Congenital Myasthenic Syndrome (CHRNB1 gene)

Congenital myasthenic syndrome (CMS) is a rare genetic disorder that disrupts communication between nerves and muscles, causing muscle weakness. The condition is inherited and its symptoms, which typically appear in infancy, can be managed with specific drug treatments and supportive therapies.

Congenital myasthenic syndrome (CMS) is a group of rare inherited disorders that affect the neuromuscular junction, the point of chemical communication between nerve cells and muscle fibers. Mutations in the CHRNB1 gene are one of the causes of postsynaptic CMS. This gene contains the instructions for producing the beta subunit of the acetylcholine receptor (AChR), a crucial protein in the muscle cell membrane. When a nerve signal reaches the muscle, it releases acetylcholine, which binds to these receptors to trigger muscle contraction. Pathogenic variants in CHRNB1 can lead to a deficiency of receptors or alter their function, resulting in ineffective neuromuscular transmission and, consequently, muscle weakness and fatigability.

The global incidence of congenital myasthenic syndromes is estimated at 1-5 per 500,000 people, classifying them as rare diseases. No clear predisposition by sex or ethnicity has been described, affecting populations worldwide. Prevalence may be higher in specific communities due to founder effects, where a particular mutation is transmitted more frequently in an isolated or highly inbred population. The inheritance of mutations in CHRNB1 is typically autosomal recessive, meaning an individual must inherit two copies of the altered gene, one from each parent, to develop the disease. There are reported cases of autosomal dominant inheritance in which affected individuals

Symptoms of CMS associated with CHRNB1 usually manifest from birth to early childhood. The main characteristic is muscle weakness that worsens with exertion and improves with rest (fatigability). The most common signs include drooping of the eyelids (ptosis), double vision (diplopia) due to involvement of the eye muscles, difficulty sucking, swallowing, and speaking (bulbar weakness), and generalized weakness in the limbs. In the most severe cases, respiratory failure requiring ventilatory support may occur.

Management of the disease is personalized according to the underlying genetic abnormality. In the case of CMS due to mutations in CHRNB1 that cause receptor deficiency, treatment typically includes drugs that increase the amount of acetylcholine at the neuromuscular junction, such as acetylcholinesterase inhibitors (e.g., pyridostigmine). Sometimes, other medications such as 3,4-diaminopyridine or adrenergic agonists like salbutamol are used to improve muscle function. Accurate genetic diagnosis is crucial, as some treatments beneficial for one type of CMS can be detrimental in others. Management is complemented by physical therapy, occupational therapy, speech therapy, and respiratory support if necessary.