In this report, we have shown that LOH of the mouse Hipk2 gene occurs frequently in radiation-induced thymic lymphomas. Most tumors retain and express one copy of the wild-type allele, demonstrating that the Hipk2 gene has a haploinsufficient role in mouse lymphoma development. Deletion analysis of lymphoma DNA samples allowed us to localize the smallest common region of deletion to a 200-kb region containing only Hipk2, thus identifying this gene as the most viable candidate tumor suppressor.
A functional role for Hipk2
in lymphomagenesis was demonstrated using mice carrying germline mutations in the Hipk2
heterozygous mice are susceptible to radiation-induced tumorigenesis, but tumors retained and expressed the wild-type allele, in agreement with the hypothesis that loss of a single gene copy is sufficient to impact tumor susceptibility. Our observations of a haploinsufficient effect of Hipk2
loss in lymphomagenesis suggests that chromosome changes resulting in copy number changes on human chromosome 7q34, where HIPK2
is located, may have a role in development of human leukemias and solid tumors in which this chromosome undergoes somatic alterations (Mitelman et al., 2010
). Others have reported rare HIPK2
mutations in AML (Li et al., 2007
), but there are also reports of amplification of HIPK2
in some cancers (Deshmukh et al., 2008
), suggesting a complex role that is context dependent. This complexity is further underlined by apparently contradictory results on the effects of Hipk2
on cell growth. Pierantoni et al. (2001)
demonstrated that Hipk2
can act as a suppressor of proliferation, but nevertheless cells from Hipk2
knockout (KO) mice have variously been reported to exhibit decreased (Trapasso et al., 2009
) or increased (Wei et al., 2007
) cell growth in vitro
. The reasons or these discrepancies are unclear, and may be related to the specific nature of the KO alleles or the genetic background from which the cells were derived. Our data demonstrate conclusively that Hipk2
is a tumor suppressor gene in vivo
; however, the fact that tumors retain and express one wild-type allele suggests that complete Hipk2
loss may in some way be detrimental to tumor growth. The mechanistic basis for the positive or negative effects of Hipk2
on cell growth and tumor development remains to be established.
Although HIPK2 has been implicated in regulating p53-dependent cell growth and apoptosis (D'Orazi et al., 2002
; Hofmann et al., 2002
), our studies on radiation-induced tumor development in the Hipk2
double heterozygous mice show that the activities of these two genes in lymphomagenesis are independent, as germline deletion of one copy of both genes shows additive effects on lymphoma susceptibility.
Independent effects of loss of p53
and of Hipk2
were also evident during the somatic events involved in tumor progression, as loss of one Hipk2
allele was also found in tumors from p53
−/− mice. Documented p53-independent functions of HIPK2 include induction of apoptosis upon DNA damage, through proteasomal degradation of CtBP (Zhang et al., 2003
), but whether this mechanism is linked to the tumor suppressor role of Hipk2
in lymphoma susceptibility and progression remains to be determined.