Multiple myeloma is a neoplasm characterized by a clonal expansion of abnormal post germinal center plasma cells and accounts for ∼10 % of all hematologic malignancies. According to Surveillance, Epidemiology and End Results (SEER) database estimate, approximately 35,780 new multiple myeloma cases will be diagnosed in 2024, accounting for 1.8 % of all new cancers in the United States.1 Rapid advances in the field of cancer therapeutics have reshaped diagnostic criteria, risk stratification and management of myeloma patients. According to the International Myeloma Working Group (IMWG), symptomatic myeloma can be diagnosed in the presence of one or more myeloma defining events in addition to evidence of either ≥10 % clonal plasma cells on bone marrow examination or a biopsy-proven plasmacytoma. Myeloma defining events include the well-established CRAB criteria (hypercalcemia, renal failure, anemia, or lytic bone lesions) and the SLiM criteria which include ≥ 60 % clonal bone marrow plasma cells, serum free light chain (FLC) ratio ≥ 100 (provided involved FLC level is ≥100 mg/L), and more than one focal lesion on magnetic resonance imaging(MRI) scan.2

A comprehensive workup of a myeloma patient involves a complete blood count with differential, serum assessment for laboratory measures for renal function, serum calcium, lactate dehydrogenase levels and measurement of serum proteins including albumin, globulin, and beta-2 microglobulin. Crucial additional laboratory testing includes serum and urine protein electrophoresis with immunofixation to evaluate for the presence of a monoclonal protein, measurement of serum free light chains and documenting the presence or absence of amyloidosis. Incorporating myeloma-defining events into the diagnosis of myeloma necessitates an initial workup that includes imaging studies and a bone marrow assessment.3

Myeloma is genetically a highly heterogeneous disease but detection depends in part on the methods of analysis. Approximately one-third of myeloma cases assessed by conventional karyotyping show cytogenetic abnormalities. By contrast, interphase fluorescence in situ hybridization (FISH) analysis has a much higher rate of detection and this can be further enhanced to >90 % of myeloma cases when FISH is performed on CD138-enriched plasma cells.4,5 Thus, FISH analysis on enriched myeloma cells is recommended for assessment of the initial bone marrow specimen because of enhanced sensitivity, however, many investigators advocate for both karyotyping and FISH.6 Conventional karyotyping provides a glimpse of global abnormalities and a positive result is usually indicative of a highly proliferative tumor. Karyotyping may also detect clonal relationship, secondary myeloid neoplasm or other unexpected findings besides myeloma related aberrations not covered by FISH probes.6 While FISH remains the predominant technique in clinical diagnostic laboratories, there exists considerable diversity in methodologies employed. These discrepancies encompass factors such as the inclusion or exclusion of CD138+ cell selection and specific probes employed.

Translocations involving the immunoglobulin heavy chain (IGH) gene locus at chromosome 14q32 and trisomies of odd numbered chromosomes (hyperdiploidy) are considered early or primary cytogenetic events. These primary alterations have been identified in cases of monoclonal gammopathy of uncertain significance (MGUS) supporting the model of acquisition of sequential genetic aberrations leading to disease progression and treatment resistance.7 Primary cytogenetic events are seen in around 50 % of new cases.8IGH translocation partners include CCND1 as a part of t(11;14) (q13;q32), CCND3/t(6;14) (p21;q32), C-MAF/t(14;16) (q32;q23), MAF-B/t(14;20) (q32;q11) and NSD2/t(4;14)(p16;q32), which are considered to be mutually exclusive. Two other rare partners are CCND2/t(12;14)(p13;q32)(<1 % cases), and MAF-A/ t(8;14)(q24;q32) each in <1 % of myeloma cases.

