Telomere dysfunction promotes non-reciprocal translocations and epithelial cancers in mice

Aged humans sustain a high rate of epithelial cancers such as carcinomas of the breast and colon, whereas mice carrying common tumour suppressor gene mutations typically develop soft tissue sarcomas and lymphomas. Among the many factors that may contribute to this species variance are differences in telomere length and regulation. Telomeres comprise the nucleoprotein complexes that cap the ends of eukaryotic chromosomes and are maintained by the reverse transcriptase, telomerase. In human cells, insufficient levels of telomerase lead to telomere attrition with cell division in culture and possibly with ageing and tumorigenesis in vivo. In contrast, critical reduction in telomere length is not observed in the mouse owing to promiscuous telomerase expression and long telomeres. Here we provide evidence that telomere attrition in ageing telomerase-deficient p53 mutant mice promotes the development of epithelial cancers by a process of fusion-bridge breakage that leads to the formation of complex non-reciprocal translocations--a classical cytogenetic feature of human carcinomas. Our data suggest a model in which telomere dysfunction brought about by continual epithelial renewal during life generates the massive ploidy changes associated with the development of epithelial cancers.

Reduction of cardiac volume in left-breast treatment fields by respiratory maneuvers: a CT study

PURPOSE

A previous study of healthy female volunteers suggested that deep inspiratory breath holding can reduce the cardiac volume in the treatment portals for left-breast cancer treatment. The reduction of irradiated cardiac volume may be important considering the reported late cardiac morbidity and mortality and the frequent coexistent use of potentially cardiotoxic chemotherapy in breast cancer patients. In the present study, we evaluated the heart volume in the fields and, thus, the true benefit of this respiratory maneuver in breast cancer patients undergoing CT simulation.

MATERIALS AND METHODS

Fifteen patients (median age, 53) were studied. For each patient, CT scans were performed both when the patient breathed normally (quiet respiration) and when the patient held her breath after a deep inspiration. Tangential fields were planned using the same medial, lateral, superior, and inferior borders on skin for the normal breathing and the breath-holding configurations. The cardiac and left-lung volumes within the tangential fields were calculated for both breathing configurations. Multiple scan series were performed for the breath-holding configuration to provide a more accurate delineation of the cardiac tissue and to study the reproducibility of the patient's position between different cycles of deep inspiration.

RESULTS

None of the patients had difficulty holding her breath for 20 s. The cardiac volume in the field was reduced (-86 +/- 24%; p < 0.001) when patients held their breath after a deep inspiration compared to when breathing normally. For 7 patients (47%), deep inspiration moved the heart completely out of the radiation fields. The expansion of the lung tissue due to deep inspiration also increased the absolute lung volume in the tangential fields (183 cm(3) vs 97 cm(3), p < 0.001). However, the fractional volume of the left lung in the field was essentially unchanged. For all but 1 patient, the maximum difference between the external body contours from different breath holding cycles was 5 mm and occurred at the lateral aspect of the breast. At the medial aspect, as indicated by the position of the midline marker, the variations were well within the currently accepted tolerance for patient positioning during tangential treatment.

CONCLUSIONS

Deep-inspiration breath holding substantially reduces cardiac volume in the tangential fields for left-sided breast cancer treatment. The variation between patient positions at different cycles of breath holding was found to be reasonably small. Therefore, it appears feasible to reduce cardiac radiation by treating patients with intratreatment minifractions lasting 10-15 s while patients hold their breath.

Flipping the oncogene switch: illumination of tumor maintenance and regression

The genetic construction of cancer-prone mice, combined with the capacity to control transgene expression in vivo, provides new opportunities to study the role of oncogenes in the maintenance of fully formed tumors. These inducible cancer models provide a means to dissect how specific oncogenic signals influence host-tumor symbiosis, to validate the importance of a given oncogenic lesion in established advanced tumors, and to predict the biological response and adaptations to therapies targeted to that cancer-causing genetic alteration.

