Hypophosphatasia

Hypophosphatasia (HPP) is a rare genetic disorder that affects how your bones and teeth develop and strengthen. It causes problems with bone mineralization, meaning minerals like calcium and phosphate do not properly harden your bones. This can lead to soft, weak bones that are prone to fractures and other skeletal issues, varying greatly in severity.

What is Hypophosphatasia?

Hypophosphatasia (HPP) is a rare inherited condition that impairs the mineralization of bones and teeth, making them soft and weak. This happens because the body has low levels of an enzyme called tissue-nonspecific alkaline phosphatase (TNSALP). The severity of HPP can range widely, from life-threatening forms in infants to milder forms that appear in adulthood or only affect teeth.

Your bones need minerals like calcium and phosphate to become hard and strong, a process called bone mineralization. In hypophosphatasia, a genetic defect prevents your body from making enough functional tissue-nonspecific alkaline phosphatase (TNSALP), an enzyme crucial for this process. Without enough TNSALP, certain substances, like pyrophosphate, build up. These substances then block calcium and phosphate from properly depositing into your bones. Imagine your bones as a building frame that needs concrete to harden it. In HPP, the 'concrete' (minerals) cannot properly set, leaving the frame weak and unstable. This leads to conditions like rickets in children, where bones are soft and bend, and osteomalacia in adults, where bones soften and weaken. There are several forms of HPP, categorized by when symptoms first appear and their severity. These include perinatal (before or at birth), infantile (before 6 months), childhood (after 6 months), adult, and odontohypophosphatasia (affecting only teeth). The specific type determines the range of symptoms and the overall outlook for the condition.

Symptoms

The symptoms of hypophosphatasia vary significantly depending on the age of onset and the severity of the condition. Common issues include soft, weak bones leading to fractures, bone pain, and problems with teeth. More severe forms can cause life-threatening complications, especially in infants, affecting breathing and brain function.

In the most severe forms, known as perinatal hypophosphatasia, symptoms can appear before birth or shortly after. These infants may have very soft bones, skeletal deformities, and severe breathing problems due to a soft rib cage. Sadly, this form is often fatal. Infantile hypophosphatasia typically appears before 6 months of age. Babies may have poor feeding, slow weight gain, soft spots on the skull that don't close (fontanelles), and bowed legs. They often experience frequent fractures and can develop kidney problems or seizures. Respiratory issues are a serious concern in this form. Childhood hypophosphatasia presents later, often after 6 months. Children may have short stature, bone pain, muscle weakness, and a waddling gait. A hallmark symptom is the premature loss of baby teeth, sometimes even before age 5, with the roots still intact. They are also prone to rickets and fractures. Adult hypophosphatasia usually develops in mid-adulthood and is often milder. Symptoms include bone pain, stress fractures, and softening of the bones (osteomalacia). Adults may also experience early loss of permanent teeth. Odontohypophosphatasia is the mildest form, affecting only the teeth, causing premature tooth loss without other bone problems.

Causes & risk factors

Hypophosphatasia is caused by genetic changes (mutations) in the *ALPL* gene, which provides instructions for making the alkaline phosphatase enzyme. This condition is inherited, meaning it is passed down through families. Most severe forms are inherited in an autosomal recessive pattern, while milder forms can sometimes be autosomal dominant.

The primary cause of hypophosphatasia is a mutation in the *ALPL* gene. This gene is responsible for producing tissue-nonspecific alkaline phosphatase (TNSALP), an enzyme vital for bone and tooth mineralization. When the *ALPL* gene has a mutation, it either produces a faulty enzyme or not enough of it, leading to the characteristic symptoms of HPP. Most severe forms of HPP, such as perinatal and infantile, are inherited in an autosomal recessive pattern. This means a person must inherit two copies of the mutated *ALPL* gene—one from each parent—to develop the condition. Parents who each carry one copy of the mutated gene are typically unaffected but are carriers. If two carriers have a child, there is a 1 in 4 chance (25%) that the child will inherit two mutated copies and develop HPP. Milder forms of HPP, including some cases of childhood and adult hypophosphatasia, can sometimes be inherited in an autosomal dominant pattern. In this case, inheriting just one copy of the mutated *ALPL* gene from one parent is enough to cause the condition. A parent with autosomal dominant HPP has a 1 in 2 chance (50%) of passing the condition to each child. Family history of HPP or unexplained early tooth loss can be a risk factor, as it suggests the presence of these genetic mutations within the family.

How it's diagnosed

Diagnosing hypophosphatasia typically involves a combination of blood tests, urine tests, X-rays, and genetic testing. A key indicator is unusually low levels of alkaline phosphatase (ALP) in the blood. These tests help confirm the diagnosis and determine the specific type and severity of the condition.

The diagnostic process usually begins with a physical examination and a review of your medical history, including any family history of bone or dental problems. If HPP is suspected, your doctor will likely order several tests. Blood tests are crucial. They will measure the level of alkaline phosphatase (ALP) in your blood. In people with HPP, ALP levels are typically much lower than normal. Blood tests may also show high levels of pyridoxal 5'-phosphate (PLP), a form of vitamin B6, which accumulates due to low ALP activity. Urine tests may be used to detect high levels of phosphoethanolamine (PEA), another substance that builds up when ALP is deficient. X-rays are also important for diagnosis. They can reveal characteristic signs of impaired bone mineralization, such as rickets in children or osteomalacia in adults, and show areas of bone softening or fractures. Finally, genetic testing can confirm the diagnosis by identifying mutations in the *ALPL* gene. This test is often used to confirm findings from blood and urine tests and can help determine the specific type of HPP and its inheritance pattern.

