premature chromosome condensation: evidence for in vivo cell fusion in human malignant tumours

Int. J. Cancer: 36, 637-641 (1985) 0 1985 Alan R. Liss, Inc.

PREMATURE CHROMOSOME CONDENSATION: EVIDENCE FOR IN VIVO CELL FUSION IN HUMAN MALIGNANT TUMOURS Gyula KOVACS Laboratory of Cytogenetics, Institute of Pathology, Medical School Hanover, P. 0. Box 610140, 0-3000, Hanover 61, Fed. Rep. of Germany.

Premature chromosome condensation (PCC) was studied in direct preparations of tissues from patients with haematological diseases and carcinomas of various histological types. PCC was found in 6 out of 166 malignancies (128 haematological cases, 35 carcinomas and 3 malignant effusions) analysed with the GTG- technique. Chromosome analysis revealed S-phase and G,-phase PCC in each case; the frequency of PCC varied between I, 4 and 8.6% of the metaphases analysed. It is suggested that PCC chromosomes, which represent cell fusion in vivo, are not very rare in naturally-occurring human malignancies, and that cell fusion may affect the malignant phenotype. In con- junction with other factors they may also explain the heterogeneity of tumour cell populations.

Many human tumours are heterogeneous in their metastatic potential, in their response to chemotherapy, and in their growth rate, cell surface antigens, karyotype, etc. (Dexter and Calabresi, 1982). The de- velopment of this heterogeneity is not completely un- derstood, but may involve genetic instability and selective advantage (Nowell, 1976). Some investigators be- lieve that epigenetic factors are responsible (Rubin, 1980). Recently, the nature of genetic changes leading to a malignant phenotype has been extensively studied by means of somatic cell hybrids. The possible role of cell fusion in tumour heterogeneity and progression has also been discussed (Harris, 1979; Croce, 1980; Sabin, 1981; Stanbridge et al., 1982; Goldenberg and Pavia, 1982). Most observations have been made with intra- and interspecies hybrids derived from cell fusion in vitro and only a few studies are reported of in vivo hybridisation (Wiener et al., 1972, 1974; Fenyo et al., 1973; Goldenberg et al., 1974, 1981, 1982; Hu and Pasztor, 1975; Kao and Hartz, 1977; Aviles et al., 1977; Ber et al., 1978; Lala et al., 1979; Kerbel et al., 1983; Ber and Lanir, 1984; De Baetselier et al., 1984). Most experiments were carried out on cell lines and tumours characterised by cytogenetic and/or biochemical gene markers, and thus the hybrid cells were easily recognised. However, in the “intraspecies human system”, namely in direct studies on naturally-occurring human tumours, evidence of cell fusion in vivo is very difficult to obtain; a suitable assay for routine observation of such fusion remains to be devel- oped.

It is suggested that premature chromosome condensation (PCC) may be used to indicate cell fusion in such cases. PCC are induced when 2 cells fuse, one of which is in mitosis while the other is in interphase. The morphology of PCC depends on the interphase cells: the PCC of GI cell consists of single chromatids; whereas the S-phase PCC consists of “pulverized or fragmented” chromatids and chromosomes and the G2-PCC consists of 2 chromatids which are longer than the mitotic chromosomes (Johnson and Rao, 1970). PCC chromosomes are also easily detectable in

direct chromosome preparations from malignant human tumours and thus can be correctly interpreted as being secondary to in vivo cell fusion.

In human tumours “ pulverisation and fragmenta- tion” of chromosomes and PCC have been reported in malignant effusions (Miles and Wolinska, 1973), acute leukaemias (Williams et al., 1976), and carcinomas (Atkin, 1979; Reichmann and Levin, 1981). We now report the finding of PCC in 6 human tumours including leukaemias, carcinomas and a malignant effusion, and discuss the possible role of cell fusion in the progression and clonal diversity of human malignant tumours.

