Association between CpG island methylation and microsatellite instability in colorectal cancer

De novo methylation of promoter region CpG islands has been increasingly associated with transcriptional inactivation of important genes in neoplasia. To study the potential mechanisms underlying aberrant methylation in cancer, we have determined the methylation patterns of selected genes in colorectal cancers with and without microsatellite instability (MI), which results from defects in one of several base mismatch repair genes. A total of 47 colorectal cancers were analyzed, of which 15 were MI+ (32%). We now report that both the frequency and the extent of de novo methylation are strikingly increased in MI+ cancers. Hypermethylation of the p16 gene was found in 60% of MI+ cancers, compared to only 22% in MI- cancers (P = 0.02). Similarly, hypermethylation of the thrombospondin-1 (TSP-1) gene, an angiogenesis inhibitor, was increased in MI+ cancers (27% versus 0%; P = 0.008). Extensive methylation of insulin-like growth factor II (IGF2) and hypermethylated in cancer-1 (HIC-1) genes was observed in 60 and 80% of MI+ cancers, respectively, as contrasted with 6 and 38% of MI- cancers (P = 0.0002 and 0.01, respectively). Furthermore, 60% of the MI+ cancers displayed the hypermethylation events at two or more loci in a concordant manner compared to only 9% of the MI- cancers (P < 0.001). These results demonstrate a strong link between promoter hypermethylation and genetic instability due to deficient DNA repair.

Abrogation of the Rb/p16 tumor-suppressive pathway in virtually all pancreatic carcinomas

The Rb/p16 tumor-suppressive pathway is abrogated frequently in human tumors, either through inactivation of the Rb or p16INK4a/CDKN2/MTS1 tumor-suppressor proteins, or through alteration or overexpression of the cyclin D1 or cyclin-dependent kinase 4 oncoproteins. We reported previously that the p16 gene was genetically inactivated in 82% of pancreatic carcinomas. Nearly half of these inactivations were by intragenic mutation of p16, and the remainder were by homozygous deletion of the gene. Here, we analyzed pancreatic carcinomas for additional mechanisms by which the Rb/p16 pathway might be inactivated. Transcriptional silencing of the p16 gene in association with methylation of its 5'-CpG island was examined by methylation-specific PCR in 18 pancreatic carcinomas. Nine of these were known to harbor an intragenic mutation in p16, and nine had a wild-type p16 coding sequence. Seven of the 18 tumors were hypermethylated, and all 7 were p16 wild-type (P = 0.001). Complete silencing of transcription from methylated wild-type gene sequences was demonstrated. Immunohistochemical analysis revealed normal expression levels of the Rb protein in all carcinomas studied. None of the carcinomas had genomic amplification of the cyclin D1 or CDK4 genes, and none had mutation of the p16-binding domain of CDK4. An additional p16 mutation was identified. In total, the Rb/p16 pathway was abrogated in 49 of the 50 carcinomas (98%) studied, all through inactivation of the p16 gene. Similar results were obtained in an independently analyzed series of 19 pancreatic carcinomas. These data demonstrate the central role of the Rb/p16 pathway in the development of pancreatic carcinoma.

Tissue-specific expression of human achaete-scute homologue-1 in neuroendocrine tumors: transcriptional regulation by dual inhibitory regions

Malignancies with neuroendocrine (NE) features such as medullary thyroid cancer (MTC) and small cell lung cancer (SCLC) are prototypic neoplasms arising from peripheral endocrine cells. The mechanisms that regulate the NE phenotype in these tumors and their cellular precursors are not well understood. However, a basic helix-loop-helix transcription factor that is homologous to Drosophila neural fate determination proteins may have a central role. Human achaete-scute homologue-1 (hASH1), a human homologue of the Drosophila achaete-scute complex, is highly expressed in MTC, SCLC, and pheochromocytomas. To determine what mechanisms allow constitutive expression of hASH1 in NE tumors, we cloned human genomic DNA fragments containing the hASH1 gene and characterized its promoter region. We show that hASH1 expression is restricted to NE cell lines by a transcriptionally regulated mechanism. Dual promoters initiate hASH1 transcription, with the predominant site being an evolutionarily conserved initiator (INR) element. Transient transfection studies provide evidence for a generalized enhancer region that has high activity in all cell lines tested. Restriction of hASH1 expression to NE tumor cells depends on two tissue-specific repressor regions, present in the proximal and distal (> 13.5 kb) 5'-flanking region. Understanding the mechanisms of tissue-specific control of hASH1 gene expression provides a useful model to explore regulatory cascades influencing both normal nervous system development and the NE phenotype of tumors such as MTC and SCLC.

