Clonally transmissible cancer

Clonally Transmissible Cancer is a rare type of cancer that can be spread from one individual to another within the same species. Unlike most cancers, which are caused by genetic mutations within an individual's own cells, clonally transmissible cancers are infectious, with the cancer cells themselves acting as the agents of transmission. This phenomenon challenges the traditional understanding of cancer as a non-transmissible disease. The most well-known examples of clonally transmissible cancers are the Devil Facial Tumour Disease (DFTD) affecting the Tasmanian devil and the Canine Transmissible Venereal Tumor (CTVT) in dogs.

Mechanism of Transmission

Clonally transmissible cancers are transmitted between individuals through physical contact, often during mating or aggressive interactions. The cancer cells are transferred directly from the affected individual to the recipient, where they evade the immune system and establish a new tumor. This evasion is possible because these cancer cells often express low levels of Major Histocompatibility Complex (MHC) molecules, making them less recognizable as foreign cells to the host's immune system.

Examples

Devil Facial Tumour Disease (DFTD)

DFTD is a fatal condition affecting the Tasmanian devil, a carnivorous marsupial native to Tasmania. The disease is characterized by the appearance of tumors around the face and neck. Since its discovery in 1996, DFTD has led to a significant decline in the Tasmanian devil population, threatening the species with extinction.

Canine Transmissible Venereal Tumor (CTVT)

CTVT is a sexually transmitted cancer that affects dogs worldwide. The tumors typically appear on the external genitalia but can also develop in other areas. Unlike DFTD, CTVT is not usually fatal and can often regress spontaneously or be treated effectively with chemotherapy.

Genetic Characteristics

The cancer cells in clonally transmissible cancers are genetically distinct from the host's cells but are clonally derived from a single original cancer cell line. Genetic studies have shown that CTVT, for example, originated from a single individual possibly thousands of years ago, making it one of the oldest known continuously living cell lines.

Implications for Conservation and Veterinary Medicine

The existence of clonally transmissible cancers poses unique challenges for conservation efforts, particularly for species like the Tasmanian devil, which is already at risk due to its limited genetic diversity and habitat loss. In veterinary medicine, understanding the mechanisms of transmission and immune evasion in these cancers can inform the development of vaccines and treatments.

Research and Future Directions

Research into clonally transmissible cancers is focused on understanding how these cancers evade the immune system, the genetic and epigenetic changes that enable their transmission, and the origins and evolution of these diseases. There is also interest in exploring the potential for these cancers to inform human cancer research, particularly in the areas of metastasis and immune evasion.