[Animal models for bone and joint disease. ttw (tiptoe walking), a model mouse of OPLL (ossification of the posterior longitudinal ligament of the spine)].

ttw (tiptoe walking), a known model mouse of OPLL (ossification of the posterior longitudinal ligament of the spine) is reviewed. ttw is a natural mutant which shows generalized ectopic calcification followed by ossification in the Achiles tendon, ear and spinal ligaments, etc. The trait is caused by a homozygous nonsense mutation in the gene for NPPS (nucleotide pyrophosphatase) , a cell-membrane enzyme that produces pyrophosphate. Its pathomechanism is the decrease of extracellular pyrophosphate due to insufficiency of NPPS. ttw is a excellent model for ectopic calcification and ossification. Several interesting genes related to ectopic calicification have been identified by studies using this mouse.