Potential pathological role of single nucleotide polymorphism (c.787TC) in alkaline phosphatase (ALPL) for the phenotypes of hypophosphatasia

Hypophosphatasia (HPP; OMIM 241510, 241500, and 146300) is an inherited metabolic disease characterized by defects of bone and tooth mineralization, which is caused by loss-of-function mutations in the ALPL gene encoding tissue non-specific alkaline phosphatase (TNSALP). In the last three decades, several studies have focused on the genotype-phenotype correlation in hypophosphatasia (HPP). In particular, functional tests based on in vitro analysis for the residual enzymatic activities of mutations have revealed a clear but imperfect genotype-phenotype correlation, suggesting that multiple potential factors modulate the phenotype. One of the missense variants identified in the tissue non-specific alkaline phosphatase (ALPL) gene, c.787TC, has been considered as a benign polymorphism in HPP; however, its pathogenicity and role in disease manifestation remain controversial. We here report our recent experience of three unrelated families harboring the c.787TC variant, suggesting clinical implications regarding the controversial pathogenicity of c.787TC. First, despite the lack of obvious clinical phenotypes, homozygous c.787TC would decrease the serum level of ALP activity. Second, c.787TC might deteriorate phenotypes of a patient harboring another ALPL variant, especially one that has thus far presumed to be benign, e.g., the c.1144GA variant. These cases contribute to the recent advances in understanding HPP to facilitate clinical recognition of more subtle phenotypes, further providing insights into the pathogenesis of HPP.