Blood, Vol. 95 No. 2 (January 15), 2000:
pp. 724-725
CORRESPONDENCE
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Letter |
To the editor:
Horizontal transfer of DNA and the "genometastasis
hypothesis"
In a recent issue of Blood, Holmgren et
al1 raised the question of whether DNA can be transferred
from one cell to another via the phagocytosis of apoptotic
bodies. Their data show conclusively that DNA can be rescued and reused
from apoptotic bodies by somatic cells. Their results have
considerable relevance to the possible role in metastasis of tumor DNA
that is circulating in the plasma.2
Circulating tumor DNA in the plasma of cancer patients has been
detected by the polymerase chain reaction with primers specific for
different mutations in the K-ras oncogene.3
However, it remains to be determined whether mutant K-ras DNA
in the plasma represents extracellular DNA released from a tumor, DNA
released from necrotic or apoptotic cells, or DNA released as a result of the lysis of fragile, circulating cancer cells. Mutant K-ras DNA can be detected in plasma even when it is undetectable in the
cellular fraction of blood. This finding suggests that circulating cancer cells might not be responsible for the presence of such DNA.3 Apoptosis or cellular necrosis would be expected to
yield detectable amounts of extracellular DNA. However, it is also
possible that detectable extracellular DNA might have been shed by
viable tumor cells.4 It is commonly assumed that
extracellular DNA in the plasma of normal individuals is susceptible to
digestion by DNases and it has been proposed, moreover, that the
elevated levels of circulating DNA in the plasma of cancer patients
might be due to the presence of circulating inhibitors of DNases in these patients.5 Whatever the explanation for the presence of the DNA, it is now clear that oncogenes can circulate in the plasma
fraction of the blood. We must now ask whether this phenomenon might
have important implications in cancer patients.
Using a rat model, we have demonstrated the presence of tumor DNA in
plasma using cancer cells (DHD/K12-PROb cells; abbreviated as DHD
cells) with a genome-associated tag that have been stably transfected
with pCDNA3.1CAT (Invitrogen, Groningen, Netherlands). This expression plasmid includes a bacterial gene for chloramphenicol acetyl transferase (CAT), as well as a neomycin-resistance
gene.2,6
We demonstrated that native DHD cells (lacking the CAT tag
sequences) could be transfected with circulating DNA simply as a result
of incubation with plasma from rats that had been rendered cancerous by
injection of DHD-CAT cells several weeks previously. We found that
non-tagged tumor cells (DHD) became genomically tagged cells (DHD-CAT)
when they were cultured for a week in a medium that contained 10%
(v/v) plasma from rats with cancer (Figure 1).2 Also, when we inoculated
healthy rats intraperitoneally with plasma from tumor-bearing rats, the
marker gene for CAT was found in extracts of the lungs of all tested
animals some weeks later (Figure 1).2 These results are
supported in part by those of Pulciani et al,7 who
demonstrated the presence of dominant oncogenes in tumor cells as a
result of transmission of the malignant phenotype from tumor cells to
normal cells via transfection with purified genomic DNA.
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| Fig 1.
Schematic representation of CAT-transfection experiments.
DHD cells were converted to tagged cells (DHD-CAT cells) in two ways:
as a result of direct transfection or as a result of culture in medium
supplemented with plasma from rats with DHD-CAT cancerous tumors.
Reprinted with permission from Histol Histopathol.
1999;14:1089.
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Moreover, 1 of the 11 surviving cultures showed
neomycin-resistance2; thus, the genome of some of the
cultured cells not only had incorporated a neomycin-resistance gene but
also had expressed it. Recent experiments have shown that by modifying the protocol used for the addition of geneticin to cultures
(adding it just after removing the plasma from medium,
instead of waiting for several days), all cultures have expressed the
neomycin-resistance gene (unpublished data).
Considering the available experimental and clinical evidence,
we suggested the following hypothesis. Metastasis might occur via
transfection of susceptible cells located in distant target organs with
dominant oncogenes that are derived from the primary tumor and are
circulating in the plasma. We tentatively proposed the term
genometastasis to describe this putative
phenomenon.2
Holmgren et al1 demonstrated that genomic DNA from
apoptotic bodies is transferred to the nuclear compartment of
phagocytosing cells and that this transferred DNA is stable over time.
Their findings might be closely related to our observations and might provide an explanation for the way in which tumor DNA that is circulating in the plasma might be transferred to cultured cells.
It is possible that apoptotic bodies, derived from tumor cells
and circulating in the plasma, might be taken up by phagocytosing cells
and that this phenomenon might be associated with the dissemination of
cancer. The horizontal transfer of DNA might occur between cancer cells
and other somatic cells, and such transfer might provide a putative
mechanism for genometastasis.
The evidence in support of the "genometastasis
hypothesis" is very provocative and suggests that further
investigations are warranted.
Damián García-Olmo
Dolores C. García-Olmo
Jesús Ontañón
Esperanza Martinez
Experimental Research Unit and Dept. of General Surgery
Albacete General Hospital Albacete, Spain
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References |
1.
Holmgren L, Szeles A, Rajnavölgyi E, et al.
Horizontal transfer of DNA by the uptake of apoptotic bodies.
Blood.
1999;93:3956-3963[Abstract/Free Full Text].
2.
García-Olmo D, García-Olmo DC, Ontañón J, Martinez E, Vallejo M.
Tumor DNA circulating in the plasma might play a role in metastasis. The hypothesis of the genometastasis.
Histol Histopathol.
1999;14:1159-1164[Medline]
[Order article via Infotrieve].
3.
Kopreski MS, Benko FA, Kwee C, et al.
Detection of mutant K-ras DNA in plasma or serum of patients with colorectal cancer.
Br J Cancer.
1997;76:1293-1299[Medline]
[Order article via Infotrieve].
4.
Leon SA, Shapiro B, Sklaroff DM, Yaros MJ.
Free DNA in the serum of cancer patients and the effects of therapy.
Cancer Res.
1977;37:646-650[Abstract/Free Full Text].
5.
Leon SA, Shapiro B, Servi P, Parsons RG.
A comparison of DNA and DNA-binding protein levels in malignant disease.
Eur J Cancer.
1981;17:533-538.
6.
García-Olmo D, Ontañón J, García-Olmo DC, Atiénzar M, Vallejo M.
Detection of genomically-tagged cancer cells in different tissues at different stages of tumor development: lack of correlation with the formation of metastasis.
Cancer Lett.
1999;140:11-20[Medline]
[Order article via Infotrieve].
7.
Pulciani S, Santos E, Lauver AV, Long LK, Aaronson SA, Barbacid M.
Oncogenes in solid human tumours.
Nature.
1982;300:539-542[Medline]
[Order article via Infotrieve].
