Juan Martín-Caballero

DISEASE STATES ASSOCIATED WITH TELOMERASE DEFICIENCY APPEAR EARLIER IN MICE WITH SHORT TELOMERES

Mice deficient for the mouse telomerase RNA (mTR−/−) and lacking telomerase activity can only be bred for approximately six generations due to decreased male and female fertility and to an increased embryonic lethality associated with a neural tube closure defect. Although late generation mTR−/− mice show defects in the hematopoietic system, they are viable to adulthood, only showing a decrease in viability in old age. To assess the contribution of genetic background to the effect of telomerase deficiency on viability, we generated mTR−/− mutants on a C57BL6 background, which showed shorter telomeres than the original mixed genetic background C57BL6/129Sv. Interestingly, these mice could be bred for only four generations and the survival of late generation mTR−/− mice decreased dramatically with age as compared with their wild‐type counterparts. Fifty percent of the generation 4 mice die at only 5 months of age. This decreased viability with age in the late generation mice is coincident with telomere shortening, sterility, splenic atrophy, reduced proliferative capacity of B and T cells, abnormal hematology and atrophy of the small intestine. These results indicate that telomere shortening in mTR−/− mice leads to progressive loss of organismal viability.

Increased epidermal tumors and increased skin wound healing in transgenic mice overexpressing the catalytic subunit of telomerase, mTERT, in basal keratinocytes

Telomerase transgenics are an important tool to assess the role of telomerase in cancer, as well as to evaluate the potential use of telomerase for gene therapy of age‐associated diseases. Here, we have targeted the expression of the catalytic component of mouse telomerase, mTERT, to basal keratinocytes using the bovine keratin 5 promoter. These telomerase‐transgenic mice are viable and show histologically normal stratified epithelia with high levels of telomerase activity and normal telomere length. Interestingly, the epidermis of these mice is highly responsive to the mitogenic effects of phorbol esters, and it is more susceptible than that of wild‐type littermates to the development skin tumors upon chemical carcinogenesis. The epidermis of telomerase‐transgenic mice also shows an increased wound‐healing rate compared with wild‐type littermates. These results suggest that, contrary to the general assumption, telomerase actively promotes proliferation in cells that have sufficiently long telomeres and unravel potential risks of gene therapy for age‐associated diseases based on telomerase upregulation.

Limited overlapping roles of P15INK4b and P18INK4c cell cycle inhibitors in proliferation and tumorigenesis

Entry of quiescent cells into the cell cycle is driven by the cyclin D‐dependent kinases Cdk4 and Cdk6. These kinases are negatively regulated by the INK4 cell cycle inhibitors. We report the generation of mice defective in P15INK4b and P18INK4c. Ablation of these genes, either alone or in combination, does not abrogate cell contact inhibition or senescence of mouse embryo fibroblasts in culture. However, loss of P15INK4b, but not of P18INK4c, confers proliferative advantage to these cells and makes them more sensitive to transformation by H‐ras oncogenes. In vivo, ablation of P15INK4b and P18INK4c genes results in lymphoproliferative disorders and tumor formation. Mice lacking P18INK4c have deregulated epithelial cell growth leading to the formation of cysts, mostly in the cortical region of the kidneys and the mammary epithelium. Loss of both P15INK4b and P18INK4c does not result in significantly distinct phenotypic manifestations except for the appearance of cysts in additional tissues. These results indicate that P15INK4b and P18IKN4c are tumor suppressor proteins that act in different cellular lineages and/or pathways with limited compensatory roles.

The cell cycle inhibitor p21 controls T-cell proliferation and sex-linked lupus development.

Here we show that the cell-cycle regulator p21 is involved in immune system function. T lymphocytes from p21−/− mice exhibit significant proliferative advantage over wild-type cells following prolonged stimulation, but not after primary activation. Consistent with this, p21-deficient mice accumulate abnormal amounts of CD4+ memory cells, and develop loss of tolerance towards nuclear antigens. Similar to human lupus, female p21-deficient mice develop antibodies against dsDNA, lymphadenopathy, and glomerulonephritis, leading to decreased viability. These data demonstrate a specialized role for p21 in the control of T-cell proliferation, tolerance to nuclear antigens, and female-prone lupus. These findings could be the basis for new therapeutic approaches to lupus.

Shorter telomeres, accelerated ageing and increased lymphoma in DNA-PKcs-deficient mice.

