Identification of a single chromosome in the normal human genome essential for suppression of hamster cell transformation

New insight into function and dysfunction of gut microfold cells

Microfold cells (M cells), derived from intestinal crypt Lgr5⁺ stem cells, are distributed in gut-associated lymphoid tissue (GALT), nasopharyngeal-associated lymphoid tissue (NALT), and bronchial-associated lymphoid tissue (BALT). The basement membrane of mature M cells protrudes upward, showing a "pocket-like" shape. M cell differentiation is mainly regulated by two pathways, one is the non-canonical NF-κB pathway, and the other is the canonical NF-κB pathway. The differentiation and maturation of M cells are closely related to RANKL and S100A4. M cells can not only transport antigens and trigger an immune response, but also are the gateway for various pathogens to invade the body. The occurrence and development of tuberculosis, prion disease, and Crohn's disease are closely related to M cells.

Genomic Alterations in Human Breast Cancer: A Review

We review and discuss data on the genetic alterations documented in human breast carcinomas at the molecular level. These alterations may result in: 1) deletion of genetic material (chromosome 11p, 13q, 3p, 1q, 17p); 2) amplification of genes or entire chromosomal segments (c-myc, c-erb-B2, locus DF3/PUM, loci on 11q13); 3) rearrangements (c-myc); 4) point mutations (c-ras). Presently available informations do not allow the development of cohesive pathogenetic models but indicate that the molecular basis of human breast cancer is heterogeneous.

Horizontal transmission of malignancy: in-vivo fusion of human lymphomas with hamster stroma produces tumors retaining human genes and lymphoid pathology

We report the in-vivo fusion of two Hodgkin lymphomas with golden hamster cheek pouch cells, resulting in serially-transplanted (over 5-6 years) GW-532 and GW-584 heterosynkaryon tumor cells displaying both human and hamster DNA (by FISH), lymphoma-like morphology, aggressive metastasis, and retention of 7 human genes (CD74, CXCR4, CD19, CD20, CD71, CD79b, and VIM) out of 24 tested by PCR. The prevalence of B-cell restricted genes (CD19, CD20, and CD79b) suggests that this uniform population may be the clonal initiating (malignant) cells of Hodgkin lymphoma, despite their not showing translation to their respective proteins by immunohistochemical analysis. This is believed to be the first report of in-vivo cell-cell fusion of human lymphoma and rodent host cells, and may be a method to disclose genes regulating both organoid and metastasis signatures, suggesting that the horizontal transfer of tumor DNA to adjacent stromal cells may be implicated in tumor heterogeneity and progression. The B-cell gene signature of the hybrid xenografts suggests that Hodgkin lymphoma, or its initiating cells, is a B-cell malignancy.

Neuronal apoptosis by prolyl hydroxylation: implication in nervous system tumours and the Warburg conundrum

Oxygen sensing is mediated partly via prolyl hydroxylation. The EglN prolyl hydroxylases are well characterized in regulating the hypoxia inducible factor alpha (HIF-alpha) hypoxic response, but also are implicated in HIF-independent processes. EglN3 executes apoptosis in neural precursors during development and failure of EglN3 developmental apoptosis can lead to certain forms of sympathetic nervous system tumours. Mutations in metabolic/mitochondrial enzymes (SDH, FH, IDH) impair EglN activity and predisposes to certain cancers. This is because the EglNs not only require molecular oxygen to execute hydroxylation, but also equally require the electron donor alpha-ketoglutarate, a metabolite from the Krebs cycle. Therefore EglN enzymes are considered oxygen, and also, metabolic sensors. alpha-Ketoglutarate is crucial for EglN hydroxylation activity, whereas the metabolites succinate and fumarate are inhibitors of the EglN enzymes. Since EglN activity is dependent upon metabolites that take part in the Krebs cycle, these enzymes are directly tied into the cellular metabolic network. Cancer cells tend to convert most glucose to lactate regardless of whether oxygen is present (aerobic glycolysis), an observation that was first made by Otto Warburg in 1924. Despite the striking difference in ATP production, cancer cells might favour aerobic glycolysis to escape from EglN hydroxylation, resulting in the accumulation of oncogenic HIFalpha and/or resistance to EglN3-mediated apoptosis.

The kinesin KIF1Bbeta acts downstream from EglN3 to induce apoptosis and is a potential 1p36 tumor suppressor

VHL, NF-1, c-Ret, and Succinate Dehydrogenase Subunits B and D act on a developmental apoptotic pathway that is activated when nerve growth factor (NGF) becomes limiting for neuronal progenitor cells and requires the EglN3 prolyl hydroxylase as a downstream effector. Germline mutations of these genes cause familial pheochromocytoma and other neural crest-derived tumors. Using an unbiased shRNA screen we found that the kinesin KIF1Bbeta acts downstream from EglN3 and is both necessary and sufficient for neuronal apoptosis when NGF becomes limiting. KIF1Bbeta maps to chromosome 1p36.2, which is frequently deleted in neural crest-derived tumors including neuroblastomas. We identified inherited loss-of-function KIF1Bbeta missense mutations in neuroblastomas and pheochromocytomas and an acquired loss-of-function mutation in a medulloblastoma, arguing that KIF1Bbeta is a pathogenic target of these deletions.

Inv(1)(p22q25) in chronic myeloproliferative disease: constitutional or clonal defect?

Pericentric inversion of chromosome 1 is uncommon in chronic myeloproliferative disease. We report the occurrence of an isolated inv(1)(p22q25) in two patients with BCR/ABL-negative chronic myeloproliferative disease. The inv(1) is an acquired clonal abnormality in one patient and a constitutional defect in the other. To our knowledge, this is the first report on the occurrence of inv(1)(p22q25) in hematolymphoid malignancies.

Deletion mapping of chromosome 1 in early onset and late onset breast tumors--a comparative study in eastern India

Younger women exhibit more aggressive pathologic features of breast cancer (BC) compared to their older counterparts. Young age has been shown to be an independent predictor of adverse prognosis. These findings have raised the question of whether these differences are also present at the genetic level. Twenty-five early onset (age < or = 40 years) tumors including 4 bilateral tumors, and 26 late onset (>40 years) breast tumors, including 2 bilateral tumors, were examined for loss of heterozygosity (LOH) at chromosome 1 using 11 polymorphic microsatellite markers. A comparative study revealed high frequencies of LOH in chr. 1p36 (61%), 1p31.3 (40%), 1p21.3 (50%) and 1q22-23.2 (56%) in a younger group, and chr. 1p36 (46%), 1p34.2 (48%), and 1q22-23.2 (52%) in an older group. These differences in LOH frequency in these two age groups were significant for chr. 1p21.3 (p = 0.025) only. These data suggest that the deletion pattern in early onset breast tumors is not fully identical to late onset breast tumors. Similar differential deletion patterns of LOH in the 5 highly deleted regions were seen in premenopausal and postmenopausal groups. An association was seen between LOH at chr. 1p34.2 and chr. 1q22-23.2 and higher grade of the tumors in older women. Among the highly deleted regions, the deletion at chr. 1p36 was found to occur early in both groups because of common allelic loss in the bilateral tumors.

