The p21(WAF1/CIP1) promoter is methylated in Rat-1 cells: stable restoration of p53-dependent p21(WAF1/CIP1) expression after transfection of a genomic clone containing the p21(WAF1/CIP1) gene.

Rat-1 cells are used in many studies on transformation, cell cycle, and apoptosis. Whereas UV treatment of Rat-1 cells results in apoptosis, X-ray treatment does not induce either apoptosis or a cell cycle block. X-ray treatment of Rat-1 cells results in both an increase of p53 protein and expression of the p53-inducible gene MDM2 but not the protein or mRNA of the p53-inducible p21(WAF1/CIP1) gene, which in other cells plays an important role in p53-mediated cell cycle block. The lack of p21(WAF1/CIP1) expression appears to be the result of hypermethylation of the p21(WAF1/CIP1) promoter region, as p21(WAF1/CIP1) protein expression could be induced by growth of Rat-1 cells in the presence of 5-aza-2-deoxycytidine. Furthermore, sequence analysis of bisulfite-treated DNA demonstrated extensive methylation of cytosine residues in CpG dinucleotides in a CpG-rich island in the promoter region of the p21(WAF1/CIP1) gene. Stable X-ray-induced p53-dependent p21(WAF1/CIP1) expression and cell cycle block were restored to a Rat-1 clone after transfection with a P1 artificial chromosome (PAC) DNA clone containing a rat genomic copy of the p21(WAF1/CIP1) gene. The absence of expression of the p21(WAF1/CIP1) gene may contribute to the suitability of Rat-1 cells for transformation, cell cycle, and apoptosis studies.

Chimeric amplicons containing the c-myc gene in HL60 cells.

The major amplicon present in HL60 cells is chimeric in nature being composed of 70 kb of DNA sequence derived from the MYC locus linked to 80 kb of novel DNA sequence derived from a non contiguous region located telomeric to the c-myc gene at 8q24 (Feo et al., 1996). Here we show by fluorescence in situ hybridization (FISH) that these coamplified sequences, MCR (Myc Coamplified Region), are derived from a locus located 3-4 Mb telomeric to the c-myc gene in the q24.2-24.3 region of chromosome 8. Genomic cloning and Southern blot analysis indicate the arrangement of chimeric amplicons are in tandem arrays. Analysis of the DNA sequences at the juncture of the MYC locus and the MCR suggest that these non syntenic regions were joined by nonhomologous recombination events. Visualization of the organization of the amplified DNA by fiber-FISH analysis illustrates we have cloned the complete amplicon. This is the first complete mammalian amplicon to be cloned and have its structure visualized. In addition to the major class of tandemly repeated amplicons, a second class of amplicons was detected by fiber-FISH in which the extent of the MCR component is about twice the size of the MCR component in the major amplicon. These longer amplicons most likely contain inverted repeats of MCR and MYC region sequences. Whether the amplicons contain mixtures of these two types of structures or separate amplicons only contain one type of structure has not yet been resolved. Properties of the MCR sequences responsible for retention in the chimeric HL60 amplicons upon long term passage are discussed.

The human Surfeit locus.

The organization of the human Surfeit locus containing the six sequence-unrelated housekeeping genes Surf-1 to Surf-6 (HGMW-approved symbols SURF1-SURF6) has been determined. The human surfeit locus occupies about 60 kb of DNA, and the tightly clustered gene organization and the juxtaposition of the human genes are similar to the mouse and chicken surfeit loci with the 5' end of each gene associated with a CpG-rich island. Whereas in the mouse the Surf-2 and Surf-4 genes overlap at their 3' ends, the human Surf-2 and Surf-4 genes have been found to be separated by 302 bp due to a much shorter 3' untranslated region in the human Surf-2 gene. The distance between the 3' ends of the human Surf-1 and Surf-3 genes is 374 bp, and the distance between the 5' ends of the human Surf-3 and Surf-5 genes is only 112 bp. Unusually the human Surf-5 gene contains an intron in its 5' untranslated region not found in the mouse or rat Surf-5 genes. This additional intron is also found in the Surf-5 gene of both Old and New World monkeys, being generated before the divergence of human and prosimians but after the divergence of primates and rodents. A contig of 200 kb containing the human Surfeit locus has been constructed from overlapping cosmid, P1, and PAC clones. Approximately 40 kb proximal to the 3' end of the Surf-6 gene, the 5' region of the ABO glycosyltransferase gene has been detected. This allows us to determine the orientation of the Surfeit and ABO loci with respect to each other and to the telomere and centromere of human chromosome 9.