Deletional, mutational, and methylation analyses of CDKN2 (p16/MTS1) in primary and metastatic prostate cancer

The tumor suppressor gene CDKN2 (p16/MTS1) resides on chromosome 9p21 and encodes a 16 kDa inhibitor of the cyclin-dependent kinases. Inactivation of CDKN2 by homozygous deletion, point mutation, and recently described aberrant methylation in the 5' promoter region may increase progression through the cell cycle in tumors. In this study, we examine the CDKN2 gene for the presence of inactivating alterations in human prostate cancer. Sequence analysis of cell lines revealed no mutation in LNCaP, PC3, and TSU-PR1 and a missense mutation, GAC-->TAC (asp to tyr), in exon 2 of the DU145 cell line at codon 76. No mutations were identified in three primary prostate cancers or in seven lymph node metastases. Loss of heterozygosity (LOH) was analyzed by analysis of microsatellite markers in the vicinity of the CDKN2 gene. LOH was detected in 12 (20%) of 60 primary tumors at one or more loci and in 13 (46%) of 28 metastases. Methylation analysis of the CpG-rich promoter region revealed a dense methylation of CDKN2 in cell lines PC3, PPC1, and TSU-PR1, and this was found to correlate with a lack of mRNA expression by reverse transcription-polymerase chain reaction. A demethylating agent, 5-aza-2'-deoxycytidine, induced reexpression when cells were exposed in vitro. DU145 and LNCaP expressed the CDKN2 transcript and were unmethylated in the promoter region. Three of twenty-four (13%) primary prostate cancers and 1 of 12 metastatic tumors demonstrated promoter methylation. No normal prostate tissues were methylated at the CDKN2 gene promoter. One tumor was found to contain concomitant LOH and promoter methylation indicative of biallelic inactivation. A comprehensive analysis of CDKN2 in prostate cancer reveals that point mutations are infrequent, but gene deletion and methylation combine to inactivate CDKN2 in a subset of tumors. Moreover, alterations in this gene may represent a late event in prostate cancer progression.

Conservation of the Drosophila lateral inhibition pathway in human lung cancer: a hairy-related protein (HES-1) directly represses achaete-scute homolog-1 expression

The achaete-scute genes encode essential transcription factors in normal Drosophila and vertebrate nervous system development. Human achaete-scute homolog-1 (hASH1) is constitutively expressed in a human lung cancer with neuroendocrine (NE) features, small cell lung cancer (SCLC), and is essential for development of the normal pulmonary NE cells that most resemble this neoplasm. Mechanisms regulating achaete-scute homolog expression outside of Drosophila are presently unclear, either in the context of the developing nervous system or in normal or neoplastic cells with NE features. We now provide evidence that the protein hairy-enhancer-of-split-1 (HES-1) acts in a similar manner as its Drosophila homolog, hairy, to transcriptionally repress achaete-scute expression. HES-1 protein is detected at abundant levels in most non-NE human lung cancer cell lines which lack hASH1 but is virtually absent in hASH1-expressing lung cancer cells. Moreover, induction of HES-1 in a SCLC cell line down-regulates endogenous hASH1 gene expression. The repressive effect of HES-1 is directly mediated by binding of the protein to a class C site in the hASH1 promoter. Thus, a key part of the process that determines neural fate in Drosophila is conserved in human lung cancer cells. Furthermore, modulation of this pathway may underlie the constitutive hASH1 expression seen in NE tumors such as SCLC, the most virulent human lung cancer.