Low-frequency loss of heterozygosity in Moloney murine leukemia virus-induced tumors in BRAKF1/J mice

To identify potential involvement of tumor suppressor gene inactivation during leukemogenesis by Moloney murine leukemia virus (M-MuLV), a genome-wide scan for loss of heterozygosity (LOH) in tumor DNAs was made. To assess LOH, it is best to study mice that are heterozygous at many loci across the genome. Accordingly, we generated a collection of 52 M-MULV-induced tumor DNAs from C57BR/cdJ x AKR/J F1 (BRAKF1) hybrid mice. By using direct hybridization with oligonucleotides specific for three different classes of endogenous MuLV-related proviruses, 48 markers on 16 of 19 autosomes were simultaneously examined for allelic loss. No allelic losses were detected, with the exception of a common loss of markers on chromosome 4 in two tumors. The three autosomes that lacked informative endogenous proviral markers were also analyzed for LOH by PCR with simple-sequence length polymorphisms (SSLPs); one additional tumor showed LOH on chromosome 15. Further screening with chromosome 4 SSLPs identified one additional tumor with LOH on chromosome 4. Therefore, in total, the average fractional allelic loss was quite low (0.002), but the LOH frequency of 6% on chromosome 4 was highly statistically significant (P < 0.0005). Detailed SSLP mapping of the three tumors with LOH on chromosome 4 localized the region of common LOH to the distal 45 centimorgans, a region syntenic with human chromosomes 1 and 9. Candidate tumor suppressor genes, Mts1 (p16INK4a) and Mts2 (p15INK4b), have been mapped to this region, but by Southern blot analysis, no homozygous deletions were detected in either gene. One of three tumors with LOH on chromosome 4 also showed a proviral insertion near the c-myc proto-oncogene. These results suggested that tumor suppressor inactivation is generally infrequent in M-MuLV-induced tumors but that a subset of these tumors may have lost a tumor suppressor gene on chromosome 4.

MYCL genotypes and loss of heterozygosity in non-small-cell lung cancer

Some studies have suggested that the S allele of the MYCL oncogene, which results from an intragenic EcoRI restriction fragment length polymorphism (RFLP), may be associated with cancer susceptibility. In addition, this allele has also been linked to metastases and adverse survival in certain cancers, although studies of lung cancer patients from different populations have yielded controversial results. We studied 108 cases of surgical resected non-small-cell lung cancer (NSCLC) and found no evidence that MYCL genotypes were associated with tumour progression or a worse prognosis. However, the presence of loss of heterozygosity (LOH) at this chromosome 1p32 locus correlated significantly with regional lymph node involvement, as well as advanced TNM stage. These data indicate the existence of a chromosome 1p candidate tumour-suppressor gene(s), possibly in linkage disequilibrium with the EcoRI RFLP in specific populations, which appears to play a role in determining tumour progression in NSCLC. Refined mapping of the critical region of loss should help attempts to identify and clone the candidate gene.

Genomic instability in 1p and human malignancies

Both cytogenetic and molecular genetic approaches have unveiled non-random genomic alterations in 1p associated with a number of human malignancies. These have been interpreted to suggest the existence of cancer-related genes in 1p. Earlier studies had employed chromosome analysis or used molecular probes mapped by in situ hybridization. Further, studies of the various tumor types often involved different molecular probes that had been mapped by different technical approaches, like linkage analysis, radioactive or fluorescence in situ hybridization, or by employing a panel of mouse x human radiation reduced somatic cell hybrids. The lack of maps fully integrating all loci has complicated the generation of a comparative and coherent picture of 1p damage in human malignancies even among different studies on the same tumor type. Only recently has the availability of genetically mapped, highly polymorphic loci at (CA)n repeats with sufficient linear density made it possible to scan genomic regions in different types of tumors readily by polymerase chain reaction (PCR) with a standard set of molecular probes. This paper aims at presenting an up-to-date picture of the association of 1p alterations with different human cancers and compiles the corresponding literature. From this it will emerge that the pattern of alterations in individual tumor types can be complex and that a stringent molecular and functional definition of the role that Ip alterations might have in tumorigenesis will require a more detailed analysis of the genomic regions involved.

Monochromosome transfers to Syrian hamster BHK cells via microcell fusion provide functional evidence for suppressor genes on human chromosome 9 both for anchorage independence and for tumorigenicity

We previously identified an anchorage independence-suppressor gene, SAII, on rat chromosome (RNO) 5. RNO5 is homologous to human chromosomes (HSA) 1 and 9. In order to find the human homolog of the SAII gene, we transferred HSA1 and HSA9 to an anchorage-independent and tumorigenic Syrian hamster BHK 191-5C cell line by microcell fusion. For HSA9, we used a t(X;9)-derivative chromosome to force the retention of this chromosome in hybrids by hypoxanthine-aminopterin-thymidine (HAT) selection. To study the possible effect of the X portion of the der(9)t(X;9), we also transferred a normal X to 191-5C cells. For HSA1, a neo-tagged chromosome was introduced. Following the transfer of der(9)t(X;9) to 191-5C cells, the hybrid cells became anchorage dependent and nontumorigenic, and, upon the loss of this chromosome, the cells regained their tumorigenic and anchorage-independent phenotypes. The transfer of HSAX or HSA1, on the other hand, affected neither of these phenotypes. These results provide functional proof of suppressor genes on HSA9 involving both anchorage independence and tumorigenicity. In addition, our data suggest the presence of another gene on HSA9 that causes a negative growth effect and whose phenotypic expression, contrary to the suppressor genes, is dosage dependent.

