About pyknodysostosis

What is pyknodysostosis?

Pycnodysostosis facts

  • Pycnodysostosis is an inherited disorder of the bone.
  • Pycnodysostosis causes short stature.
  • Pycnodysostosis causes abnormally dense brittle bones.
  • Pycnodysostosis causes the "soft spot" of a baby's skull to stay widely open.
  • Pycnodysostosis is due to a defect in an enzyme: cathepsin K.
  • Growth hormone treatment may increase growth in pycnodysostosis.

What is pycnodysostosis?

Pycnodysostosis is perhaps best known as the diagnosis given retrospectively to the late 19th century French artist Henri de Toulouse-Lautrec (portrayed by Jose Ferrer in the 1952 film "Moulin Rouge").

Pycnodysostosis is a genetic (inherited) disease of the bone. Its pattern of inheritance follows the classic rules of genetics (see below).

Pycnodysostosis consistently causes short stature. The height of adult males with the disease is less than 150 cm (59 inches, or 4 feet 11 inches). Adult females with pycnodysostosis are even shorter.

Pycnodysostosis causes the bones to be abnormally dense (osteosclerosis); the last bones of the fingers (the distal phalanges) to be unusually short; and delays the normal closure of the connections (sutures) of the skull bones in infancy, so that the "soft spot" (the fontanel) on top of the head remains widely open.

Pycnodysostosis causes brittle bones which easily break (fracture). The bones in the legs and feet tend to fracture. The jaw and collar bone (clavicles) are also particularly prone to fractures.

The precise frequency of pycnodysostosis has never been determined. Pycnodysostosis can be classified in the large group of genetic diseases that are individually uncommon, but collectively important because of the sum of their numbers, their heavy impact upon affected individuals, and the equally heavy burden they place upon their families.

What is basis for the name of this disease?

The name for this disease was coined by the French physicians Maroteaux and Lamy in 1962. They described the disorder in a report entitled "La pycnodysostose." (They were not the only discoverers of the disease. Andren and colleagues independently described the condition in 1962.) Maroteaux and Lamy put "pyknos," from the Greek meaning "dense" together with the compound word "dysostosis" meaning abnormal bone formation. The name "pycnodysostosis" was designed to convey the abnormally dense bone that is a hallmark of the disease.

Although the original Maroteaux and Lamy spelling was with a 'c', it has been written variably with a 'c' or a 'k'. Here we use the original spelling with a 'c'. No matter which way it is spelt, the name has stuck and is utilized worldwide today to designate this disease.

What are the characteristics of pycnodysostosis?

Pycnodysostosis causes abnormalities other than short stature, dense brittle bones, short fingers, and the wide open soft spot of the skull (see above). These other abnormalities involve the head and face, teeth, collar bones, skin, and nails. The front and back of the head are prominent. Within the open sutures of the skull, there may be many small bones (called wormian bones). The midface is less full than usual. The nose is prominent. The jaw can be small. The palate is narrow and grooved. The baby teeth are late coming in and may be lost much later than usual. The permanent teeth can also be slow to appear. The permanent teeth are commonly irregular and teeth may be missing (hypodontia). The collar bones are often underdeveloped and malformed. The skin over the back of the fingers is very wrinkled. The nails are flat and grooved.

Pycnodysostosis also causes problems that may become evident with time. Aside from the broken bones, the last bones of the fingers (the distal phalanges) and the collar bone can undergo slow progressive deterioration. Vertebral defects may permit the spine to curve laterally (resulting in scoliosis). The tooth problems often require orthodontic care and dental cavities are common.

What are the symptoms for pyknodysostosis?

Although researchers have been able to establish a clear syndrome with characteristic or “core” symptoms, much about the disorder is not fully understood. Several factors including the small number of identified cases, the lack of large clinical studies, and the possibility of other genes influencing the disorder prevent physicians from developing a complete picture of the potential associated symptoms and prognosis. Therefore, it is important to note that affected individuals may not have all the symptoms discussed below. Parents should talk to their children’s physician and medical team about their specific case, associated symptoms and overall prognosis.

Infants with pycnodysostosis may experience delayed closure of the sutures of the skull. An infant’s skull has seven bones and several joints called sutures. Sutures are made of tough, elastic fibrous tissue and separate the bones from one another. Sutures meet up (intersect) at two spots on the skull called fontanelles, which are better known as an infant’s “soft spots”. The seven bones of an infant’s skull usually fuse together until around age two or later. In infants with pycnodysostosis, the sutures take longer to fuse together, which can affect the proper development of the skull. The skull is often larger than normal.

Distinctive Facial features are common and include an abnormally Prominent forehead (frontal bossing), a pointed nose, a highly-arched roof of the mouth (palate), prominent eyes with blue discoloration to the whites of the eyes (bluish sclerae), and a Small jaw due to underdevelopment of the bones of the jaw (Hypoplasia of the maxilla and mandible). Affected individuals may have several dental problems including delayed eruption of baby (deciduous) teeth, and then persistence of these teeth after they come in. They also experience delayed eruption of the permanent teeth. Some individuals may be missing teeth because the teeth fail to develop (hypodontia).

