High-Risk Gene Expression Subtype Provides Molecular Basis for Different Clinical Presentation of Primary and Secondary Plasma Cell Leukemia

Plasma cell leukemia (PCL) is rare but represents an aggressive, advanced form of multiple myeloma where neoplastic plasma cells (PCs) lose dependence on the bone marrow (BM) and circulate in the peripheral blood (PB). PCL is clinically defined by diagnosis of myeloma with ≥20% circulating plasma cells (CPCs), however, several groups have proposed a ≥5% CPC cutoff. PCL is classified as primary (pPCL) if it presents at myeloma diagnosis or secondary (sPCL) if it arises at a later progression event. These presentations of PCL are clinically distinct, with sPCL patients responding poorly to novel therapies and having inferior outcomes compared to pPCL patients. Despite recent advances in myeloma therapy, PCL prognosis remains poor, and the molecular drivers of PCL remain poorly understood.

The MMRF CoMMpass study (NCT01454297) is a longitudinal, observational clinical study of 1171 newly-diagnosed myeloma patients. Tumors were characterized using whole genome (WGS), exome (WES), and RNA (RNAseq) sequencing at diagnosis and each progression event. PCs were isolated from BM-derived tumors and when >5% CPCs were detected, PCs were also isolated from the PB, creating a subcohort of patients with sequencing data from both the BM and PB compartments, with some patients assayed longitudinally.

The percent CPCs determined using flow cytometry was reported for 982 patients at myeloma diagnosis and 194 patients at progression. Patients with 5-20% CPCs (median = 19 months) at diagnosis had poor overall survival (OS) outcomes compared to those with less than 5% CPCs (median = 95 months, p<0.001). No outcome difference was observed between patients with 5-20% and >20% CPCs (median = 41 months), confirming the findings of previous independent studies. A ≥5% CPC cutoff identified 947 myeloma, 29 pPCL, and 6 sPCL patients in the CoMMpass cohort. Compared to myeloma, pPCL and sPCL patients had poor OS (p<0.001), and after sPCL detection patients had a median OS of only 53 days (range = 0-169 days). For 10 pPCL patients, the percent CPCs was reported at diagnosis and at least one progression event, and patients with persistent CPCs (n = 5, median = 16 months, p<0.01) had poor OS compared to patients with no detectable CPCs at progression (n = 5, median not met, median follow up = 64 months). This underscores the benefit of early eradication of CPCs and repeated CPC measurements in pPCL.

The proliferative (PR) gene expression subtype of myeloma has been previously described and defines a high-risk group of patients with diverse genetic backgrounds and inferior outcomes. For PCL patients, we determined the subtype of all BM and PB tumor samples characterized using RNAseq. There was high subtype concordance between paired BM and PB tumor samples (12/13, 92.3%). Overall, 6/23 (26.1%) pPCL patients were in the PR subtype, and PR pPCL patients had poor OS outcomes (median = 10 months, p<0.001) compared to non-PR pPCL patients (median = 55 months). PR emerged as a robust predictor of risk in pPCL, outperforming other molecular and clinical variables including high BM or PB PCs, plasmacytomas, renal failure, high LDH, high B2M, low platelets, t(11;14), del(1p), amp(1q), del(13q), and del(17p), suggesting that RNA subtyping CPCs may represent a non-invasive tool to predict risk in pPCL.

At myeloma diagnosis, all sPCL patients with RNAseq data were classified in non-PR subtypes. However, at sPCL, 5/6 (83.3%) patients were in the PR subtype, indicating that sPCL is associated with transition to PR. Two sPCL patients that transitioned to PR acquired biallelic deletion of CDKN2C, and a third acquired biallelic deletion of RB1. Overall, a subset of pPCL (26.1%) but the majority of sPCL (83.3%) patients were in the PR subtype at PCL diagnosis, providing a molecular basis for the different clinical presentations observed between these two groups, including the highly-aggressive nature of sPCL. In summary, this study supports using a lower percent CPC cutoff to clinically define PCL and highlights the importance of repeated CPC measurements in prognosticating pPCL patients. Further, PR RNA subtype emerged as a predictor of risk in pPCL and, given that the majority of sPCL patients were in the PR subtype, provides a molecular basis for the different clinical features observed between pPCL and sPCL patients.