NORD gratefully acknowledges Michael P. Whyte, MD, Medical-Scientific Director, Center for Metabolic Bone Disease and Molecular Research, Shriners Hospital for Children; Professor of Medicine, Pediatrics, and Genetics, Division of Bone and Mineral Diseases, Washington University School of Medicine, for assistance in the preparation of this report.
HPP is an extremely variable disorder. Six major clinical forms have been identified based primarily upon the age of onset of symptoms and diagnosis. These are known as perinatal, infantile, childhood (severe or mild), adult, and odontohypophosphatasia. Generally, the severity of these different forms of HPP correlates to the residual alkaline phosphatase activity in the body, with less enzyme activity causing more severe disease. Because HPP has broad-ranging serenity, it is important to note that affected individuals may not have all of the symptoms discussed below and that every individual case is actually unique. Some children will develop severe complications early in life; others have mild disease that may improve during young adult life. Parents should talk to their child’s physician and medical team about the specific symptoms and overall prognosis.
Perinatal HPP is associated with profound inactivity of alkaline phosphatase and markedly impaired mineralization. Consequently, the skeleton fails to form properly in the womb. Specific skeletal malformations may vary, but short, bowed arms and legs and underdeveloped ribs often occur. Some pregnancies end in stillbirth. In other cases, affected newborns survive for several days, but pass away from respiratory failure due to deformities of the chest and underdeveloped lungs.
Prenatal benign HPP is associated with bowed limbs at birth. The skeletal deformities can be apparent by ultrasound studies before birth. The skeletal malformations associated with this form improve after birth, eventually resembling the signs and symptoms of individuals who range from infantile HPP to odontohypophosphatasia.
Infantile HPP may have no noticeable abnormalities at birth, but symptoms may become apparent at any time within the first six months. The initial symptom may be the failure to gain weight and grow at the expected rate for age and gender referred to as “failure to thrive.” Some affected babies later exhibit early fusion of the bones of the skull (craniosynostosis), which can result in the head appearing disproportionately wide (brachycephaly). Craniosynostosis may be associated with increased pressure of the fluid that surrounds the brain (cerebrospinal fluid), a condition known as “intracranial hypertension.” This complication can cause headaches, swelling of the optic disk (papilledema), and bulging of the eyes (proptosis). Affected infants have softened, weakened bones that lead to the skeletal malformations of rickets. Rickets is a general term from the bone disease that occurs during growth with softening of bone and characteristic bowing deformities of the legs from growth plate abnormalities. Enlarged wrist and ankle joints may occur. Affected infants may also have deformities in the chest and ribs and rib fractures, predisposing them to pneumonia. Varying degrees of pulmonary insufficiency and breathing difficulties may develop, potentially progressing to life-threatening respiratory failure. Episodes of fever and bone pain, or tender bones may occur. Some infants have diminished muscle tone (hypotonia), so that a baby appears “floppy”, attributable to elevated levels of calcium in the blood (hypercalcemia). Hypercalcemia can cause vomiting, constipation, weakness, and poor feeding. Increased excretion of calcium may lead to kidney (renal) damage. In rare cases, vitamin B6-dependent seizures may occur. Sometimes there is spontaneous improvement in mineralization during early childhood. Short stature and skeletal deformities may persist lifelong.
Childhood HPP is highly variable, and severe and mild forms should be considered, these are less severe than the infantile form. Affected children may sometimes have craniosynostosis and exhibit signs of intracranial hypertension. Skeletal malformations that resemble rickets may become apparent at 2 to 3 years of age. Bone and joint pain and fractures may occur. Typically, one or more baby teeth fall out earlier than normal. Some children are weak and experience delays in walking, and when they do learn to walk may have a distinct, waddling gait. Spontaneous remission of bone symptoms has been reported in young adult life, but such symptoms can recur during middle-age or late adulthood.
Adult HPP is characterized by a wide variety of symptoms. Affected individuals have osteomalacia, a softening of the bones in adults. Some individuals have a history of rickets during childhood, or have experienced premature loss of their baby teeth. Individuals with adult HPP can suffer fractures, especially stress fractures in the feet or pseudofractures of the thigh. Repeated fractures can result in chronic pain and debility. Fractures of the spine seem less common, but also occur. Bone pain is a common complication. Some affected adults develop joint inflammation and pain near or around certain joints due to the accumulation of calcium crystals (calcific periarthritis) or a condition called chondrocalcinosis, characterized by the accumulation of calcium crystals within the cartilage of joints, sometimes damaging the joint. Others have sudden, severe pain in a joint (pseudogout). Affected adults may experience loss of adult teeth.
Odontohypophosphatasia is characterized by the premature loss of deciduous teeth in childhood, or loss of teeth in adulthood. The dental problems are an isolated finding that does not occur along with the characteristic bone symptoms of other forms of HPP.
