Serum TK1 as a marker of malignity in veterinary medicine
Increased cell growth is a characteristic of cancer. Thymidine kinase 1 (TK1) is an enzyme involved in the DNA precursor synthesis. In human medicine it has been used as a serum biomarker for increased cell growth and turnover since the 1980s.
High levels of TK1 has been detected in sera from patients with various malignancies, especially those with hematologic diseases. Today there are no other established blood marker which is directly correlated to the cell growth rate of tumor diseases.
In Germany, to mention one example, approx. 200 000 samples of TK1 serum are analyzed every year, mainly with an assay from DiaSorin (TK REA, TK Liaison; www.diasorin.com) measuring the enzymatic activity of TK1.
In veterinary medicine these TK activity methods have been used for diagnosing and monitoring dogs with malignant lymphoma since 1996. Several studies of tumor diseases in dogs have been performed at SLU and we have been able to show elevated values of serum TK1 activity in connection with leukemia and lymphoma. In recent years the clinical usefulness of the TK1 marker for prognostic use and for monitoring of chemotherapy in dogs with lymphoma has also been clarified in several other studies (www.vdilab.com).
A common tumor disease in dogs is mammary tumors. However, serum from dogs with mammary tumors has often displayed low TK1 activity. Because of this the TK1 activity assays have had limited use in veterinary medicine.
A new immunochemical method
Instead of determining the enzyme activity of TK1 it has relatively recently become possible to measure the TK1 protein in human serum (www.arocell.com). This is by means of the immunochemical methods which has been developed for humane TK1 in serum (AroCell TK210). The crucial benefit of this method is that it is making it possible to detect elevated levels of TK1 even if it is enzymatically inactive, which is often the case in solid tumor diseases.
Furthermore, the test is based on the ELISA platform, subsequently it can be analyzed in all types of clinical chemistry laboratories.
A disadvantage is that the method, so far, has only been applicable to human serum TK1 determinations. Veterinary use demands species-specific antibodies.
Developed for dogs
In the last few years, however, immunochemical methods for measuring TK1 serum from dog based on dog-specific antibodies have been developed at SLU. Recently an ELISA has also been established. By using these methods significantly elevated levels of serum TK1 protein have been detected in serum from dogs with malignant lymphoma, but also in serum from dogs with other tumor diseases, e.g. mammary tumors and skin tumors – diseases that do not result in significantly elevated TK activities.
These studies constitute the first definitive evidence that a large fraction of serum TK1 protein is enzymatically inactive in serum from dogs with mammary tumors, compared to what has been observed in healthy dogs and dogs with hematologic malignancies.
Our conclusion is that evaluation of TK1 protein can provide valuable information regarding prognosis and early detection of relapse of hematologic as well as solid tumors in dogs. With utmost certainty this also applies to malignant diseases in other species of veterinary interest, e.g. in horse and cat. A prototype ELISA was developed in 2013 using a polyclonal rabbit antibody and a monoclonal anti-human TK1 antibody. A summary of the results on a number of common tumor types is now published. A patent application is also submitted.
Ready for routine clinical use
Funding was obtained from Vinnova through SLU Holding AB in order to verify the results obtained with a prototype form of TK ELISA. To convert this to a clinical diagnostic assay production of the unique antibodies in a larger scale was necessary. This is now done and the antibodies are currently evaluated for the establishment of an robust ELISA suited for commercialization.
Professor Emeritus in Medical and Physiological Chemistry, Department of Anatomy, Physiology and Biochemistry at SLU, Uppsala, Sweden.
Serum: Transparent fluid which is the result when the blood corpuscles and clotting proteins have been removed from blood
Malignancy: Malignant cancer
Hematological cancer diseases: mainly leukemia and lymphoma
Immunochemical methods: Techniques utilizing reactions between antibody and antigen
Monoclonal antibodies: Antibodies derived from daughter cells of one B cell clone, thereby being identical
Polyclonal antibodies: Antibodies which derive from many different B cells and thereby differ from each other
Antigen: A substance foreign to the body, which provokes a reaction of the immune system when it enters into an organism
Rabbit antibody: Antibody that has been produced with serum from rabbits
ELISA (Enzyme-Linked ImmunoSorbent Assay): A routine assay method for detection and quantification of an antibody or antigen