ALPL and osteogenesis imperfecta: It is not clear how low TNSALP activity leads to muscular complications in HPP.4 Studies in a murine model of HPP suggest that elevated inorganic pyrophosphate levels may cause muscle weakness.57 The bone deformity, fractures, and muscular weakness in HPP are clinically similar to those seen in patients with osteogenesis imperfecta (OI).