Essential role for oncogenic Ras in tumour maintenance

Advanced malignancy in tumours represents the phenotypic endpoint of successive genetic lesions that affect the function and regulation of oncogenes and tumour-suppressor genes. The established tumour is maintained through complex and poorly understood host-tumour interactions that guide processes such as angiogenesis and immune sequestration. The many different genetic alterations that accompany tumour genesis raise questions as to whether experimental cancer-promoting mutations remain relevant during tumour maintenance. Here we show that melanoma genesis and maintenance are strictly dependent upon expression of H-RasV12G in a doxycycline-inducible H-Ras12G mouse melanoma model null for the tumour suppressor INK4a. Withdrawal of doxycycline and H-RasV12G down-regulation resulted in clinical and histological regression of primary and explanted tumours. The initial stages of regression involved marked apoptosis in the tumour cells and host-derived endothelial cells. Although the regulation of vascular endothelial growth factor (VEGF) was found to be Ras-dependent in vitro, the failure of persistent endogenous and enforced VEGF expression to sustain tumour viability indicates that the tumour-maintaining actions of activated Ras extend beyond the regulation of VEGF expression in vivo. Our results provide genetic evidence that H-RasV12G is important in both the genesis and maintenance of solid tumours.

Involvement of the Ink4a gene (p16 and p19arf) in murine tumorigenesis

The INK4A and INK4B genes map to 9p21, with the INK4A gene encoding two products, p16 and p19ARF. Many neoplasms in which INK4A and INK4B genes are altered show deletions involving both genes. Mice carrying a targeted Ink4a deletion develop tumors at an early age. In the present study we examined the genetic alterations affecting the remaining Ink4a allele and the Ink4b gene in tumors arising in heterozygous Ink4a mice. We identified deletion of the remaining Ink4a allele in 7 of 18 (39%) tumors. We also observed deletion of the exon 1beta in 3 cases, one of them presenting this deletion as a unique alteration. In conclusion, the deletion of the remaining Ink4a allele was the alteration most frequently observed, representing the inactivation of two proteins capable of arresting the cell cycle through different pathways that involve the tumor suppressors pRB and p53.

Virtual light field projection for CT-simulation

We report a method to project a virtual light field over the patient during CT-simulations of external beam radiotherapy. It can be used to perform all the tasks associated with the physical light field of a conventional simulator. The system consists of a three-dimensional sonic digitizer interfacing with a window-based software on a personal computer. The digitizer can provide the three-dimensional coordinates of any point in space accessible by the digitizer probe. When these coordinates are transformed into the beam's eye view, the position of the digitized point relative to the beam can readily be displayed. Thus, the system establishes a virtual light field in space, which can be "seen" only by the digitizer probe. For any digitized point, the system can immediately show, by the beam's eye view display, whether the point is inside the field, outside of the field, or on the field border. Moreover, this virtual field projection allows one to evaluate external target coverage (or external normal tissue sparing) conveniently and interactively. By simply digitizing the concerned area and viewing its position in the beam's eye view display, one can immediately assess the coverage and if necessary, modify the treatment field accordingly. The system also provides an efficient and essential procedure in CT-simulations for marking treatment portals on the patient. By cruising the digitizer probe on the patient's skin surface under visual and audio guidances, one can promptly find the projection of the field center, field corners, etc., on the patient. Measurements have been performed to study the accuracy of the GP-12 sonic digitizer using rigid phantoms. Based on the measured data, the overall accuracy of the portal localization system is estimated to be +/-2 mm. The system has been in clinical use for our CT simulator.

p53 deficiency rescues the adverse effects of telomere loss and cooperates with telomere dysfunction to accelerate carcinogenesis

Maintenance of telomere length and function is critical for the efficient proliferation of eukaryotic cells. Here, we examine the interactions between telomere dysfunction and p53 in cells and organs of telomerase-deficient mice. Coincident with severe telomere shortening and associated genomic instability, p53 is activated, leading to growth arrest and/or apoptosis. Deletion of p53 significantly attenuated the adverse cellular and organismal effects of telomere dysfunction, but only during the earliest stages of genetic crisis. Correspondingly, the loss of telomere function and p53 deficiency cooperated to initiate the transformation process. Together, these studies establish a key role for p53 in the cellular response to telomere dysfunction in both normal and neoplastic cells, question the significance of crisis as a tumor suppressor mechanism, and identify a biologically relevant stage of advanced crisis, termed genetic catastrophe.