HIC1 hypermethylation is a late event in hematopoietic neoplasms

HIC1, a candidate tumor suppressor gene on 17p13.3, is hypermethylated and silenced in a large number of solid tumors. To determine its potential role in leukemias, we studied its methylation status in normal and neoplastic hematopoietic cells. We found HIC1 to be unmethylated in peripheral blood cells, bone marrow cells, and CD34+ cells. HIC1 was rarely methylated in newly diagnosed acute myelogenous leukemias (10%) but was relatively frequently methylated in newly diagnosed non-Hodgkin's lymphoma (25%), acute lymphocytic leukemia (ALL; 53%), and chronic-phase chronic myelogenous leukemia (50%). By contrast, HIC1 was hypermethylated in 100% of recurrent ALL and 100% of blast crisis chronic myelogenous leukemia. In two patients with ALL for whom paired diagnosis/relapse samples were available, HIC1 was unmethylated at diagnosis but was highly methylated at relapse after a chemotherapy-induced complete remission. HIC1 methylation, therefore, seems to be a progression event in hematopoietic neoplasms.

An achaete-scute homologue essential for neuroendocrine differentiation in the lung

In Drosophila and in vertebrates, the achaete-scute family of basic helix-loop-helix transcription factors plays a critical developmental role in neuronal commitment and differentiation. Relatively little is known, however, about the transcriptional control of neural features in cells outside a neuronal context. A minority of normal bronchial epithelial cells and many lung cancers, especially small-cell lung cancer, exhibit a neuroendocrine phenotype that may reflect a common precursor cell population. We show here that human achaete-scute homologue-1 (hASH1) is selectively expressed in normal fetal pulmonary neuroendocrine cells, as well as in the diverse range of lung cancers with neuroendocrine features. Strikingly, newborn mice bearing a disruption of the ASH1 gene have no detectable pulmonary neuroendocrine cells. Depletion of this transcription factor from lung cancer cells by antisense oligonucleotides results in a significant decrease in the expression of neuroendrocrine markers. Thus, a homologue of Drosophila neural fate determination genes seems to be necessary for progression of lung epithelial cells through a neuroendocrine differentiation pathway that is a feature of small-cell lung cancer, the most lethal form of human lung cancer.

Protein kinase C-beta 2 inhibits cycling and decreases c-myc-induced apoptosis in small cell lung cancer cells

The overexpression of c-myc frequently accompanies the relapse of small cell lung cancer (SCLC) cells and contributes to the poor prognosis of this tumor. In this study, we confirm that transfected c-myc results in decreased homotypic cell aggregation and increased proliferative capacity of SCLC cells when nutrient conditions are adequate. We also find that c-myc contributes to apoptosis when cells are nutrient depleted, and flow cytometry suggests that this enhanced apoptosis is associated with a failure to halt cell cycling, consistent with the experience in other cell types. We previously found that protein kinase C-beta (PKC-beta) expression in NCI H209 (209) SCLC cells increases markedly with c-myc transfection (L. F. Barr et al., Cancer Res., 51: 5514-5519, 1991), and we hypothesized that PKC-beta may mediate some of the effects of c-myc in these cells. We test this hypothesis by transfection of rat PKC-beta 1 and bovine PKC-beta 2 isoforms into 209 cells before and after transfection with c-myc. PKC-beta 1 transfection has no effect on these cells. However, PKC-beta 2 expression has distinct phenotypic consequences. In the parental cells, PKC-beta 2 expression results in increased homotypic cell aggregation and a prolonged doubling time. Furthermore, PKC-beta 2 expression increases the fraction of these cells in G0-G1. In the cells which express a transfected c-myc gene, PKC-beta 2 expression improves the survival of cells in low serum by decreasing myc-induced apoptosis. This effect was associated with, and may be mediated by, a selection for cells in the G0-G1 fraction. We postulate that transfection of c-myc into SCLC cells may select for those expressing the PKC-beta 2 gene because this signal transduction event protects against myc-induced apoptosis.

