The major findings of the present study from 858 type-2 LQTS subjects with genetically confirmed KCNH2 mutations derived from 4 LQTS Registries are that 1) there is a significant mutation type-location interaction; specifically that the relative risk between C-terminus and the regions is different for missense versus non-missense locations, 2) patients with missense mutations in the transmembrane pore region have significantly higher cardiac event rates than those with missense mutations in either N-terminus, transmembrane non-pore or C-terminus regions, 3) patients with non-missense mutations were at significantly higher risk than those with missense mutations in the C-terminus region, 4) patients with mutations located in putative α-helical domains have significantly higher cardiac event rates than in those with mutations in either the β-sheet domains or other uncategorized locations, and these higher event rates are independent of traditional clinical risk factors and of β-blocker therapy. Our data indicate that risk stratification and specific management or treatment by distinct location, coding type and topology of the channel mutation in addition to classical risk factors such as QTc, gender or history of prior syncope may be possible in patients with type-2 LQTS, although further studies are definitely required.
A total of 12 forms of congenital LQTS have been reported,2,4,17-20
and clinical studies for genotype-phenotype correlations have been rigorously investigated in the type-1, type-2, and type-3 LQTS, which constitute more than 90 % of genotyped patients with LQTS.2,21-25
More recently, mutation-location specific differences in the severity of clinical phenotype have been investigated in each genotype. 9,11,12,26,27
As to the type-1 LQTS, a large cohort of 600 patients with KCNQ1
mutations has demonstrated that location and biophysical function of mutations were independent risk factors influencing the clinical course.11
However, the distribution of mutation location as well as the frequency of mutation type are reported to be different in each of 3 major genotypes.9,11,12,26,27
More recently, putative secondary structures of α-helices or β-sheet are reported to have an important role on the channel function in the type-2 LQTS.8
Therefore, a larger cohort of patients having a spectrum of KCNH2
mutations is required to test the hypothesis that the location, coding type and topology of mutations would influence the clinical course in the type-2 LQTS.
In contrast to our cohort of 600 type-1 LQTS patients where the majority of mutations were found in the transmembrane region (66.2%),11
in the present study mutations in KCNH2
were more evenly distributed in the N-terminus, the transmembrane domain, and the C-terminus. As to the type of mutation, missense mutations dominated (80.5%), and only 13% of the mutations were frameshift/nonsense mutations in our type-1 LQTS cohort.11
In contrast, missense mutations accounted for 61.7%, and frameshift/nonsense mutations were more frequently observed (35.8%) in this type-2 LQTS cohort. Interestingly, most of the mutations located in the transmembrane pore region were missense mutations (46/52, 88.5%) in the present study, a finding concordant with the previous type-2 LQTS cohort by Moss et al. (13/14, 92.9%).9
This indicated that the severe phenotype in patients with mutations located in the transmembrane pore region was probably due to the fact that missense mutations that are expected to cause dominant negative effects were predominant in this region. However, our type-2 LQTS patients with missense mutations located in the N-terminus, transmembrane non-pore and C-terminus regions were at significantly less risk than those with missense mutations in the transmembrane pore region. These data suggest that location of mutation, i.e. transmembrane pore region, itself was an independent risk in type-2 LQTS patients with KCNH2
On the other hand, patients with non-missense mutations, mainly frameshift/nonsense mutations were at significantly higher risk than those with missense mutations in the C-terminus region, and the event rates in patients with frameshift/nonsense mutations were not different among the transmembrane pore, transmembrane non-pore, N-terminus, and C-terminus regions. Gong et al. recently suggested that most frameshift/nonsense mutations would cause nonsense mediated decay (NMD) thereby producing less mRNA from the mutant alleles.28
This potentially would allow for the wild type allele to express more normal channels. Therefore, it is expected that the type-2 LQTS patients with frameshift/nonsense mutation causing NMD would have a mild phenotype. In contrast, the type-2 LQTS patients with frameshift/nonsense mutation without NMD would be expected to have a more severe phenotype because a truncated protein would be produced. Thus, the fact that some frameshift/nonsense mutations show NMD, whereas the other mutations do not, makes the clinical phenotype in the type-2 LQTS patients with frameshift/nonsense mutations more complicated, although this scenario is only a speculation. The present study confirmed the higher risk in patients with non-missense mutations than in those with missense mutations in C-terminus region, suggesting that more careful follow-up is required in type-2 LQTS patients with non-missense mutations in the C-terminus region.
With regard to the topology of mutation, Anderson and co-workers recently reported that missense mutations located in a highly ordered structure as α-helices or β-sheet correlated with a class 2 trafficking-deficient phenotype in the type-2 LQTS patients.8
In the present cohort, mutations located in the α-helical domains were associated with a significantly higher risk compared to those with mutations in either the β-sheet domains or other uncategorized locations. It is possible that missense mutations in α-helices, where secondary protein structure is thought to the highly ordered, lead to altered secondary and tertiary channel protein structure and abnormal trafficking. This new analysis considering putative secondary structures of mutated channel would be a useful approach in stratifying risk of cardiac events in patients with LQTS.
β-Blockers have long been the first choice of therapy in patients with congenital LQTS.2,29
However, it has been shown in previous studies that the protection that β-blockers provide against cardiac events for type-2, and type-3 LQTS patients is somewhat less effective than for type-1 LQTS patients.23,30
A variety of experimental data also support the genotype-specific efficacy of β-blockers in type-1 LQTS.31
In the present study, time-dependent β-blocker use significantly reduced the risk of first cardiac events by 63% (p<0.001), confirming the efficacy of β-blockers as a first line of therapy in patients with type-2 LQTS as well as suggesting more prophylactic use of β-blockers especially in high risk patients with type-2 LQTS. On the other hand, β-blocker use was associated with less protection (29%) in the prevention of lethal cardiac events compared to first cardiac events (mostly syncope), indicating that additional treatment such as potassium supplement or an ICD implantation may be considered in high risk patients with type-2 LQTS. The patients who have aborted cardiac arrest/sudden death may have a more malignant pathophysiology that is more resistant to β-blockers than are syncopal episodes. We purposely included “ECG missing” in the Cox model so that the β-blocker effect is actually adjusted for subjects with “ECG missing” who probably did not receive β-blockers.
We did not evaluate the risk associated with distinct type of biophysical ion-channel dysfunction (dominant-negative or haplotype insufficient), since only a small percentage of the mutations present within our patient population have been studied extensively in identical cellular expression experiments. There were 60 patients who were not genotyped, and they had an increased risk for events mainly because their fatal events occurred at a young age before they were genotyped. When these patients were excluded from the analysis, the pattern of risk in the missense and non-missense subgroups remains similar to the total population, but the significance of the effect is attenuated due to the reduced number of events.