Pyknodysostosis

Dwarfism with osteosclerosis. Congenital sclerosing osteodysplasia.

Toulouse-Lautrec Disease.

The disease is named after the French painter Henri de Toulouse-Lautrec, since Maroteaux and Lamy, the first describers of pyknodysostosis, concluded from complaints found in letters to his friends and relatives that he was suffering from pyknodysostosis, although this has been the subject of vivid debate.

NB: Some authors consider pyknodysostosis synonymous with Maroteaux-Lamy Syndrome. Although it is true that these two French physicians were the first to describe and name pyknodysostosis, the name Maroteaux-Lamy Syndrome (which is used for at least four different syndromes) most often refers to mucopolysaccharidosis type VI.

Nearly 150 cases have been reported.

Autosomal recessive with parental consanguinity being a known risk factor. The mutation has been mapped to 1q21.

Pyknodysostosis results from a gene defect affecting cathepsin K, the only lysosomal cysteine protease with high expression in osteoclasts. This fact, in combination with the highest type I collagenolytic, elastinolytic, and gelatinolytic activities of all cysteine proteases, suggests that cathepsin K plays a key function in bone matrix resorption. Pyknodysostosis can therefore be considered a skeletal dysplasia secondary to cathepsin K deficiency, which makes it a lysosomal disease. The principal site of action for cathepsin K is the subosteoclastic space into which the enzyme is secreted for bone matrix degradation. The process of bone resorption is characterized by solubilization of inorganic mineral and subsequent proteolytic degradation of the organic matrix, primarily type I collagen. The osteoclast number in pyknodysostosis is normal, but the area of demineralization surrounding each individual osteoclast is enlarged. Ultrastructural examination of these osteoclasts shows big, abnormal intracytoplasmic vacuoles filled with collagen fibrils from bone. It seems therefore that step one (demineralization) of normal bone resorption is intact, whereas step two (degradation of the organic matrix) is defective. The inadequate resorption and remodeling of bones in pyknodysostosis leads to abnormally dense and brittle bones.

Clinical features and radiologic appearance of bones (osteosclerosis, increased thickness of the trabecular bone as a result of increased density).

Short stature (adult height usually less than 150 cm [59 inches] for males) caused by short limbs, large, disproportionate head with frontal and occipital bossing, delayed suture closure and prolonged persistence of fontanels, and lack of frontal sinuses. The midface is hypoplastic with a prominent nose, grooved palate, and hypoplasia of the mandibular angles, making these patients prone to obstructive sleep apnea. The clavicles are affected by partial or complete aplasia. Progressive acro-osteolytic dysplasia affects the distal phalanges of fingers and toes with dystrophic, flattened, grooved, and brittle nails. Spondylolysis may occur at the L4-L5 level and scoliosis is a common finding. Dentition anomalies include delayed eruption and irregularities of the permanent teeth, with or without partial anodontia. The brittle bone results in fractures secondary to minimal trauma and the sometimes blue appearance of the sclera may initially result ...

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