P-Glycoprotein Expression in Canine Lymphomas

P-glycoprotein (P-gp) is a membrane glycoprotein which can act as an efflux pump for certain chemotherapeutic drugs and thereby confers resistance to these drugs on the host cell. The P-gp isoform encoded by the human mdrl gene has been implicated as a factor in clinical resistance in human tumours. To investigate the role of P-gp in acquired clinical resistance, dogs with malignant lymphoma which were referred to Glasgow University Veterinary College for treatment, were given an anthracycline based protocol until first relapse. Tumour samples were obtained from these dogs at diagnosis and at time of disease progression. P-gp expression in these samples was assessed by immunohistochemistry using a monoclonal antibody which detects all known P-gp isoforms and by dot-blot analysis using a human mdrl specific probe. Multiple members of the mdr gene family exist in every species studied and only the mdrl isoform(s) is implicated in drug resistance. Investigations of the canine mdr gene family revealed that the dog has four potential members of this family. The homologue of the human mdrl gene was expressed in normal canine liver, kidney and adrenal and at a lower level in the caudal gastro-intestinal tract. Skeletal and cardiac muscle tissue has strong P-gp expression but this did not appear to be of the mdrl homologue isoform. Immunohistochemistry revealed that normal, reactive and lymphomatous nodes contain a P-gp positive dendritic cell population. These P-gp positive cells are morphologically identical to S-100 positive cells and so may represent an antigen presenting cell population. The presence of these cells could confound accurate interpretation of the dot- blot analysis and so imunohistochemistry was used as the main criterion to determine P-gp positivity in the malignant cell population. Within tumour cells, P-gp expression at time of diagnosis was a rare occurrence (less than 5%) but was significantly more common in drug resistant tumours (p=0.0113). However, even within the drug refractory tumours, P-gp expression was not ubiquitous; only 25% of tumours were P-gp positive and therefore P-gp does not appear to be a major cause of treatment failure in canine lymphoma. Statistical analysis of the results revealed that P-gp was associated with advanced stage disease. Advanced stage at presentation seemed a more important predictor of subsequent P-gp expression than the amount or type of drugs received prior to relapse. Treatment with corticosteroids prior to chemotherapy had a profound effect on the ability to achieve remission but again this was not related to the presence of P-gp. Canine T cell lymphomas are reported to have a poorer clinical performance than B cell tumours. T cell tumours were positively identified in this group of dogs by genotyping using a feline constant region probe of the T cell receptor p chain gene. 10/45 tumours were identified as having p chain rearrangements by this technique. The T cell genotype was a poor prognostic indicator by univariate analysis but none of the T cell tumours expressed P- gp, either before treatment or at relapse, indicating that P-gp is not a major cause of treatment failure in T cell lymphomas