Ion Channel Disease Channelopathy: Myotonia Congenita - Term Paper Example

We have several genetic diseases that are caused by abnormalities in ion channels (channelopathies). This paper will focus on the voltage gated chloride channel channelopathy called Myotonia congenita that affects skeletal muscles. The paper will examine how defects in the chloride channel ion genes contribute to this disease, related cellular effects and how it affects the skeletal muscles of affected individuals. Excitability of membranes is vital for muscle function and is usually regulated by Voltage gated ion channels.

Therefore, it is not surprising that ion channels are involved in the physiopathology of disorders of the skeletal muscle (Rott, Lerche & Horn, 2002). It will also exemplify research that is currently being conducted on this disorder. According to Zhang (1999), myotonia occurs due to delayed relaxation of muscle after a voluntary contraction or mechanical stimulation. There are dystrophic and non dystrophic forms of myotonia which can be identified according to their clinical features. The non dystrophic myotonia includes three main types: myotonia congenita, paramyotonia congenita and hyperkalemic periodic paralysis.

Clinical electrophysiological studies of patients with these diseases revealed repetitive electrical discharges (myotonic runs). Normally, the myotonic runs occur in response to electrical and mechanical stimulation in muscles. Myotonia is the stiffness noticed upon initiation of movement. Myotonic dystrophy is also another myotonic disorder characterized by multisystem involvement. Patients with myotonic dystrophy have cataracts, cardiac arrhythmias, skeletal muscle problems and other endocrine abnormalities (Harper & Ptacek et al.

as cited in Zhang 1999). However, Myotonia congenital (MC), a subtype of Myotonia, is a genetic muscle disease associated with defects in the musculoskeletal chlorine voltage gated ion channels C1C-1. Pathologically, MC is an inheritable skeletal muscle disorder that results from the diminished activity of the sarcolemmal voltage gated chloride ion channels. The syndrome may be transmitted by either an autosomal dominant (Thomsen’s disease) or recessive generalized myotonia (Becker’s myotonia) mode of inheritance (Thomsen & Becker as cited in Zhang 1999).

Ptacek et al. argued that patients with MC normally have painless myotonia that is improved by exercise, a phenomenon termed as “warm up exercise” (Zhang 1999). Individuals affected by this disorder usually have well developed muscles. Histochemical studies of fiber sub types have demonstrated a complete absence of type 2B muscle fibers in MC patients with Thomsen’s disease (Crews et al. as cited in Zhang 1999). The onset of Thomsen’s disease occurs within the first two years of life and muscle stiffness is usually the predominant symptom.

These patients do not have periodic paralysis, which distinguishes Thomsen’s disease, from hyperkalemic periodic paralysis and paramyotonia congenita. Cloning has showed that, there is a substitution of glycin 230 by glutamic acid (G230E), between two crucial segments (D3 andD4) of the voltage gated chloride ion channel. This dramatically affects the properties of the ion channel.the G230E mutation causes a substantial change in the selectivity of anions and cations; as well as a key change in the rectification of current-voltage relationship (Fahlke, Beck & George 1997).

Another type of autosomal dominant Myotonia congenita called Acetalozamide Responsive MC, also exits (Trudell et al. & Rudel as cited in