HPP is caused by mutations in the tissue nonspecific alkaline phosphatase (TNSALP) gene, also called the ALPL gene. This is the only gene established to be involved in HPP. Genes provide instructions for creating proteins that play a critical role in many functions of the body. When a mutation of a gene occurs, the protein product may be faulty, inefficient, or absent. Depending upon the functions of the particular protein, one or more organ systems of the body can be compromised.
For HPP, mutations in the ALPL gene can be inherited in an autosomal recessive or autosomal dominant manner. The perinatal and infantile forms of HPP are inherited in an autosomal recessive manner. The childhood form can be either autosomal recessive or autosomal dominant. The adult form and odontohypophosphatasia typically are autosomal dominant disorders, but in rare cases can be inherited as an autosomal recessive trait.
Most genetic diseases are determined by the status of the two copies of a gene, one received from the father and one from the mother. Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary to cause a particular disease. The abnormal gene can be inherited from either parent or can be the result of a new mutation (gene change) in the affected individual. The risk of passing the abnormal gene from an affected parent to an offspring is 50% for each pregnancy. The risk is the same for males and females.
Recessive genetic disorders occur when an individual inherits two copies of an abnormal gene for the same trait, one from each parent. If an individual inherits one normal gene and one gene for the disease, the person will be a carrier for the disease but usually will not show symptoms. The risk for two carrier parents to both pass the altered gene and have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents is 25%. The risk is the same for males and females.
The ALPL gene creates (encodes) an enzyme known as tissue nonspecific alkaline phosphatase or TNSALP. Enzymes are specialized proteins that break down other chemicals in the body. TNSALP is essential for the proper development and health of bones and teeth and is expressed in the liver and kidneys as well as bone. Mutations in the ALPL gene result in insufficient levels of functional TNSALP, which, in turn, leads to the accumulation of certain chemicals in the body including phosphoethanolamine, pyridoxal 5’-phosphate, and inorganic pyrophosphate. Inorganic pyrophosphate helps to regulate skeletal mineralization. Elevated levels of inorganic pyrophosphate can indirectly lead to elevated levels of calcium in the body and insufficient calcification of bone. Generally, TNSALP enzyme activity correlates with HPP severity, typically with less residual enzyme activity when there is more severe disease.
Individuals with an extremely rare form of HPP called pseudohypophosphatasia have normal blood levels of alkaline phosphatase as detected with routine clinical laboratory testing.
HPP affects males and females in equal numbers. In Canada, the severe forms of HPP are estimated to trouble approximately 1 in 100,000 live births. The overall incidence and prevalence of all forms of HPP is unknown. Milder cases can go undiagnosed or misdiagnosed, making it difficult to determine the true frequency of HPP in the general population. HPP occurs with greatest frequency in the Mennonite population in Canada, is relatively prevalent in Japan, and is relatively rare in black individuals.
A diagnosis of HPP is based upon identification of characteristic signs and symptoms, a detailed patient history, a thorough clinical evaluation, and a variety of laboratory tests including routine x-ray and biochemical studies. Proper diagnosis of HPP is easy for physicians who are familiar or experienced with this disorder. However, most physicians have little or no knowledge of HPP. Consequently, affected individuals and families may face a frustrating delay in diagnosis. Now, mutation analysis of the ALPL gene is available from commercial laboratories.
Clinical Testing and Workup
The diagnosis is rarely first suspected from a routine panel of biochemical tests that includes measuring the activity of alkaline phosphatase in blood. Instead signs and symptoms have led to this routine test where the low levels of alkaline phosphatase must be recognized. Individuals with HPP have reduced serum alkaline phosphatase activity for their age, except for the extremely rare individual with pseudohypophosphatasia who has normal activity levels. Identification of deficient alkaline phosphatase activity is consistent with HPP, but not conclusive since other conditions can result in this finding. Additionally, some individuals who are genetic carriers of HPP, but who do not develop any symptoms of the disorder, may also have low blood ALP levels.
Importantly, the range of serum ALP activity varies by age. Healthy children normally have higher ALP levels than healthy adults. If the laboratory doing the testing only gives the normal range of ALP activity in adults in its report, a diagnosis of HPP in a child can be missed because the child’s ALP activity will mistakenly be believed to be normal.
In the U.S. and elsewhere, a suspected diagnosis of HPP can be further supported by measuring the serum level of vitamin B6. This test is performed by several commercial laboratories. Individuals with HPP have elevated levels of pyridoxal 5’-phosphate (PLP: the active form of vitamin B6) in the blood because PLP is normally broken down by TNSALP. PLP is elevated even in individuals with mild HPP. However, some genetic carriers of HPP who do not develop any symptoms can have an elevated PLP level as well. In the past, blood or urine was tested for increased amounts of phosphoethanolamine (PEA), another chemical normally broken down by TNSALP. However, this finding is not specific to HPP and can occur because of other metabolic bone diseases. Additionally, some individuals with HPP have normal PEA levels. Screening for elevated PLP is preferred over screening for PEA because it is more sensitive, more precise, and less expensive.
