Abstract
Hypophosphatasia (HOPS) is a heritable disorder
characterized by defective skeletal mineralization,
deficiency of tissue-nonspecific alkaline phosphatase
(TNSALP) activity and premature loss of
deciduous teeth. In a previous study, we detected
missense mutations in the TNSALP gene of a
patient who inherited the F310L and the V365I
mutation with severe periodontitis and childhood
HOPS. Expression of the mutant V365I TNSALP
gene using COS-1 cells demonstrated that the
protein translated from the mutant had undetectable
ALP activity. In the present study, we
characterized another ALP enzyme translated
from the mutant F310L and compared it with the
ALP in the patient’s serum. The COS-1 cells transfected
with the F310L and co-transfected with
F310L and V365I (F310L/V365I) exhibited levels of
67% and 31%, respectively, with the enzymatic
activity of the wild-type taken as 100%. In the thermostability
test, TNSALPs in the COS-1 cells
transfected with the mutant F310L or F310L/V365I
were significantly more heat labile compared with
that of the wild-type. Moreover, ALP from the
patient’s serum was also more heat labile than normal
ALP. These results suggest that the protein
translated from the mutant F310L, in addition to the
mutant V365I, may be responsible for the expression
of symptoms of the childhood-type HOPS.
