Rigid spine muscular dystrophy (RSMD) is a form of congenital muscular dystrophy (CMD) characterized by weakness and wasting (atrophy) of the muscles of the torso and the neck.
RSMD is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.
Nearly 40% of RSMD cases are caused by mutations in the SELENON gene (also known as SEPN1), which provides instructions for making a protein called selenoprotein N. The genetic mechanisms for the remaining RSMD cases have not been determined.
Selenoprotein N and RSMD
Selenoprotein N is a glycoprotein (protein with glucose residues) that is found in many tissues, especially the skeletal muscles, heart, lungs, and placenta. It also is found in large amounts in fetal muscle cells and tissues, leading  scientists to think it plays an important role in the formation and development of muscle tissue.
Selenoprotein N also is thought to be involved in protecting cells from the damage caused by reactive oxygen species (molecules that contain unstable oxygen) that can react and oxidize cellular proteins, membrane lipids, and genetic material (DNA).
Finally, it is thought to play an active role in muscle contraction because it can bind to calcium ions, which are involved in muscle contraction and relaxation.
Given these essential roles, the absence of a functional selenoprotein N can significantly affect muscle growth, differentiation, survival, and function, and therefore lead to the symptoms of RSMD.
Types of SELENONÂ mutations
Several nonsense and missense mutations in the SELENONÂ gene, which produce truncated or non-functional selenoprotein N, have been identified in RSMD patients.
A nonsense mutation is a single point mutation in a nucleotide that converts a sense codon (a set of three nucleotides that code for one of the 22 amino acids that make up a protein) to a stop codon that signals the protein-making machinery of the cell to stop making the protein. The result is a shorter (truncated) protein with reduced or no function.
A missense mutation is a single point mutation that changes the coding sequence of SELENON resulting in the production of a protein with reduced function.
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Last updated: Aug. 27, 2019
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