Canine transmissible venereal tumor - From general to molecular characteristics: A review

Cancer is a group of complex diseases resulting from the accumulation of genetic and epigenetic changes affecting control and activity of several genes, especially those involved in cell differentiation and growth processes, leading to an abnormal proliferation. When the disease reaches an advanced stage, cancer can lead to metastasis in other organs. Interestingly, recent studies have shown that some types of cancer spread not only through the body, but also can be transmitted among individuals. Therefore, these cancers are known as transmissible tumors. Among the three types of transmissible tumors that occur in nature, the canine transmissible venereal tumor (CTVT) is known as the oldest cancer in the world, since it was originated from a single individual 11 000 years ago. The disease has a worldwide distribution, and its occurrence has been documented since 1810. The CTVT presents three types of cytomorphological classification: lymphocytoid type, mixed type, and plasmacytoid type, the latter being chemoresistant due to overexpression of the ABCB1 gene, and consequently increase of the P-glycoprotein. More knowledge about the epidemiology and evolution of CTVT may help to elucidate the pathway and form of the global spread of the disease.

Somatic evolution and global expansion of an ancient transmissible cancer lineage

The canine transmissible venereal tumor (CTVT) is a cancer lineage that arose several millennia ago and survives by "metastasizing" between hosts through cell transfer. The somatic mutations in this cancer record its phylogeography and evolutionary history. We constructed a time-resolved phylogeny from 546 CTVT exomes and describe the lineage's worldwide expansion. Examining variation in mutational exposure, we identify a highly context-specific mutational process that operated early in the cancer's evolution but subsequently vanished, correlate ultraviolet-light mutagenesis with tumor latitude, and describe tumors with heritable hyperactivity of an endogenous mutational process. CTVT displays little evidence of ongoing positive selection, and negative selection is detectable only in essential genes. We illustrate how long-lived clonal organisms capture changing mutagenic environments, and reveal that neutral genetic drift is the dominant feature of long-term cancer evolution.

Molecular Signatures of Regression of the Canine Transmissible Venereal Tumor

The canine transmissible venereal tumor (CTVT) is a clonally transmissible cancer that regresses spontaneously or after treatment with vincristine, but we know little about the regression mechanisms. We performed global transcriptional, methylation, and functional pathway analyses on serial biopsies of vincristine-treated CTVTs and found that regression occurs in sequential steps; activation of the innate immune system and host epithelial tissue remodeling followed by immune infiltration of the tumor, arrest in the cell cycle, and repair of tissue damage. We identified CCL5 as a possible driver of CTVT regression. Changes in gene expression are associated with methylation changes at specific intragenic sites. Our results underscore the critical role of host innate immunity in triggering cancer regression.

Transmissible [corrected] dog cancer genome reveals the origin and history of an ancient cell lineage

Canine transmissible venereal tumor (CTVT) is the oldest known somatic cell lineage. It is a transmissible cancer that propagates naturally in dogs. We sequenced the genomes of two CTVT tumors and found that CTVT has acquired 1.9 million somatic substitution mutations and bears evidence of exposure to ultraviolet light. CTVT is remarkably stable and lacks subclonal heterogeneity despite thousands of rearrangements, copy-number changes, and retrotransposon insertions. More than 10,000 genes carry nonsynonymous variants, and 646 genes have been lost. CTVT first arose in a dog with low genomic heterozygosity that may have lived about 11,000 years ago. The cancer spawned by this individual dispersed across continents about 500 years ago. Our results provide a genetic identikit of an ancient dog and demonstrate the robustness of mammalian somatic cells to survive for millennia despite a massive mutation burden.

Extensive conservation of genomic imbalances in canine transmissible venereal tumors (CTVT) detected by microarray-based CGH analysis

Canine transmissible venereal tumor (CTVT) is an intriguing cancer that is transmitted naturally as an allograft by transplantation of viable tumor cells from affected to susceptible dogs. At least initially, the tumor is able to evade the host's immune response; thus, CTVT has potential to provide novel insights into tumor immunobiology. The nature of CTVT as a "contagious" cancer, originating from a common ancestral source of infection, has been demonstrated previously by a series of studies comparing geographically distinct tumors at the molecular level. While these studies have revealed that apparently unrelated tumors share a striking degree of karyotypic conservation, technological restraints have limited the ability to investigate the chromosome composition of CTVTs in any detail. We present characterization of a strategically selected panel of CTVT cases using microarray-based comparative genomic hybridization analysis at ~one-megabase resolution. These data show for the first time that the tumor presents with an extensive range of non-random chromosome copy number aberrations that are distributed widely throughout the dog genome. The majority of abnormalities detected were imbalances of small subchromosomal regions, often involving centromeric and telomeric sequences. All cases also showed the sex chromosome complement XO. There was remarkable conservation in the cytogenetic profiles of the tumors analyzed, with only minor variation observed between different cases. These data suggest that the CTVT genome demonstrates a vast degree of both structural and numerical reorganization that is maintained during transmission among the domestic dog population.

