What is Hypophosphatasia?
Hypophosphatasia (HPP) is a rare inherited metabolic bone disorder caused by loss-of-function mutations in the ALPL gene, resulting in deficient tissue-nonspecific alkaline phosphatase (TNSALP) enzyme activity, which leads to impaired skeletal and dental mineralization due to accumulation of pyrophosphate, a potent inhibitor of calcification 1, 2.
Genetic Basis and Pathophysiology
HPP results from pathogenic variants in the ALPL gene that encodes TNSALP, an enzyme essential for physiological bone and tooth mineralization 1, 3. The inheritance pattern varies:
- Severe forms (perinatal and infantile): autosomal recessive
- Moderate forms: can be either autosomal recessive or autosomal dominant
- Mild forms: typically autosomal dominant 3
The enzyme deficiency causes extracellular pyrophosphate (PPi) to accumulate, which powerfully inhibits calcium phosphate crystal formation and prevents proper mineralization of bones and teeth 2.
Clinical Spectrum
The disease presentation is remarkably heterogeneous, ranging from life-threatening to mild forms 1, 3:
Severe Forms
- Perinatal/infantile HPP: Life-threatening with profound skeletal hypomineralization, respiratory impairment, seizures (from pyridoxine-dependent seizures), developmental delay, and historically high mortality 4, 3
Moderate to Mild Forms
- Childhood HPP: Rickets-like bone deformities, short stature, bone pain, premature loss of deciduous teeth, delayed walking
- Adult HPP: Recurrent fractures, pseudofractures, osteomalacia, osteopenia, musculoskeletal pain, arthropathy, chondrocalcinosis, enthesopathies 5, 6
- Odontohypophosphatasia: Affects only dentition with premature tooth loss, dental abscesses, and periodontitis 3
Diagnostic Criteria
Biochemical Hallmark
The diagnostic cornerstone is persistently low serum alkaline phosphatase (ALP) activity for age and sex 1, 5, 3. This must be interpreted with age-appropriate reference ranges, as normal ALP levels vary significantly across life stages.
Supporting Laboratory Findings
- Elevated pyridoxal 5'-phosphate (PLP, vitamin B6)
- Elevated phosphoethanolamine (PEA) in urine and plasma
- These are natural substrates of TNSALP that accumulate when the enzyme is deficient 3, 7
Diagnostic Criteria for Adults
Based on the HPP International Working Group analysis of 119 studies, diagnosis requires either two major criteria OR one major criterion plus two minor criteria 5:
Major Criteria:
- Low serum ALP activity (for age/sex)
- Elevated PLP
- Radiographic evidence of osteomalacia/rickets
- Genetic confirmation of pathogenic ALPL variant(s)
Minor Criteria:
- Recurrent fractures or pseudofractures
- Dental manifestations (premature tooth loss)
- Arthropathy/chondrocalcinosis
- Muscle weakness
- Bone/joint pain 5
Genetic Confirmation
DNA sequencing of ALPL identifies disease-causing pathogenic variants. The most frequent variants globally are c.1250A>G, c.571G>A, and c.1133A>T 1. Genetic testing confirms the diagnosis and enables family screening.
Treatment
Enzyme Replacement Therapy
Asfotase alfa is a fully humanized, bone-targeted recombinant TNSALP enzyme replacement therapy that has transformed outcomes for severe HPP 4, 7. It has been approved multinationally since 2015 for pediatric-onset HPP, particularly perinatal and infantile forms 2, 4.
The therapy:
- Remarkably increases lifespan in severe forms
- Improves skeletal mineralization
- Enhances respiratory function
- Improves quality of life 2, 4
Supportive Management
- Calcium restriction in infants/children with hypercalcemia 3
- Multidisciplinary care including orthopedics, dentistry, pain management, and physical therapy 3, 7
- Management of complications: fractures, dental abscesses, pain, mobility issues
Clinical Pitfalls
HPP in adults is frequently underdiagnosed or misdiagnosed because symptoms are often mild and non-specific 5. Clinicians unfamiliar with this rare disorder may attribute fractures to osteoporosis, joint pain to arthritis, or dental problems to poor hygiene without recognizing the underlying metabolic defect. Any patient with unexplained low ALP, recurrent fractures, premature tooth loss, or pseudofractures should be evaluated for HPP 5, 6.
The absence or delay in diagnosis prevents optimal management and access to enzyme replacement therapy when indicated 5, 7.