Angiogenesis: models and modulators

Angiogenesis in vivo is distinguished by four stages: subsequent to the transduction of signals to differentiate, stage 1 is defined as an altered proteolytic balance of the cell allowing it to digest through the surrounding matrix. These committed cells then proliferate (stage 2), and migrate (stage 3) to form aligned cords of cells. The final stage is the development of vessel patency (stage 4), generated by a coalescing of intracellular vacuoles. Subsequently, these structures anastamose and the initial flow of blood through the new vessel completes the process. We present and discuss how the available models most closely represent phases of in vivo angiogenesis. The enhancement of angiogenesis by hyaluronic acid fragments, transforming growth factor beta, tumor necrosis factor alpha, angiogenin, okadaic acid, fibroblast growth factor, interleukin 8, vascular endothelial growth factor, haptoglobin, and gangliosides, and the inhibition of the process by hyaluronic acid, estrogen metabolites, genestein, heparin, cyclosporin A, placental RNase inhibitor, steroids, collagen synthesis inhibitors, thrombospondin, fumagellin, and protamine are also discussed.

Frequent deletion in chromosome 4 and duplication of chromosome 15 in liver epithelial cells derived from long-term culture of C3H mouse hepatocytes

Prolonged culture of hepatocytes isolated from mouse liver results in the spontaneous development of colonies of liver epithelial cells that can proliferate indefinitely in vitro. We established 5 such cell lines from C3H/HeJ mice (C3H) and 22 cell lines from C3H/HeJ x C57BL/6J F1 mice (C3B6F1) to investigate whether any specific karyotype alterations may be associated with the development of such cells. These lines retained some properties of hepatocytes as well as showing bile-duct-cell characteristics, and comprised mainly near-diploid and/or hypotetraploid cells. Karyotypic analysis of the C3H cell lines indicated that most cells have loss of chromosome 4 or deletion involving the C7 portion, while at least 1 (for near-diploid cells) or 2 (for hypotetraploid cells) copies of chromosome 4 were usually intact. In addition, gain of an extra chromosome 15 was frequently observed in these cell lines. Analysis of the microsatellite DNA polymorphic markers in 22 C3B6F1 lines revealed that a majority of them showed loss of heterozygosity (LOH) for, at least, 1 of 3 polymorphic loci on chromosome 4, but not for 2 loci on chromosomes 7 and 11. Mouse chromosomes 4 and 15, therefore, may contain genes related to the ability of such liver epithelial cells to grow indefinitely in vitro [The locus on chromosome 4 was designated as liver-cell immortalization (LCI) locus].

Increased actin cable organization after single chromosome introduction: association with suppression of in vitro cell growth rather than tumorigenic suppression

We previously showed that introduction of a single human chromosome 1, 6, or 9 derived from normal fibroblasts into HHUA endometrial carcinoma cells resulted in suppression of tumorigenicity. The tumorigenic suppression was accompanied by remarkable morphological changes in the microcell hybrids containing an extra copy of chromosome 1. The study presented here was undertaken to search for target cytoskeletal components affected by chromosome 1 transfer into endometrial carcinoma cells. We found that the microcell hybrids containing an extra copy of chromosome 1 were characterized by intracellular actin bundle formation and an excessive accumulation of actin and vinculin. The latter was a result of increased stabilization of the proteins. Additionally, chromosome 3 introduction into RCC23 human renal carcinoma cells resulted in prolongation of cell division and in senescence of a significant proportion of the microcell hybrids. In these microcell hybrids, the intracellular actin network was also reorganized, but the amounts of actin and vinculin protein were not increased. These findings suggest that the increased actin organization, which appeared not to cause tumorigenic suppression in the microcell hybrids, is associated with complementation of tumor suppressor genes and senescence by multiple mechanisms.

Loss of thrombospondin transcriptional activity in nickel-transformed cells

mRNA from normal Chinese hamster embryo (CHE) cells was transcribed to cDNA and subtracted with an excess of mRNA from Chinese hamster embryo cells transformed by nickel compounds. Here we report the recovery of a sequence found to be highly homologous to the mouse thrombospondin 1 gene that was obtained by this subtraction procedure. Since thrombospondin is antiangiogenic, cancer cells expressing high levels of thrombospondin cannot grow in vivo because capillaries will not proliferate to cells secreting thrombospondin. To examine expression of thrombospondin, normal CHE cells were stained with monoclonal antibodies to human thrombospondin. The protein was present abundantly in the cytoplasm of normal cells but at greatly reduced levels in Ni-transformed cells. Analysis of mRNA by Northern (RNA) blot revealed transcripts in normal cells but little thrombospondin mRNA in Ni-transformed cells. Loss of thrombospondin mRNA expression was related to Ni treatment rather than transformation, since Ni-resistant cells also exhibited fewer thrombospondin transcripts than did wild-type cells. Digestion of genomic DNA with various combinations of restriction enzymes revealed thrombospondin gene patterns that were identical in both cell types, suggesting that there were no major deletions or rearrangements of the gene in the nickel-transformed cells. The inactivation of the thrombospondin gene was further investigated by analyzing the promoter activity of this gene linked to a chloramphenicol acetyltransferase (CAT) reporter plasmid that was transfected into normal and Ni-transformed cells. The CAT activity in normal cells was significantly higher than in Ni-transformed cells, suggesting that the promoter region of thrombospondin was less efficiently transcribed in Ni-transformed cells. We studied the consequences of enhanced expression of the retinoblastoma (Rb) gene, a known tumor suppressor gene, on CAT transcription driven by the human thrombospondin promoter. Cotransfection of an expression vector containing the mouse Rb gene greatly enhanced the transcription from the thrombospondin promoter such that the expression was higher in normal cells than in transformed cells.

Loss of thrombospondin transcriptional activity in nickel-transformed cells

mRNA from normal Chinese hamster embryo (CHE) cells was transcribed to cDNA and subtracted with an excess of mRNA from Chinese hamster embryo cells transformed by nickel compounds. Here we report the recovery of a sequence found to be highly homologous to the mouse thrombospondin 1 gene that was obtained by this subtraction procedure. Since thrombospondin is antiangiogenic, cancer cells expressing high levels of thrombospondin cannot grow in vivo because capillaries will not proliferate to cells secreting thrombospondin. To examine expression of thrombospondin, normal CHE cells were stained with monoclonal antibodies to human thrombospondin. The protein was present abundantly in the cytoplasm of normal cells but at greatly reduced levels in Ni-transformed cells. Analysis of mRNA by Northern (RNA) blot revealed transcripts in normal cells but little thrombospondin mRNA in Ni-transformed cells. Loss of thrombospondin mRNA expression was related to Ni treatment rather than transformation, since Ni-resistant cells also exhibited fewer thrombospondin transcripts than did wild-type cells. Digestion of genomic DNA with various combinations of restriction enzymes revealed thrombospondin gene patterns that were identical in both cell types, suggesting that there were no major deletions or rearrangements of the gene in the nickel-transformed cells. The inactivation of the thrombospondin gene was further investigated by analyzing the promoter activity of this gene linked to a chloramphenicol acetyltransferase (CAT) reporter plasmid that was transfected into normal and Ni-transformed cells. The CAT activity in normal cells was significantly higher than in Ni-transformed cells, suggesting that the promoter region of thrombospondin was less efficiently transcribed in Ni-transformed cells. We studied the consequences of enhanced expression of the retinoblastoma (Rb) gene, a known tumor suppressor gene, on CAT transcription driven by the human thrombospondin promoter. Cotransfection of an expression vector containing the mouse Rb gene greatly enhanced the transcription from the thrombospondin promoter such that the expression was higher in normal cells than in transformed cells.

