Molekularbiologisches Screening des CHRM2-Gens bei Patienten mit dilatativer Kardiomyopathie

Dilated cardiomyopathy (DCM) is a heterogeneous heart muscle disease characterized by ventricular dilatation and impaired function of the heart. Despite autoimmune and infectious factors genetic changes play a major role in the pathogenesis of this disease. In the last few years mutations in over 30 different genes were identified in patients with DCM. Most were found in genes coding for proteins of the sarcomere or cytoskeletal proteins but mutations in receptors or their regulatory proteins were detected as well. The CHRM2-gen is a potential new disease gene of DCM. It is localized on chromosome 731q-35q and is coding for the muscarinic acetlycholin receptor type 2 (m2AChR). The m2AChR is the main muscarinic receptor in the mammalian heart and mediates a reduction of the intracellular cAMP concentration through the Gi-protein. Calcium influx into the myocardial cell is reduced resulting in negative inotrope and chronotrope effects. In a study of Zhang et al. one missense mutation in the CHRM2-gene was found in all affected members of a family with familial DCM. The purpose of this study was to identify further mutations in the coding region of the CHRM2-gene in patients with DCM and to explore the influence of these mutations on the clinical course of the disease. Therefore we screened the coding region of the CHRM2-gen with polymerase chain reaction (PCR) and single-stranded-conformation-polymorphism gelelectrophoresis (SSCP) in 337 patients with DCM. The DNA of patients with conspicuous findings in the SSCP was analyzed by direct sequencing. Informations of an echocardiographic examination at the time of admission and at an one year follow-up are documented. In the investigated group of patients with DCM we could identify two new heterozygous missense mutations and one polymorphism in the CHRM2-gene. As a result of the mutation G885A the aminoacid valin is changed to isoleucin at position 231 of the receptor protein. The effect of the mutation G1268C is a substitution of glutamin by histidine at position 358. The diagnosis of familial DCM was confirmed for the carrier of this mutation because the diagnosis of DCM was established in one son of the indexpatient. The mutation G1268C was found in the DNA of the son as well. A correlation between the identified mutations and the results of the echocardiographic examination of the carriers, especially at the one year follow-up, could not be observed. In a following screening of 300 healthy blood donors from the Marburger Blutbank no further carriers of the mutations were found. The effect of both mutations is an aminoacid change at the third intracellular loop of the m2AChR. This area is important for the receptor-G-protein-coupling and for the regulation of the receptor concentration at the cell surface or rather the regulation of the receptor function. An influence on the intracellular signal transduction and calcium homeostasis is possible. Taken together, the results of this study support the role of the CHRM2-gene in the pathogenesis of DCM. A prevalence of 0,6 % is comparable with the prevalence of established disease genes in DCM. For a complete understanding of the role of the CHRM2-gen in the pathogenesis of DCM and to clarify the effect of the mutations on the function of the m2AChR further functional analysis of the mutated receptor is required.