Loss of DMP1 causes rickets and osteomalacia and identifies a role for osteocytes in mineral metabolism

The osteocyte, a terminally differentiated cell comprising 90%-95% of all bone cells(1,2), may have multiple functions, including acting as a mechanosensor in bone (re) modeling(3). Dentin matrix protein 1 (encoded by DMP1) is highly expressed in osteocytes(4) and, when deleted in mice, results in a hypomineralized bone phenotype(5). We investigated the potential for this gene not only to direct skeletal mineralization but also to regulate phosphate (Pi) homeostasis. Both Dmp1-null mice and individuals with a newly identified disorder, autosomal recessive hypophosphatemic rickets, manifest rickets and osteomalacia with isolated renal phosphate-wasting associated with elevated fibroblast growth factor 23 (FGF23) levels and normocalciuria. Mutational analyses showed that autosomal recessive hypophosphatemic rickets family carried a mutation affecting the DMP1 start codon, and a second family carried a 7-bp deletion disrupting the highly conserved DMP1 C terminus. Mechanistic studies using Dmp1-null mice demonstrated that absence of DMP1 results in defective osteocyte maturation and increased FGF23 expression, leading to pathological changes in bone mineralization. Our findings suggest a bone-renal axis that is central to guiding proper mineral metabolism.