The spontaneous mouse grey-lethal (gl) mutation is responsible for a coat color defect and for the development of the most severe autosomal recessive form of osteopetrosis. Using a positional cloning approach, we have mapped and isolated the gl locus from a approximately 1.5 cM genetic interval. The gl locus was identified in a bacterial artificial chromosome (BAC) contig by functional genetic complementation in transgenic mice. Genomic sequence analysis revealed that the gl mutation is a deletion resulting in complete loss of function. The unique approximately 3 kb wild-type transcript is expressed primarily in osteoclasts and melanocytes as well as in brain, kidney, thymus and spleen. The gl gene is predicted to encode a 338-amino acid type I transmembrane protein that localizes to the intracellular compartment. Mutation in the human GL gene leads to severe recessive osteopetrosis. Our studies show that mouse Gl protein function is absolutely required for osteoclast and melanocyte maturation and function.

Summary Osteoporosis is a multifactorial genetic disease characterized by reduction of bone mass due to dysregulation of osteoclast differentiation or maturation. Herein, we identified a novel regulator of osteoclastogenesis, the murine homologue of inositol polyphosphate 4-phosphatase type IIa (Inpp4bα). Expression of Inpp4bα is detected from early osteoclast differentiation to activation stage. Targeted expression of native Inpp4bα ex-vivo repressed whereas phosphatase-inactive Inpp4ba stimulated osteoclast differentiation. Inpp4bα acts on intracellular calcium level that modulates NFATc1 nuclear translocation and activation. In vivo mice deficient in Inpp4b displayed increased osteoclast differentiation rate and potential resulting in decreased bone mass and osteoporosis. Importantly, INPP4B in human was identified as a susceptibility locus for osteoporosis. This study defined Inpp4b as a major modulator of the osteoclast differentiation and as a gene linked to variability of bone mineral density in mice and humans.