Hypophosphatasia - pathophysiology and treatment.

Abstract

Hypophosphatasia (HPP) is the inborn-error-of-metabolism caused by loss-of-function mutation(s) in the gene that encodes the tissue-nonspecific isozyme of alkaline phosphatase (TNAP). The disease has been classified according to patient age when the first signs and symptoms manifest; i.e., perinatal, infantile, childhood, adult HPP. Other types include odonto HPP and perinatal benign. Babies with the perinatal/infantile forms of HPP often die with severe rickets and respiratory insufficiency and sometimes hypercalcemia and vitamin B₆-responsive seizures. The primary biochemical defect in HPP is a deficiency of TNAP activity that leads to elevated circulating levels of substrates, in particular inorganic pyrophosphate (PP_i), a potent calcification inhibitor. To-date, the management of HPP has been essentially symptomatic or orthopedic. However, enzyme replacement therapy with mineral-targeting TNAP (sALP-FcD₁₀, also known as ENB-0040 or asfotase alfa) has shown promising results in a mouse model of HPP (Alpl^-/- mice). Administration of mineral-targeting TNAP from birth increased survival and prevented the seizures, rickets, as well as all the tooth abnormalities, including dentin, acellular cementum, and enamel defects in this model of severe HPP. Clinical trials using mineral-targeting TNAP in children 3 years of age or younger with life-threatening HPP was associated with healing of the skeletal manifestations of HPP as well as improved respiratory and motor function. Improvement is still being observed in the patients receiving continued asfotase alfa therapy, with more than 3 years of treatment in some children. Enzyme replacement therapy with asfotase alfa has to-date been successful in patients with life-threatening HPP.