Genetic mapping of tumor susceptibility genes involved in mouse plasmacytomagenesis

Plasmacytomas (PCTs) were induced in 47% of BALB/cAnPt mice by the intraperitoneal injection of pristane, in 2% of (BALB/c x DBA/2N)F1, and in 11% of 773 BALB/cAnPt x (BALB/cAnPt x DBA/2N)F1 N2 backcross mice. This result indicates a multigenic mode of inheritance for PCT susceptibility. To locate genes controlling this complex genetic trait, tumor susceptibility in backcross progeny generated from BALB/c and DBA/2N (resistant) mice was correlated with alleles of 83 marker loci. The genotypes of the PCT-susceptible progeny displayed an excess homozygosity for BALB/c alleles within a 32-centimorgan stretch of mouse chromosome 4 (> 95% probability of linkage) with minimal recombination (12%) near Gt10. Another susceptibility gene on mouse chromosome 1 may be linked to Fcgr2 (90% probability of linkage); there were excess heterozygotes for Fcgr2 among the susceptible progeny and excess homozygotes among the resistant progeny. Regions of mouse chromosomes 4 and 1 that are correlated with PCT susceptibility share extensive linkage homology with regions of human chromosome 1 that have been associated with cytogenetic abnormalities in multiple myeloma and lymphoid, breast, and endocrine tumors.

Chromosome 1 alterations in breast cancer: allelic loss on 1p and 1q is related to lymphogenic metastases and poor prognosis

The development of human breast cancer is characterized by a variety of genetic alterations, and cytogenetic analyses have documented the consistent involvement of both arms of chromosome 1. In the present study, molecular markers detecting restriction fragment length polymorphisms were used in pairwise screening of normal and tumor DNA to determine the frequency of allelic imbalance in breast tumors. Loss of heterozygosity (LOH) in the polymorphic epithelial mucin (PEM or MUCI) gene at 1q21 was found in 16% of 89 informative (constitutionally heterozygous) cases, whereas gain in intensity of one allelic band was more frequent (37%), a total of 47% of cases manifesting either allelic loss or gain. Three additional tumors manifested a structural alteration. Allelic loss or gain in the PEM gene was not associated with other prognostic factors, e.g., tumor size, lymph node status, steroid receptors. DNA ploidy, S phase fraction, protooncogene amplification, histological type, or patient age. However, LOH in the PEM gene was significantly correlated with early disease recurrence (P = 0.006). LOH on 1p was found in 27% of 117 informative cases, using probes for either D1S57 or D1Z2 located at 1p33-p35 and 1p36, respectively. Somatic allelic imbalance on 1p and 1q seemed to be independent events and not the effect of loss of a whole chromosome 1. LOH on 1p was significantly correlated to the presence of lymph node metastasis, to larger tumor size, and to DNA nondiploidy, but not correlation was found to disease outcome at this limited duration of follow-up (median 29 months).

Suppression of properties associated with malignancy in murine melanoma-melanocyte hybrid cells

Murine and human melanoma cells differ relatively reliably from non-tumorigenic melanocytes in certain biological properties. When cultured at low pH, melanocytes tend to be pigmented and melanoma cells unpigmented. The growth of virtually all metastatic melanoma cells is inhibited by phorbol esters such as TPA (12-O-tetradecanoyl phorbol-13-acetate), which stimulate melanocyte growth. Melanocytes fail to grow in suspension culture or produce tumours when implanted in animals, while many melanoma lines can do both. Here we studied which of these properties were dominant in hybrid cells formed by fusion of drug-resistant murine B16-F10RR melanoma cells to melanocytes of the albino and brown lines, melan-c and melan-b. The albino melanocytes are unpigmented but well-differentiated, the brown melanocytes produce pale brown pigment and the melanoma cells are unpigmented under the conditions used. All hybrid colonies observed produced black pigment, except some melan-b/melanoma hybrids when growing sparsely with TPA. Thus pigmentation was generally dominant. 14/15 hybrid lines showed stimulation of proliferation by TPA, as do melanocytes. Most hybrid lines showed no or reduced capacity for growth in suspension, though some grew better in suspension when TPA was present. There was marked suppression of the tumorigenicity of the parental melanoma cells in 4/8 hybrids examined, and tumorigenicity was reduced in the others, despite considerable chromosome loss by the passage level tested. Thus most properties of the non-tumorigenic pigment cells were dominant, as often observed for other cell lineages, and providing further evidence for gene loss in the genesis of malignant melanoma.

Growth and transformation suppressor genes for BHK Syrian hamster cells on human chromosomes 1 and 11

To map putative tumor suppressor genes for the near-diploid baby hamster kidney fibrosarcoma cell line BHK, we transferred five different normal human chromosomes (1, 3, 7, 11, and 12) into these tumor cells by microcell-mediated chromosome transfer. Transfer of human chromosome 1 into BHK cells resulted in suppression of cell growth both on plastic and in soft agar, indicating that chromosome 1 has a generalized effect on cell growth and thereby suppresses anchorage-independent growth. Selection against cells with an intact chromosome 1 was observed. In contrast, the introduction of chromosome 11 into BHK cells resulted in suppression of anchorage independence but not growth on plastic. Most chromosome-11 growth-suppressed BHK hybrids retained intact copies of human chromosome 11. Tumorigenic derivatives of chromosome 11 hybrids had lost this chromosome. Transfer of human chromosome 3, 7, or 12 into BHK cells did not correlate with growth suppression of BHK cells on plastic or in soft agar. Thus, we conclude that genes that suppress BHK-cell growth in general or in agar reside on human chromosomes 1 and 11, respectively.