Short dysfunctional telomeres impair tumorigenesis in the INK4a(delta2/3) cancer-prone mouse

Maintenance of telomere length is predicted to be essential for bypass of senescence and crisis checkpoints in cancer cells. The impact of telomere dysfunction on tumorigenesis was assessed in successive generations of mice doubly null for the telomerase RNA (mTR) and the INK4a tumor suppressor genes. Significant reductions in tumor formation in vivo and oncogenic potential in vitro were observed in late generations of telomerase deficiency, coincident with severe telomere shortening and associated dysfunction. Reintroduction of mTR into cells significantly restored the oncogenic potential, indicating telomerase activation is a cooperating event in the malignant transformation of cells containing critically short telomeres. The results described here demonstrate that loss of telomere function in a cancer-prone mouse model possessing intact DNA damage responses impairs, but does not prevent, tumor formation.

Differential effects of the widely expressed dMax splice variant of Max on E-box vs initiator element-mediated regulation by c-Myc

dMax, a naturally occurring splice variant of the Myc binding protein Max, lacks the DNA binding basic region and helix 1 of the Helix-Loop-Helix domain; dMax interacts with c-Myc in vitro and in vivo, and inhibits E-box Myc site driven transcription in transient transfection assays. Here we have investigated the expression, function and interactions of dMax. RT/PCR analyses detected dmax mRNA in multiple tissues of the developing, newborn and adult mouse. Functionally, dMax reduced the ability of c-Myc to cooperate with the progression factor A-Myb to promote S phase entry of quiescent smooth muscle cells. In contrast, dMax failed to ablate inhibition of initiator element (Inr)-mediated transcription by c-Myc in Jurkat T cells. In in vitro protein:protein association assays, dMax interacted with c-Myc, N-Myc, L-Myc, Mad1, Mxi1, Mad3 and Mad4, but not with itself or wild-type Max. These interactions required an intact leucine zipper. Inhibition of E-box-mediated transactivation by induction of dMax overexpression resulted in apoptosis of WEHI 231 B cells. Thus, dMax is a widely expressed, naturally occurring protein, with the capacity to bind most members of the Myc/Max superfamily; dMax has little effect on Inr-mediated repression by c-Myc, but can significantly decrease E-box-mediated events promoting proliferation and cell survival.

Telomerase reverse transcriptase gene is a direct target of c-Myc but is not functionally equivalent in cellular transformation

The telomerase reverse transcriptase component (TERT) is not expressed in most primary somatic human cells and tissues, but is upregulated in the majority of immortalized cell lines and tumors. Here, we identify the c-Myc transcription factor as a direct mediator of telomerase activation in primary human fibroblasts through its ability to specifically induce TERT gene expression. Through the use of a hormone inducible form of c-Myc (c-Myc-ER), we demonstrate that Myc-induced activation of the hTERT promoter requires an evolutionarily conserved E-box and that c-Myc-ER-induced accumulation of hTERT mRNA takes place in the absence of de novo protein synthesis. These findings demonstrate that the TERT gene is a direct transcriptional target of c-Myc. Since telomerase activation frequently correlates with immortalization and telomerase functions to stabilize telomers in cycling cells, we tested whether Myc-induced activation of TERT gene expression represents an important mechanism through which c-Myc acts to immortalize cells. Employing the rat embryo fibroblast cooperation assay, we show that TERT is unable to substitute for c-Myc in the transformation of primary rodent fibroblasts, suggesting that the transforming activities of Myc extend beyond its ability to activate TERT gene expression and hence telomerase activity.