The T963C mutation of TP53 gene does not participate in the clonal origin of canine TVT

In dogs, the canine transmissible venereal tumor (CTVT) is the only neoplasm which is not produced by neoplastic transformation of normal cells; the tumor is transmitted from the affected dog to healthy dogs by implantation of one or various clones of cancer cells. Thus, the CTVT of dogs analyzed in various countries reveals similar genetic characteristics and consequently CTVT is considered to have a clonal origin. The CTVTs obtained from dogs in Korea showed the T963C mutation on TP53 gene; this mutation was thought to be a molecular alteration which participates in the origin of the ancestral clone, CTVT. Nonetheless, this supposed mutation has not been identified in other studies which were carried out for the purpose of clarifying the clonal origin of CTVT. Thus we have considered it important to identify the role of the T963C mutation of the TP53 gene in the clonal origin of CTVT in dogs. Consequently the region which includes the mutation of the TP53 gene in twenty samples of CTVT obtained from various canine breeds was PCR amplified and afterwards its sequence of nucleotides was determined. We conclude that this mutation did not participate in the clonal origin of the tumor, but was acquired at a later stage.

[Is cancer a transmittable disease?]

A direct, horizontal and natural transmission of neoplasic cells has only recently been accepted by the biomedical community. There are three known examples in mammals: the Tasmanian Devil Tumor Disease, the Canine Transmissible Venereal Tumor and a similar disease in Sirian Hamsters. These diseases are not anecdotic cases only, but provide support for the cancer clonal evolution hypothesis.

The singular history of a canine transmissible tumor

In this issue of Cell, Murgia et al. (2006) confirm that the infectious agent of canine transmissible venereal tumor is the cancer cell itself and that the tumor is clonal in origin. Their findings have implications for understanding the relationship between genome instability and transmissible cancer and for conservation biology, canine genomics, and companion animal medicine.

Clonal origin and evolution of a transmissible cancer

The transmissible agent causing canine transmissible venereal tumor (CTVT) is thought to be the tumor cell itself. To test this hypothesis, we analyzed genetic markers including major histocompatibility (MHC) genes, microsatellites, and mitochondrial DNA (mtDNA) in naturally occurring tumors and matched blood samples. In each case, the tumor is genetically distinct from its host. Moreover, tumors collected from 40 dogs in 5 continents are derived from a single neoplastic clone that has diverged into two subclades. Phylogenetic analyses indicate that CTVT most likely originated from a wolf or an East Asian breed of dog between 200 and 2500 years ago. Although CTVT is highly aneuploid, it has a remarkably stable genotype. During progressive growth, CTVT downmodulates MHC antigen expression. Our findings have implications for understanding genome instability in cancer, natural transplantation of allografts, and the capacity of a somatic cell to evolve into a transmissible parasite.

Cancer biology: infectious tumour cells

New evidence is mounting that cancer cells can evolve to become infectious agents and be transmitted between individuals.

Sequence analysis of canine LINE-1 elements and p53 gene in canine transmissible venereal tumor

LINEs (long interspersed nuclear elements or long interspersed repeated DNA elements) contains two open reading frames (ORFs), ORF1 and ORF2. We analysed the ORF2 located in the 5' region to the first exon of oncogene c-myc in canine transmissible venereal tumor (TVT) cell. We also showed the transcription activation was induced by this TVT-LINE sequence using CAT assay. To identify the mutation of tumor suppressor gene, sequence analysis of p53 from TVT cell was performed. We identified the point mutation of 964 nucleotide (T-C) resulting in the change of amino acid (Phe-Ser) of p53 tumor suppressor protein.

Use of a murine xenograft model for canine transmissible venereal tumor

OBJECTIVE

To develop a murine model for canine transmissible venereal tumor (CTVT).

ANIMALS

Thirty-three 6-week-old NOD/LtSz-scid (NOD/SCID) mice and seven 6-week-old C57BL/6J mice.

PROCEDURE

Samples of CTVT were excised from a 3-year-old dog and inoculated SC into ten 6-week-old NOD/SCID mice to induce growth of xenograft transmissible venereal tumor (XTVT). To establish mouse-to-mouse transmission, samples of XTVT were removed and inoculated SC into 4 groups of 6-week-old NOD/SCID mice and into a control group. Samples of CTVT were also inoculated into immunocompetent C57BL/6J mice for a mouse antibody production (MAP) test. The canine and xenografted tumors were evaluated cytologically and histologically, and polymerase chain reaction was performed for detection of the rearranged LINE/c-MYC junction.

RESULTS

8 of 10 NOD/SCID mice that were inoculated with CTVT developed tumors 3 to 10 weeks after inoculation. In the second-generation xenograft, all mice developed tumors by postinoculation day 47; 1 X 10(6) of XTVT cells were enough to create a xenograft. Metastases developed in 4 of 20 mice. Xenografted and metastatic tumors retained cytologic, histologic, and molecular characteristics of CTVT. Results of the MAP test were negative for all pathogens.

