Pycnodysostosis

Pycnodysostosis (also known as pyknodysostosis, Toulouse-Lautrec disease, or Maroteaux-Lamy syndrome — not to be confused with MPS VI which shares the same eponym) is a rare autosomal recessive skeletal dysplasia caused by mutations in the CTSK gene on chromosome 1q21, which encodes cathepsin K, a lysosomal cysteine protease essential for osteoclast-mediated bone resorption. The deficiency of cathepsin K leads to impaired bone remodeling, resulting in increased bone density (osteosclerosis) with paradoxically fragile bones prone to fractures. The French painter Henri de Toulouse-Lautrec is widely believed to have been affected by this condition. The hallmark clinical features include short stature (adult height typically under 150 cm), generalized osteosclerosis, frequent pathological fractures (particularly of the lower extremities), open fontanelles and cranial sutures that persist into adulthood, hypoplasia of the distal phalanges (acro-osteolysis) with short and broad fingertips, dental abnormalities including delayed eruption, malocclusion, and premature tooth loss, and a characteristic facial appearance with a prominent forehead, beaked nose, and micrognathia (small jaw). The clavicles may be hypoplastic or dysplastic. Osteomyelitis of the mandible is a recognized complication, often following dental procedures. There is currently no specific curative treatment for pycnodysostosis. Management is supportive and multidisciplinary, involving orthopedic care for fracture prevention and treatment, dental and orthodontic management, and monitoring of growth and development. Fractures generally heal normally but may recur. Genetic counseling is recommended for affected families. Research into potential therapies targeting the cathepsin K pathway is ongoing, but no approved disease-modifying treatments are currently available.

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