tumorigenicity
of the cells. Second, the derived mouse system for studying
IPSC tumorigenicity bears no resemblance to how cells would
be used in regenerative medicine where they would either be
focally injected in a site to be repaired or administered intra-
venously (IV). Both focal injection and IV administration of
IPSC as a means for studying IPSC tumorigenicity have not
been reported in the literature. Unfortunately, these assays are
expected to show higher rates of tumorigenicity than the
derived mouse assay, but more accurately reflect