MATERIAL AND METHODS

Chromosome analysis was performed on 128 patients with various haematological diseases including 41 chronic myeloid leukaemias, 17 chronic megakary- ocytic-myelocytic myeloses, 9 myeloscleroses, 9 pri- mary thrombocythaemias, 2 1 cases of polycythaemia Vera, and 31 acute leukaemias. PCC was noted in two cases (Table I). Successful direct preparations for banding analysis were obtained from 35 carcinomas of various histological types and origins and from 3 pleural effusions of breast cancer. PCC was observed in 4 cases (Table I). Of the patients with PCC chromosomes, only the one with malignant effusion had received chemotherapy before chromosome analysis. Cytogenetic studies

In haematological cases, chromosome preparations were made from bone-marrow and/or peripheral blood cells by the direct preparation technique using RPMI 1640 medium supplemented with 15 % heat-inactivated foetal calf serum. Colchicine was added only in the hypotonic treatment at a final concentration of 0.01 pg/ml.

Carcinomas were processed by a direct technique for chromosome analysis as described earlier (Kovacs, 1978). Briefly, fresh tumour material was finely minced with a scalpel and the cell suspension was washed in RPMI 1640 medium. The cells were then incubated in a medium with colchicine at a final concentration of 10 pglml for 45 min to 2 hr. Air-dry preparations were made from both leukaemias and carcinomas and the metaphases were stained by using GTG- and occasion- ally QFQ- and CBG-techniques.

In the cases with PCC, all metaphases were analysed and the cell cycle position of each PCC was recorded (Table I) .

Received: January 2 2 , 1985.

638 KOVACS

TABLE I ~ PERTINENT CLINICAL AND CYTOGENETIC DATA FROM PATIENTS WITH PCC CHROMOSOMES

Case number Tumour Age Sex

1 Acute non-lymphocytic 67 F

2 Acute myelofibrosis 57 F

leukaemia, FAB, M2

(myeloblastic- megakaryoblastic)

breast carcinoma

noma of the bladder, grade I11

noma of the breast

noma of the breast

3 Pleural effusion of a 42 F

4 Transitional-cell carci- 49 M

5 Invasive ductal carci- 81 F

6 Invasive ductal carci- 50 F

FIGURE I - (u ) G-banded metaphase with 46 mitotic and about 46 condensed GI-chromosomes from an acute leukemia. FAB M2. (b) G-banding of an early G,-PCC with very condensed singlc chromatids from the acute myelofibrosis. Note the presence of the long marker (arrow) in the mitotic leukemic cell.

Modal Number of metaphases Number ot PCC number Abnormal Normal (frequency)

584? 3 GI

2 (32

45 145 7 5 GI

- 46 3 S (1.4%)

7 S (8.6%)

39 38 8 46 10

45

46 92

51

38 8

9

-

RESULTS

Some pertinent data on the tumours with PCC chromosomes are given in Table I.

Case I Thirty metaphases were karyotyped from enlarged

photos of GTG-banded preparations of very good qual- ity from peripheral blood. No structural or numerical aberrations were found in prometaphasic chromosome preparations. PCC was seen in 8 out of 592 metaphases (1.4%) recorded by microscopy. In 3 cells the PCC chromosomes appeared to be pulverised, 2 showed G2-phase and 3 GI-phase PCC. In each case of PCC the mitotic cells and G2 cells displayed 46 chromosomes with no aberration and the GI cells also appeared to be in the diploid region (Fig. la). Case 2

Twenty-five cells analysed with GTG-banding revealed the following karyotype: 45,XX,del (5) (q21), -8,i (17q), -20, +marl. Chromosome counts of 145 cells with marker chromosomes showed a modal number of 45, but 7 cells, presumably normal marrow cells, showed 46 chromosomes with no structural aberrations. PCC was found in 13 of 152 metaphases (8.6%). In each PCC the mitotic cell appeared to be a leukaemic cell showing marker chromosomes characteristic for the stemline karyotype (Fig. lb).