Tissue-specific processing of the Surf-5 and Surf-4 mRNAs.

The mouse surfeit locus is an unusually tight cluster of at least six "housekeeping" genes that do not share any sequence homology and whose gene organization may play a role in gene expression. The transcription of each of the five well-characterized genes (Surf-1 to -5) alternates with respect to its neighbor(s) and no more than 159 bp separates any two adjacent genes with the Surf-4 and Surf-2 genes overlapping at their 3' ends by 133 bp. In this work, the expression of the Surf-5 and Surf-4 genes has been examined in various mouse tissues. In addition to the ubiquitously expressed 3.5-kb Surf-5 mRNA, a second alternatively spliced Surf-5 mRNA, Surf-5b, was discovered that was highly expressed in the brain, heart, testis, and skeletal muscle. The alternative splice donor site of the Surf-5b mRNA is similar to splice donor sites found in neuron-specific mRNAs. Surf-5b encodes a unique protein, which, like the ubiquitous Surf-5 protein, has been found to be primarily located in the soluble fraction of the cytoplasm. The expression of the Surf-5b protein was also found to increase in embryonal carcinoma cells differentiated into neuronal cultures. Although the Surf-5 gene is highly conserved through evolution, the presence of the Surf-5b alternative splice may be restricted to higher vertebrates. The Surf-4 gene was ubiquitously expressed in eight different mouse tissues; however, the ratios of the three previously reported Surf-4 mRNAs (two of which are known to derive from different sites of polyadenylation) altered dramatically between tissues. The use of different forms of mRNA processing for regulation of tissue-specific expression of ubiquitously expressed genes is discussed.

Surf5: a gene in the tightly clustered mouse surfeit locus is highly conserved and transcribed divergently from the rpL7A (Surf3) gene.

The four previously characterized genes (Surf1 to 4) of the mouse Surfeit locus do not share any sequence homology, and the transcription of each gene alternates with respect to its neighbor(s). Adjacent Surfeit genes are separated by very small distances, and two of the genes overlap at their 3' ends. In this work we have further defined the Surfeit gene cluster by the isolation of Surf5, a fifth gene of the locus, and determination of its relationship to the other Surfeit genes. Surft5 does not share any sequence homology with the four cloned Surfeit genes. The transcription of Surf5 is divergent with respect to its neighbor the Surf3 gene, and the 5' ends of Surf5 and Surf3 are separated by only 159 bp, suggesting the presence of a second bidirectional promoter in the locus. The 3' end of Surf5 maps only 68 bp away from the processed 3' end of a pseudogene. The human and partial chicken Surf5 coding regions show greater than 95% identity, and a Caenorhabditis elegans homologue shows 38% identity and 56% similarity with the mouse Surf5 amino acid sequence. The 3.5-kb transcript of Surf5 encodes a small hydrophilic protein of 140 amino acid residues, which differs from the ribosomal protein L7a encoded by the Surf3 gene or the integral membrane protein encoded by the Surf4 gene. Subcellular fractionation located the Surf5 protein to the soluble fraction of the cytoplasm. The surfeit locus appears to represent a novel type of gene cluster in which the genes are unrelated by sequence or function; however, their organization may play a role in their gene expression.

Spatial and temporal expression of an epithelial mucin, Muc-1, during mouse development.

