Malignant autosomal recessive osteopetrosis caused by spontaneous mutation of murine Rank

We report the first case of lethal autosomal recessive osteopetrosis in mice caused by a spontaneous 8-bp deletion in exon 2 of the Rank gene. The phenotype, including a block in RANKL-dependent osteoclast differentiation and lymph node agenesis, copies that of Rank(-/-) mice, which have been produced by targeted recombination.

Commitment of osteoclast progenitors to the osteoclast lineage requires RANKL/RANK-mediated intercellular signals. Gene-targeted defects in this signaling pathway resulted in osteoclast deficiency and severe osteopetrosis in mice, but to date, there have been no reports of spontaneous mutations in Rankl or Rank resulting in osteopetrosis.

Mice with malignant osteopetrosis and absent lymph nodes appeared spontaneously in a highly inbred colony. Appropriate crosses were analyzed to establish the pattern of inheritance. Tissues from affected pups and littermates were evaluated grossly, histopathologically, and radiographically. Osteoclast development from splenocytes was tested in vitro under a variety of conditions, including after infection with RANK-encoding retrovirus. Rank mutational analysis was performed by direct sequencing of RT-PCR products and genomic DNA.

The inheritance pattern was consistent with autosomal recessive inheritance, and the phenotype resembled that of either Rankl or Rank knockout mice with the exception of as yet unexplained death of most mice 2-3 weeks after weaning. Osteoclast precursors from the spleens of affected pups failed to form osteoclasts in vitro when stimulated with macrophage-colony stimulating factor (M-CSF) and RANKL, unless they were forced to express wildtype Rank cDNA. Molecular genetic studies identified an 8-bp deletion in exon 2 of the Rank gene. The resulting allele, termed Rank(del8), encodes only a small portion of the RANK extracellular domain, which is probably nonfunctional.

The phenotypic similarities between Rank(del8) and mice previously described with a combined insertion and deletion in Rank confirm the role of this receptor in osteoclastogenesis and lymph node development and suggest that some forms of malignant osteopetrosis in humans could result from a similar defect.