Treatment options

Treatment for hypophosphatasia focuses on managing symptoms and, for severe forms, replacing the missing enzyme. Enzyme replacement therapy (ERT) is available for pediatric-onset HPP and can significantly improve bone health and overall function. Supportive care, including pain management, physical therapy, and dental care, is also vital for all forms.

For severe forms of hypophosphatasia that begin in childhood, an enzyme replacement therapy (ERT) called asfotase alfa is available. This medication works by providing a synthetic version of the missing alkaline phosphatase enzyme, helping to restore proper bone mineralization. Asfotase alfa is given by injection under the skin and has been shown to improve bone strength, respiratory function, and motor skills in infants and children with HPP. Beyond ERT, supportive care is a cornerstone of treatment for all individuals with HPP. This includes managing pain with appropriate medications, engaging in physical therapy to maintain muscle strength and mobility, and receiving specialized dental care to address premature tooth loss and other oral health issues. Orthopedic surgery may be necessary to correct bone deformities or repair fractures. Regular monitoring by a team of specialists, including endocrinologists, geneticists, orthopedic surgeons, and dentists, is essential. This comprehensive approach helps to address the wide range of symptoms and complications that can arise from HPP, aiming to improve quality of life and prevent further deterioration of bone health.

Recovery & outlook

The recovery and outlook for individuals with hypophosphatasia vary widely, depending on the severity and age of onset. Severe forms, especially in infants, can be life-threatening without treatment. However, with enzyme replacement therapy and comprehensive supportive care, many individuals, particularly those with pediatric-onset HPP, can experience significant improvements and lead more fulfilling lives.

For individuals with the most severe perinatal and infantile forms of hypophosphatasia, the outlook was historically poor, with high mortality rates due to respiratory failure. However, the introduction of enzyme replacement therapy (asfotase alfa) has dramatically improved survival rates and quality of life for many infants and children with these severe forms. Treatment can lead to better bone mineralization, improved breathing, and enhanced motor development. People with childhood and adult forms of HPP generally have a better prognosis, though they may still experience chronic bone pain, frequent fractures, and dental problems. With ongoing supportive care, including pain management, physical therapy, and dental interventions, many can manage their symptoms effectively. Regular monitoring by a healthcare team is crucial to address any new complications and adjust treatment as needed. While HPP is a lifelong condition, advancements in treatment mean that the long-term outlook is improving. It's important to work closely with your healthcare providers to develop a personalized management plan. This helps to optimize bone health, minimize complications, and support overall well-being throughout life.

When to see a doctor

You should see a doctor if you or your child experience symptoms suggestive of hypophosphatasia, such as unexplained bone pain, frequent fractures, or premature loss of baby teeth. Seek immediate medical attention for severe symptoms like difficulty breathing, seizures, or significant skeletal deformities, especially in infants.

If you notice any persistent or concerning symptoms that could indicate hypophosphatasia, it's important to consult your doctor. For children, this includes bowed legs, an unusually large head, soft spots on the skull that don't close, or baby teeth falling out very early with their roots intact. For adults, unexplained bone pain, frequent stress fractures, or early loss of permanent teeth warrant medical evaluation. If there is a family history of hypophosphatasia, even mild symptoms should prompt a discussion with your doctor about potential genetic testing or screening. Early diagnosis can lead to earlier intervention and better management of the condition. Seek immediate medical attention or go to the emergency room if you or your child experience severe symptoms. These include significant difficulty breathing, especially in infants, which could indicate respiratory failure. Seizures, severe muscle weakness, or sudden, intense bone pain after a minor injury are also reasons to seek urgent medical care. These could be signs of severe complications requiring prompt treatment.

Frequently asked questions

Is Hypophosphatasia a common condition?

No, hypophosphatasia is considered a rare genetic disorder. The exact prevalence varies by population and severity, but it is not commonly encountered. Milder forms may sometimes go undiagnosed or be mistaken for other conditions, making precise figures challenging to obtain.

Can adults develop Hypophosphatasia?

Yes, adults can develop hypophosphatasia. This form, known as adult hypophosphatasia, typically presents in mid-adulthood with symptoms like bone pain, stress fractures, and premature loss of permanent teeth. It is generally milder than the forms seen in infants and children.

Is there a cure for Hypophosphatasia?

Currently, there is no cure for hypophosphatasia, as it is a genetic condition. However, enzyme replacement therapy (ERT) with asfotase alfa is an effective treatment for pediatric-onset forms. This therapy helps manage the condition by replacing the missing enzyme, significantly improving bone health and overall outcomes.

How is Hypophosphatasia inherited?

Hypophosphatasia is primarily inherited in an autosomal recessive pattern, meaning a person needs to inherit two mutated copies of the *ALPL* gene (one from each parent) to develop severe forms. Milder forms can sometimes be inherited in an autosomal dominant pattern, requiring only one mutated gene copy from a parent.

What is the role of alkaline phosphatase (ALP) in Hypophosphatasia?

Alkaline phosphatase (ALP) is an enzyme crucial for bone mineralization, the process where minerals harden bones and teeth. In hypophosphatasia, there are abnormally low levels of a specific type of ALP, leading to impaired mineralization and soft, weak bones and teeth.

Can Hypophosphatasia affect organs other than bones and teeth?

Yes, especially in severe forms, hypophosphatasia can affect other organs. Infants with severe HPP may experience respiratory problems due to a soft rib cage, kidney issues, and sometimes seizures due to vitamin B6-dependent epilepsy. Muscle weakness is also a common symptom across various forms.

Sources

• MedlinePlus — Hypophosphatasia
• Mayo Clinic — Hypophosphatasia
• Cochrane Library — Hypophosphatasia