Angiogenesis in Multiple Myeloma (MM): Angiogenic Switch or Reflexion of Plasma Cell Number? A Gene Expression Based Survey in Primary Myeloma Cells and the Bone Marrow Microenvironment.

BACKGROUND. Angiogenesis is a hallmark of active multiple myeloma. However, two etiologic hypotheses have been proposed:

1. an angiogenic switch (i.e. differential or de novo expression of pro/antiangiogenic genes in MM), and, alternatively

2. an effect of increased plasma cell number.

AIM of this study was to investigate the angiogenic signature of multiple myeloma cells (MMC), normal bone marrow plasma cells (BMPC), the bone marrow microenvironment (BMME) and cellular subfractions therein.

PATIENTS AND METHODS. 128 newly diagnosed MM-patients (65 training (TG) / 63 independent validation group (VG)) and 14 normal donors (ND) were included. Bone marrow aspirates were CD138-purified by activated magnetic cell sorting. Whole bone marrow (n=49) and FACSAria sorted subfractions thereof (n=5) were investigated. RNA was in-vitro transcribed and hybridised to Affymetrix HG U133 A+B GeneChip (TG) and HG U133 2.0 plus arrays (VG). Expression data were gcrma-normalised and the empirical Bayes algorithm used. p-Values were adjusted using the Benjamini-Hochberg method (Bioconductor). iFISH was performed on purified MM-cells using probesets for chromosomes 1q21, 9q34, 11q23, 11q13, 13q14, 15q22, 17p13, 19q13, 22q11 and the translocations t(4;14) and t(11;14). HGF expression was verified by real time RT-PCR and western blotting. Based on Medline review, we established a list of 89 pro- and 56 antiangiogenic genes and investigated their expression according to the stage of disease: BMPC vs. MGUS, SD stage I (asymptomatic myeloma) vs. SD stage II/III (symptomatic myeloma requiring therapy).

RESULTS.

1. BMPC express pro- (e.g. VEGFA) and antiangiogenic genes (e.g. TIMP2).

2. Only one pro-angiogenic gene (hepatocyte growth factor, HGF) is significantly overexpressed in MMC compared to BMPC. HGF has previously been linked with myeloma progression and induction of angiogenesis.

3. Six antiangiogenic genes (TIMP2, SERPINF1, COL18A1, PF4, THBS1, CXCL14) are downregulated in MMC compared with BMPC.

4. Compared to healthy donors, the BMME of MM shows a significant downregulation of PLAU (urokinase, antiangiogenic) and upregulation of TNF(proangiogenic).

CONCLUSION. Upregulation of HGF-expression, downregulation of TIMP2, SERPINF1, COLA18A1, PF4, THBS1 and CXCL14 expression in MMC as well as downregulation of PLAU and upregulation of TNFα in the BMME seem to indicate an “angiogenic switch”. However, given the relatively low number of differentially expressed genes (7/145) and the expression of angiogenic genes by BMPC, an effect caused by an increasing number of plasma cells might be evenly important.