Hypophosphatasia

Hypophosphatasia (HPP) is an inherited metabolic disease caused by loss-of-function mutations in the tissue non-specific alkaline phosphatase (TNAP) gene. Reduced activity of TNAP leads to the accumulation of its substrates, mainly inorganic pyrophosphate and pyridoxal-5΄-phosphate, metabolic aberrations that largely explain the musculoskeletal and systemic features of the disease. More than 400 ALPL mutations, mostly missense, are reported to date, transmitted by either autosomal dominant or recessive mode. Severe disease is rare, with incidence ranging from 1:100,000 to 1:300,000 live births, while the estimated prevalence of the less severe adult form is estimated to be between 1:3100 to 1:508, in different countries in Europe. Presentation largely varies, ranging from death in utero to asymptomatic adults. In infants and children, clinical features include skeletal, respiratory and neurologic complications, while recurrent, poorly healing fractures, muscle weakness and arthropathy are common in adults. Persistently low serum alkaline phosphatase is the cardinal biochemical feature of the disease. Management requires a dedicated multidisciplinary team. In mild cases, treatment is usually symptomatic. Severe cases, with life-threating or debilitating complications, can be successfully treated with enzyme replacement therapy with asfotase alfa.

Altered Thyroid Function Tests Observed in Hypophosphatasia Patients Treated with Asfotase Alfa

Background. Asfotase alfa is the only approved treatment that can normalize mineralization in patients with hypophosphatasia (HPP). Its interference in alkaline phosphatase (ALP) dependent immunoassays has been reported. Objective. To describe thyroid function tests interfered with by asfotase alfa and elucidate the underlying mechanism. Patients and Methods. Three patients with HPP treated with asfotase alfa were included. Thyroid hormone levels measured using five different immunoassays with or without ALP as a labeling enzyme during asfotase alfa treatment were evaluated. Results. After the initiation of asfotase alfa, three HPP patients showed low free triiodothyronine (FT3) and free thyroxine (FT4) measured with AIA-2000 (Tosoh, Tokyo, Japan), an enzyme immunoassay system that uses ALP as a labeling enzyme, but their thyroid-stimulating hormone (TSH) levels were within the normal range. The other CLEIA system using ALP as a label, AIA-CL2400 (Tosoh, Tokyo, Japan), and ALP-independent immunoassay systems demonstrated normal FT3 and FT4 levels. These data suggested that although the thyroid function of these three patients was normal, asfotase alfa interfered with the thyroid hormone measurements made with AIA-2000. AIA-2000 and AIA-CL2400 adopted one-step and delayed one-step measurements, respectively, and the same antibody was used for both immunoassays. However, asfotase alfa may be absorbed on the magnetic beads used in the AIA reagent with the AIA-2000 system but not absorbed on the microparticles used in AIA-CL2400. Conclusion. Clinicians should be aware of the possible interference in thyroid function measurements by adopting specific types of immunoassays in asfotase alfa-treated HPP patients.

Bilateral atypical femoral fractures during denosumab therapy in a patient with adult-onset hypophosphatasia

Summary Hypophosphatasia (HPP) is a rare and under-recognised genetic defect in bone mineralisation. Patients presenting with fragility fractures may be mistakenly diagnosed as having osteoporosis and prescribed antiresorptive therapy, a treatment which may increase fracture risk. Adult-onset HPPhypophosphatasia was identified in a 40-year-old woman who presented with bilateral atypical femoral fractures after 4 years of denosumab therapy. A low serum alkaline phosphatase (ALP) and increased serum vitamin B6 level signalled the diagnosis, which was later confirmed by identification of two recessive mutations of the ALPL gene. The patient was treated with teriparatide given the unavailability of ALP enzyme-replacement therapy (asfotase alfa). Fracture healing occurred, but impaired mobility persisted. HPP predisposes to atypical femoral fracture (AFF) during antiresorptive therapy; hence, bisphosphonates and denosumab are contraindicated in this condition. Screening patients with fracture or ‘osteoporosis’ to identify a low ALP level is recommended. Learning points Hypophosphatasia (HPP) is a rare and under-recognised cause of bone fragility produced by impaired matrix mineralisation that can be misdiagnosed as a fragility fracture due to age-related bone loss. Antiresorptive therapy is contraindicated in HPP. Low serum alkaline phosphatase (ALP) provides a clue to the diagnosis. Elevated serum vitamin B6 (an ALP substrate) is indicative of HPP, while identification of a mutation in the ALPL gene is confirmatory. Enzyme therapy with recombinant ALP (asfotase alfa) is currently prohibitively costly. Treatment with anabolic bone agents such as teriparatide has been reported, but whether normally mineralized bone is formed requires further study.

Hypophosphatasia Misdiagnosed as Osteoporosis in a Young Girl

Introduction: Hypophosphatasia (HPP) is a rare inherited disease of mineral metabolism characterized by low activity of the tissue-nonspecific isoenzyme of alkaline phosphatase (TNALP), which causes an inability to liberate inorganic phosphate for hydroxyapatite crystal propagation as well as toxic accumulation of inorganic pyrophosphate, pyridoxal-5’-phosphate and urinary phosphoethanolamine. It has a prevalence of 1/100,000 to 1/900,000, although milder forms have an estimated prevalence of 1/6,370. Variable mutations in TNALP cause clinical expressions ranging from a severe perinatal form, which is often fatal after birth from pulmonary complications, to an infantile form, which can cause vitamin B6-responsive seizures, to an asymptomatic adult form. Case: A 27-year-old, ventilator-dependent female with osteoporosis, hypothyroidism, cerebral palsy with previous spinal fusion, seizure disorder and nephrocalcinosis presented with surgical site infection from a right femur ORIF she underwent a month ago. She had a history of microfractures and low-impact fractures of both femurs requiring several surgeries. Osteoporosis was diagnosed at age 5 and she had been on Fosamax ever since. She did not meet any developmental milestones as a baby and never reached menarche. Various diagnoses by multiple specialists did not fully explain her clinical presentation. Her medications included alendronate 5 mg, calcium carbonate 600 mg, ergocalciferol 400 U and levothyroxine 50 mcg. Physical exam showed poor dentition, a misshapen skull and bowed legs with contractures of her extremities. Her labs revealed Ca 9.1 mg/dL (8.4–10.2 mg/dL), albumin 3.2 gm/dL (3.5–5.2 gm/dL), phosphorus 5.1 mg/dL (2.5–4.5 mg/dL), alkaline phosphatase 32 U/L (35–105 U/L), PTH 28 pg/mL (15–65 pg/mL), vitamin D 33.5 ng/mL (30–100 ng/mL), C-telopeptide 509 pg/mL (34–635 pg/mL). A right knee X-Ray reported diffusely gracile and demineralized bones with muscular atrophy. She recently transitioned care from a pediatric endocrinologist to an adult endocrinologist, who tested her positive for heterozygous ALPL pathogenic variant hypophosphatasia and was considering her for asfotase alfa enzyme replacement therapy. Discussion: Our patient had infantile HPP, but due to misdiagnosis as osteoporosis, she was inappropriately treated with a bisphosphate for over 20 years. Treatment of HPP had been supportive until the approval of asfotase alfa (Strensiq) in October 2015. It is a bone-targeted human recombinant enzyme replacement therapy approved for infantile- and juvenile-onset HPP and has been shown to decrease mortality from 73% to 16% at age 5. With improvement in life-sustaining technology, more HPP patients are able to survive into adulthood. Awareness of the complex and polymorphic presentation of HPP by adult endocrinologists is paramount for accurate diagnosis, thus avoiding inappropriate treatments.