DNA methylation changes in hematologic malignancies: biologic and clinical implications

DNA methylation changes are among the most common detectable abnormalities in human neoplasia. Hypermethylation within the promoters of selected genes appears to be especially common in all types of human hematopoietic neoplasms, and is usually associated with inactivation of the involved gene(s). Such hypermethylation-associated silencing of gene expression has been shown for several genes regulating the growth and differentiation of hematopoietic cells, including the estrogen receptor (ER) gene, P15, P16 and others. Hypermethylation within the promoters of some genes appear to be an early event in the pathogenesis of neoplasia (ER, P15), while other genes seem to become methylated during the progression of leukemias (HIC1, c-abl). The high prevalence of promoter methylation suggests that this molecular abnormality can be used to monitor disease activity during therapy. In addition, new technology allows the sensitive identification of gene hypermethylation in a background of normal cells, suggesting possible new strategies for the detection of minimal residual disease. Finally, reactivation of tumor-suppressor gene expression through pharmacologic inhibition of DNA methyltransferase and resultant DNA demethylation appears to be a promising new avenue of therapy in acute leukemia.

Frequent aberrant methylation of p16INK4a in primary rat lung tumors

The p16INK4a (p16) tumor suppressor gene is frequently inactivated by homozygous deletion or methylation of the 5' CpG island in cell lines derived from human non-small-cell lung cancers. However, the frequency of dysfunction in primary tumors appears to be significantly lower than that in cell lines. This discordance could result from the occurrence or selection of p16 dysfunction during cell culture. Alternatively, techniques commonly used to examine tumors for genetic and epigenetic alterations may not be sensitive enough to detect all dysfunctions within the heterogeneous cell population present in primary tumors. If p16 inactivation plays a central role in development of non-small-cell lung cancer, then the frequency of gene inactivation in primary tumors should parallel that observed in cell lines. The present investigation addressed this issue in primary rat lung tumors and corresponding derived cell lines. A further goal was to determine whether the aberrant p16 gene methylation seen in human tumors is a conserved event in this animal model. The rat p16 gene was cloned and sequenced, and the predicted amino acid sequence of its product found to be 62% homologous to the amino acid sequence of the human analog. Homozygous deletion accounted for loss of p16 expression in 8 of 20 cell lines, while methylation of the CpG island extending throughout exon 1 was observed in 9 of 20 cell lines. 2-Deoxy-5-azacytidine treatment of cell lines with aberrant methylation restored gene expression. The methylated phenotype seen in cell lines showed an absolute correlation with detection of methylation in primary tumors. Aberrant methylation was also detected in four of eight primary tumors in which the derived cell line contained a deletion in p16. These results substantiate the primary tumor as the origin for dysfunction of the p16 gene and implicate CpG island methylation as the major mechanism for inactivating this gene in the rat lung tumors examined. Furthermore, rat lung cancer appears to be an excellent model in which to investigate the mechanisms of de novo gene methylation and the role of p16 dysfunction in the progression of neoplasia.

Distinct patterns of inactivation of p15INK4B and p16INK4A characterize the major types of hematological malignancies

Inactivation of the cyclin-dependent kinase inhibitors p16INK4A and p15INK4B are frequent alterations in neoplasia, often resulting from homozygous deletion or promoter region hypermethylation. We have analyzed both modes of inactivation of p15INK4B and p16INK4A in the major types of adult and pediatric hematological malignancies. Hypermethylation of p15INK4B, without alteration of p16INK4A, was an almost universal finding in adult acute myelogenous leukemia, and occurred very frequently in adult acute lymphocytic leukemia and pediatric acute myelogenous leukemia and acute lymphocytic leukemia. In contrast, neither p15INK4B nor p16INK4A were inactivated in any stage of chronic myelogenous leukemia. Hypermethylation of p16INK4A, often without alterations of p15INK4B, was found in non-Hodgkin's lymphoma and was much more frequent in cases with high-grade than low-grade histology. Enriched normal bone marrow stem cells had no detectable promoter region methylation of these genes, as analyzed by a newly developed PCR method. Remarkably distinct patterns of inactivation of p15INK4B and p16INK4A characterize different types of hematological malignancy, and alterations in these tumor suppressor genes are one of the most common alterations in hematological malignancies.

