Plasma cell leukemia (PCL) is a plasma cell dyscrasia, i.e. a disease involving the malignant degeneration of a subtype of white blood cells called plasma cells. It is the terminal stage and most aggressive form of these dyscrasias, constituting 2% to 4% of all cases of plasma cell malignancies. PCL may present as primary plasma cell leukemia, i.e. in patients without prior history of a plasma cell dyscrasia or as secondary plasma cell dyscrasia, i.e. in patients previously diagnosed with a history of its predecessor dyscrasia, multiple myeloma. The two forms of PCL appear to be at least partially distinct from each other. In all cases, however, PCL is an extremely serious, life-threatening, and therapeutically challenging disease.
Signs and symptoms
Primary PCL The clinical presentation of primary PCL (pPCL) indicates a far more aggressive disease than that of a typical multiple myeloma case with its clinical features being a combination of those found in multiple myeloma and acute leukemia. Like multiple myeloma patients, pPCL patients exhibit pathologically high levels of monoclonal plasma cells in their bone marrow plus a malignant plasma cell-secreted circulating monoclonal myeloma protein, either IgG, IgA, a light chain, or none in 28-56%, 4-7%, 23-44%, or 0-12% of cases, respectively. Similar to B cell leukemias, but unlike multiple myeloma, pPCL patients exhibit relative high frequencies of splenomegaly, lymphadenopathy, hepatomegaly, kidney failure, bone marrow failure (i.e. thrombocytopenia, anemia, and/or, rarely, leukopenia), central nervous system defects, and peripheral neuropathies due to the invasion of these tissues by plasma cells and/or the deposition of their circulating monoclonal immunoglobulin in them. Compared to multiple myeloma patients, pPCL patients also: exhibit 1) high rates of developing a hypercalcemic crisis, i.e. a potentially life-threatening episode of high ionic calcium (Ca2+) levels in the blood due to excess bone re-absorption and/or renal failure; b) higher levels of serum lactate dehydrogenase and Beta-2 microglobulin; and c) lower rates of bone but higher rates of soft tissue plasma cell tumors termed plasmacytomas.
Secondary PCL Secondary PCL (sPCL) is diagnosed in 1-4% of patients known to have had multiple myeloma for a median time of ~21 months. It is the terminal phase of these patients' myeloma disease. sPCL patients typically are highly symptomatic due to extensive disease with malignant plasma cell infiltrations in, and failures of, not only the bone marrow but also other organs. They have failed or broken through one or more treatment regimens and therefore may also show some of the toxic effects of these treatments.
Cause PCL is caused by the development of an excessively high number of genetic abnormalities in plasma cells or, more particularly, their precursor B cells and plasmablasts (see plasma cells). This genetic instability is due to a myriad of acquired abnormalities including gene mutations; single nucleotide polymorphisms; depletions and duplications of parts of a gene, larger portion of a chromosome, or even an entire arm of a chromosome; translocations, deletions, and duplications of entire chromosomes; and increases and decreases in the expression of intact genes due to, e.g. the methylation of gene promotors and various less direct effects. These genetic abnormalities effect the Wnt signaling pathway, regulation of the cell cycle, RNA metabolism, protein folding, and cadherin-related cell adherence to extracellular matrix. These effects in turn control plasma cell proliferation, survival, apoptosis, adhesion to bone marrow, genome stability, and secretion of monoclonal immunoglobulins. Secondary plasma cell leukemia (sPCL) results from the comparatively slow development of plasma cell/plasma cell precursor genetic abnormalities which initially create a clone of cells that cause the premalignant condition of monoclonal gammopathy of undetermined significance. In a very small percentage of these cases, the progressive development of further genetic abnormalities serially create a clone(s) of plasma cells that cause the more serious but still premalignant disorder of smoldering multiple myeloma, overt myeloma cancer, and ultimately sPCL. In contrast to sPCL, pPCL presents de novo with a broad range of genetic abnormalities. For example, advanced methods for examining the genome viz., whole-exome sequencing and gene expression profiling, have identified 166 non-silent gene variants per pPCL patient sample at the time of diagnosis. These abnormalities are similar but not identical to those detected in sPCL while the abnormalities detected in sPCL more closely resemble those detected in multiple myeloma than do those of pPCL: the genetic data support the clinical data in suggesting that sPCL and pPCL are distinct diseases with sPCL among the two PCLs being more closely related to multiple myeloma. Examination of plasma cell immunophenotype by measuring certain of their cell surface antigens, particularly Cluster of differentiation. CD markers on plasma cells from patients with pPCL differ from those taken from multiple myeloma or sPCL patients. For example: pPCL plasma cells more often express CD20 antigen, which is considered important in anchoring plasma cells to the bone marrow stroma, than do those on plasma cells taken from myeloma patients (50% vs. 17%); pPCL plasma cells often lack CD56 antigen which is present on the majority of plasma cells taken from multiple myeloma patients; and pPCL plasma cells more frequently express CD28 than do sPCL plasma cells. Thus, immunophenotyping supports that notion that multiple myeloma, sPCL, and pPCL show critically important fundamental differences that may explain their different clinical presentations, courses, responses to therapy, and prognoses.
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