The Muc-1 mucin is found as a transmembrane protein in the apical surface of glandular epithelia. To provide insight into possible functions, we have assessed the timing of expression and the distribution of the Muc-1 protein during mouse embryogenesis using three different techniques: RT-PCR, northern blots and immunohistochemistry. Our results indicate that Muc-1 expression correlates with epithelial differentiation in stomach, pancreas, lung, trachea, kidney and salivary glands. Once started, Muc-1 synthesis continually increases with time, mainly due to epithelial area growth. Our data suggest that expression of the Muc-1 gene is under spatial and temporal control during organogenesis. Although Muc-1 is present in different organs, its expression is not induced systemically, but according to the particular onset of epithelial polarization and branching morphogenesis of each individual organ. It is of particular interest that Muc-1 protein can be detected lining the apical surfaces of the developing lumens when the epithelium of these organs is still undergoing folding and branching, and glandular activity has not yet started. We speculate that Muc-1 may participate in epithelial sheet differentiation/lumen formation during early development of the organs known to express it. This speculation is based on: (1) the detection of Muc-1 expression early during organogenesis, (2) the defined apical localization in different epithelia, (3) the decrease in cell-cell interactions when Muc-1 protein is highly expressed and (4) the possible interaction of its cytoplasmic tail with the actin cytoskeleton. However, it remains to be established using in vitro systems, whether the temporal and local expression of the Muc-1 gene coincident with the morphogenetic events described here is relevant for the process.

Tissue-specific expression of a human polymorphic epithelial mucin (MUC1) in transgenic mice.

The human MUC1 gene codes for the core protein of a mucin which is expressed by glandular epithelia and the carcinomas which develop from these tissues. The core protein is aberrantly glycosylated in cancers, and some antibodies show specificity in their reactions with the cancer-associated mucin, which also contains epitopes recognized by T-cells from breast and pancreatic cancer patients. For evaluating the potential use of mucin-reactive antibodies and mucin-based immunogens in cancer patients, a mouse model, expressing the MUC1 gene product PEM (polymorphic epithelial mucin) as a self antigen, would be extremely useful. To this end, we have developed transgenic mouse strains expressing the human MUC1 gene product in a tissue-specific manner. The TG4 mouse strain was established using a 40-kilobase fragment containing 4.5 kilobases of 5' and 27 kilobases of 3' flanking sequence. The TG18 strain was developed using a 10.6-kilobase SacII fragment from the 40-kilobase fragment; this fragment contained 1.6 kilobases of 5' sequence and 1.9 kilobases of 3' flanking sequence. Both strains showed tissue specificity of expression of the MUC1 gene, which was very similar to the profile of expression seen in human tissues. The antibody SM-3 is directed to a core protein epitope, which is selectively exposed in breast cancers and which shows a more restricted distribution on normal human tissues. It was established that the distribution of the SM-3 epitope of PEM in the tissues of the transgenic mice is similar to that seen in humans. The transgenic mouse strains described here should form the basis for the development of a preclinical model for the evaluation of PEM-based antigens and of antibodies directed to PEM in cancer therapy.

Structure and biology of a carcinoma-associated mucin, MUC1.

Although mucins have been studied at the biochemical and biophysical level for some time, attempts to define their structures in detail were only partially successful because of their size and complexity. The advent of monoclonal antibodies reactive with these molecules introduced a new approach to structural studies by defining antigenic epitopes, by allowing purification of the mucin molecules by affinity chromatography, and by providing a means to clone genes coding for the core proteins. By their profile of reactivity with the normal and cancer-associated mucin in a particular tissue, the antibodies also defined a difference in the mucin derived from the two sources. It is now clear that this difference lies in the carbohydrate side chains, as the core proteins are identical. Because the mucins are tumor-associated antigens and the cancer mucins can express epitopes that are relatively tumor specific, this family of molecules is now being intensively studied. There is also considerable interest in elucidating the normal function of the mucin and in determining whether, through an altered structure, this function is subverted in malignancy. In the next few years we should expect that the structure of other mucins will be defined in the same detail as the product of the MUC1 gene. We should also expect to see the continued application of mucin-reactive antibodies in the clinic and the investigation of mucins as agents for immunotherapy of some cancers. As to the function(s) of these molecules, perhaps we will learn enough in the future to make a critical reappraisal of the name.

