Jansen's metaphyseal chondrodysplasia

Jansen's metaphyseal chondrodysplasia or JMC is a disease that results from ligand-independent activation of the type 1 of the parathyroid hormone receptor (PTHR1), due to one of three reported mutations (activating mutation).

JMC is extremely rare and as of 2007 there are less than 20 reported cases worldwide.

Presentation
Blood levels of parathryoid hormone (PTH) are undetectable, but the mutation in the PTHR1 leads to auto-activation of the signaling as though the hormone PTH was present. Severe JMC produces a dwarfing phenotype, or short stature. Examination of the bone reveals normal epiphyseal plates but disorganized metaphyseal regions. Hypercalcemia (elevated levels of calcium in the blood) )and hypophosphatemia (reduced blood levels of phosphorus), and elevated urinary calcium and phosphorus are generally found in JMC. The absence of hypercalcemia does not eliminate the disease from consideration.

Physical irregularities often associated with Jansen's include: prominent or protruding eyes, a high arched palate, micrognathia or abnormal smallness of the jaws - particularly the lower (mandible) jaw, choanal stenosis, wide cranial sutures and irregular formation of the long bones which can resemble rickets. Nephrocalcinosis (accumulation of calcium in the interstitum of the kidney) is seen commonly as well.

Treatment
There is no known treatment at present, although some investigators have tried to lessen the hypercalcemia with various forms of bisphosphonates.

Note
There is one known family in which the disease was transmitted mother to daughter.

Synonyms

 * Murk Jansen Type Metaphyseal Chondrodysplasia
 * Jansen Metaphyseal Dysostosis
 * Jansen Disease
 * JMC

Eponym
It is named for Murk Jansen.