The INK4a/ARF tumor suppressor: one gene--two products--two pathways

Functional inactivation of the retinoblastoma (RB) and p53 pathways appears to be a rite of passage for all cancerous cells and results in disruption of cell-cycle regulation and deactivation of the apoptotic response that normally ensues. The INK4a/ARF locus sits at the nexus of these two growth-control pathways, by virtue of its ability to generate two distinct products: the p16INK4a protein, a cyclin-dependent kinase inhibitor that functions upstream of RB; and the p19ARF protein, which blocks MDM2 inhibition of p53 activity. This 'one gene--two products--two pathways' arrangement provides a basis for the prominence of INK4a/ARF in tumorigenesis.

Expression of mouse telomerase reverse transcriptase during development, differentiation and proliferation

We have identified the mouse telomerase reverse transcriptase component (mTERT) and demonstrate both substantial sequence homology to the human ortholog (hTERT), and the presence of reverse transcriptase and telomerase specific motifs. Furthermore, we show functional interchangeability with hTERT in in vitro telomerase reconstitution experiments, as mTERT produces strong telomerase activity in combination with the human telomerase RNA component hTR. The mouse TERT is widely expressed at low levels in adult tissues, with greatest abundance during embryogenesis and in adult thymus and intestine. The mTERT component mRNA levels were regulated during both differentiation and proliferation, while mTR levels remained constant throughout both processes. Comparison of mTERT and mTR levels to telomerase activity indicates that mTERT expression is more tightly linked to the regulation of telomerase activity during these processes than is mTR. In contrast to the situation in human cell cultures, mTERT transcript levels are present at readily detectable levels in primary cultured cells and are not upregulated following crisis. The widespread expression of mTERT in primary cells and mouse tissues could explain the increased frequency of spontaneous immortalization of mouse cells in culture and tumorigenesis in vivo.

The Ink4a tumor suppressor gene product, p19Arf, interacts with MDM2 and neutralizes MDM2's inhibition of p53

The INK4a gene encodes two distinct growth inhibitors--the cyclin-dependent kinase inhibitor p16Ink4a, which is a component of the Rb pathway, and the tumor suppressor p19Arf, which has been functionally linked to p53. Here we show that p19Arf potently suppresses oncogenic transformation in primary cells and that this function is abrogated when p53 is neutralized by viral oncoproteins and dominant-negative mutants but not by the p53 antagonist MDM2. This finding, coupled with the observations that p19Arf and MDM2 physically interact and that p19Rrf blocks MDM2-induced p53 degradation and transactivational silencing, suggests that p19Arf functions mechanistically to prevent MDM2's neutralization of p53. Together, our findings ascribe INK4a's potent tumor suppressor activity to the cooperative actions of its two protein products and their relation to the two central growth control pathways, Rb and p53.

Planning model of resource utilization in an academic pediatric emergency department

This study describes a field observation study and use of simulation to quantify the effect of patient arrival rate and physician practices on physician idle time and patient wait time. The observation study measured actual service (diagnosis, therapy, and charting) times for 126 patients. Subsequently, a FORTRAN simulation model examined effects of physician practices and patient arrival rate on physician utilization and patient wait time. Observations were taken in the emergency department of an urban, university-affiliated pediatric teaching hospital. Although times for initial diagnostic evaluation (diagnosis), therapy, and charting averaged 13.3, 13.8, and 11.6 minutes, respectively, maximum patient visits approached six hours. The simulation model showed that, during times of frequent patient arrivals, maximum patient wait times increased greatly. Additionally, the model predicted that physician idle time persists even during periods of frequent patient arrivals and long maximum visit times. Emergency department (ED) senior staff (fellows and attendings) often begin treating new patients when current patients leave for tests external to the ED. This practice increases physician utilization and makes more physician capacity available, but it may lead to small additional patient waits if the physician is treating another patient when the first patient returns from the external test before their original physician becomes free. On the other hand, during periods of high patient arrivals, increased physician utilization and reduced idle time result in reduced average and expected maximum patient visit times. Unfortunately the variability in visit times increased. A very small percentage of patients wait longer, owing to additional waits incurred upon returning from external testing. Overall, most patients benefit from shorter visits. Finally, the study suggested maximum rather than average wait time be considered as a measure of emergency department capacity and quality of service provided. Although average wait times seemed reasonable, maximum wait times were at times quite long and could impact both physician's and patient's perceptions of service quality.

