Lower number of 5-hydroxymethylcytosine-expressing cells in plasma cell myeloma than in reactive plasma cell hyperplasia: a useful immunohistochemical approach for identification of neoplastic plasma cells

摘要

Plasma cell myeloma (PCM) is a haematological malignancy defined by aberrant proliferation of plasma cells in the bone marrow. For the diagnosis of PCM, immuno-staining of light chains or surface marker analyses by flow cytometry is needed but sometimes difficult. Aberrant methylation at the carbon-5 position of cytosine results in 5-methylcytosine (5-mC), which is a well-known epigenetic hallmark of cancer in mammals. 5-mC is oxidised to 5-hydroxymethylcytosine (5-hmC), and low 5-hmC levels occur in several cancers and haematopoietic malignancies. We compared the expression of 5-hmC by immunohistochemistry (IHC) in neoplastic plasma cells in 31 PCM cases and in non-neoplastic plasma cells in 14 benign reactive lesions. The mean percentage of 5-hmC-positive cells was 7.8% and 81.5% in PCM and plasma cell hyperplasia, respectively. Thus, the frequency of 5-hmC-positive cells in PCM specimens was significantly lower than that in reactive plasma cell hyperplasia (p 0.001 by Student's t-test). IHC of 5-hmC is a useful method for differentiating neoplastic plasma cells from non-neoplastic plasma cells in reactive lesions even in tiny tissue samples unsuitable for flow cytometric analyses or for immunoglobulin light chain IHC.