Functional analysis of the single nucleotide polymorphism (787T>C) in the tissue-nonspecific alkaline phosphatase gene associated with BMD.
Polymorphisms of the TNSALP gene have not previously been studied as a possible determinant for variations in BMD or as a predisposing genetic factor for osteoporosis. This study showed a significantly higher association between the 787T>C (Tyr246His) TNSALP gene and BMD among 501 postmenopausal women. Furthermore, the effects of amino acid substitution on the catalytic property of the protein translated from the 787T>C gene were examined.


BACKGROUND
Alkaline phosphatase (ALP) is present mainly on the cell membrane in various tissues and hydrolyzes a variety of monophosphate esters into inorganic phosphoric acid and alcohol. Human ALPs are classified into four types: tissue-nonspecific, intestinal, placental, and germ cell types. Based on studies of hypophosphatasia, which is a systemic skeletal disorder resulting from a tissue-nonspecific ALP (TNSALP) deficiency, TNSALP was suggested to be indispensable for bone mineralization.


METHODS
We explored the possibility that the TNSALP gene may contribute to age-related bone loss in humans by examining the association between TNSALP gene polymorphisms and BMD in 501 Japanese postmenopausal women. To analyze the protein translated from the TNSALP gene associated with BMD, we constructed a TNSALP cDNA expression plasmid.


RESULTS
We genotyped two single nucleotide polymorphisms (787T>C[Tyr246His] and 876A>G[Pro275Pro]), which proved to be in complete linkage disequilibrium. There was a significant difference in BMD and the BMD score adjusted for age and body weight (Z score) among haplotypes (p = 0.041), which was lowest among 787T/876A homozygotes, highest among 787T>C/876A>G homozygotes, and intermediate among heterozygotes. In subgroups divided by age, haplotypes were significantly associated with BMD in older postmenopausal women (>74 years; p = 0.001), but not in younger postmenopausal women (<74 years; p = 0.964). Expression of the 787T>C TNSALP gene using COS-1 cells showed that the protein translated from 787T>C had ALP-specific activity similar to that of 787T. Interestingly, the K(m) value for TNSALP in cells transfected with the 787T>C TNSALP gene was decreased significantly compared with that of cells bearing the 787T gene, reflecting the higher affinity.


CONCLUSIONS
These results suggest that variation in TNSALP may be an important determinant of age-related bone loss in humans and that the phosphate metabolism pathway may provide a novel target for the prevention and treatment of osteoporosis.
Goseki-Sone
Masae
M
Department of Food and Nutrition, Japan Women's University, Tokyo, Japan. goseki@fc.jwu.ac.jp
Sogabe
Natsuko
N
Fukushi-Irie
Mariko
M
Mizoi
Lena
L
Orimo
Hideo
H
Suzuki
Takao
T
Nakamura
Hiroyuki
H
Orimo
Hajime
H
Hosoi
Takayuki
T
D000368:Aged; D000369:Aged, 80 and over; D000469:Alkaline Phosphatase / Q000235:genetics* / Q000378:metabolism; D000818:Animals; D015153:Blotting, Western; D015519:Bone Density; D019556:COS Cells; D002384:Catalysis; D002460:Cell Line; D045744:Cell Line, Tumor; D018076:DNA, Complementary / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D020022:Genetic Predisposition to Disease; D005838:Genotype; D006239:Haplotypes; D006720:Homozygote; D006801:Humans; D006868:Hydrolysis; D007700:Kinetics; D015810:Linkage Disequilibrium; D008958:Models, Molecular; D010024:Osteoporosis; D010957:Plasmids / Q000378:metabolism; D011110:Polymorphism, Genetic; D020641:Polymorphism, Single Nucleotide; D017698:Postmenopause; D014176:Protein Biosynthesis; D011487:Protein Conformation; D014162:Transfection
D018076:DNA, Complementary; D000469:Alkaline Phosphatase