Suppression of the chemically transformed phenotype of BHK cells by a human cDNA

Transformation of the baby hamster kidney cell line BHK SN-10 by chemical carcinogens such as nitrosylmethylurea (NMU) is mediated by the loss of a gene product critical for the suppression of malignant transformation. Somatic cell hybrids between chemically transformed BHK SN-10 cells and either normal hamster kidney or human fibroblast cells are nontransformed; therefore, a recessive mechanism underlies the malignant transformation of BHK SN-10 cells after chemical carcinogenesis (A. Stoler and N. P. Bouck, Proc. Natl. Acad. Sci. USA 82:570-574, 1985). A human fibroblast cDNA library was constructed and introduced into NMU-transformed BHK SN-10 cells (NMU 34m) in order to identify a human cDNA capable of suppressing cellular transformation. NMU-transformed BHK cells were analyzed for reversion to an anchorage-dependent normal cellular phenotype after transfection with human cDNA. The human cDNA capable of inducing stable reversion of NMU 34m cells encodes the intermediate filament protein vimentin, which is apparently required for maintenance of the normal phenotype in BHK SN-10 cells.

Suppression of the chemically transformed phenotype of BHK cells by a human cDNA

Transformation of the baby hamster kidney cell line BHK SN-10 by chemical carcinogens such as nitrosylmethylurea (NMU) is mediated by the loss of a gene product critical for the suppression of malignant transformation. Somatic cell hybrids between chemically transformed BHK SN-10 cells and either normal hamster kidney or human fibroblast cells are nontransformed; therefore, a recessive mechanism underlies the malignant transformation of BHK SN-10 cells after chemical carcinogenesis (A. Stoler and N. P. Bouck, Proc. Natl. Acad. Sci. USA 82:570-574, 1985). A human fibroblast cDNA library was constructed and introduced into NMU-transformed BHK SN-10 cells (NMU 34m) in order to identify a human cDNA capable of suppressing cellular transformation. NMU-transformed BHK cells were analyzed for reversion to an anchorage-dependent normal cellular phenotype after transfection with human cDNA. The human cDNA capable of inducing stable reversion of NMU 34m cells encodes the intermediate filament protein vimentin, which is apparently required for maintenance of the normal phenotype in BHK SN-10 cells.

Loss of a tumor suppressor function during neoplastic progression of epithelial cells in vitro

In vitro transformation of rat urothelial cells is a multi-step process. We have used cell fusion to analyse the role of recessive events during in vitro progression of an immortal urothelial cell line. Somatic cell hybrids were made between the transformed cell line RM2T and a series of immortal urothelial cell lines, including the progenitor line RM2AD, from which RM2T was isolated. The ability to produce colonies in soft agar (anchorage independence) was used as an in vitro marker of transformation, and a series of 10 hybrid clones and 4 mass populations of hybrids were assessed for suppression of this phenotype. Hybrids between early-passage (less than passage 35, anchorage-dependent) RM2AD cells and late-passage (greater than passage 35, anchorage-independent) RM2T cells, showed suppression of anchorage independence when tested early after fusion (4/4 mass populations, 7/10 clones). This indicates that in vitro progression of this cell line is associated with loss of a function which can suppress growth in soft agar. Fusions between anchorage-independent RM2T cells and a series of other anchorage-dependent immortal urothelial cell lines generated hybrids which showed no suppression of anchorage independence, indicating that these anchorage-dependent cells have lost the suppressor function identified in RM2AD. Our results indicate that loss of a suppressor function can contribute to urothelial transformation in vitro and that clonal populations of immortal cells, at apparently the same stage of transformation, differ in their ability to suppress anchorage independence of the cell line RM2T. These differences provide the basis for suppressor-gene cloning experiments based on gene transfer.

Normal human chromosome 1 carries suppressor activity for various phenotypes of a Kirsten murine sarcoma virus-transformed NIH/3T3 cell line

In order to identify chromosomes that carry putative tumor-suppressor genes for the various phenotypes of Kirsten sarcoma virus-transformed NIH/3T3 (DT) cells, we performed microcell-mediated chromosome transfer into DT cells. We first isolated mouse A9 clones, containing a single human chromosome 1, 11 or 12 tagged with pSV2-neo plasmid DNA. Then, chromosome 1, 11 or 12 was transferred from the A9 clones into DT cells by microcell fusion. The growth rate, colony-forming ability in soft agar and tumorigenicity of the DT cells were controlled by chromosome 1, but not by chromosome 11 or 12, indicating that normal human chromosome 1 carries a putative tumor-suppressor gene(s) that affects various transformed phenotypes of DT cells.

A tumor suppressor-dependent inhibitor of angiogenesis is immunologically and functionally indistinguishable from a fragment of thrombospondin

A secreted inhibitor of angiogenesis that is controlled by a tumor suppressor gene in hamster cells has been found to be similar to a fragment of the platelet and matrix protein thrombospondin. The two proteins were biochemically similar and immunologically crossreactive and could substitute for one another in two functional assays. Human thrombospondin inhibited neovascularization in vivo and endothelial cell migration in vitro, as does the hamster protein, gp140. gp140 sensitized smooth muscle cells to stimulation by epidermal growth factor, as does human thrombospondin. The thrombospondin gene has been localized on human chromosome 15. These results demonstrate a function for the ubiquitous adhesive glycoprotein thrombospondin that is likely to be important in the normal physiological down-regulation of neovascularization. In addition, they raise the possibility that thrombospondin may be one of a number of target molecules through which a tumor suppressor gene could act to restrain tumor growth.

A gene that encodes for a leukemia-associated phosphoprotein (p18) maps to chromosome bands 1p35-36.1

The cytosolic protein p18 which is expressed in increased amounts in acute leukemia cells is variably phosphorylated as a function of growth and differentiation. Proteins with identical amino acid sequence were independently found to be highly expressed in normal brain tissue and neuroendocrine tumor cells. Here we described the mapping of the recently cloned p18 gene to chromosome 1, band p35-36.1 by Southern blot analysis of human-rodent somatic cell hybrid DNA and by chromosome in situ hybridization using a p18 genomic probe. This region of the distal short arm of chromosome 1 is a frequent site of deletions or loss of heterozygosity in tumors derived from neural crest cells, particularly neuroblastomas and melanomas. The high levels of expression of p18 in brain and neuroendocrine tumor cells, its possible role in growth regulation, and its chromosomal location in a region frequently deleted in neuroectodermal tumors suggest that this gene may be involved in common genetic events occurring in these tumors.