In the most severe cases of HPP, specifically the perinatal and infantile forms, x-ray studies can reveal diagnostic changes within the bones. However, these changes may not be recognized as being associated with HPP, except by radiologists familiar with the disorder.
Molecular genetic testing can support a diagnosis of HPP. Molecular genetic testing can detect mutations in the ALPL gene known to cause the disorder, but it is only available as a diagnostic service at specialized laboratories. The test is often expensive and often not necessary to confirm a diagnosis of HPP.
In 2015, the U.S. Food and Drug Administration (FDA) approved Strensiq (asfotase alfa) as the first approved medical treatment for perinatal, infantile, and juvenile-onset HPP. Patients of all ages with pediatric onset HPP are eligible for treatment with this bone-targeted form of TNSALP replacement therapy given by subcutaneous injection.
Other treatment of HPP is directed toward the specific symptoms and complications that may differ from individual to individual. Treatment may require the coordinated efforts of a team of specialists. Pediatricians, orthopedic surgeons, dental specialists (e.g. pediatric dentist), pain management specialist, and other healthcare professionals may need to systematically and comprehensively plan treatment.
Non-steroidal anti-inflammatory drugs (NSAIDs) may be given to treat bone and joint pain. NSAIDs require caution and monitoring when used because they can cause side effects (e.g. they can hurt the stomach and kidneys), especially when given in excess doses and for prolonged amounts of time. If craniosynostosis causes intracranial pressure, surgery may be necessary to relieve pressure.
Vitamin B6 can help to control specific seizures in severely affected babies. Severely affected infants that develop elevated levels of calcium (hypercalcemia) in the blood may be treated with dietary calcium restriction, hydration, certain diuretics, and perhaps calcitonin, but hypercalcemia is often difficult to manage as it occurs in severely affected patients.
Regular dental care beginning early on is recommended. Physical and occupational therapy may be recommended in some cases.
Adults with repeated long bone fractures may be treated by an orthopedic internal fixation, which is also known as “rodding.” During this procedure, an orthopedic surgeon places a metal rod through the center opening of a bone to make it more stable and stronger. Special medical devices designed for the feet (foot orthotics) may be used by adults to help manage foot (tarsal) fractures.
Affected individuals should avoid bisphosphonates, a class of drugs used to treat other bone disorders such as osteoporosis. These medications may worsen HPP or cause symptoms in individuals with undiagnosed HPP. Examples of bisphosphonate drugs include alendronate, ibandronate, pamidronate, risedronate, and zolendronate.
Genetic counseling may be of benefit for affected individuals and their families. With children with HPP, psychosocial support for the entire family may be helpful as well.
Bone marrow transplantation, specifically hematopoietic stem cell transplantation, was used to treat two unrelated infant girls with life-threatening HPP. They improved. One was also treated with bone fragments and cultured osteoblasts, which are bone-forming cells. ‘Cultured cells’ refers to cells that are grown under specific conditions outside of their natural environment (the body) and instead within a laboratory. According to the medical literature, both of these patients demonstrated significant sustained, but incomplete improvement, although no more formal studies have been conducted on these procedures.
The drug teriparatide (parathyroid hormone 1-34) has been given “off-label” to several adults with HPP with metatarsal stress fractures or femoral pseudofractures, resulting in healing of the fractures. The drug is not permitted for use in children. More research is necessary to determine the long-term safety and effectiveness of teriparatide in the treatment of HPP.
Preliminary short-term results have also been reported from the use for HPP of an anti-sclerostin antibody. Sclerostin is a protein found in the star-shaped bone cells known as osteocytes. Sclerostin helps to reduce or suppress (downregulate) bone-forming cells called osteoblasts. Antibodies that act against sclerostin have been shown to increase bone mass in osteoporosis.
For additional information regarding HPP, contact:
Michael P. Whyte, MD
Center for Metabolic Bone Disease and Molecular Research,
Shriners Hospital for Children;
4400 Clayton Ave
St. Louis, MO 63110
Email: [email protected]
The Center for Metabolic Bone Disease and Molecular Research at Shriners Hospital for Children in St. Louis, Missouri, is a unique research center that diagnoses, treats, and investigates more than 100 rare bone diseases. The Center is known worldwide for its expertise in several rare diseases including HPP. The research team has led clinical trials for the new treatment for HPP (asfotase alfa), and they follow more pediatric and adult patients with HPP than elsewhere worldwide. The Center serves as a global resource for patients and physicians seeking information about rare, genetic bone diseases such as HPP. For more information, contact:
Center for Metabolic Bone Disease and Molecular Research
Shriners Hospital for Children
4400 Clayton Ave
St. Louis, MO 63110
Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. government funding, and some supported by private industry, are posted on this government web site.
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