Reprinted from The Horse Report with permission from the Center for Equine Health, School of Veterinary Medicine, University of California, Davis.
Horses are living longer than ever, much like their human counterparts, but longevity also brings its own set of problems. Although cancer is not as common in horses compared with other species, the number of horses that do develop cancer has been increasing with the growing population of geriatric horses.
The Equine Oncology Program at the University of California, Davis (UC Davis), School of Veterinary Medicine, led by Alain Théon, DVM, MS, professor and chief of radiation oncology at the William R. Pritchard Veterinary Medical Teaching Hospital, is a unique group that has evolved from an interest in horses with a high risk for developing tumors. Many of these horses are aging recreational and companion horses or horses that have passed their prime athletic abilities.
The program offers all aspects of diagnosis and conventional treatments as well as experimental treatments for horses with malignancies. Théon has had a long-standing commitment to research in equine oncology and to the development of improved treatment options, particularly for equine melanomas.
Melanoma is a cancer that arises from melanocytes, the cells producing pigments in the skin and hair. These tumors often occur in the area of the tail, perineum, perianal region, external genitalia, and below haired skin on the head, neck, and trunk.
Melanoma is commonly thought to be benign because it grows relatively slowly, but nearly all forms of melanoma in the horse are malignant because, with time, there is a chance that cancer cells can spread to other internal organs. In addition, there is a subset of particularly aggressive melanomas with a fast growth rate that usually arises in younger horses or ponies, which if left untreated has the potential to significantly compromise the health of the horse and even lead to its death.
The current proven method for control of these tumors is removal, through surgery, by laser, or with cautery. However, sometimes complete removal of melanoma is not possible. For those cases, the Equine Oncology Program has been involved in research on immunotherapy, where tumor vaccines are used to immunize the patient against its own tumor cells. Unlike most vaccines, which prevent specific infections, the goal of therapeutic cancer vaccines is to train the body's immune system to 1) recognize and destroy cancer cells that already exist within its tissues, and 2) continue killing malignant cells long after treatment has ended.
A Collaborative Research Project
Another important research area that offers promise for improving the diagnosis and treatment of melanoma is in the area of genetics, made possible by the 2009 completion of the horse genome project. With genetic mapping of the horse, it is now possible to identify mutations that may be linked to melanoma (and other diseases) by comparing the genetic profiles from healthy and affected horses.
Théon and his group now have a unique opportunity to study genetic alterations associated with graying horses, which have altered pigmentation of the hair and skin.
At UC Davis, known for its collaborative approach to solving many different kinds of problems, oncology researchers will collaborate with the UC Davis Genome Center to carry out the research described below.
The Connemara Pony Breed
It is well established that about 80% of graying horses and ponies with dark skin over the age of 15 years have or will develop melanomas. The association of graying of the hair coat with a mutation on chromosome 25 has been demonstrated recently.
It is also known that the incidence of melanomas is relatively low in young animals and increases with aging. Because the Connemara Pony has had a significant increase in graying hair coat over the last few years as a result of selective breeding to control the blue-eyed cream phenotype, and the fact that the registered breed population is aging, it is reasonable to be concerned that this will lead to an increased incidence of melanoma in the breed when compared with other breeds.
The closed breeding process imposed on the Connemara Pony has produced a homogenous population of individuals with common morphological and behavioral traits.
This genetic uniformity presents a tremendous opportunity to detect individual genetic alterations associated with melanoma. In other breeds, the separate variations of a few genes associated with the disease would be lost in the huge background of genetic variations between individuals.
Our research project will compare and analyze by genome-wide association studies the entire genome (DNA) of melanoma-bearing and melanoma-free Connemara ponies. Altered genes and their expression will be analyzed to determine which ones promote the disease. As importantly, we will identify pathways that could vary from pony to pony and could explain the multitude of clinical presentations of melanomas in horses.
How Pony Owners Can Help
A necessary component of this project is to establish a tissue bank to provide quality biological samples for genomic analysis. Tissue samples will be needed from:
- Ponies less than 6 years old with melanomas;
- Ponies over 6 years old with melanomas;
- Older graying ponies (20 years old and older) without melanoma; and
- Middle-age (approximately 10 years old) non-graying ponies without melanoma.
Once ponies have been identified and entered in the study, Dr. Théon's laboratory will send a shipping kit to the owner/breeder for tissue collection and shipment. The kit will include a veterinary medical questionnaire, informed consent form, and collection protocols for blood and urine as well as tumor (affected ponies only) samples obtained by their local veterinarian.
Horse owners and breeders who are interested in participating in the study are asked to e-mail Dr. Théon at firstname.lastname@example.org, or Teri Guerrero, oncology clinical trials coordinator, at email@example.com or 530/752-0125.
Interested parties will be provided with more information about the study and whether their pony is eligible for the study.
Benefits of This Research
This research will benefit not only the Connemara Pony industry but also all graying horse breeds and might provide information about melanomas in other animal species, including humans. The results of the study will identify targets for improving the treatment of melanomas. Identification of defective genes will also be used to prevent or minimize melanoma risk through genetic screening and careful breeding.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.