Osteolytic bone disease (OBD) in multiple myeloma (MM) is known to be caused by a combination of osteoclast hyperactivation and osteoblast inhibition. One of the pathways known to be involved in osteoblast inhibition from in vitro studies is the HGF pathway consisting of HGF, its receptor MET, the co-receptor Syndecan-1 (SDC-1), the partial MET antagonist Decorin and HGF activator responsible for HGF processing to its active form. So far, gene expression studies in MM have been performed on isolated MM plasma cells or bone marrow (BM) aspirates, which are not completely representative of the cell composition in the BM micro-environment. We used a novel strategy, whereby gene expression of factors associated with the HGF pathway was evaluated in snap-frozen BM biopsies, and moreover we determined the protein levels in matched BM plasma samples.
An additional BM core biopsy obtained during the diagnostic procedure of MM patients was snap-frozen. Biopsies were cut, homogenized and RNA was purified and analyzed by qRT-PCR using low density arrays (Applied Biosystems). The relative quantitative gene expression was calculated using 3 internal reference genes (ABL, GAPDH and GUS). OBD was evaluated using standard radiographs. All patients were untreated and did not receive medicine that could influence bone remodeling. We examined 10 healthy volunteers (HV), 35 monoclonal gammopathy of unknown significance (MGUS) and 65 untreated MM patients, which according to radiographic findings were divided into NO/LOW and advanced OBD, i.e. OBD in ≥2 regions.
ELISA was performed on a total of 31 matched BM plasma samples of HV, MGUS and MM obtained at the same time point as the biopsies. In addition, extra samples without gene data (N=52) were analyzed. Commercial kits for SDC-1 (Diaclone), HGF (RnD, Quantikine) and Decorin (RnD, Duoset) were run in duplicates according to manufacturer's instructions.
Gene expression of HGF, SDC1, and MET were significantly different comparing HV, MGUS, no/low and advanced OBD (p<0.05) (For HGF, see figure 1). Decorin was not associated to OBD. HGF activator was not expressed in any of our samples, but only in the positive control.
A significant correlation between gene and protein expression levels measured by ELISA was found for SDC-1 (Spearman's rho= 0.463, p=0.0058) and HGF (Spearman's rho=0.45, p=0.01). No correlation was found between Decorin gene levels and BM plasma levels (Spearman's rho =-0.24, p=0.22).
The protein level of SDC-1 and HGF in BM plasma were both upregulated in MM and associated significantly to OBD level (p<0.05), while Decorin were significantly downregulated in MGUS and MM samples compared to HVs (p<0.05). A significant difference in HGF BM plasma levels were found between patients with no/limited OBD (median: 1.7ng/mL) and advanced OBD (median: 6.2ng/mL) in BM plasma.
In our expression study reflecting the in vivo situation in MM patients, genes in the HGF pathway and proteins were significantly associated to OBD. The use of whole snap-frozen BM biopsies is a novel strategy for evaluation of gene expression in MM making it possible to investigate patients independent of degree of MM plasma cell infiltration. In addition to the dys-regulated gene expression levels alteration of SDC-1 and HGF was also observed at protein level, supporting the gene expression findings, and underscoring the usefulness of BM biopsies for gene expression studies in MM. Furthermore, our study for the first time shows up regulation of HGF in association with OBD at both gene and protein level in a large clinical material.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.