Mutations are seen less frequently affecting domains I and II, both of which encode sections of the N-terminal region of the protein. RYR2 mutations associated with CPVT that occur outside these four domains are very rare, being responsible for as few as 10% of reported cases. RYR2 mutations are most often missense mutations, such as single nucleotide substitutions causing one amino acid to be replaced by another, although in-frame substitutions and duplications have been described. More damaging nonsense mutations have not been reported in association with CPVT, potentially because these variants may lead to different cardiac diseases such as cardiomyopathies. CPVT2: CASQ2 mutations
Mutations in the CASQ2 gene are associated with an autosomal recessive form of typical CPVT known as CPVT2. This gene encodes calsequestrin, the major calcium-binding protein and calcium buffer within the sarcoplasmic reticulum. Mutations in CASQ2 account for only 3-5% of cases of CPVT. Fourteen mutations in CASQ2 have been identified in association with CPVT. Two of these are nonsense mutations causing the protein to be abnormally short, and two are deletion mutations, while ten are missense mutations that substitute one amino acid for another in the chain forming the protein.Mutations in CASQ2 cause a decrease in sarcoplasmic reticulum calcium-buffering capacity. This means that abrupt changes in sarcoplasmic total calcium will be buffered less and therefore translate to larger shifts in free calcium.
Sarcoplasmic reticulum calcium content increases in response to stimulation from catecholamines, explaining why arrhythmias in those with CPVT occur at times when catecholamine levels are elevated. Some suggest that the increased sensitivity to calcium occurs only when the ryanodine receptor is phosphorylated by protein kinase A, while other suggest that the increased sensitivity also occurs under resting conditions.Two theories have been proposed for the underlying mechanism by which mutations in RYR2 promote store-overload induced calcium release: domain unzipping and FKBP12.6 unbinding. Domain unzipping refers to the separation of two important regions of the ryanodine receptor, the N-terminus and the central domain. Through this mechanism, a mutation might destabilise the ryanodine receptors closed state and increase its sensitivity to calcium. A second potential mechanism involves the regulatory protein FKBP12.6, a protein that binds to and stabilises the ryanodine receptor. The binding of FKBP12.6 to the ryanodine receptor is regulated by phosphorylation. Phosphorylation by protein kinase A leads to the dissociation of FKBP12.6, rendering the ryanodine receptor more sensitive to cytosolic calcium. RYR2 mutations may interfere with the binding of FKB12.6 to the ryanodine receptor and thereby increase sensitivity to calcium. It is likely that FKBP12.6 plays a role in some CPVT mutations but not others.RYR2 mutations responsible for CPVT are mainly found in four major domains of the gene. Mutations affecting domains III and IV of the gene (corresponding to the N-terminal region of the protein and cytosolic linker respectively) occur in 46% of cases.