Klaus Heidorn

Telomerase activity in `immortal' fish1

Eukaryotic chromosome termini consist of telomeres, short sequence repeats. According to the telomere hypothesis, DNA replication leads to telomere shortening, resulting in a cellular mitotic clock. Telomerase resets it by telomere synthesis. In mammals with a limited growth phase, telomerase activity in somatic tissues is restricted to stem cell derivatives with high proliferation potential. But other animals, like some fish, grow throughout their life with little senescence. All somatic cells require a high proliferation capacity and telomerase should be active in all cells, irrespective of fish age. Indeed, we detected high telomerase activities in all analyzed organs of rainbow trout (Oncorhynchus mykiss).

Longevity of lobsters is linked to ubiquitous telomerase expression

Mammals have high growth rates in embryonic and juvenile phases and no growth in adult and senescent phases. We analyzed telomerase activity in a fundamentally different animal which grows indeterminately. Lobsters (Homarus americanus) grow throughout their life and the occurrence of senescence is slow. A modified TRAP assay was developed and the lobster telomeric repeat sequence TTAGG was determined. We detected telomerase activities which were dependent on RNA and protein components, required dGTP, dATP and dTTP, but not dCTP. Telomerase products with a five nucleotide periodicity were generated. High telomerase activities were detected in all lobster organs. We conclude that telomerase activation is a conserved mechanism for maintaining long‐term cell proliferation capacity and preventing senescence, not only in cellular models or embryonic life stages but also in adult multicellular organisms.

Telomerase Activity in Melanocytic Lesions : A Potential Marker of Tumor Biology

Telomerase activation, being a cardinal requirement for immortalization, is a crucial step in the development of malignancy. With a view toward diagnostic and biological aspects in melanocytic neoplasia, we investigated the relative levels of telomerase activity in 72 nevi and 16 malignant melanomas by means of a modified telomeric repeat amplification protocol (TRAP) assay, including an internal amplification standard. We further compared telomerase activity with the expression of two different proliferation-specific proteins, Ki-67 and repp86, a protein expressed exclusively in the cell cycle phases S, G2, and M. Telomerase activity was associated with the overall growth fraction (Ki-67) but showed a closer correlation with the expression of repp86. Both telomerase activity and proliferation indices discriminated clearly between malignant melanomas and nevi, but not between common and dysplastic nevi. Nonetheless, a portion of nevi exhibited markedly elevated telomerase activity levels without proportionally increased proliferation. This was independent of discernible morphological changes. Clinicopathological correlations showed an association between high telomerase activity and early metastatic spread in melanomas, linking telomerase to tumor biology. Our results provide arguments in favor of an occasional progression from nevi to melanomas and imply that proliferation measurements in combination with telomerase assays may help to elicit early malignant transformation that is undetectable by conventional morphology.

Expression of the c-kit receptor characterizes a subset of neuroblastomas with favorable prognosis.

Expression of the c-kit proto-oncogene product in neuroblastomas has been reported, but its clinical relevance is unclear. We determined the expression of c-kit by immunohistochemistry in a series of 155 neuroblastomas with long-term follow-up. The specificity of the reaction was verified by Western blot analysis and quantitative RT–PCR, and exon 11 of the kit gene was screened for mutations by PCR and capillary electrophoresis. No mutations were detected, and transcription of the kit gene correlated with protein expression. c-kit expression was associated with lower tumor stages and a low rate of MYCN amplification. More importantly, it coincided with tumor differentiation (P<0.0001), and portended a favorable outcome with a relative risk of 0.18 (P<0.0001). In a multivariate analysis of event-free survival, loss of c-kit (relative risk 4.25, P<0.0001) was an independent prognostic factor next to INSS stage 4 and before MYCN amplification. It is concluded that c-kit is transcriptionally regulated in neuroblastomas. Its expression likely identifies a subset of neuroblastomas with conserved capacity for differentiation, which may represent the embryonal variety of the disease. Assessment of c-kit may improve prognostic models for neuroblastoma and provide a basis for new therapy concepts.

