Murine Rankl-/- Mesenchymal Stromal Cells Display an Osteogenic Differentiation Defect Improved by a RANKL-Expressing Lentiviral Vector

Autosomal recessive osteopetrosis (ARO) is a severe bone disease characterized by increased
bone density due to impairment in osteoclast resorptive function or differentiation. Hematopoietic
stem cell transplantation is the only available treatment; however, this therapy is not effective
in RANKL-dependent ARO, since in bone this gene is mainly expressed by cells of
mesenchymal origin. Of note, whether lack of RANKL production might cause a defect also in
the bone marrow (BM) stromal compartment, possibly contributing to the pathology, is
unknown. To verify this possibility, we generated and characterized BM mesenchymal stromal
cell (BM-MSC) lines from wild type and Rankl2/2 mice, and found that Rankl2/2 BM-MSCs displayed
reduced clonogenicity and osteogenic capacity. The differentiation defect was significantly
improved by lentiviral transduction of Rankl2/2 BM-MSCs with a vector stably expressing
human soluble RANKL (hsRANKL). Expression of Rankl receptor, Rank, on the cytoplasmic membrane
of BM-MSCs pointed to the existence of an autocrine loop possibly activated by the
secreted cytokine. Based on the close resemblance of RANKL-defective osteopetrosis in humans
and mice, we expect that our results are also relevant for RANKL-dependent ARO patients.
Data obtained in vitro after transduction with a lentiviral vector expressing hsRANKL would suggest
that restoration of RANKL production might not only rescue the defective osteoclastogenesis
of this ARO form, but also improve a less obvious defect in the osteoblast lineage, thus
possibly achieving higher benefit for the patients, when the approach is translated to clinics.