Pathophysiology of hypophosphatasia and the potential role of asfotase alfa

Abstract

Hypophosphatasia (HPP) is an inherited systemic bone disease that is characterized by bone hypomineralization. HPP is classified into six forms according to the age of onset and severity as perinatal (lethal), perinatal benign, infantile, childhood, adult, and odontohypophosphatasia. The causative gene of the disease is the ALPL gene that encodes tissue-nonspecific alkaline phosphatase (TNAP). TNAP is expressed ubiquitously, and its physiological role is apparent in bone mineralization. A defect in bone mineralization can manifest in several ways, including rickets or osteomalacia in HPP patients. Patients with severe forms suffer from respiratory failure because of hypoplastic chest, which is the main cause of death. They sometimes present with seizures due to a defect in vitamin B6 metabolism resulting from the lack of alkaline phosphatase activity in neuronal cells, which is also lethal. Patients with a mild form of the disease exhibit rickets or osteomalacia and a functional defect of exercise. Odontohypophosphatasia shows only dental manifestations. To date, 302 mutations in the ALPL gene have been reported, mainly single-nucleotide substitutions, and the relationships between phenotype and genotype have been partially elucidated. An established treatment for HPP was not available until the recent development of enzyme replacement therapy. The first successful enzyme replacement therapy in model mice using a modified human TNAP protein (asfotase alfa) was reported in 2008, and subsequently success in patients with severe form of the disease was reported in 2012. In 2015, asfotase alfa was approved in Japan in July, followed by in the EU and Canada in August, and then by the US Food and Drug Administration in the USA in October. It is expected that therapy with asfotase alfa will drastically change treatments and prognosis of HPP.

Keywords: alkaline phosphatase; asfotase alfa; hypophosphatasia; mutation; respiratory failure; rickets.

Figure 1

Structure and mutation sites of TNAP. Notes: The 3D structure of human TNAP is obtained using a simulation model based on human PLAP. Blue and green ribbons show each monomer. Mutations located in the active site and its vicinity, the homodimer interface, the crown domain, and the calcium-binding domain result in severe phenotypes, whereas mutations in the active site valley result in less severe phenotype. The simulation model was based on human PLAP and was provided by Dr T Matsumura, Nippon Medical School. Abbreviations: 3D, three-dimensional; PLAP, placental alkaline phosphatase; TNAP, tissue-nonspecific alkaline phosphatase.

Figure 2

Mineralization in and surrounding a matrix vesicle. Notes: In a matrix vesicle, inorganic phosphate provided by hydrolysis of phospholipid and by the sodium–phosphate cotransporter, together with calcium provided by annexin Ca²⁺ channels, forms hydroxyapatite. Hydroxyapatite then penetrates through the matrix vesicle membrane and elongates using extracellular Pi and Ca²⁺. Extracellular PPi inhibits hydroxyapatite formation. Nucleotide pyrophosphate phosphodiesterase 1 provides PPi, and tissue-nonspecific alkaline phosphatase hydrolyzes PPi to yield Pi. Extracellular PPi is also provided by ANKH, a plasma membrane PPi transporter. Mineralization is regulated by the balance of these three molecules, NPP1, ANK(H), and TNAP. Abbreviations: NPP1, nucleotide pyrophosphate phosphodiesterase 1; Pi, inorganic phosphate; PPi, inorganic pyrophosphate; TNAP, tissue-nonspecific alkaline phosphatase; PCho, phosphocholine; PEA, phosphoethanolamine; Na/Pi transporter, sodium-phosphate transporter; PLC, phospholipase C.

Figure 3

Structure of asfotase alfa. Notes: Asfotase alfa is composed of a soluble form of tissue-nonspecific alkaline phosphatase, the Fc region of human IgG, and deca-aspartate. The glycosylphosphatidylinositol anchor of TNAP is removed to make the soluble form, which is fused with the IgG Fc region but maintains enzymatic activity. The IgG Fc domain is added for rapid purification of the protein, while D10 is added because it binds to hydroxyapatite with high affinity. TNAP is bioengineered as a homodimer with the presence of two disulfide bridges in the hinge domain of two monomeric Fc regions. Abbreviations: D10, deca-aspartate; TNAP, tissue-nonspecific alkaline phosphatase.