Mutation detection of cardiac ion channel genes in sudden death cases

在一例形态学检查无法解释且既往史未见的青壮年猝死中，由于长QT综合征（LQT）是由多种离子通道基因突变引起的，我们构建了心脏传导系统离子通道表达可能参与猝死的工作理论。分析8例猝死病例和100例健康对照的突变导致LQT的HERG、KVLQT和SCN5A，以及Kir6.2、GIRK1、GIRK4和MAXIK的突变。基于GIRK4相关通道的功能（I_<KACh>），发现GIRK4内含子中的C到T转换是猝死的唯一候选突变。这种转变被认为改变了剪接供体位点，导致在下一个外显子过早引入终止密码子。该mRNA预计比正常mRNA长5bp。从健康个体纯合子中恢复的白细胞中异位表达的mRNA的RT-PCR未能显示更长的mRNA，但这一结果并不意味着假定的剪接变体不会在脉冲传导系统中产生。只有一小部分截断的GIRK4必须通过显性负作用降低I_<KACh>通道，从而导致I_<KACh>通道功能损伤。除了GIRK4突变外，在其他通道基因中也发现了一些多态性突变，但它们都是中性变异体。在这项研究中，没有沿着基因的总长度寻找突变。需要进一步筛选基因突变。

In the sudden death of a young adult, which is unexplained by a morphorogical examination and is unexpected by past history, we contructed a working theory that ion channels expressed in cardiac conducting system may be involved in the sudden death because long QT syndrome (LQT) has been shown to be caused by mutations in various ion channel genes.HERG, KVLQT, and SCN5A of which mutations cause LQT, and Kir6.2, GIRK1, GIRK4, and MAXIK were analyzed for mutations in eight sudden death cases and 100 healthy controls. A C to T transition in an intron of GIRK4 was found to be the only candidate mutation for sudden death on the basis of the function of the GIRK4 associated channel (I_<KACh>). The transition was assumed to change the splice donor site, leading to premature introduction of stop codon in the following exon. The mRNA was expected to be 5 bp longer than usual one. RT-PCR of ectopically expressed mRNA in leucocytes recovered from a healthy individual homozygous for the mutation failed to show the longer mRNA but this result does not mean that the presumed splice variant is not produced in impulse' conducting system. Only a small fraction of the truncated GIRK4 must decrease the I_<KACh> channel by dominant negative effect, thus resulting in functional impairment of the I_<KACh> channel.In addition to the GIRK4 mutation, several polymorphic mutations were identified in the other channel genes but all of them were found to be neutral variants. In this study, mutation has not been searched for along the total length of the genes. Further screening for mutation of the genes will be required.