New 5' regions of the murine and human genes for DNA (cytosine-5)-methyltransferase

DNA (cytosine-5)-methyltransferases (EC 2.1.1.37) maintain patterns of methylated cytosine residues in the mammalian genome; faithful maintenance of methylation patterns is required for normal development of mice, and aberrant methylation patterns are associated with certain human tumors and developmental abnormalities. The organization of coding sequences at the 5'-end of the murine and human DNA methyltransferase genes was investigated, and the DNA methyltransferase open reading frame was found to be longer than previously suspected. Expression of the complete open reading frame by in vitro transcription-translation and by transfection of expression constructs into COS7 cells resulted in the production of an active DNA methyltransferase of the same apparent mass as the endogenous protein, while translation from the second in-frame ATG codon produced a slightly smaller but fully active protein. Characterization of mRNA 5' sequences and the intron-exon structure of the 5' region of the murine and human genes indicated that a previously described promoter element (Rouleau, J., Tanigawa, G., and Szyf, M. (1992) J. Biol. Chem. 267, 7368-7377) actually lies in an intron that is more than 5 kilobases downstream of the transcription start sites.

Switch from monoallelic to biallelic human IGF2 promoter methylation during aging and carcinogenesis

We have previously linked aging, carcinogenesis, and de novo methylation within the promoter of the estrogen receptor (ER) gene in human colon. We now examine the dynamics of this process for the imprinted gene for insulin-like growth factor II (IGF2). In young individuals, the P2-4 promoters of IGF2 are methylated exclusively on the silenced maternal allele. During aging, this promoter methylation becomes more extensive and involves the originally unmethylated allele. Most adult human tumors, including colon, breast, lung, and leukemias, exhibit increased methylation at the P2-4 IGF2 promoters, suggesting further spreading during the neoplastic process. In tumors, this methylation is associated with diminished or absent IGF2 expression from the methylated P3 promoter but maintained expression from P1, an upstream promoter that is not contained within the IGF2 CpG island. Our results demonstrate a remarkable evolution of methylation patterns in the imprinted promoter of the IGF2 gene during aging and carcinogenesis, and provide further evidence for a potential link between aberrant methylation and diseases of aging.

RREB-1, a novel zinc finger protein, is involved in the differentiation response to Ras in human medullary thyroid carcinomas

An activated ras oncogene induces a program of differentiation in the human medullary thyroid cancer cell line TT. This differentiation process is accompanied by a marked increase in the transcription of the human calcitonin (CT) gene. We have localized a unique Ras-responsive transcriptional element (RRE) in the CT gene promoter. DNase I protection indicates two domains of protein-DNA interaction, and each domain separately can confer Ras-mediated transcriptional inducibility. This bipartite RRE was also found to be Raf responsive. By affinity screening, we have cloned a cDNA coding for a zinc finger transcription factor (RREB-1) that binds to the distal RRE. The consensus binding site for this factor is CCCCAAACCACCCC. RREB-1 is expressed ubiquitously in human tissues outside the adult brain. Overexpression of RREB-1 protein in TT cells confers the ability to mediate increased transactivation of the CT gene promoter-reporter construct during Ras- or Raf-induced differentiation. These data suggest that RREB-1 may play a role in Ras and Raf signal transduction in medullary thyroid cancer and other cells.

Methylation-specific PCR: a novel PCR assay for methylation status of CpG islands

Precise mapping of DNA methylation patterns in CpG islands has become essential for understanding diverse biological processes such as the regulation of imprinted genes, X chromosome inactivation, and tumor suppressor gene silencing in human cancer. We describe a new method, MSP (methylation-specific PCR), which can rapidly assess the methylation status of virtually any group of CpG sites within a CpG island, independent of the use of methylation-sensitive restriction enzymes. This assay entails initial modification of DNA by sodium bisulfite, converting all unmethylated, but not methylated, cytosines to uracil, and subsequent amplification with primers specific for methylated versus unmethylated DNA. MSP requires only small quantities of DNA, is sensitive to 0.1% methylated alleles of a given CpG island locus, and can be performed on DNA extracted from paraffin-embedded samples. MSP eliminates the false positive results inherent to previous PCR-based approaches which relied on differential restriction enzyme cleavage to distinguish methylated from unmethylated DNA. In this study, we demonstrate the use of MSP to identify promoter region hypermethylation changes associated with transcriptional inactivation in four important tumor suppressor genes (p16, p15, E-cadherin, and von Hippel-Lindau) in human cancer.