Individuals with pycnodysostosis may also have malformation of the collarbone (clavicular dysplasia) and breakdown of bone (osteolysis), particularly the outermost bones of the fingers and toes (acroosteolysis). The fingers may be small and stubby, and the fingernails may be absent or abnormally small. People with pycnodysostosis experience abnormal hardening and density of bone (osteosclerosis). Osteosclerosis causes bone to be fragile and affected individuals are prone to repeated fractures with minimal trauma or without trauma (spontaneously). The legs are most commonly affected by fracturing. Some people may have abnormal curvature of the spine such as having an s-shaped spine (scoliosis). Lower Back Pain due to stress fractures of the lower spinal bones (vertebrae) can occur. There may be deformities of chest development, which can cause breathing (respiratory) issues. Some individuals may snore, while others may have sleep apnea. Sleep apnea is a condition where a person will momentarily stop breathing during sleep. Sometimes, sleep apnea can be severe.

Some individuals with pycnodysostosis have growth hormone deficiency and deficiency of another hormone called insulin-like growth factor 1 (IGF-1), which also plays a role in growth during childhood. Affected individuals reach an adult height that is shorter than would otherwise be expected (short stature). The arms and legs of affected individuals are usually abnormally short.

Less often, pycnodysostosis may occur with other symptoms such as loose (lax) joints, underdevelopment (hypoplasia) of the sinuses, enlargement of the liver and spleen (hepatosplenomegaly), and underdevelopment of the pituitary gland.

What are the causes for pyknodysostosis?

Pycnodysostosis is caused by a change (mutation) in the cathepsin K (CTSK) gene. 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, this can affect many organ systems of the body.

Pycnodysostosis is inherited in an autosomal recessive manner. 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. 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.

In certain populations, pycnodysostosis has occurred in families where the parents are closely related. All individuals carry 4-5 abnormal genes. Parents who are close relatives (consanguineous) have a higher chance than unrelated parents to both carry the same abnormal gene, which increases the risk to have children with a recessive genetic disorder.

The CTSK gene produces an enzyme called cathepsin K. This enzyme is a lysosomal enzyme and its job is to break down certain substances in the body include bone matrix proteins, certain types of collagen and other substances. This enzyme is predominantly found in osteoclasts, which are bone cells that absorb bone tissue during growth and healing. Basically, osteoclasts break down bone, a normal process called resorption. Bone is a dynamic system continuously engaged in a remodeling process of resorption and formation. Bone resorption refers to the breakdown of bone followed by the formation of new bone. In pycnodysostosis, some of these materials are not completely broken down and build up in lysosomes of cell. When enough of these materials build up, they become toxic to the affected cells and tissues of the body.

What are the treatments for pyknodysostosis?

The treatment of pycnodysostosis is directed toward the specific symptoms that are apparent in an individual. Treatment may require the coordinated efforts of a team of specialists. Pediatricians, specialists who diagnose and treat bone disorder (orthopedists), orthopedic surgeons, specialists who diagnose and treat hormonal disorders (endocrinologists), dental specialists, and other healthcare professionals may need to systematically and comprehensively plan an affected child’s treatment. Genetic counseling may be of benefit for affected individuals and their families. Psychosocial support for the entire family is essential as well.

There are no standardized treatment protocols or guidelines for affected individuals. Due to the rarity of the disease, there are no treatment trials that have been tested on a large group of patients. Various treatments have been reported in the medical literature as part of single case reports or small series of patients. Treatment trials would be very helpful to determine the long-term safety and effectiveness of specific medications and treatments for individuals with pycnodysostosis.

Individuals with pycnodysostosis will need orthopedic care when fractures occur. There are no standard guidelines on the best method or surgical intervention for fracture treatment in pycnodysostosis. Specialized dental care should be provided and craniofacial surgery may be warranted.

Pycnodysostosis can potentially complicate the use of anesthesia in some people because of abnormalities affecting the jaw and the roof of the mouth (palate). Affected individuals should be evaluated before undergoing procedures that require anesthesia.

Investigational Therapies

Some children with pycnodysostosis have been treated with growth hormone therapy because short stature is a common finding in this disorder. Initial results have shown positive results in individuals’ final adult height.

Researchers are studying specific medications called enzyme inhibitors for the treatment of pycnodysostosis and similar bone disorders. These medications work by inhibiting the activity of a specific enzymes in the body to block negative or harmful effects that these enzymes may cause because of an underlying genetic abnormality.

What are the risk factors for pyknodysostosis?

Pycnodysostosis is an extremely rare disorder that affects both males and females. About 200 people have been reported in the medical literature with this disorder. It is estimated to affect about 1 in 1.7 million people in the general discussion. Pycnodysostosis may go undiagnosed or misdiagnosed, making it difficult to determine the true frequency in the general population.

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