Identity Noise and Adipogenic Traits Characterize Dermal Fibroblast Aging.

During aging, stromal functions are thought to be impaired, but little is known whether this stems from changes of fibroblasts. Using population- and single-cell transcriptomics, as well as long-term lineage tracing, we studied whether murine dermal fibroblasts are altered during physiological aging under different dietary regimes that affect longevity. We show that the identity of old fibroblasts becomes undefined, with the fibroblast states present in young skin no longer clearly demarcated. In addition, old fibroblasts not only reduce the expression of genes involved in the formation of the extracellular matrix, but also gain adipogenic traits, paradoxically becoming more similar to neonatal pro-adipogenic fibroblasts. These alterations are sensitive to systemic metabolic changes: long-term caloric restriction reversibly prevents them, whereas a high-fat diet potentiates them. Our results therefore highlight loss of cell identity and the acquisition of adipogenic traits as a mechanism underlying cellular aging, which is influenced by systemic metabolism.

Exploring Adaptive Phenotypes for the Human Calcium-Sensing Receptor Polymorphism R990G.

Rainforest hunter-gatherers from Southeast Asia are characterized by specific morphological features including a particularly dark skin color (D), short stature (S), woolly hair (W), and the presence of steatopygia (S)-fat accumulation localized in the hips (DSWS phenotype). Based on previous evidence in the Andamanese population, we first characterized signatures of adaptive natural selection around the calcium-sensing receptor gene in Southeast Asian rainforest groups presenting the DSWS phenotype and identified the R990G substitution (rs1042636) as a putative adaptive variant for experimental follow-up. Although the calcium-sensing receptor has a critical role in calcium homeostasis by directly regulating the parathyroid hormone secretion, it is expressed in different tissues and has been described to be involved in many biological functions. Previous works have also characterized the R990G substitution as an activating polymorphism of the calcium-sensing receptor associated with hypocalcemia. Therefore, we generated a knock-in mouse for this substitution and investigated organismal phenotypes that could have become adaptive in rainforest hunter-gatherers from Southeast Asia. Interestingly, we found that mouse homozygous for the derived allele show not only lower serum calcium concentration but also greater body weight and fat accumulation, probably because of enhanced preadipocyte differentiation and lipolysis impairment resulting from the calcium-sensing receptor activation mediated by R990G. We speculate that such differential features in humans could have facilitated the survival of hunter-gatherer groups during periods of nutritional stress in the challenging conditions of the Southeast Asian tropical rainforests.

Distinct roles for PARP-1 and PARP-2 in c-Myc-driven B-cell lymphoma in mice.

Dysregulation of the c-Myc oncogene occurs in a wide variety of hematologic malignancies, and its overexpression has been linked with aggressive tumor progression. Here, we show that poly (ADP-ribose) polymerase 1 (PARP-1) and PARP-2 exert opposing influences on progression of c-Myc-driven B-cell lymphoma. PARP-1 and PARP-2 catalyze the synthesis and transfer of ADP-ribose units onto amino acid residues of acceptor proteins in response to DNA strand breaks, playing a central role in the response to DNA damage. Accordingly, PARP inhibitors have emerged as promising new cancer therapeutics. However, the inhibitors currently available for clinical use are not able to discriminate between individual PARP proteins. We found that genetic deletion of PARP-2 prevents c-Myc-driven B-cell lymphoma, whereas PARP-1 deficiency accelerates lymphomagenesis in the Eµ-Myc mouse model of aggressive B-cell lymphoma. Loss of PARP-2 aggravates replication stress in preleukemic Eµ-Myc B cells, resulting in accumulation of DNA damage and concomitant cell death that restricts the c-Myc-driven expansion of B cells, thereby providing protection against B-cell lymphoma. In contrast, PARP-1 deficiency induces a proinflammatory response and an increase in regulatory T cells, likely contributing to immune escape of B-cell lymphoma, resulting in an acceleration of lymphomagenesis. These findings pinpoint specific functions for PARP-1 and PARP-2 in c-Myc-driven lymphomagenesis with antagonistic consequences that may help inform the design of new PARP-centered therapeutic strategies, with selective PARP-2 inhibition potentially representing a new therapeutic approach for the treatment of c-Myc-driven tumors.

A mouse model uncovers LKB1 as an UVB-induced DNA damage sensor mediating CDKN1A (p21WAF1/CIP1) degradation.