Structure and expression of the human polymorphic epithelial mucin gene: an expressed VNTR unit.

The human polymorphic epithelial mucin (PEM) is expressed apically by glandular epithelium and by the carcinomas that develop from these tissues. Previously isolated cDNA clones revealed that the core protein contained a large domain consisting of variable numbers of 60 bp tandem repeats (TR), making it an expressed minisatellite. We now report the full genomic sequence of the PEM gene, including 803 bp of 5' flanking sequence. The gene is composed of 7 exons and varies in size from approximately 4 to approximately 7 kb, depending on the number of tandem repeats in exon 2. Expression of PEM was obtained from a genomic clone in an Epstein-Barr virus based vector, after transfection into a human epithelial cell line, indicating the presence of effective regulatory sequences in this clone.

Molecular cloning and expression of human tumor-associated polymorphic epithelial mucin.

Human mammary cells present on the cell surface a polymorphic epithelial mucin (PEM) which is developmentally regulated and aberrantly expressed in tumors. PEM carries tumor-associated epitopes recognized by the monoclonal antibodies HMFG-1, HMFG-2, and SM-3. Previously isolated partial cDNA clones revealed that the core protein contained a large domain consisting of variable numbers of 20-amino acid repeat units. We now report the full sequence for PEM, as deduced from cDNA sequences. The encoded protein consists of three distinct regions: the amino terminus consisting of a putative signal peptide and degenerate repeats; the major portion of the protein which is the tandem repeat region; the carboxyl terminus consisting of degenerate tandem repeats and a unique sequence containing a transmembrane sequence and a cytoplasmic tail. Potential O-glycosylation sites (serines or threonines) make up more than one-fourth of the amino acids. Length variations in the tandem repeat result in PEM being an expressed variable number tandem repeat locus. Tandem repeats appear to be a general characteristic of mucin core proteins.

The locus of the polymorphic epithelial mucin (PEM) tumour antigen on chromosome 1q21 shows a high frequency of alteration in primary human breast tumours.

Tumour and blood leukocyte DNAs from sporadic breast cancer patients were examined for chromosome 1 loss of heterozygosity using a probe for a polymorphic epithelial mucin, PEM, which is expressed in greater than 92% of breast carcinomas as well as in normal lactating breast tissue. Expression is detected by the monoclonal antibodies (MAbs) HMFG-1, -2 and SM-3 which react with epitopes in the 20 amino-acid repeat unit of the core protein. The PEM probe has been mapped to the chromosome band 1q21, a region that is often incriminated in chromosomal rearrangements in breast tumours. Loss of heterozygosity or alteration at the PEM locus was detected in 34% of the 70 informative patients examined. Twenty of the 24 individuals showed loss of an allele, whereas 4 showed gain of an additional allele or amplification of an existing allele. Twenty-eight percent of informative cases exhibited alterations at the MS32 locus, 1q42-43, and 20% had alterations at the short arm locus MS1 at 1p33-35. These findings identify the long arm of chromosome 1 and in particular the region around the PEM gene for localization of a gene whose loss or alteration may, in some tumours, contribute to the progression of disease in breast cancer patients.

A short sequence, within the amino acid tandem repeat of a cancer-associated mucin, contains immunodominant epitopes.

The polymorphic epithelial mucin (PEM) appears to be the target molecule for many monoclonal antibodies (MAbs) which react with tumour-associated and epithelium-specific antigens. PEM contains a large domain made up of 20 amino-acid tandem repeats which are highly immunodominant as many of the antibodies reactive with this molecule recognize epitopes within this area. Using overlapping peptide octamers, we have precisely mapped the epitopes of 4 MAbs reactive with the tandem repeats including one, SM-3, which shows enhanced tumour specificity. We report that the core of the SM-3 epitope corresponds to the continuous amino acid sequence Pro-Asp-Thr-Arg-Pro. We also show that the epitopes recognized by 3 other antibodies, which show reactivity with normal and malignant tissues, map to within this area of the tandem repeat. However, none of these epitopes contain the proline found at the amino end of the SM-3 determinant. These results are consistent with the idea that, in the cancer-associated mucin, premature termination of the carbohydrate side-chains results in the exposure of the SM-3 epitope.

