Genetic variants for long QT syndrome among infants and children from a statewide newborn hearing screening program cohort

Long QT and Hearing Loss in High-Risk Infants Prospective Study Registry

The objective of this study is to determine the prevalence of an abnormal electrocardiogram showing a prolonged QTc greater than 450 ms in infants with unilateral or bilateral sensorineural hearing loss. We conducted a prospective study of healthy term infants (≥37 weeks gestational age) who failed their newborn auditory brainstem response hearing screen, were seen by an audiologist and diagnosed as having sensorineural hearing loss during follow-up to 1 year of age. In infants with a diagnosis of hearing loss, we collected a detailed family history and performed an ECG between 2 and 6 months of age. We obtained follow-up for 1 year by calling the parent requesting the hearing and cardiac status of their child. Two of the 40 infants with sensorineural hearing loss (5%) had a QTc greater than 450 ms. Both had mild bilateral hearing loss and genetic testing did not identify a known mutation for long QT syndrome. The remaining 38 infants had QTc intervals of ≤ 450 ms. One patient diagnosed with bilateral severe sensorineural hearing loss had a normal ECG (QTc = 417 ms). Several months after the ECG was performed, the infant's mother contacted the study cardiologist after she learned that the infant's maternal grandmother was diagnosed with a cardiomyopathy and arrhythmias. Genetic testing was recommended even though the child was asymptomatic and was positive for a pathogenic mutation in the KCNQ1 gene. We speculate that molecular genetic testing in infants with hearing loss may become the standard of care rather than targeted electrocardiograms.Clinical Trial Registration NCT02082431 https://www.clinicaltrials.gov/ct2/show/NCT02692521?cond=NCT02692521&rank=1 .

Genomic and Non-Genomic Regulatory Mechanisms of the Cardiac Sodium Channel in Cardiac Arrhythmias

Nav1.5 is the predominant cardiac sodium channel subtype, encoded by the SCN5A gene, which is involved in the initiation and conduction of action potentials throughout the heart. Along its biosynthesis process, Nav1.5 undergoes strict genomic and non-genomic regulatory and quality control steps that allow only newly synthesized channels to reach their final membrane destination and carry out their electrophysiological role. These regulatory pathways are ensured by distinct interacting proteins that accompany the nascent Nav1.5 protein along with different subcellular organelles. Defects on a large number of these pathways have a tremendous impact on Nav1.5 functionality and are thus intimately linked to cardiac arrhythmias. In the present review, we provide current state-of-the-art information on the molecular events that regulate SCN5A/Nav1.5 and the cardiac channelopathies associated with defects in these pathways.

Mutational and phenotypic spectra of KCNE1 deficiency in Jervell and Lange-Nielsen Syndrome and Romano-Ward Syndrome

KCNE1 encodes a regulatory subunit of the KCNQ1 potassium channel-complex. Both KCNE1 and KCNQ1 are necessary for normal hearing and cardiac ventricular repolarization. Recessive variants in these genes are associated with Jervell and Lange-Nielson syndrome (JLNS1 and JLNS2), a cardio-auditory syndrome characterized by congenital profound sensorineural deafness and a prolonged QT interval that can cause ventricular arrhythmias and sudden cardiac death. Some normal-hearing carriers of heterozygous missense variants of KCNE1 and KCNQ1 have prolonged QT intervals, a dominantly inherited phenotype designated Romano-Ward syndrome (RWS), which is also associated with arrhythmias and elevated risk of sudden death. Coassembly of certain mutant KCNE1 monomers with wild-type KCNQ1 subunits results in RWS by a dominant negative mechanism. This paper reviews variants of KCNE1 and their associated phenotypes, including biallelic truncating null variants of KCNE1 that have not been previously reported. We describe three homozygous nonsense mutations of KCNE1 segregating in families ascertained ostensibly for nonsyndromic deafness: c.50G>A (p.Trp17*), c.51G>A (p.Trp17*), and c.138C>A (p.Tyr46*). Some individuals carrying missense variants of KCNE1 have RWS. However, heterozygotes for loss-of-function variants of KCNE1 may have normal QT intervals while biallelic null alleles are associated with JLNS2, indicating a complex genotype-phenotype spectrum for KCNE1 variants.

