The Nicotinic Acetylcholine Receptor (nAChR) is a membrane receptor protein found on skeletal muscles, which is responsible for signal transduction between nerves and muscles. The binding of the ligand acetylcholine to the nAChR molecule, leads to a conformation change resulting in the opening of an ion-conducting channel across the plasma membrane. The mature functional protein is composed of four subunits - alpha, beta, gamma/epsilon, and delta. The gamma polypeptide of this protein is coded for by the CHRNG gene. In fetal and denervated muscles, the nAChR protein contains the gamma polypeptide. This forms a low-conductance, long open-end channel, which is distributed throughout the fiber length. In adults, however, the CHRNG is expressed in only very small amounts, so that the channel contains the epsilon polypeptide, and is of high-conductance, brief open type.
Mutations in the CHRNG gene have been implicated in the development of the lethal type and Escobar variant forms of multiple pterygium syndrome.
The CHRNG gene is located on chromosome 2, where it spans a length of approximately 8 Kb. The protein product of the gene contains 517 amino acids and weighs approximately 57 KDa. The gamma subunit is present as part of the functional nAChR protein before the 33rd week of gestation. Reports suggest that this fetal form of the nAChR helps not only in neuromuscular signal transduction, but also in neuromuscular organogenesis.
Hoffmann et al. (2006) included three consanguineous Lebanese families in their linkage study to identify the locus responsible for Escobar Syndrome. All three families had at least one affected child. Having localized the causative loci to the CHRNG gene in an Omani family, Hoffmann et al. (2006) detected three different mutations in each of the Lebanese families. These included: an insertional mutation, 807insT (del248-274, 275X), which caused a frame shift in the protein, a putative splice site mutation, G1249C (del395-418, 419X), and a mutation predicted to truncate major parts of the polypeptide, C715T (p.R217C). All mutations were found in homozygous condition in the affected individuals. The p.R217C mutation was found to be especially severe, considering that the family that harbored this mutation had two children affected with the severe form of the disease, and also had a history of a neonatal death [See also: Oman > Hoffmann et al., 2006].
In affected members of a family with Escobar syndrome reported by Rajab et al. (2005), Hoffman et al. (2006) performed linkage studies to identify mutations within the gamma subunit gene (CHRNG). Genomewide scanning with 10K Affymetrix SNP chip, and fine mapping with microsatellites were done, and then functional candidate genes within the linkage interval were sequenced by the standard sequencing techniques. All described CHRNG mutations were tested in control chromosomes and for correct segregation within the patient's family. Expression studies of the detected mutations were performed in HEK cell lines. The families under study included one from Oman, which consisted of eight unaffected and two affected (a 12-year old girl, and a 5-year old boy) children born to consanguineous parents. Genomic scanning, fine mapping and haplotyping narrowed the region which had significant linkage to chromosome 2q37 to a 10cM interval within which nonsense gamma Q-18X mutation in CHRNG was detected at exon 1. The mutation cosegregated with the phenotype in the family and was not found in the control chromosomes, thus making polymorphism unlikely. The result of transfection of HEK cells with expression vectors for all subunits was regular AChR assembly and positioning at the cellular surface, which was absent from the surface, with partial subunit assembly in the endoplasmic reticulum, when the gamma subunit-expression vector was not used. This state of affairs corresponded to the gamma Q-18X mutation. In situ hybridization at mouse E14 showed the expression of gamma subunit in skeletal muscles of the limbs, head, paravertebral trunk, and in the diaphragm which corresponded to the major sites of human disease, while postnatally, gamma expression decreased along with a reciprocal increase in the epsilon subunit, indicating that the disease is mainly relevant for intrauterine neuromuscular development. Hoffman et al. (2006) concluded that the observed postnatal symptoms (arthrogryposis, pterygia, and scoliosis) were results of improper AChR function in utero with no progression of the disease after birth, and that Escobar syndrome is an inherited fetal myasthenic disease but with no myasthenic symptoms later in life as gamma expression is restricted to early development [See also: Lebanon > Hoffmann et al., 2006].
Morgan et al. (2006) undertook a genomewide linkage scan of a large consanguineous Saudi Arabian family with both lethal and the non-lethal (Escobar) variants of multiple pterygium syndrome. This family has five affected individuals (all males) as well as two affected individuals who died in the neonatal period. Genetic linkage confirmed a region of homozygosity at 2q36-q37 in the five living affected individuals from this family as well as other family of Pakistani origin, also studied by Morgan et al. (2006). DNA analysis revealed a homozygous missense mutation in the exon 4 of the CHRNG gene (c.320T>G, p.Val107Gly) in the Saudi Arabian family. Morgan et al. (2006) did not find this mutation in 384 Asian and whites (192 of each) and 84 Arabic control chromosomes. The val107 residue is conserved in the CHRNG proteins of 6 other species.
In a large consanguineous UAE family of Pakistani origin in which two sisters had the Escobar variant of multiple pterygium syndrome, Morgan et al. (2006) found a homozygous nonsense mutation, c.136C>T (p.R46X).