Spontaneous and cAMP-dependent induction of a resting phase and neurite formation in cell hybrids between human neuroblastoma cells and thymidine auxotrophs of rat nerve-like cells

Cell hybrids (BIM) were produced between human neuroblastoma cells (IMR-32) and thymidine auxotrophs (B3T) of rat nerve-like cells (B103) in order to obtain cell lines undergoing stable neuronal differentiation. BIM cells exhibited the growth properties of partial transformation: 1) When the cell growth reached a plateau, BIM cells ceased to proliferate and expressed a differentiated phenotype. The shape of the cells changed from flat to round and they extended neurites. 2) When cultured in methylcellulose, BIM cells formed colonies, indicating that BIM cells have the ability of anchorage-independent growth. By Southern blot analysis, BIM cells had both human and rat types of N-myc genes. The human N-myc genes were amplified, but the extent of the amplification was lower in BIM cells than that in the parental cell line IMR-32. The rat N-myc gene was detected at a similar level in BIM, B3T, B103, and rat fibroblastic cells 3Y1. Therefore, the decrease in amplification of human N-myc genes may be involved in the properties of partial reverse-transformation in BIM cells. When treated with various drugs such as db-cAMP, forskolin, and cAMP with isobutyl-methylxanthine, BIM cells expressed a nerve-like phenotype. These findings indicate that cell hybridization yielded partial normalization of transformed nerve-like cells.

Loss of heterozygosity of the L-myc oncogene in human breast tumors

Recent studies suggest that loss of heterozygosity may play an important role in various human neoplasia. Cytogenetic abnormalities detected in primary breast tumors led us to examine breast tumor DNAs for deletions. In the present study, we demonstrate, using restriction fragment length polymorphism (RFLP) analysis at the L-myc proto-oncogene (chromosome 1p32), a frequent loss of heterozygosity in primary breast tumor DNAs (55 out of 152 informative tumor DNAs). Most of these deletions appear to be limited to chromosome 1p. No correlation was observed between this genetic alteration and several parameters of each patient's history or characteristics of the tumor. However, a significantly (P = 0.011) shorter survival period after relapse was observed for patients with loss of heterozygosity at L-myc in primary tumor DNAs compared with patients with tumor DNAs lacking this alteration.

Suppression of the progression phenotype in somatic cell hybrids occurs in the absence of altered adenovirus type 5 gene expression

In the present study we have analyzed the genetic regulation of increased expression of transformation-associated traits, a process termed progression, in adenovirus type 5 (Ad5)-transformed secondary rat embryo cells. Somatic cell hybrids were formed between a highly progressed neomycin-resistant Ad5-transformed cloned cell line (E11-NMTneo) and an untransformed chloramphenicol-resistant rat embryo fibroblast cell line (CREFcap). Parental E11-NMTneo cells grew with high efficiency in agar, exhibited reduced 125I-epidermal growth factor (EGF) binding, and were tumorigenic in nude mice. Parental CREFcap cells exhibited phenotypes opposite to those of E11-NMTneo cells. A high proportion (84%) of the presumptive hybrid cell types obtained after fusion and genetic selection (G418 and chloramphenicol) displayed a flat morphological phenotype intermediate between CREFcap and E11-NMTneo cells, suggesting that a trans-dominant extinction phenomenon had occurred. Two hybrids with a round morphology (R), which still exhibited the progressed phenotype, and two hybrids with a flat morphology (F), which had lost the progressed phenotype, were chosen for detailed analysis. Both R hybrids grew efficiently in agar, exhibited low 125I-EGF binding, and were tumorigenic in nude mice, whereas both F hybrids grew poorly in agar, displayed increased 125I-EGF binding in comparison with E11-NMTneo and R hybrids, and were nontumorigenic in nude mice. An analysis of the viral DNA integration patterns and the rates of transcription, steady-state mRNA accumulation, and relative levels of the Ad5 E1A and E1B gene products revealed no differences among the parental and hybrid cells. These studies indicate that normal CREF cells may contain a suppressor gene(s) which can inhibit the expression of specific traits of the progression phenotype in Ad5-transformed cells and that this suppression is not associated with changes in the expression of Ad5 transforming genes.

Suppression of the progression phenotype in somatic cell hybrids occurs in the absence of altered adenovirus type 5 gene expression

In the present study we have analyzed the genetic regulation of increased expression of transformation-associated traits, a process termed progression, in adenovirus type 5 (Ad5)-transformed secondary rat embryo cells. Somatic cell hybrids were formed between a highly progressed neomycin-resistant Ad5-transformed cloned cell line (E11-NMTneo) and an untransformed chloramphenicol-resistant rat embryo fibroblast cell line (CREFcap). Parental E11-NMTneo cells grew with high efficiency in agar, exhibited reduced 125I-epidermal growth factor (EGF) binding, and were tumorigenic in nude mice. Parental CREFcap cells exhibited phenotypes opposite to those of E11-NMTneo cells. A high proportion (84%) of the presumptive hybrid cell types obtained after fusion and genetic selection (G418 and chloramphenicol) displayed a flat morphological phenotype intermediate between CREFcap and E11-NMTneo cells, suggesting that a trans-dominant extinction phenomenon had occurred. Two hybrids with a round morphology (R), which still exhibited the progressed phenotype, and two hybrids with a flat morphology (F), which had lost the progressed phenotype, were chosen for detailed analysis. Both R hybrids grew efficiently in agar, exhibited low 125I-EGF binding, and were tumorigenic in nude mice, whereas both F hybrids grew poorly in agar, displayed increased 125I-EGF binding in comparison with E11-NMTneo and R hybrids, and were nontumorigenic in nude mice. An analysis of the viral DNA integration patterns and the rates of transcription, steady-state mRNA accumulation, and relative levels of the Ad5 E1A and E1B gene products revealed no differences among the parental and hybrid cells. These studies indicate that normal CREF cells may contain a suppressor gene(s) which can inhibit the expression of specific traits of the progression phenotype in Ad5-transformed cells and that this suppression is not associated with changes in the expression of Ad5 transforming genes.

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.

Induction of cellular senescence in immortalized cells by human chromosome 1

The control of cellular senescence by specific human chromosomes was examined in interspecies cell hybrids between diploid human fibroblasts and an immortal, Syrian hamster cell line. Most such hybrids exhibited a limited life span comparable to that of the human fibroblasts, indicating that cellular senescence is dominant in these hybrids. Karyotypic analyses of the hybrid clones that did not senesce revealed that all these clones had lost both copies of human chromosome 1, whereas all other human chromosomes were observed in at least some of the immortal hybrids. The application of selective pressure for retention of human chromosome 1 to the cell hybrids resulted in an increased percentage of hybrids that senesced. Further, the introduction of a single copy of human chromosome 1 to the hamster cells by microcell fusion caused typical signs of cellular senescence. Transfer of chromosome 11 had no effect on the growth of the cells. These findings indicate that human chromosome 1 may participate in the control of cellular senescence and further support a genetic basis for cellular senescence.

