Osteopetrosis

Osteopetrosis is a rare disorder that causes bones to grow abnormally and become overly dense. When bones become overly dense, they are brittle and can fracture (break) easily. In addition, bones may be misshapen and large, causing other problems in the body. For example, changes to the bone size and density of the skull can cause pressure on nerves leading to vision loss, hearing loss, and paralysis of the facial muscles.

As bone increases in size, the amount of space for the bone marrow (soft, sponge-like tissue in the center of most bones that produces blood cells) becomes crowded. Decreases in the amount of space for bone marrow can lead to low levels of cells in the body that fight infection, carry oxygen to the body’s cells, or control bleeding.

Osteopetrosis is a genetic disease that a child inherits from their parents. The disorder may be mild to severe, and symptoms may appear early in infancy or later in adulthood. The treatment options depend on the type of osteopetrosis the person has.

What happens in osteopetrosis?

Normally, old bone is broken down and new bone is formed. However, this process does not work properly in people with osteopetrosis. Instead, old bone is not broken down as new bone is formed. This causes the bone to develop incorrectly, leading to dense, thick bones that can fracture easily.

Who Gets Osteopetrosis?

People with a family history of the disease are at greater risk of inheriting it through an abnormal gene that is passed on from one or both parents.

Types of Osteopetrosis

The types of osteopetrosis are broken into categories based on inheritance pattern, age the disease develops, and the severity of the disease. The types include:

  • Autosomal recessive osteopetrosis is the severe infantile form of the disorder. The symptoms of this type develop at or shortly after birth and shorten life expectancy.
  • Autosomal dominant osteopetrosis is the noninfantile form of the disorder and is also known as Albers-Schönberg disease. The symptoms of this type develop later than the infantile form and have a wide range of disease severity. Even within the same family, some people have little or no impact from the disease, while others are severely affected by the disease and have significant disability and shortened life expectancy. People with this type of osteopetrosis may not be diagnosed until adolescence or adulthood.
  • X-linked osteopetrosis is a rare form of the disease and can affect many areas of the body.

Symptoms of Osteopetrosis

The severity of symptoms can vary depending on the type of osteopetrosis. They include:

  • Bone fractures from brittle, dense bones.
  • Nasal congestion from narrowing sinus cavities.
  • Vision and hearing change from enlarged bones pressing on nerves.
  • Dental problems due to bone changes in the skull and jaw and because teeth come in later than normal.
  • Low blood cell levels due to smaller amounts of the bone marrow because of crowding in the center of bones can lead to anemia and infection.
  • Infections in the bone (osteomyelitis).
  • Chronic bone pain.

Cause of Osteopetrosis

Osteopetrosis is a genetic disease that, in most cases, a child inherits from one or both parents. Genes carry information that determines which features are passed to you from your parents. We have two copies of most of our genes, one from each parent.

People with osteopetrosis have a gene that causes the body to make too few or abnormal cells called osteoclasts. When the osteoclasts are missing, old bone is not broken down as new bone is made, leading to dense, weak bones.

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The inheritance patterns help determine the type of osteopetrosis the person has.

  • Autosomal recessive inheritance pattern happens when the parents each carry one copy of the mutated (changed) gene but do not show any signs or symptoms of the disease. It’s important to note that the mutated gene may not be passed from both parents to every child. Some children will not have any copies of the gene, and others may only have one copy, making them a carrier.
  • Autosomal dominant inheritance pattern happens when only one mutated copy of the gene appears. This happens when the child inherits one copy of a mutated gene from one parent; however, sometimes the mutated gene appears with no family history.
  • X-linked inheritance pattern causes a rare form of the disease and happens when a mutation occurs in genes on the X chromosome, one of the two sex chromosomes in each cell. This type of osteopetrosis usually affects boys.

Genetic research has found this disease of osteoclastic dysfunction to have an association with at least 8 gene mutations.

Six of these eight genes are associated with a malignant, autosomal recessive form of the disease. Loss of function mutations in TCIRG1, CLCN7, OSTM1, PLEKHM1, AND SNX10 lead to an osteoclast-rich version of autosomal recessive osteopetrosis. In this version, osteoclasts are plentiful. However, the osteoclasts that are present are unable to resorb bone due to defective ruffled border formation effectively. Loss of function mutations in TNFSF11 and TNFRSF11A lead to disrupted osteoclast development and a condition of osteoclast poor osteopetrosis.

