Non-Invasive Liquid Biopsy to Quantify and Molecularly Characterize Circulating Multiple Myeloma Cells in the Assessment of Precursor Disease Pathology

Introduction: Multiple Myeloma (MM) is an incurable hematologic malignancy characterized by the abnormal growth of clonal plasma cells in the bone marrow (BM). In most cases MM develops from early, asymptomatic disease stages known as Monoclonal Gammopathy of Undetermined Significance (MGUS) and Smoldering Multiple Myeloma (SMM). Despite effective new therapies, most MM patients inevitably relapse and require further treatment, highlighting the need for better early detection methods for precursor patients and targeted interventions to prevent early disease from progressing.

The initial diagnosis of MGUS/SMM remains an incidental process following the identification of increased clonal immunoglobulin in the blood. BM biopsy is the gold standard for diagnosis and monitoring of MM progression, but is intrusive, painful, and comes with possible secondary complications for patients. Consequently, repeated assessment is not a feasible option for MGUS and SMM patients who are asymptomatic. Here we tested the utility of circulating multiple myeloma cells (CMMCs) from non-invasive blood biopsy to accompany BM as a method to monitor disease development, by enumerating CMMCs from MGUS/SMM patients.

Methods: Peripheral blood from 185 precursor patients (75 MGUS and 110 SMM) from the Dana-Farber Cancer Institute observational PCROWD study (IRB #14-174) was collected in CellRescue ^TM Preservative Tubes and processed on the CellSearch CellTracks Autoprep system using the CMMC assay kit using 4mL of blood. This assay employs the enrichment of CMMCs through the immunophenotype of CD138 ⁺CD45 ^-19 ^-, and leukocyte exclusion based on CD45 ⁺CD19 ⁺. Nucleated cells were identified using DAPI staining. The CellTracks Analyzer II fluorescence microscope system was subsequently used to scan captured CMMC cartridges, with software allowing the automated scoring and enumeration of CMMCs. Additional molecular analyses were carried out on SMM patients. Briefly, minipools of CMMCs were sorted by DEPArray and underwent whole genome amplification using Ampli1 kit, PCR-free library construction, quantification and low pass whole genome sequencing (~0.5x) on the Illumina HiSeq2500. To assess whether molecular analyses can be performed to detect hyperdiploidy as a genomic biomarker of MM disease, ichorCNA analyses was performed to determine copy number variant (CNV) events and infer tumour fraction.

Results: CMMCs were detected in 27% of MGUS patients collected, with a median count of 2 CMMCs (range 0 to 1328). Comparably, CMMCs were detected in 57% of SMM patients, with a median enumeration of 13 CMMCs (range 0 to 43836). Enumeration of CMMCs illustrated a correlation with clinical measure of disease including the International Myeloma Working Group 2/20/20 risk stratification model. A higher CMMC count was associated with increasing risk group based on the 3-risk factor model, with a median of 5, 29 and 59 CMMCs detected at low, intermediate, and high-risk SMM groups, respectively. CMMC counts were significantly increased at intermediate (P = 5.0 x 10 ^-4) and high-risk stages (P = 3.7 x 10 ^-3) compared to low-risk.

While enumeration provides a correlative measure of CMMCs that may be of tumor origin, downstream molecular characterization can confirm MM-associated genetic alterations. At the precursor stages, a low tumour burden is evident clinically, thus both normal and malignant plasma cells are present. Therefore, to determine the concordance between bone marrow and peripheral blood CMMCs, we performed genomic analyses to identify arm level gain or loss events. Molecular analyses of CMMCs was carried out in patients who had matched BM and clinical fluorescent in situ hybridization (FISH) results. We showed that CMMCs can capture 100% of clinically annotated BM FISH CNV events. Furthermore, CMMC samples identified additional yield, with further CNVs identified that were not observed by FISH. In cases that did not have BM biopsy results, sequencing of CMMCs revealed the existence of genetic aberrations.

Conclusion:

Our results demonstrate clinical correlation and molecular characterization of CMMCs from MGUS/SMM patients. This study provides a foundation for non-invasive detection, enumeration and genomic interrogation of rare CMMCs from the peripheral blood of MGUS/SMM, illustrating the clinical potential of using liquid biopsies for monitoring and managing disease in the precursor setting of MM.