Case 3 In a malignant pleural effusion of a treated breast

carcinoma, 32 metaphases with a modal chromosome number of 65 (range 64 to 69) were analysed. Exact karyotyping was not possible due to complex aberrations which varied from cell to cell. In this case a relatively high percentage of mitotic cells showed a normal karyotype representing activated pleural cells and macrophages. PCC was found in 4 % of metaphases. Since all mitotic cells with PCC chromosomes were in the triploid region, this could indicate that they were of tumour origin. Case 4

This turnour showed a complex karyology with diploid, pseudodiploid and hypodiploid karyotypes as fol-

IN VIVO CELL FUSION IN HUMAN MALIGNANCIES 639

Case 6 Forty-six GTG-banded metaphases were karyo-

typed. Chromosome analysis revealed several structural aberrations, thus the exact description of marker chromosomes was not possible. Thirty-eight metaphases showed the stemline karyotype as follows:

+9mar. One of the markers was a long submetacentric chromosome, which could be identified in the GI-PCC in the mitotic cell as well as in the GI cell, showing that fusion between two tumour cells occurred (Fig. 2b). In 8 metaphases with a chromosome number of 92 exactly two sets of stemline karyotype occurred. No normal karyotype was found.

46,XX, -3, -5, -6, -6, -8 , -9, -13, -19, -22,

FIGURE 2 - (a) Early phase Gl-PCC from a breast cancer with G-banding. (b) G-banding patterns in a PCC from the breast cancer induced by the fusion between a mitotic and G I phase tumour cell. Note the same long marker chromosome with characteristic banding pattern in the mitotic cell (thick arrow) as well as in the G I cell (thin arrow).

lows: 46,XY=8/46,XY, -4,+mar3= 10/39,XY, -2,

-22, +marl, +mar2, +mar3, +mar4=38. Marl was a long marker of unknown origin. PCC chromosomes were seen in 4 of 56 metaphases (7.1 %). Each mitotic cell contained the long marker showing at least one of the fused cells to be a tumour cell. Case 5

Fifty-one GTG-banded metaphases showed a stemline karyotype with a ring chromosome as follows: 45,XX,-8,del (11) (q21), -12,+r(8?;12) (?; p13 +

q24). In 9 metaphases with 46 chromosomes no structural aberrations were detected. These cells are prob- ably lymphocytes or plasma cells, which were abundantly detected in histological sections of the tumour. PCC was seen in 2 of 60 metaphases (3.3%). The mitotic cells in GI-PCC showed a loss of some chromosomes due to a technical error (Fig. 2a).

-4, -6, -14, -15, -15, -17, -18, -20, -21,

DISCUSSION

We have observed PCC chromosomes in direct preparations of 6 human malignancies including leukaemias and carcinomas. The cytogenetical analysis revealed not only S-phase PCC, but also GI-PCC with characteristic single-stranded chromosomes in each case, and in the 2 haematological cases G2-PCC was also seen. Four tumours presented here showed a chromosome number in the diploid region and in Case 1 the tumour cells showed a normal karyotype, thus correct identification of the origin of GI cells was very difficult. However, using the presence of marker chromosomes as a criterion (Case 2-6), at least one of the two cells fused was a tumour cell. The GI-PCC of Case 6 is further very convincing evidence that fusion between two pseudodiploid tumour cells occurred.

Although the phenomenon of PCC in the experimental cell system has been extensively studied and well documented (for review see: Rao et al., 1982), there are only a few reported observations on PCC in vivo in human malignancies. Our present findings suggest that PCC chromosomes are not rare in human tumours; it is more likely the case that observers do not pay enough attention to the phenomenon of PCC dur- ing cytogenetic analysis of human leukaemias and carcinomas.