Intermediate autosomal recessive osteopetrosis is the result of a loss of function mutation in CAII, the gene responsible for the production of the carbonic anhydrase II protein.

Autosomal dominant osteopetrosis is the result of the dysfunction of chloride channel 7 secondary to a dominant-negative mutation of CLCN7.

Diagnosis of Osteopetrosis

Doctors diagnose osteopetrosis by:

  • Taking a family and medical history.
  • Performing an exam.
  • Ordering laboratory tests and imaging studies.

Your doctor may order one or more of the following tests to diagnose osteopetrosis:

  • X-rays evaluate the bone structure, including dense and misshapen bones.
  • Genetic testing to help determine the specific cause and type of osteopetrosis.
  • Blood tests to check:
    • Blood cell counts.
    • Vitamin and mineral levels.
    • Certain hormone levels.
  • Computerized axial tomography (CAT) scans and magnetic resonance imaging (MRI) to evaluate changes in bone marrow space and skull.

Treatment of Osteopetrosis

Knowing the type of osteopetrosis helps doctors treat the symptoms and problems from the disease. Genetic testing allows doctors to examine specific changes in the genes so treatment can be tailored depending on the cause and expected severity of the disease. Treatments can include:

  • Interferon gamma-1b is an injected drug designed to delay disease progression and is the only therapy specifically approved by the U.S. Food and Drug Administration (FDA) for the severe infantile form of the disease. It is not currently approved for the treatment of the noninfantile form of osteopetrosis.
  • Corticosteroids may help improve some blood cell levels. Because they are potent drugs, your doctor will prescribe the lowest dose possible to achieve the desired benefit.
  • Physical and occupational therapy can help children develop motor and other skills.
  • Bone care may include casts, splints, or surgery to correct fractures and misshapen bones.
  • A bone marrow transplant may be recommended for infants with the severe form of the disease and people with bone marrow failure. Doctors will recommend this type of treatment on a case-by-case basis after reviewing the risks and benefits. A bone marrow transplant can cure the disease for certain people, but it cannot reverse the damage that has already occurred.
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Regular visits with the doctor are important to check symptoms and laboratory testing results and to discuss potential treatments.

Due to the frequency of cranial nerve compression neuropathies, most frequently of the optic nerve, routine ophthalmologic evaluation is needed, and in some patients, surgical decompression of the optic nerve may be required to preserve eyesight.

Routine dental evaluation is needed in these patients to prevent the complications of abscesses, cysts, and osteomyelitis that can occur more frequently in these patients due to altered bone anatomy of the mandible .

Bone marrow transplantation of hematopoietic stem cells (HSC) is reserved for the malignant, autosomal recessive form of osteopetrosis due to the risk of rejection and other possible complications. HSC therapy from HLA-matched donors does not necessarily reverse disease complications but has been found to have 73% 5-year disease-free survival.

Interferon-gamma 1b therapy has been used in some patients found unfit for bone marrow transplantation or as bridging therapy until HSC therapy can be used. It shows benefit in increasing immune function and bone resorption. High-dose calcitriol has also been used to attempt the stimulation of host osteoclasts.

Preventive Pharmacotherapy

The ISHLT (International Society for Heart and Lung Transplantation) 2010 guidelines recommend preventive therapy for all heart transplant recipients throughout the first year after transplantation. The AASLD (The American Association for the Study of Liver Diseases) 2012 practice guidelines recommend therapy for all liver transplant patients with osteoporosis and patients with osteopenia and other risk factors for fractures.

Since studies have failed to demonstrate an association between clinical indicators such as cumulative corticosteroid dose or the rate of change of BMD and fracture risk in transplant recipients, many authors recommend preventive pharmacologic therapy in all patients undergoing heart, liver, lung, and bone marrow transplantations. In most of the prospective trials, preventive therapy has been initiated as soon as possible after transplantation and continued for at least twelve months. Many authors recommend re-assessing the patient at one year to determine the need for continued therapy. Re-assessment can be based on repeat BMD measurement, current CS dose, etc.

In kidney transplant recipients, there is no consensus regarding preventive pharmacologic therapy. Most authors recommend a comprehensive fracture risk assessment of these patients based on factors like BMD, vertebral fractures, PTH level, patient’s age, sex, body weight, etc. and consider preventive therapy for high-risk patients. Some studies also suggest the measurement of bone turnover markers such as C-telopeptide crosslink (CTX) and bone-specific alkaline phosphatase to risk-stratify patients.