Electrocardiogram Screening in Children with Congenital Sensorineural Hearing Loss: Prevalence and Follow-up of Abnormalities

Objective The purpose is to determine the prevalence of electrocardiogram (ECG) abnormalities, including borderline and prolonged QT, among screened children with sensorineural hearing loss (SNHL) and to analyze their subsequent medical workup. Study Design Institutional Review Board-approved case series with chart review. Setting Tertiary academic center. Subjects and Methods Cases from 1996 to 2014 involving pediatric patients (N = 1994) with SNHL were analyzed. Abnormal ECGs were categorized as borderline/prolonged QT or other. A board-certified pediatric cardiologist retrospectively determined the clinical significance of ECG changes. For follow-up analysis, children with heart disease, known syndromes, or inaccessible records were excluded. Results Among 772 children who had ECGs, 215 (27.8%) had abnormal results: 35 (4.5%) with QT abnormalities and 180 (23.3%) with other abnormalities. For children with QT abnormalities meeting inclusion criteria (n = 30), follow-up measures included cardiology referral (46.6%), repeat ECG by ear, nose, and throat (ENT) specialist (20%), clearance by ENT specialist with clinical correlation and/or comparison with old ECGs (20%), and pediatrician follow-up (6.7%). Documentation of further workup by ENT or referral was absent for 6.7%. For children with other ECG changes meeting inclusion criteria (n = 136), abnormalities were documented for 57 (41.9%); normal QT without other abnormality was documented for 18 (13.2%). The most common follow-up referrals were to pediatricians (16.9%) and cardiologists (10.3%). Among patients with clinically significant non-QT abnormalities mandating further evaluation (n = 122), 38 (31.1%) had documented follow-up in medical records. Conclusion There is a high prevalence of ECG abnormalities among children with congenital SNHL. If findings are confirmed by future studies, screening should be considered for congenital unilateral or bilateral SNHL, regardless of severity. We describe a standardized protocol for ECG screening/follow-up.

Infant sudden death: Mutations responsible for impaired Nav1.5 channel trafficking and function

BACKGROUND

Two genetic variants in SCN5A, encoding the Nav1.5 Na⁺ channel α-subunit, were found in a 5-month-old girl who died suddenly in her sleep. The first variant is a missense mutation, resulting in an amino acid change (Q1832E), which has been described (but not characterized) in a patient with Brugada syndrome. The second is a nonsense mutation that produces a premature stop codon and a C-terminal truncation (R1944Δ).

METHODS AND RESULTS

To investigate their functional relevance with patch clamp experiments in transfected HEK-293 cells. The Q1832E mutation drastically reduced Nav1.5 current density. The R1944Δ C-terminal truncation had negligible effects on Nav1.5 current density. Neither of the mutations affected the voltage dependence of steady activation and inactivation or influenced the late Na⁺ current or the recovery from inactivation. Biochemical and immunofluorescent approaches demonstrated that the Q1832E mutation caused severe trafficking defects. Polymerase chain reaction cloning and sequencing the victim's genomic DNA allowed us to determine that the two variants were in trans. We investigated the functional consequences by coexpressing Nav1.5(Q1832E) and Nav1.5(R1944Δ), which led to a significantly reduced current amplitude relative to wild-type.

CONCLUSIONS

These sudden infant death syndrome (SIDS)-related variants caused a severely dysfunctional Nav1.5 channel, which was mainly due to trafficking defects caused by the Q1832E mutation. The decreased current density is likely to be a major contributing factor to arrhythmogenesis in Brugada syndrome and the sudden death of this SIDS victim.