A highly immunogenic region of a human polymorphic epithelial mucin expressed by carcinomas is made up of tandem repeats.

The nucleotide sequences of partial cDNA clones coding for the core protein of a human polymorphic epithelial mucin were determined, and a large domain was found to consist of a 60-base pair tandem repeat sequence. The cDNA clones were originally selected (Gendler, S. J., Burchell, J. M., Duhig, T., Lamport, D., White, R., Parker, M., and Taylor-Papadimitriou, J. (1987) Proc. Natl. Acad. Sci. U. S. A. 84, 6060-6064) using three monoclonal antibodies which show differential reactivity with the mucin produced by normal and malignant breast. Two of the epitopes are exposed in the normally processed and cancer-associated mucin, while one epitope is unmasked only in the cancer-associated mucin (Burchell, J. M., Durbin, H., and Taylor-Papadimitriou, J. (1983) J. Immunol. 131, 508-513; Burchell, J., Gendler, S., Taylor-Papadimitriou, J., Girling, A., Lewis, A., Millis, R., and Lamport, D. (1987) Cancer Res. 47, 5476-5482). We show here that all three antibodies react with a synthetic peptide with an amino acid sequence corresponding to that predicted by the tandem repeat. Identification of the epitopes preferentially expressed on the cancer-associated mucin should allow a directed approach to the development of tumor-specific antibodies using synthetic peptides as immunogens.

Cloning of partial cDNA encoding differentiation and tumor-associated mucin glycoproteins expressed by human mammary epithelium.

Human mammary epithelial cells secrete and express on their cell surfaces complex mucin glycoproteins (Mr greater than 250,000) that are developmentally regulated, tumor-associated, and highly immunogenic. Studies using monoclonal antibodies directed to these glycoproteins suggest that their molecular structures can vary with differentiation stages in the normal gland and in malignancy. To analyze the molecular nature of these glycoproteins, milk mucin was affinity-purified and deglycosylated with hydrogen fluoride, yielding bands at 68 and 72 kDa on silver-stained gels. Polyclonal and monoclonal antibodies to the stripped core protein were developed and used to screen a lambda gt11 expression library of cDNA made from mRNA of the mammary tumor cell line MCF-7. Seven cross-reacting clones were isolated, with inserts 0.1-1.8 kilobases long. RNA blot analysis, using as a probe the 1.8-kilobase insert subcloned in plasmid pUC8 (pMUC10), revealed transcripts of 4.7 and 6.4 kilobases in MCF-7 and T47D mammary tumor cells, whereas normal mammary epithelial cells from pooled milks have additional transcripts. The expression of mRNA correlates with antigen expression as determined by binding of two previously characterized anti-mucin monoclonal antibodies (HMFG-1 and HMFG-2) to seven cell lines. Restriction enzyme analysis detected a restriction fragment length polymorphism when human genomic DNA was digested with EcoRI or HinfI.

Cyclic AMP inhibits the growth of human breast cancer cells in defined medium.

Three well-characterized human breast cancer cell lines, MCF-7, T47D and Cama-1, have been grown in defined medium in the absence of serum. Under these conditions, the growth of these cells was inhibited by a variety of cyclic AMP elevating agents, including cholera toxin, monobutyryl cyclic AMP, bromo-cyclic AMP, prostaglandin E2, and 1-methyl-3-isobutyryl xanthine. This inhibitory effect of cyclic AMP on breast cancer cells contrasts with the stimulation of proliferation induced by elevation of intra-cellular cyclic AMP in normal human mammary epithelial cells. Such a major perturbation of growth control may arise either as a result of malignant change, or may reflect a difference in the differentiation phenotype of normal and malignant human mammary epithelial cells in vitro.