Clinical and Molecular Characterization of Primary Plasma Cell Leukemia

Primary plasma cell leukemia (pPCL) is a rare form of multiple myeloma (MM) that is characterized by aggressive behavior and short overall survival (OS) despite significant improvement in anti-MM therapy. Due to its low incidence of 1-2% of all MM patients, most previous studies reported only on a small number of pPCL patients and the physiopathology as well as best treatment approaches remain unclear. In order to improve outcome in this patient population it will be necessary to evaluate molecular mechanisms underlying pPCL, which could potentially serve as therapeutic targets. The goal of the present study was to address clinical parameters and survival in pPCL patients. We further analyzed molecular characteristics by gene expression analysis and exome sequencing with the aim to detect distinct markers in pPCL.

We retrospectively evaluated 84 pPCL patients who were treated at the Myeloma Institute in Little Rock, and had at least cytogenetic (by karyotype) and outcome data available. The cytogenetic analysis showed a complex karyotype (>3 chromosomal aberrations) in 88% of the patients and hypodiploidy in 54% patients. Deletion 13 was very common and seen in 54% of patients and translocation 11;14 and deletion 17p were evident in 22% and 19% of patients respectively. These results are consistent with previous reports and show a higher prevalence of poor prognosis alterations in pPCL patients compared to non-pPCL MM patients. Clinical and biological characteristics of pPCL patients showed very high proportion of ISS 3 stage (86%) and bone marrow (BM) plasma cell infiltration of at least 70% in 77% of patients. Median overall survival (OS) of this pPCL population was 17 months. When restricting the analysis to patients <70 years of age that underwent high dose chemotherapy induction regimen followed by ASCT (n=59) the median OS was still short at 24 months.

We further evaluated gene expression profiling (GEP) from BM aspirates, which was performed on 54 patients and used the previously described 70-gene classifier (GEP70) for risk assessment. Surprisingly 40% of the patients had a low risk (LR) GEP70 profile, suggesting that pPCL might be more heterogeneous than previously thought or that some pPCL - with less aggressive biology - is caused from overcrowding of extensively involved BM space with leakage into the peripheral blood. Though median OS in LR pPCL patients was significantly higher than in HR patients (30 vs 16months, p=0.025), it still remained short suggesting that other parameters not identified by GEP70 influence outcome. In order to identify a gene signature specific to pPCL, we next studied differential gene expression of 18670 probes in BM samples from pPCL patients and compared them with healthy controls. There were 130 differentially expressed probes with at least 5 fold change at a false discovery rate of <0.01. Of these only 13 probes were upregulated with PHF19 (chr9q33.2) being the most upregulated gene in pPCL. Interestingly, when analyzing expression of PHF 19 throughout MM stages, we show that expression levels of PHF19 are similar in healthy controls and in patients with monoclonal gammopathy of undetermined significance (MGUS) as well as with smoldering MM (SMM) and increase in patients with MM and achieve highest levels in pPCL patients, suggesting that PHF19 mediates pPCL tumorigenicity and is a marker of disease aggressiveness.

We further analyzed data on targeted exome sequencing which was performed on 10 pPCL patients. A mutation in the RAS oncogene was present in 9 patients (KRAS n=5, NRAS n=3, one patient had both mutation), which is a much higher prevalence of RAS mutations than the previously described ~40% in non pPCL MM. TP53 mutations were second most common and seen in 5 patients.

In summary, this is the largest series of patients with pPCL reported so far. It confirms the poor prognosis of pPCL, even in young patients that were treated with an aggressive approach and in GEP70 defined LR disease. PHF19, a regulator of Polycomb Repressive Complex 2 (PRC2) that leads to transcriptional repression through H3K27 hypermethylation, was highly expressed in pCPL patients and its role in disease pathogenesis should be further evaluated. Further, targeted exome sequencing showed a RAS mutations in 90% of patients and though the patient number for this analysis was small, already available therapy targeting the RAS-pathway should be evaluated in pPCL patients.