Two children with hypophosphatasia with a heterozygous c.1559delT variant in the gene, the most common variant in Japanese populations.

Hypophosphatasia (HPP), a genetic disorder characterized by decreased tissue-nonspecific alkaline phosphatase (TNSALP) activity, is caused by loss-of-function mutations in the gene, which encodes TNSALP. The most frequent pathogenic variant in Japanese patients with HPP is a frameshift mutation in the gene, c.1559delT, and its carrier frequency is reported to be one in 480 in the Japanese population. We report the cases of two Japanese children with HPP who had a heterozygous c.1559delT variant in the gene. One case (involving a neonate) exhibited respiratory insufficiency associated with vitamin B6 dependent convulsions, significant defective mineralization similar to the severe form of HPP, and extremely low ALP activity. Enzyme replacement therapy (ERT) using asfotase alfa promptly improved her respiratory insufficiency, bone mineralization, and maintained her motor development during infancy. The second case involved a 10-year-old boy who demonstrated diffuse musculoskeletal pain and weakness that progressively disturbed mobility. Although he showed no bony lesions, the clinical symptoms and biochemical abnormalities were compatible with childhood HPP. ERT successfully relieved the severe generalized pain and significantly improved motor function.