What are the genetic changes related to Schwartz-Jampel syndrome ?
Schwartz-Jampel syndrome is caused by mutations in the HSPG2 gene. This gene provides instructions for making a protein known as perlecan. This protein is found in the extracellular matrix, which is the intricate lattice of proteins and other molecules that forms in the spaces between cells. Specifically, it is found in part of the extracellular matrix called the basement membrane, which is a thin, sheet-like structure that separates and supports cells in many tissues. Perlecan is also found in cartilage, a tough, flexible tissue that makes up much of the skeleton during early development. Most cartilage is later converted to bone, except for the cartilage that continues to cover and protect the ends of bones and is present in the nose and external ears.  Perlecan has multiple functions, including cell signaling and the normal maintenance of basement membranes and cartilage. The protein also plays a critical role at the neuromuscular junction, which is the area between the ends of nerve cells and muscle cells where signals are relayed to trigger muscle contraction.  The mutations that cause Schwartz-Jampel syndrome reduce the amount of perlecan that is produced or lead to a version of perlecan that is only partially functional. A reduction in the amount or function of this protein disrupts the normal development of cartilage and bone tissue, which underlies chondrodysplasia in affected individuals. A reduced amount of functional perlecan at the neuromuscular junction likely alters the balance of other molecules that signal when muscles should contract and when they should relax. As a result, muscle contraction is triggered continuously, leading to myotonia.