Sustained Osteomalacia of Long Bones Despite Major Improvement in Other Hypophosphatasia-Related Mineral Deficits in Tissue Nonspecific Alkaline Phosphatase/Nucleotide Pyrophosphatase Phosphodiesterase 1 Double-Deficient Mice

We have shown previously that the hypomineralization defects of the calvarium and vertebrae of tissue nonspecific alkaline phosphatase (TNAP)-deficient (Akp2−/−) hypophosphatasia mice are rescued by simultaneous deletion of the Enpp1 gene, which encodes nucleotide pyrophosphatase phosphodiesterase 1 (NPP1). Conversely, the hyperossification in the vertebral apophyses typical of Enpp1−/− mice is corrected in [Akp2−/−; Enpp1−/−] double-knockout mice. Here we have examined the appendicular skeletons of Akp2−/−, Enpp1−/−, and [Akp2−/−; Enpp1−/−] mice to ascertain the degree of rescue afforded at these skeletal sites. Alizarin red and Alcian blue whole mount analysis of the skeletons from wild-type, Akp2−/−, and [Akp2−/−; Enpp1−/−] mice revealed that although calvarium and vertebrae of double-knockout mice were normalized with respect to mineral deposition, the femur and tibia were not. Using several different methodologies, we found reduced mineralization not only in Akp2−/− but also in Enpp1−/− and [Akp2−/−; Enpp1−/−] femurs and tibias. Analysis of calvarial- and bone marrow-derived osteoblasts for mineralized nodule formation in vitro showed increased mineral deposition by Enpp1−/− calvarial osteoblasts but decreased mineral deposition by Enpp1−/− long bone marrow-derived osteoblasts in comparison to wild-type cells. Thus, the osteomalacia of Akp2−/− mice and the hypomineralized phenotype of the long bones of Enpp1−/− mice are not rescued by simultaneous deletion of TNAP and NPP1 functions.