Intelligent FISH for Myeloma: Enhanced Cytogenetic Aberration Detection with Immunomagnetic Bead Enrichment of Plasma Cells

Abstract 1918

The detection of cytogenetic aberrations in plasma cell neoplasms is definitive for risk stratification. However, metaphase chromosome analysis is limited, due to a low proliferative rate in vitro. Interphase florescent in situ hybridization (FISH) dramatically enhances detection. When detected by FISH, high risk aberrations include complex karyotypes, t(4;14), t(14;16), p53 gene deletions, and hypodiploidy; standard risk aberrations include hyperdiploidy and chromosome 13q deletions detected by FISH only; and low risk aberrations include normal karyotypes, t(6;14), and t(11;14). Despite the utility of interphase FISH in predicting patient outcomes, it is often difficult to detect underlying genetic changes in patients with low levels of clonal plasma cells, such as monoclonal gammopathy of undetermined significance (MGUS) and minimal residual disease (MRD). Plasma cell-targeted FISH can be achieved through 1) simultaneous cytoplasmic light chain staining and FISH, 2) sequential morphologic identification and FISH, 3) immunomagnetic bead depletion of non-plasma cells, or 4) immunomagnetic bead enrichment of plasma cells. Immunomagnetic bead enrichment is superior to other methodologies because it efficiently captures a large number of purified plasma cells for downstream analysis. In this study, we use anti-CD138 coated magnetic beads to enrich bone marrow aspirates containing 1.1% ∼ 5.0% monoclonal plasma cells by flow cytometry, to attain purified samples with approximately 90% plasma cells, for subsequent FISH analysis; we call this procedure Intelligent FISH. We present cytogenetic and morphologic findings on our first 257 cases examined with Intelligent FISH. More than 87% (225/257) of enriched cases exhibit cytogenetic aberrations. In contrast, a matched group of cases analyzed with standard FISH exhibit cytogenetic aberrations in only 31.6% (79/250). These results demonstrate that patients with low levels of plasma cells and normal FISH results may be misclassified as lower risk when they should be considered higher risk, due to lack of sensitivity of conventional FISH techniques. Among our enriched cohort, 22% (57/227) exhibit high risk aberrations, 47% (122/257) exhibit standard risk aberrations, and 30% (78/257) have low risk aberrations. We assess morphologic and immunophenotypic features observed within the different risk groups. These studies show that Intelligent FISH is an efficient and highly valuable technique for accurate risk stratification and detection of minimal disease in patients with plasma cell neoplasms. In particular, it can provide prognostic guidance previously unavailable for the clinical management of MGUS patients. In conclusion, plasma cell enrichment followed by FISH should be incorporated into the standard work-up of MGUS and MRD patients to accurately assess risk, follow-up interval, and treatment options.