What is Jansen's Metaphyseal Chondrodysplasia (JMC)?
Jansen's Metaphyseal Chondrodysplasia (JMC) is an extremely rare genetic skeletal disorder that affects bone growth and calcium regulation. It is a progressive condition, meaning skeletal abnormalities and related complications worsen over time.
JMC is marked by abnormalities in the growth plates of long bones, which disrupt normal bone development and can result in skeletal deformities and mobility challenges.
Because the condition is so rare, it may be unfamiliar even to experienced clinicians. As a result, individuals and families often face a delayed or complex diagnostic journey.
How JMC Affects the Body
JMC interferes with the normal regulation of chondrocyte growth and differentiation within the growth plates. Over time, this leads to progressive changes in the long bones and overall skeletal structure, including short stature, bowed limbs, and skeletal malalignment.
Calcium is lost from bones faster than it can be absorbed, which weakens the skeletal system and often increases the need for surgical intervention, physical therapy, and occupational therapy to support bone alignment and mobility.
Additional skeletal and craniofacial features may include craniosynostosis (fusion of skull bones), prominent or protruding eyes, high-arched palate, micrognathia (small lower jaw), impacted teeth with enamel defects, and thickening of the posterior skull, which can affect vision and hearing.
JMC can also disrupt mineral balance, leading to complications such as nephrocalcinosis, the accumulation of calcium in the kidneys.
Genetic Cause and Inheritance
JMC is caused by activating mutations in the PTH1R gene, which encodes the parathyroid hormone/parathyroid hormone"related peptide receptor. This receptor plays a critical role in bone development and calcium and phosphate regulation.
The condition follows an autosomal dominant inheritance pattern, meaning a single copy of the mutated gene can cause disease. Most cases result from de novo (new) mutations that arise spontaneously and are not inherited from a parent.
In rare cases, an individual with JMC who has children would have a 50% chance of passing the mutation to each child.
How Rare Is JMC?
Currently, there are approximately 30 genetically confirmed cases worldwide, including 11 patients in the United States.
Confirmed cases have also been identified in Brazil, Israel, Ukraine, Australia, Paraguay, Spain, and Italy.
This page provides a general overview of Jansen's Metaphyseal Chondrodysplasia. Use the navigation to explore diagnosis, imaging, education, resources, and school support in more detail.