Jansen's metaphyseal chondrodysplasia

Jansen’s metaphyseal chondrodysplasia (JMC) is a rare disease that results from ligand-independent activating mutation of the type 1 of the parathyroid hormone receptor (PTHR1).

It is named for Murk Jansen.^[1][2]

Jansen’s metaphyseal chondrodysplasia is caused by constitutively active mutations in the parathyroid hormone-1 receptor on chromosome 3p21.

Jansen’s metaphyseal chondrodysplasia is characterized by severe short stature, short bowed limbs, clinodactyly, prominent cheeks, and small mandible. Other dysmorphic features associated with JMC include: prominent or protruding eyes, hypertelorism, high skull vault, high arched palate, micrognathia or abnormal smallness of the jaws – particularly the lower (mandible) jaw, choanal atresia, wide cranial sutures and irregular formation of the long bones which can resemble rickets. Nephrocalcinosis is also commonly seen.

Hypercalcemia and hypophosphatemia occur despite undetectable serum levels of parathryoid hormone (PTH). Examination of the bone reveals normal epiphyseal plates but disorganized metaphyseal regions. Elevated urinary calcium and phosphorus are generally found in JMC. The absence of hypercalcemia does not eliminate the disease from consideration.

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

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