Miles and Wolinska (1973) were the first to describe the ‘pulverisation and degradation” of some chromosomes in 12/58 malignant effusions, and suggested that the extreme pulverisation of chromosomes may be interpreted as S-PCC due to cell fusion. Williams et al. (1976) reported 7 leukaemia patients, 6 of whom had acute myeloid leukaemias with erythroid precursor cells, and found a PCC frequency ranging from 10% to 100% of the metaphases, suggesting an extreme cell fusion tendency due to virus infection. Atkin (1979) described clear observations of PCC in human tumour. He found PCC in 5.6% of metaphases from a bladder cancer with a chromosomal mode of 64. In addition to S-phase PCC, PCC with typical GI chromosomes was found: in this PCC the mitotic cell showed 64 chromosomes and the GI cell 46 chromatids, representing a cell fusion between a mitotic tumour cell and a normal GI cell. Furthermore, 2 cells with about 110 chromosomes (64 +46) were found, indicating that viable hybrids originated from the fusion between a normal somatic cell and a tumour cell. Recently, Reichmann and Levine (1981) observed PCC in 2 patients with large-bowel cancer of near-tetraploid chromosome number, and hypothesised that the near-

640 KOVACS

tetraploid chromosome number might be the product of fusion of 2 near-diploid cells. The findings on PCC- chromosomes in our Case 6 show very clearly that near-tetraploid clone(s) can be generated by the fusion of 2 near-diploid tumour cells.

The few studies carried out have shown cell fusion in vivo between injected tumour cells and host cells to be a relatively common event (see opening para- graphs). Hybrids obtained from the fusion of tumour and host cells can produce invasive and metastatic tumours in the host, which prompted Goldenberg et al. (1974) to postulate, that the “formation of such in vivo hybrids may serve as a means for reducing certain tumour-specific antigenicity of the tumour cells and thus for escaping the host immunologic surveillance and achieving a more advanced stage of malignancy for invasion and metastasis”. Similar findings were reported from other laboratories studying various hybrid systems (Wiener et at., 1972, 1974; Fenyo et al., 1973; Kerbel et al., 1983; Larizza et al., 1984; De Baetselier et al., 1984). Such in vivo tumour X host- cell fusion appears to be relatively common. However, formal proof of cell fusion in vivo is very difficult to obtain, particularly in naturally-occurring human tumours. In experimental hybrid systems chromosomal and/or biochemical markers help to identify hybrid cells. However, it is not possible, using either cytogenetic methods or biochemical markers, to demonstrate in vivo cell fusion in naturally-occurring human tumours as shown in experimental hybrids. Only when a mitotic cell fused with an interphase cell and this was followed by the formation of PCC could it be concluded that in vivo cell fusion had occurred.

The effect of cell fusion on the malignant phenotype has been studied intensively by somatic cell hybridiza- tion. The experiments showed a partial or complete suppression of tumorigenicity (for review see Harris, 1979; Klinger, 1980). Other investigators have found that tumorigenicity was not suppressed in the hybrid cells and also that the malignant phenotype is dominant (Kucherlapati and Shin, 1979; Croce, 1980). Some investigators have concluded that cellular tumorigenicity may be both a recessive and a dominant trait (Weissman and Stanbridge, 1983; Stanbridge et al., 1982). More recently it was suggested that the reces- siveness and dominance of malignant phenotype depend on the somatic origin of the tumour and normal cells used (De Baetselier et al., 1981; Cowell and Franks, 1984). Whether the cell fusion in naturally occurring human tumours occurs generally between two turnour cells or between tumour cells and normal somatic cells, and which type of normal cells are involved in the latter case is not known, but cells with “nomadic properties” may be preferentially involved. The observation on bladder carcinoma made by Atkin (1979) has provided direct evidence for turnour x host-cell fusion and the findings on a breast cancer (Case 6 ) presented here demonstrate tumour X tumour-cell fusion. Thus, it must be postulated that cell fusion in human tumours occurs between all types of cells. However, it should be pointed out that the fusion products of two established tumour cells are immortal, whereas those of a tumour and normal host cell have a finite life span and can generate rare immortal progress by segregation processes. If a hybrid cell is viable, the change of the genome may be favourable to malignant

cell progression or the malignant phenotype may be suppressed. Another possibility is that the hybrid cells retain their malignant phenotype with some alterations in response to cellular differentiation.