Secondary chromosomal abnormalities include gain of 1q, deletion of 1p, deletion of 17p, deletion of 13q or monosomy 13 and MYC translocations. Based on the presence or absence of these cytogenetic abnormalities, patients are grouped into either the standard risk or adverse-risk categories (Table 1). These drivers of disease directly determine clinical behavior .The cytogenetic abnormalities are associated with prognosis, treatment response and have a strong correlation with gene expression profiling.9,10 Secondary events provide a survival advantage to subclones and are required for tumor progression and refractoriness to therapy. For instance, patients with myeloma associated with >3 copies of chromosome 1q have a poorer prognosis than patients with <3 copies.11 Patients with adverse risk secondary abnormalities at diagnosis, such as del(17p) or MYC related abnormalities (usually rearrangements) show inferior outcomes even after high-dose therapy and autologous stem cell transplantation.12, 13, 14

The International consensus classification (ICC) recommends the formal division of multiple myeloma (MM) into mutually exclusive diagnostic groups based on cytogenetic/FISH abnormality:1 MM, not otherwise specified and2 MM with recurrent genetic abnormalities, including MM with CCND family translocations (standard risk), MM with MAF family translocation (high- risk), MM with NSD2 translocation (high-risk), and MM with hyperdiploidy.10 In addition to large copy number changes and chromosomal translocations, focal copy number variants are also implicated in myeloma. Gain of chromosome 8q24.21 affecting MYC and gain of chromosme11q13.2 involving CCND1 are seen in about 14 % and 15 % of patients, respectively. Chromosome 11q deletion is seen in about 7 % of myeloma cases and leads to downregulation of tumor suppressor genes BIRC2 and BIRC3. Deletions of 14q32 involving the tumor suppressor TRAF3 in observed in about 10 % of patients. TRAF3 plays a crucial role in B-cell homeostasis and its loss enhances the survival of these cells.15,16

Early cytogenetic alterations (trisomies and/or one of the IGH translocations) occur in post-germinal center B-cells/plasma cells contributing to the formation of the so-called “founder-clone”. Over time, this clone expands and evolves, acquiring additional secondary cytogenetic abnormalities such as del 17p, chromosome1q gain, additional copy number changes, epigenetic changes and secondary mutations, all of which contribute to the intratumoral heterogeneity of multiple myeloma as shown in Fig. 1. This clonal evolution is a major contributor to disease progression or relapse and ultimately therapeutic refractoriness. In a study of 128 patients with myeloma assessed by FISH at initial diagnosis and at relapse after autologous stem cell transplantation, adverse-risk cytogenetic abnormalities such as de novo 1q gain and new deletion of 17p occur more frequently at relapse. This observation suggests that systemic treatment can potentially favor pre-existing aggressive subclones or induce secondary genetic events and clonal evolution in patients experiencing recurrent disease.12 In the same study, a minority of patients with hyperdiploid karyotype lost their hyperdiploid karyotype at relapse suggesting chemosensitivity of this clone.12 Other studies have suggested that biallelic TP53 inactivation leading to chromosomal instability is more common at relapse.17 Another interesting observation is that although new IGH translocations usually do not occur at relapse, a small percentage of cases can develop a new t(4;14), suggesting that t(4;14) can evolve from subclones.18 Furthermore, there is an increased incidence of IGH translocations with unknown partners at relapse evidenced by IGH break apart FISH probes. In relapsed and refractory end stage disease, it is postulated that clones can lose their dependency on bone marrow microenvironment and can survive and expand in circulation, spreading to different sites and resulting in plasma cell leukemia or extramedullary myeloma.19

Plasma cell leukemia was historically defined by the presence of >20 % plasma cells in the peripheral blood and/or circulating plasma cell count of >2 × 109/L. According to the updated criteria by the International Myeloma Working Group (IMWG), the new diagnostic criteria for plasma cell leukemia is the presence of ≥ 5 % circulating plasma cells in the peripheral blood, significantly lower than the prior threshold of ≥ 20 %.20De novo plasma cell leukemia is associated with frequent MYC translocations and t(11;14) while plasma cell leukemia associated with a pre-existing multiple myeloma (so-called secondary plasma cell leukemia) more commonly harbors del(17p).21 Most studies focused on extramedullary disease in myeloma have shown increasing cytogenetic complexity compared with bone marrow, suggesting that extramedullary disease may be composed of an evolved, more aggressive clone. Specifically, abnormalities commonly identified in extramedullary myeloma include t(4;14), del(17p13), del(13) and chromosome 1 aberrations when compared with the bone marrow.22