Correlation of Cytomorphology, Immunophenotyping, and Interphase Fluorescence in Situ Hybridization (FISH) in 381 Patients with MGUS and 310 Patients with Multiple Myeloma.
Authors: Bacher Ulrike , Haferlach Torsten , Kern Wolfgang , Weiss Tamara , Schnittger Susanne , Haferlach Claudia 
Published: October 2019

Abstract
Abstract 2812
Poster Board II-788
From cytomorphological aspects, monoclonal gammopathy of undetermined significance (MGUS) and multiple myeloma (MM) are overlapping disorders. According to the WHO classification, a threshold of 10% of plasma cells (PCs) in bone marrow (BM) aspirates separates both categories. To clarify whether this separation is justified from cytogenetic aspects, we performed comparison of interphase fluorescence in situ hybridization (FISH) patterns in 691 patients with MGUS (n=381) or MM (n=310). Also, the results of cytomorphology and immunophenotyping using multiparameter flow cytometry (MFC) were correlated. 278 females and 413 males (22.8-92.8 yrs) at first presentation of MM/MGUS were analyzed between 2005-2009 in our laboratory with a combination of cytomorphology, MFC, and FISH following magnetic activated cell sorting (MACS) of CD138+ PCs (Robosep, STEMCELL Technologies, Vancouver). According to cytomorphological WHO criteria, 381 patients were categorized as MGUS (median BM PCs, 5%), 310 as MM (median PCs, 18.5%). The number of FISH probes being applicable to samples was depending on the amount of plasma cells yielded from MACS procedure. In the MM pts, a median of 12 probes was applied (range, 0-22) which was slightly higher than in the MGUS patients (median: 11; 0-18) (p=0.00002). FISH procedure was hampered by insufficient PC numbers more frequently in MGUS (79/381, 20.7%) than in MM pts (29/310; 9.3%) (p=0.0004). In MM pts, FISH revealed a median of 2 and a maximum of 7 cytogenetic alterations per patient in contrast to a median of 0 and a maximum of 5 in MGUS (p<0.0001). In more detail, in MM the maximum number of gains of genetic material per patient was 7 (MGUS: 5), of losses 7 (MGUS: 3), and of reciprocal rearrangements 2 (MGUS: 1). Subsequently, cytogenetic alterations were compared in those 527 pts (260 MM and 267 MGUS pts; 76.3% of all pts), in whom PC numbers allowed performance of at least 5 FISH probes (t(11;14), t(4;14), t(14;16), 13q14, TP53). Abnormal FISH results were detected in 145/260 MM (55.8%) and 106/267 MGUS pts (39.7%) (p=0.0002). In MM, 31/260 (11.8%) had a t(4;14)/IGH-FGFR3 in contrast to 5/268 (1.9%) in MGUS (p<0.0001). The t(11;14)/IGH-CCND1 (MM: n=41; 15.6%; MGUS: n=50; 18.7%; n.s.) and t(14;16)/IGH-MAF were similarly frequent in both cohorts (MM: n=8; 3.1%; MGUS: n=3; 1.1%; n.s.). Monosomy13/del(13)(q14) was more frequent in MM (n=103; 39.3%) when compared to MGUS (n=59; 22.1%, p=0.0001). Deletions of TP53/17p13 were seen in 16 MM (6.1%) and in 6 MGUS pts (2.2%) (p=0.029). Notably, in 7 MGUS cases with <1% PCs in cytomorphology, FISH revealed genetic alterations in 3 cases. PCs as quantified by cytomorphology vs. MFC ranged from 0-96% vs. 0-84% (median 8.5 vs. 2.0), respectively, with a highly significant correlation between both methods (Pearson, r=0.712, p<0.0001). However, as previously reported, MFC detected lower numbers in general: the median ratio of PCs by cytomorphology:MFC amounted to 4.25 (range 0.00-178.00). In 12 MGUS cases (1.7%) as defined with cytomorphology, MFC did not detect any PCs. Conversely, in 5 MGUS cases, MFC detected PCs while cytomorphology did not. In conclusion, cytogenetic patterns showed higher genetic complexity in MM cases when compared to MGUS, and both the t(4;14) as -13/del(13)(q14) were significantly more frequent in MM when compared to MGUS. However, the cytogenetic alterations showed no specific pattern for MM or MGUS categories. Therefore, the overlaps being seen from morphological aspects do also exist on the genetic level. This suggests that a cytogenetically based categorization of these cases might correlate better with the clinical profiles than the cytomorphological separation of MM/MGUS. Finally, this study supports the performance of FISH in MGUS cases. The demonstrated detection of malignant/monoclonal PCs by MFC also in cases in which cytomorphology fails to diagnose MM/MGUS emphasizes the inclusion of MFC in a standard diagnostic procedure.

Disclosures:
Haferlach: MLL Munich Leukemia Laboratory: Equity Ownership. Kern:MLL Munich Leukemia Laboratory: Equity Ownership. Weiss:MLL Munich Leukemia Laboratory: Employment. Schnittger:MLL Munich Leukemia Laboratory: Equity Ownership. Haferlach:MLL Munich Leukemia Laboratory: Equity Ownership.