Cell-substratum interactions mediate oncogene-induced phenotype of lung cancer cells

In vivo and in vitro studies have linked small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC) cells along a differentiation continuum. The transition of a SCLC toward a NSCLC phenotype is modeled in culture by the simultaneous overexpression of myc and ras genes in cultured SCLC cells. A major phenotypic distinction between SCLC and NSCLC in culture is that SCLC cells usually grow in floating aggregates, whereas NSCLC cells and myc- plus ras-expressing SCLC cells grow as adherent spreading monolayers like other epithelial cells. The present studies examine how myc, ras, cell aggregation, and attachment to laminin may interact to modulate transitions between the SCLC and NSCLC phenotypes. We find that myc-expressing SCLC cells, which normally grow as anchorage-independent cells in plastic flasks, will adhere to laminin and exhibit an epithelial morphology. In this setting, the cells express both NSCLC and SCLC markers, thus resembling a tumor type previously termed NSCLC with neuroendocrine features. Anchorage-dependent SCLC cells simultaneously expressing the myc family and an exogenous ras oncogene move further toward the NSCLC phenotype than the above myc-expressing cells. However, forced suspension of such cells restores the expression of neuroendocrine SCLC features. These studies indicate that cell environment, as much as gene expression events, profoundly affects aspects of the SCLC cell phenotype.

Methylation of the estrogen receptor CpG island in lung tumors is related to the specific type of carcinogen exposure

Promoter methylation has recently been shown to be an alternative to mutation in inactivating tumor suppressor genes in human neoplasia. Although specific carcinogen exposures have been associated with characteristic mutation patterns in genes, the factors that lead to promoter hypermethylation remain unknown. One gene target for inactivation through promoter methylation is the estrogen receptor (ER). The purpose of this investigation was to determine the methylation status of this gene in lung tumors from smokers and those who never smoked and in rodents exposed to specific environmental carcinogens. Promoter methylation at the ER locus was detected in 4 of 11 tumors from never-smokers (36.4%) and 7 of 35 tumors from smokers (20%, P < 0.001). Lung tumors induced by the tobacco-derived carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1- butanone also had a low incidence (16.7%) of ER methylation. In marked contrast, spontaneous and plutonium-induced tumors had a very high (81.8%) incidence of ER methylation. X-ray-induced tumors had an intermediate frequency of ER methylation (38.1%). The presence of ER methylation was associated with absent ER expression in rodent lung cancer cell lines. These results show for the first time that gene-specific promoter methylation can be modulated differentially depending on carcinogen exposure.

Hypermethylation of chromosome 17P locus D17S5 in human prostate tissue

PURPOSE

Under normal conditions genomic CpG islands are not methylated. Hypermethylation of a CpG island in the 5' regulatory region of a gene has the capacity to silence gene transcription. Recently, hypermethylation of a CpG island at D17S5 on chromosome 17P13.3 has been shown to be a frequent tumor-specific event. When it has been observed, hypermethylation of D17S5 occurs solely in neoplastic tissues. Consequently, it has been hypothesized that hypermethylation of D17S5 may be an important carcinogenic event in the organs in which it occurs (colon, kidney, and brain). In this study we examine D17S5 hypermethylation in DNA from the prostate, a gland which is unique in that it undergoes hyperplastic or neoplastic growth or both in virtually all aging men.

MATERIALS AND METHODS

The methylation sensitive restriction enzyme Notl, a cDNA probe specific for the D17S5 locus, and Southern blotting were used to assay for hypermethylation of D17S5 in DNA derived from normal, benign hyperplastic and malignant prostate tissues.