Exposure to ultraviolet (UV) radiation from sunlight accounts for 90% of the symptoms of premature skin aging and skin cancer. The tumor suppressor serine-threonine kinase LKB1 is mutated in Peutz-Jeghers syndrome and in a spectrum of epithelial cancers whose etiology suggests a cooperation with environmental insults. Here we analyzed the role of LKB1 in a UV-dependent mouse skin cancer model and show that LKB1 haploinsufficiency is enough to impede UVB-induced DNA damage repair, contributing to tumor development driven by aberrant growth factor signaling. We demonstrate that LKB1 and its downstream kinase NUAK1 bind to CDKN1A. In response to UVB irradiation, LKB1 together with NUAK1 phosphorylates CDKN1A regulating the DNA damage response. Upon UVB treatment, LKB1 or NUAK1 deficiency results in CDKN1A accumulation, impaired DNA repair and resistance to apoptosis. Importantly, analysis of human tumor samples suggests that LKB1 mutational status could be a prognostic risk factor for UV-induced skin cancer. Altogether, our results identify LKB1 as a DNA damage sensor protein regulating skin UV-induced DNA damage response.

A cellular model reflecting the phenotypic heterogeneity of mutant HRAS driven squamous cell carcinoma.

Squamous cell carcinomas have a range of histopathological manifestations. The parameters that determine this clinically observed heterogeneity are not fully understood. Here, we report the generation of a cell culture model that reflects part of this heterogeneity. We have used the catalytic subunit of human telomerase hTERT and large T to immortalize primary UV-unexposed keratinocytes. Then, mutant HRAS G12V has been introduced to transform these immortal keratinocytes. When injected into immunosuppressed mice, transformed cells grew as xenografts with distinct histopathological characteristics. We observed three major tissue architectures: solid, sarcomatoid and cystic growth types, which were primarily composed of pleomorphic and basaloid cells but in some cases displayed focal apocrine differentiation. We demonstrate that the cells generated represent different stages of skin cancerogenesis and as such can be used to identify novel tumor-promoting alterations such as the overexpression of the PADI2 oncogene in solid-type SCC. Importantly, the cultured cells maintain the characteristics from the xenograft they were derived from while being amenable to manipulation and analysis. The availability of cell lines representing different clinical manifestations opens a new tool to study the stochastic and deterministic factors that cause case-to-case heterogeneity despite departing from the same set of oncogenes and the same genetic background.

Parp-2 is required to maintain hematopoiesis following sublethal γ-irradiation in mice.

Hematopoietic stem cells self-renew for life to guarantee the continuous supply of all blood cell lineages. Here we show that Poly(ADP-ribose) polymerase-2 (Parp-2) plays an essential role in hematopoietic stem/progenitor cells (HSPC) survival under steady-state conditions and in response to stress. Increased levels of cell death were observed in HSPC from untreated Parp-2-/- mice, but this deficit was compensated by increased rates of self-renewal, associated with impaired reconstitution of hematopoiesis upon serial bone marrow transplantation. Cell death after γ-irradiation correlated with an impaired capacity to repair DNA damage in the absence of Parp-2. Upon exposure to sublethal doses of γ-irradiation, Parp-2-/- mice exhibited bone marrow failure that correlated with reduced long-term repopulation potential of irradiated Parp-2-/- HSPC under competitive conditions. In line with a protective role of Parp-2 against irradiation-induced apoptosis, loss of p53 or the pro-apoptotic BH3-only protein Puma restored survival of irradiated Parp-2-/- mice, whereas loss of Noxa had no such effect. Our results show that Parp-2 plays essential roles in the surveillance of genome integrity of HSPC by orchestrating DNA repair and restraining p53-induced and Puma-mediated apoptosis. The data may affect the design of drugs targeting Parp proteins and the improvement of radiotherapy-based therapeutic strategies.

Loss of Lkb1 cooperates with BrafV600E and ultraviolet radiation, increasing melanoma multiplicity and neural-like dedifferentiation.

