Mutation in Npps in a mouse model of ossification of the posterior longitudinal ligament of the spine


Ossification of the posterior longitudinal ligament of the spine (OPLL) is a common form of human myelopathy caused by a compression of the spinal cord by ectopic ossification of spinal ligaments1,2. To elucidate the genetic basis for OPLL, we have been studying the ttw (tiptoe walking; previously designated twy) mouse, a naturally occurring mutant which exhibits ossification of the spinal ligaments very similar to human OPLL (Refs 3,4). Using a positional candidate-gene approach, we determined the ttw phenotype is caused by a nonsense mutation (glycine 568 to stop) in the Npps gene which encodes nucleotide pyrophosphatase. This enzyme regulates soft-tissue calcification and bone mineralization by producing inorganic pyrophosphate, a major inhibitor of calcification5,6,7,8. The accelerated bone formation characteristic of ttw mice is likely to result from dysfunction of NPPS caused by predicted truncation of the gene product, resulting in the loss of more than one-third of the native protein. Our results may lead to novel insights into the mechanism of ectopic ossification and the aetiology of human OPLL.

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Figure 1: Phenotypic similarity between OPLL and the ttw mouse.
Figure 2: Linkage map and physical contig of YAC, BAC and cosmid clones at the ttw locus on mouse chromosome 10.
Figure 3: The ttw mutation. a, Sequence analysis of the Npps cDNA.
Figure 4: Schematic structure of mouse nucleotide pyrophosphatase (Npps).

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We thank K. Hioki, Y. Endo and T. Shiroishi for preparing the animal crosses, and T. Tanaka for helpful discussion. This work was supported in part by grants from the Ministry of Health and Welfare and from the Ministry of Education, Culture, Sports and Science of Japan.

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Correspondence to Yusuke Nakamura.

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Okawa, A., Nakamura, I., Goto, S. et al. Mutation in Npps in a mouse model of ossification of the posterior longitudinal ligament of the spine. Nat Genet 19, 271–273 (1998).

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