Clonal relationship between Hashimoto thyroiditis and thyroid lymphoma

Background: Although Hashimoto thyroiditis (HT) is a predisposing factor for B-lineage thyroid lymphoma, clonal B-cell populations in HT are rare. Aim: To investigate whether there is a clonal relationship between HT and primary thyroid lymphoma. Methods: Clonalilty and sequence similarity was determined by PCR followed by sequencing and comparing immunoglobulin heavy chain (IgH) gene rearrangement sequences to germline sequences and to each other. Results: 12/20 patients with primary thyroid lymphoma had a previous history and histological diagnosis of HT. Clonal IgH bands associated with a polyclonal background were present in four of these 12 cases of HT; of these four, three had reproducible clonal IgH bands from the subsequently developed lymphoma. The range of similarity (homology) of multiple clonal bands in HT with the germline IgH varied from 90% to 96.3%. Multiple clonal bands in HT had sequence similarity (homology) of 62 100% with the clonal band in the lymphoma from the same patient. At least one clonal band in HT had more than 96% similarity (homology) with the clonal band of lymphoma in all three cases. Conclusion: Sequence similarity between the clonal bands in HT and subsequently developed thyroid lymphoma is supportive of the argument that primary thyroid lymphoma may evolve from HT.