PCR-based clonality analysis of antigen receptor gene rearrangements in canine cutaneous plasmacytoma

Plasmacytomas are discrete, B cell-derived, round cell tumours that sometimes are difficult to distinguish from canine cutaneous histiocytomas or T cell lymphosarcomas (lymphomas). Diagnosis of plasmacytomas relies on morphological observations and immunohistochemistry for multiple myeloma oncogene-1 (MUM-1) and cluster of differentiation 3 (CD3). Clonality testing often is used as an adjunct diagnostic tool to examine lymphoproliferative diseases. In this study, the sensitivity of PCR-based clonality analysis of antigen receptor gene rearrangements in canine cutaneous plasmacytomas was determined. Formalin-fixed paraffin-embedded sections of 29 canine plasmacytomas, 23 diffuse large B cell lymphomas (DLBCLs) and 23 lymph nodes without lymphoma were used for clonality analysis. New oligonucleotide primers for the framework (FR)2 and FR3 regions of the immunoglobulin heavy chain (IGH) V gene subgroup 3 were designed and used with previously reported FR3 primers. Although plasma cells are of B cell lineage, the detected frequency of IGH clonality in plasmacytoma was 0-34.5% with the seven primers used, whereas in DLBCLs it was 8.7-78.3%. In 23 lymph nodes without lymphoma, IGH clonality was detected in only one case with two out of the seven primers used. Sequence analysis of PCR products from plasmacytomas revealed mismatches in the annealing region of the FR3 primers. The sensitivity of detecting IGH clonality in canine plasmacytomas was lower than in DLBCLs. The low detection rate of IGH clonality in canine plasmacytoma may be due to somatic hypermutation of the variable region.