The mechanisms that work alongside BRAFV600E oncogene in melanoma development, in addition to ultraviolet (UV) radiation (UVR), are of great interest. Analysis of human melanoma tumors [data from The Cancer Genome Atlas (TCGA)] revealed that 50% or more of the samples expressed no or low amounts of serine/threonine protein kinase STK11 (also known as LKB1) protein. Here, we report that, in a mouse model, concomitant neonatal BrafV600E activation and Lkb1 tumor suppressor ablation in melanocytes led to full melanoma development. A single postnatal dose of UVB radiation had no effect on melanoma onset in Lkb1-depleted mice compared with BrafV600E-irradiated mice, but increased tumor multiplicity. In concordance with these findings and previous reports, Lkb1-null irradiated mice exhibited deficient DNA damage repair (DDR). Histologically, tumors lacking Lkb1 were enriched in neural-like tumor morphology. Genetic profiling and gene set enrichment analyses of tumor sample mutated genes indicated that loss of Lkb1 promoted the selection of altered genes associated with neural differentiation processes. Thus, these results suggest that the loss of Lkb1 cooperates with BrafV600E and UVR, impairing the DDR and increasing melanoma multiplicity and neural-like dedifferentiation.

Staphylococcus prevails in the skin microbiota of long-term immunodeficient mice.

Host-commensal relationships in the skin are a complex system governed by variables related to the host, the bacteria and the environment. A disruption of this system may lead to new steady states, which, in turn, may lead to disease. We have studied one such disruption by characterizing the skin microbiota in healthy and immunodepressed (ID) mice. A detailed anatomopathological study failed to reveal any difference between the skin of healthy and ID mice. We sequenced the 16S rDNA V1-V2 gene region to saturation in 10 healthy and 10 ID 8 week-old mice, and found than all of the healthy and two of the ID mice had bacterial communities that were similar in composition to that of human skin, although, presumably because of the uniform raising conditions, less interindividual variation was found in mice. However, eight ID mice showed microbiota dominated by Staphylococcus epidermidis. Quantitative PCR amplification of 16S rDNA gene and of the Staphylococcus-specific TstaG region confirmed the previous results and indicated that the quantitative levels of Staphylococcus were similar in both groups while the total number of 16S copies was greater in the healthy mice. Thus, it is possible that, under long-term immunodeficiency, which removes the acquired but not the native immune system, S.epidermidis may inhibit the growth of other bacteria but does not cause a pathogenic state.

Combined loss of p21(waf1/cip1) and p27(kip1) enhances tumorigenesis in mice.

The cell cycle inhibitors p21(Waf1/Cip1) and p27(Kip1) are frequently downregulated in many human cancers, and correlate with a worse prognosis. We show here that combined deficiency in p21 and p27 proteins in mice is linked to more aggressive spontaneous tumorigenesis, resulting in a decreased lifespan. The most common tumors developed in p21p27 double-null mice were endocrine, with a higher incidence of pituitary adenomas, pheochromocytomas and thyroid adenomas. The combined absence of p21 and p27 proteins delays the incidence of radiation-induced thymic lymphomas with a higher apoptotic rate, measured by active caspase-3 and cleaved PARP-1 immunoexpresion. These results provide experimental evidence for a cooperation of both cyclin-dependent kinase inhibitors in tumorigenesis in mice.

Coordinated signals from PARP-1 and PARP-2 are required to establish a proper T cell immune response to breast tumors in mice.

Poly(ADP-ribose)-polymerase (PARP)-1 and PARP-2 play an essential role in the DNA damage response. Based on this effect of PARP in the tumor cell itself, PARP inhibitors have emerged as new therapeutic tools both approved and in clinical trials. However, the interactome of multiple other cell types, particularly T cells, within the tumor microenvironment are known to either favor or limit tumorigenesis. Here, we bypassed the embryonic lethality of dually PARP-1/PARP-2-deficient mice by using a PARP-1-deficient mouse with a Cd4-promoter-driven deletion of PARP-2 in T cells to investigate the understudied role of these PARPs in the modulation of T cell responses against AT-3-induced breast tumors. We found that dual PARP-1/PARP-2-deficiency in T cells promotes tumor growth while single deficiency of each protein limited tumor progression. Analysis of tumor-infiltrating cells in dual PARP-1/PARP-2-deficiency host-mice revealed a global change in immunological profile and impaired recruitment and activation of T cells. Conversely, single PARP-1 and PARP-2-deficiency tends to produce an environment with an active and partially upregulated immune response. Our findings pinpoint opposite effects of single and dual PARP-1 and PARP-2-deficiency in modulating the antitumor response with an impact on tumor progression, and will have implications for the development of more selective PARP-centered therapies.

The cyclin-dependent kinase inhibitor, p21(WAF1), promotes angiogenesis by repressing gene transcription of thioredoxin-binding protein 2 in cancer cells.