Most human tumours, particularly carcinomas, have heteroploid chromosome numbers in the triploid and tetraploid region. This chromosome pattern is believed to be generated by endomitosis or extreme non-dis- junction. However, the possibility cannot be ruled out that divergent clones with high chromosome numbers are produced through tumour X host or tumour X tumour cell fusion followed by loss of some chromosomes. Since hybrid cells from induced fusion between different species divide and lose chromosomes it is reasonable to assume that fused human tumour cells in vivo do likewise, particularly when one of the fusion partners is a normal host cell. It is usual for many somatic cell hybrids to lose selectively the chromosomes of one of the parent cells, while the chromosomes from the other remain dominant in number. In the case of human x rodent hybrids it is the human chromosomes which are preferentially lost, and alternative combinations of chromosomes appear to be able to effect suppression of the malignant phenotype (Av- iles et al., 1977; Barski and Belehradek, 1979; Klin- ger, 1980; Franks, 1982). The suppressor chromosomes seem in the rodent X human system to be the human chromosomes 4, 7, 8, 9, 10, 11, 13, and 17 (for references see Klinger and Shows, 1983) and in the human X human system to be chromosome 11 and 14 (Stanbridge et al., 1981). These chromosomes are also frequently involved in numerical and structural aberrations specific to malignant tumours of various histological types (Mitelman and Levan, 1981). Furthermore, cellular oncogenes and genes related to cell growth have been mapped on several of these chromosomes (Yunis, 1983; LeBeau and Rowley, 1984).

Accordingly, it was suggested by Klinger and Shows (1983) that suppression of the malignant phenotype could be explained either by a dosage effect between c-one genes and unaltered c-one alleles or by a return to normal regulatory control due to transacting products of genes present on the normal chromosomes of the non-tumorigenic partner. However, an alternative combination of retained chromosomes in the hybrid may occur, particularly in interspecies hybrids, which is favourable for neoplastic cell progression through duplication of reactivated c-onc gene(s) or loss of chromosomes carrying the unaltered c-onc gene(s). This mechanism may be implied in cell fusion in vivo in human malignancies, not only in response to c-onc genes, but also in response to other genes or gene families responsible for cell function and differentiation.

In summary, previous and present findings on PCC suggest that cell fusion in vivo is not uncommon in human malignancies. It may be argued that the fusion between 2 tumour cells or between a tumour cell and a normal somatic cell, followed by the selective loss and retention of specific chromosomes carrying c-onc genes and regulatory genes, contributes to the altera- tion of the phenotype of human tumours. This mechanism, in addition to other pathways, may also explain the heterogeneity of tumour-cell populations.

IN VIVO CELL FUSION IN HUMAN MALIGNANCIES 641

knowledee the technical assistance of Miss A. Stiirmer ACKNOWLEDGEMENTS ~~~ ~

and MisFE. Rust. I also thank Dr. P. Cullen for critical reading of the manuscript, This work was supported by the Deutsche Forsch-

ungsgemeinschaft (DFG-KO 841/1-1). I gratefully ac-

REFERENCES

ATKIN, N.B., Premature chromosome condensation in carcinoma of the bladder: presumptive evidence for fusion of normal and malignant cells. Cytogenet. Cell Genet., 23, 217-219 (1979). AVILES, D., JAMI. J., ROUSSET. J-P., and RITZ, E., Tumor X host cell hybrids in the mouse: chromosomes from the normal cell parent maintained in malignant hybrid tumors. J. nut. Cancer fnst., 58, 1391-1399 (1977). BARSKI. G., and BELEHRADEK. J., Inheritance of malignancy in somatic cell hybrids. Somut. Cell Genet.. 5, 897-908 (1979). BER, R., and LANIR. N., Fusion of plasmacytoma and host cells in vivo: selection of proliferating and nonproliferating cultures. J. nut. Cancer fnst., 72,403-409 (1984). BER, R., WIENER, F., and FENYO, E.M., Proof of in vivo fusion of murine tumor cells with host cells by universal fusers. J . nut. Cancer fnsr., 60, 931-933 (1978). COWELL, J.K., and FRANKS, L.M., The ability of normal mouse cells to reduce the malignant Dotential of transformed mouse bladder epithelial cells depend; on their somatic origin. fnt. J. Cancer, 33, 657-667 (1984). CROCE. C.M., Cancer genes in cell hybrids. Biochem. Biophys.

