In individuals with the plasma cell malignancy multiple myeloma, there is evidence that bone formation rates are reduced and that increased ostoeclastic bone resorption is associated with impaired osteoblast function. We investigated the osteogenic differentiation capacity of myeloma bone marrow mesenchymal progenitor cells (MPCs). We also examined the expression levels and activity of Runx2, the transcription factor required for osteogenesis.
Bone marrow MPCs were cultured in osteogenic induction medium and assessed for bone alkaline phosphatase (bALP) expression (Sigma kit), induction of osteoblast specific genes like osteocalcin (RT-PCR) and mineralization by von Kossa staining. Immunoblot analysis and electrophoretic mobility shift assays (EMSA) were used to determine Runx2 expression and DNA-binding activity respectively. RT-PCR was used to sequence Runx2 and detect any mutations or deletions present within its domains. Transactivation ability of Runx2 was measured by its ability to activate osteocalcin promoter in transient transfection assays.
Myeloma -derived MPCs showed reduced levels of bALP and osteocalcin transcript and a lower degree of mineralization, in osteogenic induction medium, as compared to that of the healthy donors. Immunoblot analysis and EMSA indicated equivalent Runx2 expression and DNA-binding capacity, respectively, in both healthy donor and myeloma-derived MPCs. Sequence analysis of Runx2 indentified a splice variant of Runx2 lacking exon 8 (Runx2D8) in myeloma patients with reduced transactivation ability. Co-transfection of the splice variant led to reduced transcriptional activity of the full-length Runx2. The reduced transactivation ability of spliced Runx2, as well as its suppressive action on the transactivation function of full-length Runx2, likely contributes to the defective osteogenesis clinically observed in myeloma pateints.