Non‐homologous end joining (NHEJ) is the principal repair mechanism used by mammalian cells to cope with double‐strand breaks (DSBs) that continually occur in the genome. One of the key components of the mammalian NHEJ machinery is the DNA‐PK complex, formed by the Ku86/70 heterodimer and the DNA‐PK catalytic subunit (DNA‐PKcs). Here, we report on the detailed life‐long follow‐up of DNA‐PKcs‐defective mice. Apart from defining a role of DNA‐PKcs in telomere length maintenance in the context of the ageing organism, we observed that DNA‐PKcs‐defective mice had a shorter life span and showed an earlier onset of ageing‐related pathologies than the corresponding wild‐type littermates. In addition, DNA‐PKcs ablation was associated with a markedly higher incidence of T lymphomas and infections. In conclusion, these data link the dual role of DNA‐PKcs in DNA repair and telomere length maintenance to organismal ageing and cancer.

Increased p53 activity does not accelerate telomere‐driven ageing

There is a great interest in determining the impact of p53 on ageing and, for this, it is important to discriminate among the known causes of ageing. Telomere loss is a well‐established source of age‐associated damage, which by itself can recapitulate ageing in mouse models. Here, we have used a genetic approach to interrogate whether p53 contributes to the elimination of telomere‐damaged cells and its impact on telomere‐driven ageing. We have generated compound mice carrying three functional copies of the p53 gene (super‐p53) in a telomerase‐deficient background and we have measured the presence of chromosomal abnormalities and DNA damage in several tissues. We have found that the in vivo load of telomere‐derived chromosomal damage is significantly decreased in super‐p53/telomerase‐null mice compared with normal‐p53/telomerase‐null mice. Interestingly, the presence of extra p53 activity neither accelerates nor delays telomere‐driven ageing. From these observations, we conclude that p53 has an active role in eliminating telomere‐damaged cells, and we exclude the possibility of an age‐promoting effect of p53 on telomere‐driven ageing.

Constitutive Expression of Tert in Thymocytes Leads to Increased Incidence and Dissemination of T-Cell Lymphoma in Lck-Tert Mice

ABSTRACT 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.

Tumour-suppression activity of the proapoptotic regulator Par4.

The proapoptotic protein encoded by Par4 (prostate apoptosis response 4) has been implicated in tumour suppression, particularly in the prostate. We report here that Par4‐null mice are prone to develop tumours, both spontaneously and on carcinogenic treatment. The endometrium and prostate of Par4‐null mice were particularly sensitive to the development of proliferative lesions. Most (80%) Par4‐null females presented endometrial hyperplasia by 9 months of age, and a significant proportion (36%) developed endometrial adenocarcinomas after 1 year of age. Similarly, Par4‐null males showed a high incidence of prostate hyperplasia and prostatic intraepithelial neoplasias, and were extraordinarily sensitive to testosterone‐induced prostate hyperplasia. Finally, the uterus and prostate of young Par4‐null mice have increased levels of the apoptosis inhibitor XIAP (X‐chromosome‐linked inhibitor of apoptosis), supporting the previously proposed function of Par4 as an inhibitor of the ζPKC (atypical protein kinase)–NF‐κB (nuclear factor‐κB)–XIAP pathway. These data show that Par4 has an important role in tumour suppression, with a particular relevance in the endometrium and prostate.

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.

Autocrine Production of IFN-γ by Macrophages Controls Their Recruitment to Kidney and the Development of Glomerulonephritis in MRL/lpr Mice

Anti-DNA autoantibody production is a key factor in lupus erythematosus development; nonetheless, the link between glomerular anti-DNA autoantibody deposition and glomerulonephritis development is not understood. To study the inflammatory and destructive processes in kidney, we used IFN-γ+/− MRL/lpr mice which produce high anti-DNA Ab levels but are protected from kidney disease. The results showed that defective macrophage recruitment to IFN-γ+/− mouse kidney was not caused by decreased levels of monocyte chemoattractant protein-1, a chemokine that controls macrophage migration to MRL/lpr mouse kidney. To determine which IFN-γ-producing cell type orchestrates the inflammation pathway in kidney, we transferred IFN-γ+/+ monocyte/macrophages or T cells to IFN-γ−/− mice, which do not develop anti-DNA autoantibodies. The data demonstrate that IFN-γ production by infiltrating macrophages, and not by T cells, is responsible for adhesion molecule up-regulation, macrophage accumulation, and inflammation in kidney, even in the absence of autoantibody deposits. Therefore, in addition to monocyte chemoattractant protein-1, macrophage-produced IFN-γ controls macrophage migration to kidney; the degree of IFN-γ production by macrophages also regulates glomerulonephritis development. Our findings establish the level of IFN-γ secretion by macrophages as a link between anti-DNA autoantibody deposition and glomerulonephritis development, outline the pathway of the inflammatory process, and suggest potential treatment for disease even after autoantibody development.