Bisphosphonate Therapy

Bisphosphonate (antiresorptive) therapy is the initial choice of preventive therapy for transplantation osteoporosis as it directly counteracts the effects of CS. Several studies have proven their efficacy in decreasing bone loss and the number of vertebral fractures at one year after transplantation. Presently, there is no preference of one bisphosphonate over another, or of the route of administration. A study that compared the efficacies of intravenous zoledronic acid and oral alendronate for the prevention of bone loss in transplant recipients showed equal efficacy using either of these agents. It is important to exercise caution while considering bisphosphonates in premenopausal women because their safety during pregnancy has not been sufficiently established. Due to their potential of worsening adynamic bone disease, bisphosphonates are not recommended in patients with glomerular filtration rate (GFR) less than 30 ml/minute.

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Vitamin D Metabolites

Calcitriol, a vitamin D metabolite, can be used as preventive therapy when bisphosphonate therapy is contraindicated. It increases calcium absorption in the intestine and its reabsorption in the kidneys, and as a result, leads to negative feedback on PTH release. At a dose of 5 micrograms per day, calcitriol has shown efficacy in decreasing bone loss and vertebral fractures. It is essential to screen all patients on calcitriol therapy for the development of hypercalcemia or hypercalciuria by periodically measuring serum and urinary calcium levels.

Hormone Replacement Therapy (HRT)

All transplant recipients with hypogonadism should be treated with HRT as it has been shown to slow bone loss in this population. At present, HRT is used as sole preventive therapy only in select patients with hypogonadism, in whom bisphosphonates are contraindicated.

Denosumab

Denosumab, a humanized monoclonal antibody to the RANK ligand, is efficacious in the treatment of osteoporosis in kidney transplant recipients. It has also demonstrated efficacy in the treatment of osteoporosis in liver transplant recipients. In a recent study where it was used as prophylactic therapy in kidney transplant recipients, denosumab has shown efficacy in increasing BMD in the first post-transplant year. Of note, denosumab is associated with increased bone loss upon discontinuation, and in such cases, it may need to be switched to antiresorptive therapy with bisphosphonates.

Teriparatide

Teriparatide, a recombinant human PTH, has shown efficacy in steroid-induced osteoporosis. A study that used teriparatide as preventive therapy in kidney transplant recipients did not show BMD improvement in the teriparatide group compared to the placebo group. Hence, further studies are needed to delineate the role of this drug as preventive therapy for transplantation osteoporosis. As of now, no studies are available using abaloparatide, a parathyroid hormone-related peptide (PTHrP) analog, in transplant recipients.

Calcitonin

Calcitonin acts by directly inhibiting osteoclastic bone resorption. Its role in the prevention and treatment of post-transplantation osteoporosis has not been elucidated. An older study has illustrated the efficacy of combined preventive therapy with intranasal calcitonin and calcitriol in improving BMD at 18 months following heart transplantation. Nevertheless, newer studies are required to clarify the role of calcitonin in transplant recipients.

Who Treats Osteopetrosis?

Several different types of doctors diagnose and treat osteopetrosis. Often, people work with a team of doctors to provide the best treatment. These doctors may include:

  • Endocrinologists treat hormonal and metabolic disorders.
  • Dentists can help prevent dental problems and treat issues, including orthodontists and oral-maxillofacial surgeons.
  • Geneticists, study genes, especially how they are inherited and mutate.
  • Hematologists specialize in blood disorders.
  • Neurologists treat disorders and diseases of the spine, brain, and nerves.
  • Neurosurgeons, perform surgery for disorders and diseases of the spine, brain, and nerves.
  • Ophthalmologists treat disorders and diseases of the eye.
  • Orthopaedists, treat and perform surgery for bone and joint diseases.
  • Otolaryngologists treat ear, nose, and throat disorders.

Living With Osteopetrosis

Living with osteopetrosis is different for each person. Some people have few or no symptoms, while others have many symptoms that affect their ability to perform daily activities. The following tips may help.

  • See your health care providers regularly.
  • Talk to your doctor or physical therapist about which types of exercises are best for you based on the type of osteopetrosis you have.
  • Ask your doctor about nutrition.
  • Keep the lines of communication open. Ask family and friends for help when you need it.
  • Reach out to online and community support groups.