Hypokalemic Periodic Paralysis - Genetics

Genetics

Mutations in the following genes can cause hypokalemic periodic paralysis:

An association with KCNE3 (voltage-gated potassium channel) has also been described, but is currently disputed, and excluded from the disease definition in OMIM.

Action potentials from the central nervous system cause end-plate potentials at the NMJ which causes sodium ions to enter and depolarise the muscle cells. This depolarisation propagates to the T-tubules where it triggers the entry of calcium ions via Ca_v1.1 as well as from the sarcoplasmic reticulum through the associated ryanodine receptor RyR1. This causes contraction (tensing) of the muscle. Depolarisation of the motor end plate causes potassium ions to leave the muscle cells, repolarising the muscle and closing the calcium channels. Calcium is pumped away from the contractile apparatus and the muscle relaxes.

Mutations altering the usual structure and function of these channels therefore disrupts regulation of muscle contraction, leading to episodes of severe muscle weakness or paralysis. Mutations have been identified in arginine residues making up the voltage sensor of Na_v1.4. This voltage sensor comprises the S4 alpha helix of each of the four transmembrane domains (I-IV) of the protein, and contains basic residues that only allow entry of the positive sodium ions at appropriate membrane voltages by blocking or opening the channel pore. In Ca_v1.1, mutations have also been found in domains II and IV. These mutations are loss-of-function, such that the channels cannot open normally.

In patients with mutations in SCN4A or CACNA1S, therefore, the channel has a reduced excitability and signals from the central nervous system are unable to depolarise the muscle. As a result, the muscle cannot contract efficiently (paralysis). The condition is hypokalemic because a low extracellular potassium ion concentration will cause the muscle to repolarise to the resting potential more quickly, so even if calcium conductance does occur it cannot be sustained. It becomes more difficult to reach the calcium threshold at which the muscle can contract, and even if this is reached then the muscle is more likely to relax. Because of this, the severity would be reduced if potassium ion concentrations are kept high.

In contrast, hyperkalemic periodic paralysis refers to gain-of-function mutations in sodium channels that maintain muscle depolarisation and therefore are aggravated by high potassium ion concentrations.

This condition is inherited in an autosomal dominant pattern (but with a high proportion of sporadic cases), which means one copy of the altered gene in each cell is sufficient to cause the disorder.

==See also==

• Hypokalemic sensory overstimulation