Regulation of telomerase activity by alternate splicing of human telomerase reverse transcriptase mRNA in a subset of neuroblastomas

It has been proposed that the regulation of telomerase takes place at the transcriptional level, the expression of the catalytic subunit human telomerase reverse transcriptase (hTERT) being crucial for telomerase activity (TA). Recently, differential splicing of hTERT mRNA has been demonstrated in various tissues during embryonal development, and it has been suggested that only full-length transcripts translate into functionally active telomerase. With this in view, we analyzed the different hTERT transcripts by reverse transcriptase-polymerase chain reaction in neuroblastic tumors and compared the results with the TA, the tumor growth fraction, and the MYCN status. In a series of 38 neuroblastic tumors, high TA and full-length hTERT transcripts were found in nine samples, whereas nine samples showed absence of both enzymatic activity and hTERT transcripts. Interestingly, in another eight samples, low or absent TA coincided with a lack of full-length hTERT transcripts. Eleven samples contained hTERT transcripts with low or undetectable TA and one sample had low TA but no hTERT transcripts. TA correlated with MYCN amplification and was weakly associated with the proliferative activity. Moreover, a significant correlation with tumor progression was observed. Our findings point at a posttranscriptional regulation of TA in a subset of neuroblastic tumors. Because high TA was detected only in tumors with full-length hTERT transcripts, reverse transcriptase-polymerase chain reaction analysis of archival neuroblastic tumor samples might help to appraise the malignant potential in individual cases.

M-CSF and M-CSF-receptor gene expression in acute myelomonocytic leukemias.

The role of hematopoietic growth factors in the pathogenesis of human leukemias is still obscure. In this study, RNA from 24 human acute myelomonocytic leukemias (AML) was used to analyze the expression of the macrophage colony stimulating factor (M-CSF) and its corresponding receptor (c-fms). Fifty percent of AML cells exhibited c-fms transcripts of regular length but at a lower level than in normal monocytes/macrophages. In most cases the reduced c-fms expression of AML cells was not associated with autostimulatory M-CSF expression. Only a few cases of AML showed co-expression of M-CSF and c-fms, which by contrast was regularly observed in cultivated blood monocytes and some tissue macrophage subsets. Higher levels of c-fms expression could be found in AMLs with a more mature monocytic immunophenotype. Permanent myelomonocytic cell lines expressed c-fms only after induction of monocytic differentiation. Neither the M-CSF gene nor the c-fms gene were rearranged in AML cells. In AML cells the homozygote genotype of the c-fms gene predominated. Our results do not provide evidence for the involvement of M-CSF and c-fms genes in human myeloid leukemogenesis. c-fms expression appears to indicate monocytic differentiation within the myelomonocytic lineage. We found autostimulatory M-CSF expression to be a physiologic feature of some tissue macrophages and hence not necessarily associated with neoplastic proliferation.

Regulation of telomerase activity in quiescent immortalized human cells

Telomerase is a key enzyme in carcinogenesis; telomerase activity has been found in more than 90% of human tumors. Understanding the regulation of this enzyme will improve our knowledge of tumor biology and may lead to novel strategies in cancer therapy. We examined effects of growth arrest on telomerase activity in the human immortalized cell lines U 937 (lymphoma) and L 428 (Hodgkins disease). Cells were starved by serum depletion for 4 days. After readdition of serum, a recovery phase followed. Cell proliferation was monitored with the monoclonal antibody Ki-S5. In the absence of serum, telomerase levels declined fivefold. After serum readdition, recovery to threefold increased level was observed. Furthermore, the prevalence of telomerase-positive cells in normal tissues is an important issue for understanding tumorigenesis. Our TRAP assay is robust against false positives and in mixed cell samples, we found a rather limited sensitivity of the telomere repeat amplification protocol (TRAP) assay. This means that adequate screenings for telomerase-positive somatic cells have to include enrichment steps.

