Canine Transmissible Venereal Tumor - Biology

Biology

CTVT is a histiocytic tumor that may be transmitted among dogs through coitus, licking, biting and sniffing tumor affected areas. The concept that the tumor is naturally transmissible as an allograft came from three important observations. First, CTVT can only be experimentally induced by transplanting living tumor cells, and not by killed cells or cell filtrates. Second, the tumor karyotes is aneuploid but has characteristic marker chromosomes in all tumors collected in different geographic regions. Third, a long interspersed nuclear element (LINE-1) insertion near c-myc has been found in all tumors examined so far and can be used as a diagnostic marker to confirm that a tumor is CTVT.

CTVT cells have fewer chromosomes than normal cells. Dog cells normally have 78 chromosomes; CTVT tumor cells contain 57–64 chromosomes that are very different in appearance from normal dog chromosomes. All dog chromosomes except X and Y are acrocentric, having a centromere very near to the end of the chromosome, while many of the CTVT chromosomes are metacentric or submetacentric, having a centromere nearer to the middle. There is no evidence that the tumor is caused by a virus or virus-like organism. The infectious agent of canine transmissible venereal tumor is the cancer cell itself and the tumor is clonal in origin. All tumor cells of this type of cancer share extremely similar genetic code, often if not always unrelated to the DNA of their host. Specifically, the LINE-1 (Long interspersed nuclear element) element in the tumor cells is in a different location than in normal canine DNA. This demonstrates that the tumors do not arise from separate cancerous transformation in individual animals. Rather, the malignant tumor cells from one dog are transferred to another.

CTVT is most commonly seen in sexually active dogs in tropical and subtropical climates. The disease is spread when dogs mate, and it can even be transmitted to other canine species, such as foxes and coyotes. Spontaneous regression of the tumor can occur, probably due to a response from the immune system. CTVT undergoes a predictable cycle: the initial growth phase of four to six months (P phase), a stable phase, and a regression phase (R phase), although not all CTVTs will regress. The tumor does not often metastasize (occurring in about 5 percent of cases), except in puppies and immunocompromised dogs. Metastasis is most commonly to regional lymph nodes, but can also be seen in the skin, brain, eye, liver, spleen, testicle, and muscle. Biopsy is necessary for diagnosis.

The success of this single cell lineage, believed to be the longest continually propagated cell lineage in the world, can be attributed to the tumor’s mode of transmission in a specific host system. Although direct contact is generally not a highly efficient mode of transfer, CTVT takes advantage of the popular sire effect of domestic dogs. A single male can produce dozens of litters over his lifetime, allowing the tumor to affect many more females than it could if a monogamous species were the host. Understanding the epidemiology of CTVT will provide insights for populations that may experience CTVT exposure and information about disease prevalence. CTVT is more often found in temperate climates where there are large populations of stray dogs, but little is known about the details of transmission.