Tumor suppressor genes: the puzzle and the promise

Tumor suppressor genes are wild-type alleles of genes that play regulatory roles in cell proliferation, differentiation, and other cellular and systemic processes. It is their loss or inactivation that is oncogenic. The first evidence of tumor suppressor genes appeared in the early 1970s, but only within the past few years has a wealth of new information illuminated the central importance of these genes. Two or more different suppressor genes may be inactivated in the same tumors, and the same suppressors may be inactive in different tumor types (for example, lung, breast, and colon). The suppressor genes already identified are involved in cell cycle control, signal transduction, angiogenesis, and development, indicating that they contribute to a broad array of normal and tumor-related functions. It is proposed that tumor suppressor genes provide a vast untapped resource for anticancer therapy.

Tumor suppression involves down-regulation of interleukin 3 expression in hybrids between autocrine mastocytoma and interleukin 3-dependent parental mast cells

Interleukin 3 (IL-3)-dependent PB-3c mouse mastocytes can be transformed by the v-Ha-ras oncogene to generate autocrine IL-3-producing mastocytomas. Hybrid cell lines were constructed by fusing an IL-3-producing mastocytoma cell line with its IL-3-dependent normal parental cell. Unlike the mastocytoma parent cell line, hybrid cell lines required growth factor for in vitro proliferation, indicating that the IL-3-dependent phenotype is dominant. IL-3 mRNA, expressed at high levels in the tumor cells, appeared down-regulated in the cell hybrids. In contrast, p21v-Ha-ras levels were not reduced in the hybrids. The hybrid lines generated tumors in vivo with drastically prolonged latency times when compared to the tumor parent (10 versus 2 weeks). We propose that down-regulation of IL-3 mRNA production after cell fusion is responsible for the loss of growth autonomy in the hybrids and is likely to play a role in the partial suppression of tumor formation in vivo. Our data are consistent with the hypothesis that a tumor suppressor, present in PB-3c cells, acts as a negative regulator of IL-3 expression.

Distal deletion of chromosome Ip in ductal carcinoma of the breast

By use of recombinant DNA probes that correspond to genetic loci residing on human chromosome 1, DNA samples from 37 ductal breast carcinomas and constitutional DNA from the same individuals were tested for loss of heterozygosity. A high frequency (41%) of reduction to homozygosity was detected with the probe p1-79, which recognizes the highly polymorphic locus D1Z2, localized on 1p36. Loss of heterozygosity at other chromosome 1 loci was much less common, not exceeding a frequency of 10%, and was never observed in the absence of the D1Z2 loss. Somatic loss of heterozygosity at D1Z2 was more frequent in patients with a strong family history of breast cancer (60%), in patients with early diagnosis (before 45 years of age) (70%), and in those with multiple tumors or tumor foci (50%) than in patients with none of the characteristics of hereditary tumors (21%). No associations were observed between loss of heterozygosity and prognostic factors. These results suggest that inactivation of a tumor suppressor gene located on the distal portion of chromosome 1p, alone or combined with other genetic changes, may represent a fundamental step in the pathogenesis of ductal carcinoma of the breast.

Loss of alleles from the distal short arm of chromosome 1 occurs late in melanoma tumor progression

The gene for familial malignant melanoma and its precursor lesion, the dysplastic nevus, has been assigned to a region of the distal short arm of chromosome 1, which is frequently involved in karyotypic abnormalities in melanoma cells. We have examined loci on chromosome 1p for loss-of-constitutional heterozygosity in 35 melanomas and 21 melanoma cell lines to analyze the role of these abnormalities in melanocyte transformation. Loss-of-heterozygosity at loci on chromosome 1p was identified in 15/35 (43%) melanomas and 11/21 (52%) melanoma cell lines. Analysis of multiple metastases derived from the same patient and of melanoma and lymphoblastoid samples from a family with hereditary melanoma showed that the loss-of-heterozygosity at loci on distal 1p is a late event in tumor progression, rather than the second mutation that would occur if melanoma were due to a cellular recessive mechanism. Comparisons with neuroblastoma and multiple endocrine neoplasia (MEN2) suggest that the frequent 1p loss-of-heterozygosity in these malignancies is a common late event of neuroectodermal tumor progression.

Possible involvement of chromosome 1 in in vitro immortalization: evidence from progression of a human adenoma-derived cell line in vitro

We have previously reported that continuous in vitro passage in the presence of 3T3 feeders of a non-tumorigenic adenoma-derived epithelial cell line, designated PC/AA, resulted in its becoming immortal. At early passage PC/AA was normal diploid, whereas every cell of PC/AA late passage had an isochromosome 1(q) which led us to suggest that abnormalities of chromosome 1 may be involved in tumour progression. We now report the isolation of a 3T3-feeder-independent variant of early-passage PC/AA, designated PC/AA/FI, which was immortal in vitro and remained non-tumorigenic. Each cell of PC/AA/FI again has an isochromosome 1(q), like the late-passage PC/AA. However, with PC/AA/FI it is the other chromosome 1 of the homologous pair which is involved in the formation of the isochromosome 1(q). This is possible to determine because of the polymorphic centromeric heterochromatin on chromosome 1 of the early-passage PC/AA. With the late-passage PC/AA (grown with 3T3 feeders) the homologue with the large C-band has given rise to an isochromosome 1(q) whereas with PC/AA/FI it is the other homologue with the smaller C-band which has given rise to this isochromosome. Both the immortal PC/AA/FI and the immortal PC/AA late passage, therefore, have independent abnormalities involving chromosome 1. These results indicate that chromosome 1 may be involved in in vitro immortalization.

Suppression of transformation and immortality in human/Chinese hamster fibroblast hybrids--a model for suppressor gene isolation

Somatic cell hybrids were produced by fusion of normal human (foreskin) fibroblasts and a transformed Chinese hamster fibroblast line V79-8. Overall, approximately 30% of hybrid clones showed stable reversion to normal morphology and growth control in vitro as shown by serum and anchorage dependence. In one-third of these clones, senescence was observed after a number of generations similar to that required for the human fibroblast parent cells to senesce. The remainder appear to be immortal. Normal human chromosomes can therefore restore growth control with or without finite life-span to this transformed cell. V79 cells were found to be transfectable at an efficiency compatible with detection of single-copy gene transfer from genomic DNA. Furthermore, these cells were exceptionally sensitive to negative ("suicide") selection. Taken together, our data suggest that the V79 line represents an ideal system for isolation of human tumour suppressor genes.

