Correlation of cytomorphology, immunophenotyping, and interphase fluorescence in situ hybridization in 381 patients with monoclonal gammopathy of undetermined significance and 301 patients with plasma cell myeloma

To further clarify the transformation from monoclonal gammopathy of undetermined significance (MGUS) to plasma cell myeloma (PCM), we compared interphase fluorescence in situ hybridization (FISH) patterns in 381 MGUS and 301 PCM patients. According to the World Health Organization and the International Myeloma Working Group, a threshold of 10% of bone marrow plasma cells separated MGUS from PCM. After magnetic activated cell sorting for CD138+ cells, FISH succeeded in 272 of 301 (90.4%) PCM, but in only 302 of 381 (79.3%) MGUS cases (P < 0.001). Cytogenetic alterations were more frequent in PCM (237 of 272; 87.1%) than MGUS (169 of 302; 56.0%; P = 0.0002). PCM showed a median of two cytogenetic alterations (range, 0–9) and MGUS one (range, 0–6). Considering only cases with a yield of plasma cells allowing five or more FISH probes, del(13)(q14) was found in 99 of 251 (39.3%) PCM but in only 59 of 267 (22.1%) MGUS (P = 0.0001), del(17p) in 15 PCM (6.0%) and in 6 MGUS (2.2%) patients (P = 0.029). A t(4;14)/IGH-FGFR3 was detected in 28 PCM (11.1%) and 5 MGUS (1.9%; P < 0.001). The t(11;14)/IGH-CCND1 and the t(14;16)/IGH-MAF showed no significant differences. Cytomorphology detected higher numbers of plasma cells than multiparameter flow cytometry (median ratio 4.25). This study underlines the genetic heterogeneity of MGUS similar to PCM. Genetic analysis might contribute to more diversified monitoring strategies for MGUS patients.