Recurrent intrauterine fetal loss due to near absence of HERG: clinical and functional characterization of a homozygous nonsense HERG Q1070X mutation.

Background: Inherited arrhythmias may underlie intrauterine and neonatal arrhythmias. Resolving the molecular genetic nature of these rare cases provides significant insight into the role of the affected proteins in arrhythmogenesis and (extra-) cardiac development.
Objective: The purpose of this study was to perform clinical, molecular, and functional studies of a consanguineous Arabian family with repeated early miscarriages and two intrauterine fetal losses in the early part of the third trimester of pregnancy due to persistent arrhythmias.
Methods: In-depth clinical investigation was performed in two siblings, both of whom developed severe arrhythmia during the second trimester of pregnancy. Homozygosity mapping with microsatellite repeat polymorphic markers encompassing various cardiac ion channel genes linked to electrical instability of the heart was performed. Screening of the candidate gene in the homozygous locus was performed. Biochemical and electrophysiologic analysis was performed to elucidate the function of the mutated gene.
Results: Screening of the HERG gene in the homozygous locus detected a homozygous nonsense mutation Q1070X in the HERG C-terminus in affected children. Biochemical and functional analysis of the Q1070X mutant showed that although the mutant HERG had the ability to traffic to the plasma membrane and to form functional channels, it was destroyed by the nonsense-mediated decay (NMD) pathway before its translation. NMD leads to near absence of HERG in homozygous Q1070X mutation carriers, causing debilitating arrhythmias (prior to birth) in homozygous carriers but no apparent phenotype in heterozygous carriers.
Conclusion: Homozygous HERG Q1070X is equivalent to near functional knockout of HERG. Clinical consequences appear early, originating during the early stages of embryonic life. The NMD pathway renders HERG Q1070X functionless before it can form a functional ion channel.