Question: What are the genetic changes related to glycogen storage disease type VII ?
Answer: Mutations in the PFKM gene cause GSDVII. This gene provides instructions for making one piece (the PFKM subunit) of an enzyme called phosphofructokinase, which plays a role in the breakdown of glycogen. The phosphofructokinase enzyme is made up of four subunits and is found in a variety of tissues. Different combinations of subunits are found in different tissues. In muscles used for movement (skeletal muscles), the phosphofructokinase enzyme is composed solely of PFKM subunits. In skeletal muscle, the cells' main source of energy is stored as glycogen. Glycogen can be broken down rapidly into the simple sugar glucose when energy is needed, for instance to maintain normal blood sugar levels between meals or for energy during exercise. Phosphofructokinase is involved in the sequence of events that breaks down glycogen to provide energy to muscle cells. PFKM gene mutations result in the production of PFKM subunits that have little or no function. As a result, no functional phosphofructokinase is formed in skeletal muscles, and glycogen cannot be completely broken down. Partially broken down glycogen then builds up in muscle cells. Muscles that do not have access to glycogen as an energy source become weakened and cramped following moderate strain, such as exercise, and in some cases, begin to break down. In other tissues, other subunits that make up the phosphofructokinase enzyme likely compensate for the lack of PFKM subunits, and the enzyme is able to retain some function. This compensation may help explain why other tissues are not affected by PFKM gene mutations. It is unclear why some individuals with GSDVII are affected with more severe forms of the disorder than others.
Source: GHR