PHOSPHO1 and Arthritis: However, despite KO mice for TNAP (Tissue-nonspecific Alkaline Phosphatase, (Akp2−/−)), Phospho1 (phosphoethanolamine/phosphocholine phosphatase 1, (Phospho1−/−)) and osteopontin (OPN) are characterized by high circulating levels of ePPi, they did not show signs of arthritis or a phenotype compatible with CPPD (8–10).