The cyclin-dependent kinase inhibitor, p21(WAF1), induces cell-cycle arrest and can act as a tumor suppressor. However, increasing evidence indicates that p21(WAF1) can also increase resistance to some anticancer therapies and thus promote tumor growth. The mechanisms explaining this paradox have not been explained. We found that conditioned media from MCF-7 breast cancer cells transfected with a p21(WAF1)-specific small interfering RNA (siRNA) significantly reduced endothelial cell migration, invasion and vascular sprouting. Liquid chromatography/mass spectrometry analysis of the conditioned media revealed that p21(WAF1) knockdown significantly reduced secretion of thioredoxin (Trx), a redox protein known to promote tumor angiogenesis. p21(WAF1) knockdown decreased Trx enzymatic activity in cancer cells, by effects on the expression levels of intracellular thioredoxin-binding protein 2 (TBP2), known to bind and inactivate Trx. Consistent with these findings, media from cancer cells transfected with TBP2 siRNA promoted endothelial cell invasion and blocked the anti-angiogenic effect of p21(WAF1) siRNA. Addition of Trx siRNA blocked the pro-angiogenic effects of TBP2 siRNA. Chromatin immunoprecipitation assays showed p21(WAF1) bound TBP2 gene promoter. Taken together, our data suggests that p21(WAF1) can induce Trx secretion and angiogenesis in cancer cells, by direct transcriptional repression of the TBP2 promoter.

Impact of telomerase ablation on organismal viability, aging, and tumorigenesis in mice lacking the DNA repair proteins PARP-1, Ku86, or DNA-PKcs.

The DNA repair proteins poly(ADP-ribose) polymerase-1 (PARP-1), Ku86, and catalytic subunit of DNA-PK (DNA-PKcs) have been involved in telomere metabolism. To genetically dissect the impact of these activities on telomere function, as well as organismal cancer and aging, we have generated mice doubly deficient for both telomerase and any of the mentioned DNA repair proteins, PARP-1, Ku86, or DNA-PKcs. First, we show that abrogation of PARP-1 in the absence of telomerase does not affect the rate of telomere shortening, telomere capping, or organismal viability compared with single telomerase-deficient controls. Thus, PARP-1 does not have a major role in telomere metabolism, not even in the context of telomerase deficiency. In contrast, mice doubly deficient for telomerase and either Ku86 or DNA-PKcs manifest accelerated loss of organismal viability compared with single telomerase-deficient mice. Interestingly, this loss of organismal viability correlates with proliferative defects and age-related pathologies, but not with increased incidence of cancer. These results support the notion that absence of telomerase and short telomeres in combination with DNA repair deficiencies accelerate the aging process without impacting on tumorigenesis.

Coordinated signals from the DNA repair enzymes PARP-1 and PARP-2 promotes B-cell development and function.

Poly (ADP-ribose) polymerase (PARP)-1 and PARP-2 regulate the function of various DNA-interacting proteins by transferring ADP-ribose emerging from catalytic cleavage of cellular ß-NAD+. Hence, mice lacking PARP-1 or PARP-2 show DNA perturbations ranging from altered DNA integrity to impaired DNA repair. These effects stem from the central role that PARP-1 and PARP-2 have on the cellular response to DNA damage. Failure to mount a proper response culminates in cell death. Accordingly, PARP inhibitors are emerging as promising drugs in cancer therapy. However, the full impact of these inhibitors on immunity, including B-cell antibody production, remains elusive. Given that mice carrying dual PARP-1 and PARP-2 deficiency develop early embryonic lethality, we crossed PARP-1-deficient mice with mice carrying a B-cell-conditional PARP-2 gene deletion. We found that the resulting dually PARP-1 and PARP-2-deficient mice had perturbed bone-marrow B-cell development as well as profound peripheral depletion of transitional and follicular but not marginal zone B-cells. Of note, bone-marrow B-cell progenitors and peripheral mature B-cells were conserved in mice carrying either PARP-1 or PARP-2 deficiency. In dually PARP-1 and PARP-2-deficient mice, B-cell lymphopenia was associated with increased DNA damage and accentuated death in actively proliferating B-cells. Moreover, dual PARP-1 and PARP-2 deficiency impaired antibody responses to T-independent carbohydrate but not to T-dependent protein antigens. Notwithstanding the pivotal role of PARP-1 and PARP-2 in DNA repair, combined PARP-1 and PARP-2 deficiency did not perturb the DNA-editing processes required for the generation of a protective antibody repertoire, including Ig V(D)J gene recombination and IgM-to-IgG class switching. These findings provide key information as to the potential impact of PARP inhibitors on humoral immunity, which will facilitate the development of safer PARP-targeting regimens against cancer.