RESULTS

We find that methylation of Notl restriction sites at D17S5 is a very common occurrence in prostate cancers (25 of 26 cases examined). Surprisingly, we found that methylation of these sites at D17S5 also occurred in histologically normal prostate and benign hyperplastic (BPH) tissue from glands which both did and did not contain cancer. In contrast, seminal vesicle, an androgen-dependent male sex accessory tissue that rarely undergoes pathological overgrowth, was devoid of hypermethylation at this locus. CONCLUSIONS. These data demonstrate that hypermethylation of D17S5 is a tissue-specific event in prostate DNA, and we hypothesize that methylation of this and/or related loci may play a role in the extreme predilection of this gland to neoplastic growth.

De novo methylation of CpG island sequences in human fibroblasts overexpressing DNA (cytosine-5-)-methyltransferase

Recent studies showing a correlation between the levels of DNA (cytosine-5-)-methyltransferase (DNA MTase) enzyme activity and tumorigenicity have implicated this enzyme in the carcinogenic process. Moreover, hypermethylation of CpG island-containing promoters is associated with the inactivation of genes important to tumor initiation and progression. One proposed role for DNA MTase in tumorigenesis is therefore a direct role in the de novo methylation of these otherwise unmethylated CpG islands. In this study, we sought to determine whether increased levels of DNA MTase could directly affect CpG island methylation. A full-length cDNA for human DNA MTase driven by the cytomegalovirus promoter was constitutively expressed in human fibroblasts. Individual clones derived from cells transfected with DNA MTase (HMT) expressed 1- to 50-fold the level of DNA MTase protein and enzyme activity of the parental cell line or clones transfected with the control vector alone (Neo). To determine the effects of DNA MTase overexpression on CpG island methylation, we examined 12 endogenous CpG island loci in the HMT clones. HMT clones expressing > or = 9-fold the parental levels of DNA MTase activity were significantly hypermethylated relative to at least 11 Neo clones at five CpG island loci. In the HMT clones, methylation reached nearly 100% at susceptible CpG island loci with time in culture. In contrast, there was little change in the methylation status in the Neo clones over the same time frame. Taken together, the data indicate that overexpression of DNA MTase can drive the de novo methylation of susceptible CpG island loci, thus providing support for the idea that DNA MTase can contribute to tumor progression through CpG island methylation-mediated gene inactivation.

Differentiation of medullary thyroid cancer by C-Raf-1 silences expression of the neural transcription factor human achaete-scute homolog-1

BACKGROUND

Human achaete-scute homolog-1 (hASH1), a fetal neural transcription factor, is highly expressed in neuroendocrine tumors such as medullary thyroid cancer (MTC). Although hASH1 probably plays a part in the growth and development of these tumors, its precise role and mechanism are unknown.

METHODS

To further elucidate the function and regulation of hASH1 in neuroendocrine tumor differentiation, we used a model of MTC tumor differentiation mediated by the ras/raf-1 signaling pathway. The MTC TT cells alone or transduced with a beta-estradiol activatable raf-1 construct (TT: delta Raf-1:ER) were treated with beta-estradiol or carrier. Northern analysis and nuclear run-off assays were performed to determine the hASH1 messenger RNA (mRNA) levels and transcription rate, respectively.

RESULTS

The TT: delta Raf-1:ER cells treated with beta-estradiol underwent marked biochemical and morphologic changes, including cell rounding, increase in calcitonin transcription, loss of RET proto-oncogene expression, and cessation of cell growth. During this differentiation process expression of hASH1 mRNA was silenced. Nuclear run-off experiments revealed that this decrease in steady-state hASH1 mRNA by raf-1 activation resulted predominantly from transcriptional inhibition.

CONCLUSIONS

Silencing of hASH1 in parallel with loss of RET is associated with development of a mature C-cell differentiation pattern. Mechanisms leading to transcriptional silencing of hASH1 may be crucial in regulating the proliferative capacity or differentiation status of MTC. Downstream targets of hASH1 could play a role in C-cell proliferation and progression to MTC.