CONCLUSION

We established an NOD/SCID murine model for XTVT and metastasis of CTVT. This model should facilitate study of tumor transplantation, progression, and metastasis and should decrease or eliminate the need for maintaining allogenic transfer in dogs.

Molecular structure of canine LINE-1 elements in canine transmissible venereal tumor

We determined the 4251-bp sequence of open reading frame 2 (ORF2) of canine LINE-1 retroposon that encodes 1275 amino acids. The truncated LINE-1 inserts associated with transmissible venereal tumor (TVT) of dogs contained the 1378-bp LINE-1 insert (TVT-LINE) flanked by 10-bp direct repeats upstream to c-myc gene. The TVT-LINE elements were composed of 416 bp inverse sequences homologous to the complementary strand of the LINE-1, a 5-bp deletion and 962-bp sequences homologous to the 3' region of the LINE-1.

Identity of rearranged LINE/c-MYC junction sequences specific for the canine transmissible venereal tumor

The canine transmissible venereal tumor is a naturally occurring neoplastic disease that affects the external genitalia of both sexes and is transmitted during coitus. Cytogenetic and immunologic studies demonstrated that tumors from different parts of the world are very similar, suggesting that they are transferred from one animal to another by the transplantation of viable cells. We found that the c-MYC oncogene was rearranged in this tumor by the insertion of a transposable genetic element sequence (known as LINE, long interspersed element) 5' to the first exon. The amplification of a DNA segment located in the junction of the LINE genome and c-MYC upstream sequences enabled the testing of the similarity of transmissible venereal tumor samples collected independently in different parts of the world. Oligonucleotide primers flanking the LINE/c-MYC junction were used to amplify a 340-base-pair segment and nested primers amplified a 280-base-pair segment. A fifth oligonucleotide used as a probe contained the actual junction sequence. All of the tumors analyzed revealed the existence of the specific bands, which were absent in normal canine DNA samples. The amplified segments obtained from all of the tumors analyzed were identical in size and nucleotide sequence, suggesting transmission of the original rearranged cell itself, as opposed to independent events of LINE insertion in a "hot spot."

Chromosome analysis of canine transmissible sarcoma cells

The chromosomal banding patterns of canine transmissible sarcoma (CTS) cells were analyzed and compared to those of normal canine cells with four banding techniques. In addition, N-myc and N-ras oncogenes on the chromosomes of the CTS cells were investigated by the in situ hybridization method. The modal chromosome number of the CTS cells was 58:17 metacentrics and 41 acrocentrics. Based on their G- and Q-banding patterns most of the chromosomes of the CTS cells were present in normal cells except for the second largest metacentric chromosomes. It was considered that the metacentric chromosomes of the CTS cells results from Robertsonian translocation of normal chromosomes. The long arm of the second largest metacentric element of the CTS cells was stained negatively by the Q-banding technique. The C-band was found on the long arm of the second largest metacentric element and three pairs of N-bands were recognized in the same region. It was suggested that this amplification of N-bands enhanced tumorigenic properties in the CTS. No N-myc or N-ras oncogenes were detected on chromosomes of the CTS cells by the in situ hybridization method.

Canine transmissible venereal tumor (TVT): a review

The occurrence, transmission, clinical appearance, histological findings, chromosome studies, immunity, different methods of treatment and the prevention of canine transmissible venereal tumour are reviewed.

Cytogenetic observations on the canine venereal tumor in long-term culture

Cytogenetic observations are reported on five canine venereal tumor tissue culture cell lines maintained over periods of 13 to 30 months. In all instances, a karyotype was observed that is strikingly similar to those reported elsewhere for spontaneous or transplanted venereal tumors. The modal number in all lines was close to 59, containing 17 to 19 abnormal metacentric chromosomes. Three of the metacentric chromosomes were identified as markers. Assays for oncogenic properties were undertake in heterologous and homologous hosts. Attempts to induce tumor formation in homologous hosts using the cultured venereal tumor cell were uniformly unsuccessful.

Enhancement of the biological activities of deoxyribonucleic acid extracted from canine venereal tumours

Deoxyribonucleic acid (DNA) extracted from canine venereal tumours was capable of changing normal cells to morphologically abnormal cells in an in vitro system of L-fibroblasts. The observed effects were proportional to the concentration of the nucleic acid extract used; the higher the concentration, the more pronounced the effect. Inactivation of this effect by deoxyribonuclease indicated that the active nucleic acid moiety was DNA. Purified RNA did not appear to contribute to the observed activity. The activity was enhanced by diethyl-amino ethyl-dextran (DEAE-D) and dimethyl sulphoxide (DMSO) and the data showed that 20% DMSO was superior to 300 g/ml DEAE-D as an enhancing agent.