Association of the human NPPS gene with ossification of the posterior longitudinal ligament of the spine (OPLL)

OPLL (ossification of the posterior longitudinal ligament of the spine) is a common form of human myelopathy with a prevalence of as much as 4% in a v ariety of ethnic groups. To clarify the genetic factors that predispose to OPLL, we have studied ttw, (tiptoe walking), a mouse model that presents ec topic ossification of the spinal ligaments similar to OPLL and have found t hat the ttw phenotype is caused by the nonsense mutation of the gene encodi ng nucleotide pyrophosphatase (NPPS), a membrane-bound glycoprotein thought to produce inorganic pyrophosphate, a major inhibitor of calcification and mineralization, To investigate a possible role of NPPS in the etiology of OPLL, we have examined its genetic variations in OPLL patients. A total of 323 OPLL patients was screened by means of polymerase chain reaction/single -strand conformation polymorphism analysis covering all the exons and their surrounding introns, plus about 1.5-kb of the promoter region. We identifi ed ten nucleotide variations in the NPPS gene; five of the alterations caus ed amino-acid substitutions, and two of them were found specifically in OPL L patients. Subsequently, we performed an association study using these var iations and found a significant association of an allele, viz., a deletion of T at a position 11 nucleotides upstream from the splice: acceptor site o f intron 20 (IVS20-11delT), with OPLL, the proportion of the individuals ha ving this deletion was significantly higher (P = 0.0029) in OPLL patients t han in controls, indicating that those who have this variation may be more susceptible to the abnormal ossification of the spinal ligaments. Thus, our study suggests that NPPS plays an important role in the etiology of human OPLL.