Telomere shortening in uterine leiomyomas.

OBJECTIVE To gain a better understanding of proliferation control mechanisms in a common benign tumor, we investigated the mean telomere length and the clonality of uterine leiomyomas. STUDY DESIGN Deoxyribonucleic acid from uterine leiomyomas and from the adjacent normal myometrium of 51 patients (total number of uterine leiomyomas 107; 28 patients with single leiomyoma, 23 patients with multiple leiomyomas ranging from 2 to 8 myoma nodules per case) was hybridized to a telomeric oligonucleotide probe by Southern blot and chemiluminescent detection. The mean telomere length was evaluated by densitometry. Clonality was assessed with use of the phosphoglycerokinase gene polymorphism. RESULTS The mean telomere length was significantly shorter in uterine leiomyomas (median 7950 bp, interquartile range 7261 to 8372 bp) than in normal myometrium (median 9688 bp, interquartile range 8528 to 10535 bp) (P < .001). There was no correlation between tumor size and telomere attrition. Multiple uterine leiomyomas were found to have an independent clonal origin. CONCLUSIONS Telomere attrition in uterine leiomyomas reflects enhanced proliferation activity in the course of tumor evolution. The basic telomere lengths differ in the myocytes from which the uterine leiomyomas originate, probably explaining the lack of correlation between telomere attrition and tumor size.

Modulation of c‐fms Proto‐Oncogene Expression in Human Blood Monocytes and Macrophages

The gene product of the c‐fms proto‐oncogene is a transmembrane protein with tyrosine‐kinase activity that is obviously related to the receptor for the colony‐stimulating‐factor CSF‐1. By Northern blot analysis, we investigated the expression of the cellular counterpart of v‐fms in purified normal human blood mononuclear cells and different macrophage populations. The proto‐oncogene c‐fms expression was demonstrable in blood monocytes but not in blood lymphocytes. Short‐term cultivated blood monocytes exhibited an increased expression of c‐fms in comparison to freshly isolated blood monocytes, possibly due to a temporary down regulation of c‐fms during the separation procedure of blood monocytes. A comparably high rate of fms‐RNA expression was found in most of the analyzed samples of resident peritoneal macrophages, while resident alveolar macrophages showed a considerably lower level of c‐fms expression. In this, alveolar macrophages resembled long‐term cultivated adherent blood monocytes, which showed a down regulation of c‐fms expression. By correlating these data obtained by Northern blot analysis with phenotypic properties of the analyzed monocyte/macrophage populations, it is concluded that different levels of c‐fms expression in monocytes/macrophages correspond to their stage of differentiation and maturity.

Increased Methylation of the c-fms Protooncogene in Acute Myelomonocytic Leukemias

DNA methylation provides an epigenetic information possibly involved in differentiation processes and gene regulation. In this study we investigated the methylation state of the c-fms/M-CSF receptor gene in normal human blood cells and leukemias. The methylation pattern of the c-fms gene as detected by isoschizomeric restriction analysis with MspI/HpaII showed only slight interindividual variations in normal donors, but there were constant differences between granulocytes and monocytes from the same donor. Of 21 acute myelomonocytic leukemias investigated, 85% revealed a methylation pattern different from that of normal monocytes. One or more restriction sites which were at least partly unmethylated in normal monocytes proved to be methylated in leukemic cells. In contrast, leukemias of lymphocytic origin showed hypomethylation of the c-fms gene. There was no correlation between the methylation state of the c-fms gene and its expression at the RNA level, but an increase in monocyte/macrophage-specific differentiation markers could be observed in those samples which exhibited a methylation pattern similar to that of normal monocytes. The increased DNA methylation within the c-fms gene might reflect the inactivation of differentiation genes in leukemic cell clones, contributing to their maturation arrest. Furthermore, neoplastic hematopoietic cells seem to exhibit lineage-specific differences in c-fms gene methylation.