Functional Evaluation of Mutations in the Tissue-Nonspecific Alkaline Phosphatase Gene

Objective: To explore the relationship between the genotypes and phenotypes of these hypophosphatasia patients caused by gene mutations of tissue-nonspecific alkaline phosphatase (TNSALP). Methods: Based on the genotypes of these patients, site-directed mutations of TNSALP cDNA at c.1162T>C or c.1120G>A or c.668G>A or c.535G>A were performed in the expression plasmids, respectively. The plasmids were transfected into U2OS cells and the alkaline phosphatase activity of the cells were measured. Transfected U2OS cells were induced to mineral formation, and mineralisation assay were performed by Alizarin Red staining. Results: The cells transfected with mutated TNSALP (c.1162T>C, c.1120G>A, c.668G>A, and c.535G>A) showed 39.7%, 57.6%, 2.9%, and 10.9% of alkaline phosphatase activity and 48.5%, 74.4%, 10.4%, and 16.7% mineralisation ability compared to those cells transfected with the wild-type TNSALP. Conclusion: Our results suggested that the new mutation c.1162T>C would moderately decrease the function of TNSALP while the mutations c.1120G>A and c.668G>A would mildly and severely decrease the function of TNSALP, respectively. Keywords: tissue-nonspecific alkaline phosphatase, mineralisation, mutation, hypophosphatasia