Genotype-phenotype correlation in the long-QT syndrome: gene-specific triggers for life-threatening arrhythmias

BACKGROUND

The congenital long-QT syndrome (LQTS) is caused by mutations on several genes, all of which encode cardiac ion channels. The progressive understanding of the electrophysiological consequences of these mutations opens unforeseen possibilities for genotype-phenotype correlation studies. Preliminary observations suggested that the conditions ("triggers") associated with cardiac events may in large part be gene specific.

METHODS AND RESULTS

We identified 670 LQTS patients of known genotype (LQT1, n=371; LQT2, n=234; LQT3, n=65) who had symptoms (syncope, cardiac arrest, sudden death) and examined whether 3 specific triggers (exercise, emotion, and sleep/rest without arousal) differed according to genotype. LQT1 patients experienced the majority of their events (62%) during exercise, and only 3% occurred during rest/sleep. These percentages were almost reversed among LQT2 and LQT3 patients, who were less likely to have events during exercise (13%) and more likely to have events during rest/sleep (29% and 39%). Lethal and nonlethal events followed the same pattern. Corrected QT interval did not differ among LQT1, LQT2, and LQT3 patients (498, 497, and 506 ms, respectively). The percent of patients who were free of recurrence with ss-blocker therapy was higher and the death rate was lower among LQT1 patients (81% and 4%, respectively) than among LQT2 (59% and 4%, respectively) and LQT3 (50% and 17%, respectively) patients.

CONCLUSIONS

Life-threatening arrhythmias in LQTS patients tend to occur under specific circumstances in a gene-specific manner. These data allow new insights into the mechanisms that relate the electrophysiological consequences of mutations on specific genes to clinical manifestations and offer the possibility of complementing traditional therapy with gene-specific approaches.

Mutations in the skeletal muscle alpha-actin gene in patients with actin myopathy and nemaline myopathy

Muscle contraction results from the force generated between the thin filament protein actin and the thick filament protein myosin, which causes the thick and thin muscle filaments to slide past each other. There are skeletal muscle, cardiac muscle, smooth muscle and non-muscle isoforms of both actin and myosin. Inherited diseases in humans have been associated with defects in cardiac actin (dilated cardiomyopathy and hypertrophic cardiomyopathy), cardiac myosin (hypertrophic cardiomyopathy) and non-muscle myosin (deafness). Here we report that mutations in the human skeletal muscle alpha-actin gene (ACTA1) are associated with two different muscle diseases, 'congenital myopathy with excess of thin myofilaments' (actin myopathy) and nemaline myopathy. Both diseases are characterized by structural abnormalities of the muscle fibres and variable degrees of muscle weakness. We have identified 15 different missense mutations resulting in 14 different amino acid changes. The missense mutations in ACTA1 are distributed throughout all six coding exons, and some involve known functional domains of actin. Approximately half of the patients died within their first year, but two female patients have survived into their thirties and have children. We identified dominant mutations in all but 1 of 14 families, with the missense mutations being single and heterozygous. The only family showing dominant inheritance comprised a 33-year-old affected mother and her two affected and two unaffected children. In another family, the clinically unaffected father is a somatic mosaic for the mutation seen in both of his affected children. We identified recessive mutations in one family in which the two affected siblings had heterozygous mutations in two different exons, one paternally and the other maternally inherited. We also identified de novo mutations in seven sporadic probands for which it was possible to analyse parental DNA.

Mutations in the nebulin gene associated with autosomal recessive nemaline myopathy

The congenital nemaline myopathies are rare hereditary muscle disorders characterized by the presence in the muscle fibers of nemaline bodies consisting of proteins derived from the Z disc and thin filament. In a single large Australian family with an autosomal dominant form of nemaline myopathy, the disease is caused by a mutation in the alpha-tropomyosin gene TPM3. The typical form of nemaline myopathy is inherited as an autosomal recessive trait, the locus of which we previously assigned to chromosome 2q21.2-q22. We show here that mutations in the nebulin gene located within this region are associated with the disease. The nebulin protein is a giant protein found in the thin filaments of striated muscle. A variety of nebulin isoforms are thought to contribute to the molecular diversity of Z discs. We have studied the 3' end of the 20. 8-kb cDNA encoding the Z disc part of the 800-kDa protein and describe six disease-associated mutations in patients from five families of different ethnic origins. In two families with consanguineous parents, the patients were homozygous for point mutations. In one family with nonconsanguineous parents, the affected siblings were compound heterozygotes for two different mutations, and in two further families with one detected mutation each, haplotypes are compatible with compound heterozygosity. Immunofluorescence studies with antibodies specific to the C-terminal region of nebulin indicate that the mutations may cause protein truncation possibly associated with loss of fiber-type diversity, which may be relevant to disease pathogenesis.