Control of angiogenic activity in carcinogen-initiated and neoplastic hamster pouch keratinocytes and their hybrid cells

This study was undertaken to test the hypothesis that deregulated expression of the angiogenic phenotype by tumor cells is due to loss or inactivation of an angiogenesis suppressor gene(s). We used the technique of somatic cell hybridization to test the ability of untreated or chemical carcinogen-initiated hamster pouch keratinocytes, when fused to squamous epithelial neoplasms, to suppress tumor angiogenic activity by assaying hybrid-conditioned media (CM) in the avascular cornea of rat eyes. A non-angiogenic keratinocyte line, CL-2, derived from cultures of untreated epithelium and 3 lines of carcinogen-initiated keratinocytes, PN3, 5, and 7, of varying angiogenic potential were fused, using polyethylene glycol, to 3 tumorigenic, potently angiogenic, drug-resistant, hamster pouch carcinomas cell lines: HCPC-1, AW16E1-1, and AW16 E1-2. Serum-free 48-h CM from hybrid clones was prepared and assayed for angiogenic activity in rat corneas. CM from 5 hybrid clones derived from normal x neoplastic keratinocytes failed to induce an angiogenic response in 28 of 29 (97%) corneas tested. In contrast, CM from 4 hybrid clones derived from fusions between carcinogen-initiated and tumor cells were potently angiogenic in 24 of 25 (96%) corneas tested. Two angiogenesis suppressed hybrids clones were propagated in culture for an extended period of time, to permit chromosome segregation, and were found to re-express the angiogenic phenotype. These result indicate that angiogenesis is a recessive trait in normal hamster keratinocytes which is regulated in trans in these hybrid cells. It would also appear that loss or inactivation of angiogenesis suppressor function occurs early in the neoplastic process.

Recessive mechanisms of malignancy

It is increasingly recognised that recessive mutations play an important role in the pathogenesis of many forms of malignancy. Some of the affected loci may prove to be recessively-activated proto-oncogenes, but others are now known to be tumorigenic solely by virtue of their loss or inactivation and therefore form a distinct and novel family of tumour genes. Preliminary evidence suggests that such genes are likely to be functionally heterogeneous and to encode molecules involved in the inhibition of cellular proliferation and/or the induction of differentiation. Their further study is likely to illuminate fundamental mechanisms of normal cellular growth and differentiation as well as having important implications for the pathogenesis and management of cancer.

The approaching era of the tumor suppressor genes

Genes that can inhibit the expression of the tumorigenic phenotype have been detected by the fusion of normal and malignant cells, the phenotypic reversion of in vitro transformants, the induction of terminal differentiation of malignant cell lineages, the loss of "recessive cancer genes," the discovery of regulatory sequences in the immediate vicinity of certain oncogenes, and the inhibition of tumor growth by normal cell products. Such tumor suppressor genes will probably turn out to be as, if not more, diversified as the oncogenes. Consideration of both kinds of genes may reveal common or interrelated functional properties.

Growth suppression of hybrids between transformed cells and normal fibroblasts in serum-free medium: correlation with retention of human chromosomes

Somatic cell hybrids formed by crossing PG19 mouse melanoma cells with mouse embryo fibroblasts have a reduced ability to proliferate in growth factor-unsupplemented serum-free medium relative to the parental melanoma cells. The suppression of growth of the hybrid cells in serum-free medium is attributable to a strict requirement of these cells for polypeptide growth factors (insulin plus platelet-derived growth factor, fibroblast growth factor, or epidermal growth factor). In contrast, the parental melanoma cells are able to grow without exogenously added growth factors. Fifteen hybrids derived from crosses between mouse L cells and normal human skin fibroblasts also have been tested for ability to grow in growth factor-unsupplemented serum-free medium. Depending on which human chromosomes are retained, growth of these hybrids in serum-free medium is also suppressed relative to growth of the L cell parent. There appear to be several genes on different chromosomes that are involved in suppression of serum-free growth of the fibroblast x L cell hybrids. One weak suppressor gene appears to be on the human X chromosome.

Loss of heterozygosity in hypotriploid cell cultures from testicular tumours

We have established cell lines with a hypotriploid chromosome number from four testicular tumours. Each line had at least one Y chromosome and most of the informative centrosome and enzyme markers were heterozygous implying that the tumours originated from germ cells before the first meiotic division. The small metacentric marker chromosome (i12p), specific for testicular tumours, was present in all tumour cell lines and up to three copies were found in some lines. Rearrangements of chromosome 1 and 11 were each found in three out of four tumours. The rearrangements of chromosome 1 all resulted in duplication of 1q and deletion of short-arm material from the same chromosome giving loss of heterozygosity for enzyme markers on 1p. Loss of satellite material from chromosome 13 and the centromere region of chromosome 9 were found in single cases. This study shows that even where the chromosome number of tumour cells is near triploid, regions of the genome can be deleted. The chromosomes most frequently involved in rearrangements, 1, 11, and 12 all contain sites of ras oncogenes and it is suggested that loss of normal alleles could result in homozygosity for mutant oncogenes which may play a part in tumour progression.

The Ha-ras-induced transformed phenotype of rat-1 cells can be suppressed in hybrids with rat embryonic fibroblasts

Somatic cell hybrids were isolated from fusions of diploid embryonic rat fibroblasts with transformed Rat-1 cells which contained 4 to 5 copies of the transforming human Ha-ras 1 gene. In contrast to their transformed parental cells four hybrid clones showed normal morphology, long latency periods of tumorigenicity in newborn rats, anchorage requirement of proliferation, and an eightfold-reduced amount of secreted transforming growth factor activity. Thus these hybrids are called suppressed with regard to expression of the Ha-ras-induced transformed phenotype. Tumorigenic derivatives of the suppressed hybrids that had segregated chromosomes were isolated. Since two of the tumorigenic hybrid clones showed the similar low level of secreted transforming growth factors as the suppressed hybrids, decreased production of transforming growth factor activity is unlikely to be a sufficient criterion for suppression of malignancy. Whereas one of the suppressed hybrids expressed the transforming gene product p21 at a level similar to that of the transformed parental cells, other suppressed hybrids expressed less p21. This suggests that the suppressed phenotype can be regulated at the posttranslational level of p21 but that additional controls of expression of p21 are likely to exist. DNA of the suppressed hybrids transformed Rat-1 cells to proliferation in the presence of semisolid agar. Thus the activated human Ha-ras gene in the suppressed hybrids retained its biological activity even though it did not transform these cells to tumorigenicity.