Retention of unmethylated CpG island alleles in human diploid fibroblast x fibrosarcoma hybrids expressing high levels of DNA methyltransferase

The mechanisms underlying ectopic methylation of CpG islands in neoplastic cells are poorly understood. One determinant may be the increased expression of DNA methyltransferase (DNA MTase) observed frequently in neoplastic cells. To evaluate the role of DNA MTase overexpression in aberrant CpG island methylation, we assessed methylation of fibroblast-derived CpG islands in human diploid fibroblast x fibrosarcoma hybrid cell lines. Each of six independently derived, immortalized hybrid cell lines exhibited a high level of DNA MTase expression, comparable to that of the fibrosarcoma parental line. The methylation status of five CpG island loci, each of which was methylated extensively in the fibrosarcoma parental cells but not in the fibroblasts, was then determined in the hybrid cell lines. The patterns of methylation were consistent and highly locus dependent among the hybrid lines. Unmethylated alleles were retained stably at three loci. The parental origin of alleles could be determined at two other loci in the hybrid cells. Whereas no methylation of parental fibroblast-derived alleles of the HIC-1 locus was noted in hybrid cell lines, a marked increase in methylation of fibroblast-derived alleles of the estrogen receptor was observed in all hybrid cell lines. Therefore, despite high-level DNA MTase expression, widespread loss of unmethylated CpG islands was not observed in the hybrid cell lines. The nonrandom pattern of increased CpG island methylation in the hybrid cell lines suggests that locus-specific features and/or clonal selection, and not just DNA MTase expression, affect the evolution of ectopic methylation in neoplastic cells. Somatic cell hybrids may provide useful models for studying aberrant epigenetic events in neoplastic cells.

Methylation of estrogen and progesterone receptor gene 5' CpG islands correlates with lack of estrogen and progesterone receptor gene expression in breast tumors

Hormonal factors have a profound influence on the development, treatment, and outcome of breast cancer. The absence of steroid hormone receptors is highly correlated with resistance to antihormonal treatments. Work in cultured human breast cancer cell lines has shown that the absence of estrogen receptor (ER) gene expression in ER- cells is associated with extensive methylation of the ER gene 5' CpG island, and treatment with agents that demethylate the ER gene CpG island results in the production of functional ER protein. The current study shows that CpG islands in the 5' region of the ER and progesterone receptor (PR) genes are methylated in a significant fraction of primary human breast cancer tissues. The ER CpG island is methylated at the methylation-sensitive NotI restriction site in 9 of 39 (25%) of primary ER- breast cancers but remains unmethylated in 53 ER+ breast cancers and 9 normal breast specimens. Three methylation-sensitive restriction sites in the PR gene CpG island are not methylated in normal breast specimens and PR+ human breast cancers but are hypermethylated in 40% of PR- human breast tumors. These data demonstrate that methylation of the ER and PR gene CpG islands is associated with the lack of ER and PR gene expression in a significant fraction of human breast cancers.

Increased cytosine DNA-methyltransferase activity is target-cell-specific and an early event in lung cancer

The association between increased DNA-methyltransferase (DNA-MTase) activity and tumor development suggest a fundamental role for this enzyme in the initiation and progression of cancer. A true functional role for DNA-MTase in the neoplastic process would be further substantiated if the target cells affected by the initiating carcinogen exhibit changes in enzyme activity. This hypothesis was addressed by examining DNA-MTase activity in alveolar type II (target) and Clara (nontarget) cells from A/J and C3H mice that exhibit high and low susceptibility, respectively, for lung tumor formation. Increased DNA-MTase activity was found only in the target alveolar type II cells of the susceptible A/J mouse and caused a marked increase in overall DNA methylation in these cells. Both DNA-MTase and DNA methylation changes were detected 7 days after carcinogen exposure and, thus, were early events in neoplastic evolution. Increased gene expression was also detected by RNA in situ hybridization in hypertrophic alveolar type II cells of carcinogen-treated A/J mice, indicating that elevated levels of expression may be a biomarker for premalignancy. Enzyme activity increased incrementally during lung cancer progression and coincided with increased expression of the DNA-MTase activity are strongly associated with neoplastic development and constitute a key step in carcinogenesis. The detection of premalignant lung disease through increased DNA-MTase expression and the possibility of blocking the deleterious effects of this change with specific inhibitors will offer new intervention strategies for lung cancer.