DE BAETSELIER, P., GORELIK, E., ESHHAR. Z. , RON, Y., KATZAV, S., FELDMAN. M., and SEGAL, S., Metastatic properties conferred on nonmetastatic tumors by hybridisation of spleen B-lymphocytes with plasmacytoma cells. J . nut. Cancer fnst., 67, 1079- 1087 (1981). DE BAETSELIER, P., ROOS, E., BRYS, L., REMELS, L. , and FELD- MAN, M. , Generation of invasive and metastatic variants of a non-metastatic T-cell lymphoma by in vivo fusion with normal host cells. fnr. J. Cancer, 34, 731-738 (1984). DEXTER, D.L., and CALABRESI, P., Intraneoplastic diversity. Biochem. Biophys. Acta, 695, 97-112 (1982). FENYO, E.M., WIENER, F., KLEIN, G., and HARRIS, H., Selection of tumor-host cell hybrids from polyoma virus and methylchol- anthrene-induced sarcomas. J. nar. Cancer fnst., 51, 1865-1875 (1973). FRANKS, L.M., The use of cell hybrids to study phenotypic expression in ageing and tumour cells. In: J.E. Celis (ed.), Gene expression in normal and transformed cells, Plenum, New York (1982). GOLDENBERG, D.M., and PAVIA, R.A., Malignant potential of murine stromal cells after transplantation of human tumors into nude mice. Science, 212,65-67 (1981). GOLDENBERG, D.M., and PAVIA. R.A., In vivo horizontal onco- genesis by a human tumor in nude mice. Proc. nut. Acad. Sci. (Wash.), 79,2389-2392 (1982). GOLDENBERG,D.M., PAVIA,R.A., and TsAo,M.C., In vivo hybridisation of human tumour and normal hamster cells. Nature (Lond.), 250, 649-651 (1974). HARRIS, H. , Some thoughts about genetics, differentiation, and malignancy. Somut. Cell Genet., 5, 923-930 (1979). Hu, F., and PASZTOR, L.M., In vivo hybridisation of cultured melanoma cells and isogenic normal mouse cells. Diferentia- tion, 4, 93-97 (1975). JOHNSON, R.T., and RAO, P.N., Mammalian cell fusion: induction of premature chromosome condensation in interphase nuclei. Nature (Lond.), 226, 717-722 (1970). KAo. F.T., and HARTZ, J.A., Genetic and tumorigenic character- istics of cell hybrids formed in vivo between injected tumor cells and host cells. J . nut. Cancer Inst., 59,409-413 (1977). KERBEL, R.S . , LAGARDE. E.A., DENNIS, J.W., and DONAGHUE. T.P., Spontaneous fusion between normal host and tumor cells: possible contribution to tumor progression and metastasis studied with a lectin resistant mutant tumor. Mol. cell. Biol., 3, 523-538 (1983). KLINGER. H.P., Suppression of tumorigenicity in somatic cell

Acts, 605,411-430 (1980).