Macrophage-specific MHCII expression is regulated by a remote Ciita enhancer controlled by NFAT5.

MHCII in antigen-presenting cells (APCs) is a key regulator of adaptive immune responses. Expression of MHCII genes is controlled by the transcription coactivator CIITA, itself regulated through cell type-specific promoters. Here we show that the transcription factor NFAT5 is needed for expression of Ciita and MHCII in macrophages, but not in dendritic cells and other APCs. NFAT5-deficient macrophages showed defective activation of MHCII-dependent responses in CD4+ T lymphocytes and attenuated capacity to elicit graft rejection in vivo. Ultrasequencing analysis of NFAT5-immunoprecipitated chromatin uncovered an NFAT5-regulated region distally upstream of Ciita This region was required for CIITA and hence MHCII expression, exhibited NFAT5-dependent characteristics of active enhancers such as H3K27 acetylation marks, and required NFAT5 to interact with Ciita myeloid promoter I. Our results uncover an NFAT5-regulated mechanism that maintains CIITA and MHCII expression in macrophages and thus modulates their T lymphocyte priming capacity.

Constitutive expression of tert in thymocytes leads to increased incidence and dissemination of T-cell lymphoma in Lck-Tert mice.

Here we describe a new mouse model with constitutive expression of the catalytic subunit of telomerase (Tert) targeted to thymocytes and peripheral T cells (Lck-Tert mice). Two independent Lck-Tert mouse lines showed higher incidences of spontaneous T-cell lymphoma than the corresponding age-matched wild-type controls, indicating that constitutive expression of Tert promotes lymphoma. Interestingly, T-cell lymphomas in Lck-Tert mice were more disseminated than those in wild-type controls and affected both lymphoid and nonlymphoid tissues, while nonlymphoid tissues were never affected with lymphoma in age-matched wild-type controls. Importantly, these roles of Tert constitutive expression in promoting tumor progression and dissemination were independent of the role of telomerase in telomere length maintenance, since telomere length distributions on a single-cell basis were identical in Lck-Tert and wild-type thymocytes. Finally, Tert constitutive expression did not interfere with telomere capping in Lck-Tert primary thymocytes, although it resulted in greater chromosomal instability upon gamma irradiation in Lck-Tert primary lymphocytes than in controls, suggesting that Tert overexpression may interfere with the cellular response to DNA damage.

Cooperation between Cdk4 and p27kip1 in tumor development: a preclinical model to evaluate cell cycle inhibitors with therapeutic activity.

Deregulation of the G1-S transition of the cell cycle is a common feature of human cancer. Tumor-associated alterations in this process frequently affect cyclin-dependent kinases (Cdk), their regulators (cyclins, INK4 inhibitors, or p27Kip1), and their substrates (retinoblastoma protein). Although these proteins are generally thought to act in a linear pathway, mutations in different components frequently cooperate in tumor development. Using gene-targeted mouse models, we report in this article that Cdk4 resistance to INK4 inhibitors, due to the Cdk4 R24C mutation, strongly cooperates with p27(Kip1) deficiency in tumor development. No such cooperation is observed between Cdk4 R24C and p18(INK4c) absence, suggesting that the only function of p18INK4c is inhibiting Cdk4 in this model. Cdk4(R/R) knock in mice, which express the Cdk4 R24C mutant protein, develop pituitary tumors with complete penetrance and short latency in a p27Kip1-/- or p27Kip1+/- background. We have investigated whether this tumor model could be useful to assess the therapeutic activity of cell cycle inhibitors. We show here that exposure to flavopiridol, a wide-spectrum Cdk inhibitor, significantly delays tumor progression and leads to tumor-free survival in a significant percentage of treated mice. These data suggest that genetically engineered tumor models involving key cell cycle regulators are a valuable tool to evaluate drugs with potential therapeutic benefit in human cancer.

miRNA-1 and miRNA-133a are involved in early commitment of pluripotent stem cells and demonstrate antagonistic roles in the regulation of cardiac differentiation.