Cooperative effects of INK4a and ras in melanoma susceptibility in vivo

The familial melanoma gene (INK4a/MTS1/CDKN2) encodes potent tumor suppressor activity. Although mice null for the ink4a homolog develop a cancer-prone condition, a pathogenetic link to melanoma susceptibility has yet to be established. Here we report that mice with melanocyte-specific expression of activated H-rasG12V on an ink4a-deficient background develop spontaneous cutaneous melanomas after a short latency and with high penetrance. Consistent loss of the wild-type ink4a allele was observed in tumors arising in ink4a heterozygous transgenic mice. No homozygous deletion of the neighboring ink4b gene was detected. Moreover, as in human melanomas, the p53 gene remained in a wild-type configuration with no observed mutation or allelic loss. These results show that loss of ink4a and activation of Ras can cooperate to accelerate the development of melanoma and provide the first in vivo experimental evidence for a causal relationship between ink4a deficiency and the pathogenesis of melanoma. In addition, this mouse model affords a system in which to identify and analyze pathways involved in tumor progression against the backdrop of genetic alterations encountered in human melanomas.

Role for N-CoR and histone deacetylase in Sin3-mediated transcriptional repression

Normal mammalian growth and development are highly dependent on the regulation of the expression and activity of the Myc family of transcription factors. Mxi1-mediated inhibition of Myc activities requires interaction with mammalian Sin3A or Sin3B proteins, which have been purported to act as scaffolds for additional co-repressor factors. The identification of two such Sin3-associated factors, the nuclear receptor co-repressor (N-CoR) and histone deacetylase (HD1), provides a basis for Mxi1/Sin3-induced transcriptional repression and tumour suppression.

Is machine energy (4-8 MV) associated with outcome for stage I-II breast cancer patients?

PURPOSE

To assess the relationship between machine energy (4-8 MV) and treatment outcome in patients treated with conservative surgery and radiation therapy.

METHODS AND MATERIALS

Between 1968 and 1985, 1624 patients were treated for clinical Stage I or II invasive breast cancer. The study population was limited to 1380 patients who underwent complete gross excision and received greater than or equal to 60 Gy to the tumor bed. Of these, 1125 were treated on a 4 MV, 153 on a 6 MV, and 102 on an 8 MV linear accelerator. Patients were selected for treatment on the 8 MV machine based on chest wall separations greater than 24 cm. Of patients treated on the 8 MV, netting was used for 42% and bolus was used for 26%. The median dose with bolus was 14 Gy in seven fractions (range: 2-34.2 Gy). Patients treated on the 8 MV accelerator were older, had a higher percentage of clinical T2 tumors, a higher percentage of pathologically positive nodes, and a lower incidence of extensive intraductal component (EIC). Median follow-up times were 130, 153, and 102 months, respectively, for survivors treated on the 4, 6, and 8 MV machines.

RESULTS

We analyzed the site and 5-year crude incidence of first failure by machine energy and found the pattern of first failure site (local, nodal, or distant) to be virtually identical for each energy group. Of the local failures, 12 were in the skin of the treated breast, and these failures were evenly distributed by machine energy. We performed a multivariate analysis to adjust for factors known to predict for treatment failure. When adjusted for these other variables, machine energy was not associated with an increased (or decreased) risk of recurrence (RR for 8 MV vs. 4 MV = 0.94, p = 0.7; RR for 6 MV vs. 4 MV = 1.0, p = 0.9). We also analyzed the nature and incidence of treatment complications (rib fracture, radiation pneumonitis, soft tissue necrosis, and brachial plexopathy) and found no significant differences among the three treatment groups when stratified by treatment technique (tangents only vs. three-field). There was also no significant difference in cosmetic outcome at 5 years among the three groups.

CONCLUSIONS

We conclude that machine energy over the range of 4 to 8 MV does not significantly affect treatment outcome. Specifically, it was feasible to treat patients with large chest wall separations using an 8 MV machine without an increase in skin recurrences and with the improved dose homogeneity afforded by 8 MV machines as compared with those of lower energies.