hybrids. I. Suppression and reexpression of tumorigenicity in diploid human X D98AH2 hybrids and independent segregation of tumorigenicity from other cell phenotypes. Cytogenet. Cell Genet., 27, 254-266 (1980). KLINGER, H.P., and SHOWS. T.B., Suppression of tumorigenicity in somatic cell hybrids. 11. Human chromosomes implicated as suppressors of tumorigenicity in hybrids with Chinese hamster ovary cells. J. nut. Cancer fnst., 71, 559-569 (1983). KOVACS. G., Abnormalities of chromosome No. 1 in human solid malignant tumours. fnr. J. Cancer, 21, 688-694 (1978). KUCHERLAPATI, R., and SHIN, S., Genetic control of tumorigenicity in interspecific mammalian cell hybrids. Cell, 16, 639-648 (1979). LALA, P.K., SANTER. V., and RAHIL, K.S . , Spontaneous fusion between Ehrlich ascites tumor cells and host cells in vivo: kinet- ics of hybridisation and concurrent changes in the histocompati- bility profile of the tumor after propagation in different host strains. Europ. J. Cancer, 16,487-510 (1979). LARIZZA, L., SCHIRRMACHER. V., GRAF, L., PFLUGER, E. , PERES- MARTINEZ, M., and STOHR, M., Suggestive evidence that the highly metastatic variant ESb of the T-cell lymphoma Eb is derived from spontaneous fusion with a host macrophage. fnt. J. Cancer, 34,699-707 (1984). LEBEAU, M.M., and ROWLEY, J.D., Heritable fragile sites in cancer. Nature (Lond.), 308, 607-608 (1984). MILES, C.P., and WOLINSKA, W., A comparative analysis of chromosomes and diagnostic cytology in effusions from 58 cancer patients. Cancer, 32, 1458-1469 (1973). MITELMAN, F., and LEVAN, G., Clustering of aberrations to specific chromosomes in human neoplasms. IV. A survey of 1871 cases. Hereditas (Lond.), 95, 79-139 (1981). NOWELL, P.C., The clonal evolution of tumor cell populations. Science, 195,23-28 (1976). P.N. Rao, R.T. Johnson, and K. Sperling (eds.), Premature chromosome condensation, Academic Press, New York (1982). REICHMANN. A, , and LEVIN. B., Premature chromosome condensation in human large bowel cancer. Cancer Genet. Cyrogenet.,

RUBIN. H., Is somatic mutation the major mechanism of malignant transformation? J . nut. Cancer Inst., 64, 995-1000 (1980). SABIN, A.B., Suppression of malignancy in human cancer cells: issues and challenges. Proc. nat. Acad. Sci. (Wash), 78, 7129- 7133 (1981). STANBRIDGE, E.J., DER, C.J., DOERSEN, C.J., NISHIMI, R.Y., PEEHL. D.M.. WEISMANN. B.E.. and WILKINSON. J.E.. Human

3,221-225 (1981).

cell hybrids: analysis of transformation and tumorigenicity. Sci- ence, 215, 252-259 (1982). STANBRIDGE, E.J., FLANDENMEYER, R.R., DANIELS, D.W., and NELSON-REES, W.A., Specific chromosome loss associated with the expression of tumorigenicity in human cell hybrids. Som. Cell Genet., 7, 699-712 (1981). WEISSMAN, B.E., and STANBRIDGE, E.J., Complementation of the tumorigenic phenotype in human cell hybrids. J. nut. Cancer fnst., 70, 667-672 (1983). WIENER, F., FENYO., E.M., and KLEIN, G., Tumor-host-cell hybrids in radiochimeras. Proc. nut. Acad. Sci. (Wash.), 71,

WIENER. F., FENYO. E.M., KLEIN. G., and HARRIS, H. , Fusion of tumour cells with host cells. Narure (New Biol). ,238, 155-59 (1972). WILLIAMS, D.M., SCOTT, C.D., and BECK,, T.M., Premature chromosome condensation in human leukemia. Blood, 47, 687- 693. Y u ~ r s . J.J., The chromosomal basis of human neoplasia. Sci- ence, 221,227-236 (1983).

148-152 (1974).

premature chromosome condensation: evidence for in vivo cell fusion in human malignant tumours

Documents