miRNA-1 (miR-1) and miRNA-133a (miR-133a) are muscle-specific miRNAs that play an important role in heart development and physiopathology. Although both miRNAs have been broadly studied during cardiogenesis, the mechanisms by which miR-1 and miR-133a could influence linage commitment in pluripotent stem cells remain poorly characterized. In this study we analysed the regulation of miR-1 and miR-133a expression during pluripotent stem cell differentiation [P19.CL6 cells; embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs)] and investigated their role in DMSO and embryoid body (EB)-mediated mesodermal and cardiac differentiation by gain- and loss-of-function studies, as well as in vivo, by the induction of teratomas. Gene expression analysis revealed that miR-1 and miR-133a are upregulated during cardiac differentiation of P19.CL6 cells, and also during ESC and iPSC EB differentiation. Forced overexpression of both miRNAs promoted mesodermal commitment and a concomitant decrease in the expression of neural differentiation markers. Moreover, overexpression of miR-1 enhanced the cardiac differentiation of P19.CL6, while miR-133a reduced it with respect to control cells. Teratoma formation experiments with P19.CL6 cells confirmed the influence of miR-1 and miR-133a during in vivo differentiation. Finally, inhibition of both miRNAs during P19.CL6 cardiac differentiation had opposite results to their overexpression. In conclusion, gene regulation involving miR-1 and miR-133a controls the mesodermal and cardiac fate of pluripotent stem cells. Copyright © 2014 John Wiley & Sons, Ltd.

Constitutive Cyclin O deficiency results in penetrant hydrocephalus, impaired growth and infertility.

Cyclin O (encoded by CCNO) is a member of the cyclin family with regulatory functions in ciliogenesis and apoptosis. Homozygous CCNO mutations have been identified in human patients with Reduced Generation of Multiple Motile Cilia (RGMC) and conditional inactivation of Ccno in the mouse recapitulates some of the pathologies associated with the human disease. These include defects in the development of motile cilia and hydrocephalus. To further investigate the functions of Ccno in vivo, we have generated a new mouse model characterized by the constitutive loss of Ccno in all tissues and followed a cohort during ageing. Ccno-/- mice were growth impaired and developed hydrocephalus with high penetrance. In addition, some Ccno+/- mice also developed hydrocephalus and affected Ccno-/- and Ccno+/- mice exhibited additional CNS defects including cortical thinning and hippocampal abnormalities. In addition to the CNS defects, both male and female Ccno-/- mice were infertile and female mice exhibited few motile cilia in the oviduct. Our results further establish CCNO as an important gene for normal development and suggest that heterozygous CCNO mutations could underlie hydrocephalus or diminished fertility in some human patients.

PARP-1/PARP-2 double deficiency in mouse T cells results in faulty immune responses and T lymphomas.

The maintenance of T-cell homeostasis must be tightly regulated. Here, we have identified a coordinated role of Poly(ADP-ribose) polymerase-1 (PARP-1) and PARP-2 in maintaining T-lymphocyte number and function. Mice bearing a T-cell specific deficiency of PARP-2 in a PARP-1-deficient background showed defective thymocyte maturation and diminished numbers of peripheral CD4+ and CD8+ T-cells. Meanwhile, peripheral T-cell number was not affected in single PARP-1 or PARP-2-deficient mice. T-cell lymphopenia was associated with dampened in vivo immune responses to synthetic T-dependent antigens and virus, increased DNA damage and T-cell death. Moreover, double-deficiency in PARP-1/PARP-2 in T-cells led to highly aggressive T-cell lymphomas with long latency. Our findings establish a coordinated role of PARP-1 and PARP-2 in T-cell homeostasis that might impact on the development of PARP-centred therapies.

Different cooperating effect of p21 or p27 deficiency in combination with INK4a/ARF deletion in mice.

The control exerted by the INK4a/ARF locus on cellular proliferation is crucial to restrict tumor development. In agreement with this, mice with defects in this locus are highly tumor prone. However, the potential contribution of other pathways in modulating tumorigenesis in the absence of INK4a/ARF is largely unexplored. In the present study, we investigated the consequences of the combined loss of either of two cyclin-dependent kinase inhibitors, p21 and p27, in cooperation with deletion of the INK4a/ARF locus. Our results show a clear differential effect in tumorigenesis depending on the CKI that is absent. The absence of p21 produced no overt alteration of the lifespan of the INK4a/ARF-null mice, although it modified their tumor spectrum, causing a significant increase in the incidence of fibrosarcomas and the appearance of a small number of rhabdomyosarcomas. In contrast, deficiency of p27 resulted in a significant increase in lethality due to accelerated tumor development, especially in the case of T-cell lymphomas. Finally, combined deficiency of INK4a/ARF and p27 resulted in a significant increase in the number of metastatic tumors. These results demonstrate genetically the oncogenic cooperation between defects on INK4a/ARF and p27, which are common alterations in human cancer.