Clinical and molecular findings in a Moroccan family with Jervell and Lange-Nielsen syndrome: a case report

Background

Jervell and Lange-Nielsen syndrome (Online Mendelian Inheritance in Man 220400) is a rare autosomal recessive cardioauditory ion channel disorder that affects 1/200,000 to 1/1,000,000 children. It is characterized by congenital profound bilateral sensorineural hearing loss, a long QT interval, ventricular tachyarrhythmias, and episodes of torsade de pointes on an electrocardiogram. Cardiac symptoms arise mostly in early childhood and consist of syncopal episodes during periods of stress, exercise, or fright and are associated with a high risk of sudden cardiac death. Jervell and Lange-Nielsen syndrome is caused by homozygous or compound heterozygous mutations in KCNQ1 on 11p15.5 or KCNE1 on 1q22.1-q22.2.

Case presentation

We report the case of a 10-year-old Moroccan boy with congenital hearing loss and severely prolonged QT interval who presented with multiple episodes of syncope. His parents are first-degree cousins. We performed Sanger sequencing and identified a homozygous variant in KCNQ1 (c.1343dupC, p.Glu449Argfs*14).

Conclusions

The identification of the genetic substrate in this patient confirmed the clinical diagnosis of Jervell and Lange-Nielsen syndrome and allowed us to provide him with appropriate management and genetic counseling to his family. In addition, this finding contributes to our understanding of genetic disease in the Moroccan population.

Inherited arrhythmias: The cardiac channelopathies

Ion channels in the myocardial cellular membrane are responsible for allowing the cardiac action potential. Genetic abnormalities in these channels can predispose to life-threatening arrhythmias. We discuss the basic science of the cardiac action potential; outline the different clinical entities, including information regarding overlapping diagnoses, touching upon relevant genetics, new innovations in screening, diagnosis, risk stratification, and management. The special considerations of sudden unexplained death and sudden infant death syndrome are discussed. Scientists and clinicians continue to reconcile the rapidly growing body of knowledge regarding the molecular mechanisms and genetics while continuing to improve our understanding of the various clinical entities and their diagnosis and management in clinical setting. Two separate searches were run on the National Center for Biotechnology Information's website. The first using the term cardiac channelopathies was run on the PubMed database using filters for time (published in past 5 years) and age (birth-18 years), yielding 47 results. The second search using the medical subject headings (MeSH) database with the search terms “Long QT Syndrome” (MeSH) and “Short QT Syndrome” (MeSH) and “Brugada Syndrome” (MeSH) and “Catecholaminergic Polymorphic Ventricular Tachycardia” (MeSH), applying the same filters yielded 467 results. The abstracts of these articles were studied, and the articles were categorized and organized. Articles of relevance were read in full. As and where applicable, relevant references and citations from the primary articles where further explored and read in full.

Comparison of formulas for calculation of the corrected QT interval in infants and young children

OBJECTIVE

To compare 4 heart rate correction formulas for calculation of the rate corrected QT (QTc) interval among infants and young children.

STUDY DESIGN

R-R and QT intervals were measured from digital electrocardiograms. QTc were calculated with the Bazett, Fridericia, Hodges, and Framingham formulas. QTc vs R-R graphs were plotted, and slopes of the regression lines compared. Slopes of QTc-R-R regression lines close to zero indicate consistent QT corrections over the range of heart rates.

RESULTS

We reviewed electrocardiograms from 702 children, with 233 (33%) <1 year of age and 567 (81%) <2 years. The average heart rate was 122 ± 20 bpm (median 121 bpm). The slopes of the QTc-R-R regression lines for the 4 correction formulas were -0.019 (Bazett); 0.1028 (Fridericia); -0.1241 (Hodges); and 0.2748 (Framingham). With the Bazett formula, a QTc >460 ms was 2 SDs above the mean, compared with "prolonged" QTc values of 414, 443, and 353 ms for the Fridericia, Hodges, and Framingham formulas, respectively.

CONCLUSIONS

The Bazett formula calculated the most consistent QTc; 460 ms is the best threshold for prolonged QTc. The study supports continued use of the Bazett formula for infants and children and differs from the use of the Fridericia correction during clinical trials of new medications.