Growth and radiation response of cells grown in macroporous gelatin microcarriers (CultiSpher-G)

Four cell lines have been cultured in macroporous gelatin microcarrier beads (CultiSpher-G) to test this as a new model of three-dimensional cell growth for use in experimental cancer therapy. A549, KB 3-1, KB 8-5 and V79 cells were successfully grown in CultiSphers, albeit with longer doubling times than observed for the respective cell type in monolayer cultures. MTT staining and histology demonstrated three-dimensional contact of cells in the microcarriers. A549 cells populated the microcarriers more densely than KB 3-1 cells, and with both cell types there is bead-to-bead variation in occupancy by cells. [3H]TdR autoradiography reveals labelled cells throughout A549 and KB 3-1 CultiSphers, with no proliferation gradient from edge to centre. Central necrosis has not been observed. A549 cells but not KB 3-1 cells demonstrated a radiation contact effect (i.e. resistance) when irradiated in CultiSphers, compared with monolayers. We conclude that CultiSphers may be a useful model for three-dimensional growth and cell contact when investigators want to investigate these phenomena without the microenvironmental gradients of spheroids.

Role of the INK4a locus in tumor suppression and cell mortality

The cell cycle inhibitor p16INK4a is inactivated in many human tumors and in families with hereditary melanoma and pancreatic cancer. Tumor-associated alterations in the INK4a locus may also affect the overlapping gene encoding p19ARF and the adjacent gene encoding p15I1NK4b, both negative regulators of cell proliferation. We report the phenotype of mice carrying a targeted deletion of the INK4a locus that eliminates both p16INK4a and p19ARF. The mice are viable but develop spontaneous tumors at an early age and are highly sensitive to carcinogenic treatments. INK4a-deficient primary fibroblasts proliferate rapidly and have a high colony-formation efficiency. In contrast with normal cells, the introduction of activated Ha-ras into INK4a-deficient fibroblasts can result in neoplastic transformation. These findings directly demonstrate that the INK4a locus functions to suppress neoplastic growth.

Expression and activity of L-Myc in normal mouse development

To determine the role of L-Myc in normal mammalian development and its functional relationship to other members of the Myc family, we determined the normal patterns of L-myc gene expression in the developing mouse by RNA in situ hybridization and assessed the phenotypic impact of L-Myc deficiency produced through standard gene targeting methodology. L-myc transcripts were detected in the developing kidney and lung as well as in both the proliferative and the differentiative zones of the brain and neural tube. Despite significant expression of L-myc in developing mouse tissue, homozygous null L-myc mice were found to be viable, reproductively competent, and represented in expected frequencies from heterozygous matings. A detailed histological survey of embryonic and adult tissues, characterization of an embryonic neuronal marker, and measurement of cellular proliferation in situ did not reveal any congenital abnormalities. The lack of an apparent phenotype associated with L-Myc deficiency indicates that L-Myc is dispensable for gross morphological development and argues against a unique role for L-Myc in early central nervous system development as had been previously suggested. Although overlapping expression patterns among myc family members raise the possibility of complementation of L-Myc deficiency by other Myc oncoproteins, compensatory changes in the levels of c- and/or N-myc transcripts were not detected in homozygous null L-myc mice.

Functional interactions among members of the Myc superfamily and potential relevance to cutaneous growth and development

Myc family oncoproteins function as sequence-specific transcription factors that are believed to regulate the expression of genes governing cellular growth, differentiation, and programmed cell death. Activities of Myc are countered by those of Mad and Mxi1, two related members of the Myc superfamily. Mad and Mxi1 compete with Myc for common elements and interact with putative transcriptional repressors. While the precise role of the Myc superfamily in cutaneous biology remains to be determined, findings from a number of organ systems suggest that the regulated expression and function of its members are intimately correlated with proper development and physiology. Reviewed here are current data on Myc superfamily function with references where relevant to cutaneous processes with the ultimate goal of providing a framework upon which these proteins can be exploited in gene therapeutic approaches for diseases of the skin, including neoplasia.