Mutation of the gene for IsK associated with both Jervell and Lange-Nielsen and Romano-Ward forms of Long-QT syndrome

BACKGROUND

Long-QT syndrome (LQTS) is a disorder of ventricular repolarization characterized by a prolonged QT interval, syncope, seizures, and sudden death. Recently, three forms of LQTS have been shown to result from mutations in potassium or sodium ion channel genes: KVLQT1 for LQT1, HERG for LQT2, and SCN5A for LQT3. IsK, an apparent potassium channel subunit encoded by KCNE1 on chromosome 21, regulates both KVLQT1 and HERG. This relationship makes KCNE1 a likely candidate gene, because mutations of these genes are known to cause both the autosomal dominant Romano-Ward and recessive Jervell and Lange-Nielsen (JLN) forms of LQTS.

METHODS AND RESULTS

We screened 84 unrelated patients with Romano-Ward and 4 with JLN for possible mutations in KCNE1. We identified one homozygous mutation in a JLN patient that results in the nonconservative substitution of Asn for Asp at amino acid 76. The patient is congenitally deaf-mute, with recurrent syncopal events and a greatly prolonged QTc interval. The proband's mother and half-sister are both heterozygous for this mutation. Remarkably, both these family members have prolonged QTc intervals and would have been classified as Romano-Ward patients if not for the proband's diagnosis of JLN. This mutation was not identified in more than 100 control individuals.

CONCLUSIONS

These data provide strong evidence that KCNE1 mutations represent a fifth LQTS locus (LQT5). Further functional analysis, as well as the identification of more LQTS patients with KCNE1 mutations, will be important to confirm the role of IsK in LQTS.

Mutations of the slow muscle alpha-tropomyosin gene, TPM3, are a rare cause of nemaline myopathy

The alpha-tropomyosin-3 (TPM3) gene was screened in 40 unrelated patients with nemaline myopathy (NM). A single compound heterozygous patient was identified carrying one mutation that converts the stop codon to a serine and a second splicing mutation that is predicted to prevent inclusion of skeletal muscle exon IX. TPM3 mutations are a rare cause of NM, probably accounting for less than 3% of cases. The severity of cases with TPM3 mutations may vary from severe infantile to late childhood onset, slowly progressive forms.

High prevalence of V37I genetic variant in the connexin-26 (GJB2) gene among non-syndromic hearing-impaired and control Thai individuals

Hearing loss is highly prevalent with a worldwide incidence of 1-2 per 1000 newborns. Several previous studies have demonstrated that mutations of connexin 26 (Cx26 or GJB2) are responsible for most cases of the recessive non-syndromic sensorineural hearing loss (NSSHL). Certain mutations have been described frequently among various populations, which include 35delG, 167delT, and 235delC. Recently, a missense mutation, V37I, was reported as a pathogenic change in East Asian affected individuals. To identify genetic variants associated with NSSHL in Thai population, we performed mutation analysis of Cx26 in 166 unrelated probands with NSSHL and 205 controls. We identified seven novel genetic variants in Cx26. We also identified a high prevalence of the V37I mutation among both affected probands (11.1%) and control subjects (8.5%), which suggests that the pathologic role of V37I may be modified by other genes. Our data support previous studies that show heterogeneity in the frequencies and types of mutations in Cx26 within populations and among ethnicities and that before clinical significance and causality can be attributed to a genetic variant, functional characterization is necessary.

Sodium channel abnormalities are infrequent in patients with long QT syndrome: identification of two novel SCN5A mutations

Long QT syndrome (LQTS) is a heterogeneous disorder caused by mutations of at least five different loci. Three of these, LQT1, LQT2, and LQT5, encode potassium channel subunits. LQT3 encodes the cardiac-specific sodium channel, SCN5A. Previously reported LQTS-associated mutations of SCN5A include a recurring three amino acid deletion (DeltaKPQ1505-1507) in four different families, and four different missense mutations. We have examined the SCN5A gene in 88 index cases with LQTS, including four with Jervell and Lange-Nielsen syndrome and the remainder with Romano-Ward syndrome. Screening portions of DIII-DIV, where mutations have previously been found, showed that none of these patients has the three amino acid deletion, DeltaKPQ1505-1507, or the other four known mutations. We identified a novel missense mutation, T1645M, in the DIV; S4 voltage sensor immediately adjacent to the previously reported mutation R1644H. We also examined all of the additional pore-forming regions and voltage-sensing regions and discovered another novel mutation, T1304M, at the voltage-sensing region DIII; S4. Neither T1645M nor T1304M were seen in a panel of unaffected control individuals. Five of six T1304M gene carriers were symptomatic. In contrast to previous studies, QT(onset-c) was not a sensitive indicator of SCN5A-associated LQTS, at least in this family. These data suggest that mutations of SCN5A are responsible for only a small proportion of LQTS cases.

Molecular analysis of the iduronate-2-sulfatase gene in Thai patients with Hunter syndrome

Molecular defects in the gene encoding the enzyme iduronate-2-sulfatase (IDS) result in Hunter disease (mucopolysaccharidosis type II, MPS II). To determine the molecular basis of MPS II in Thailand, the IDS gene was analysed in 20 Thai patients with Hunter syndrome from 18 unrelated families. A total of 19 different mutations, including 9 missense mutations, 3 nonsense mutations, 3 splice site alterations, 1 deletion, 2 indels, and 1 rearrangement were identified, 8 of which were novel (p.R101C, p.D148V, p.G224A, p.K227E, p.E254X, p.W337X, c.440_442delinsTT and c.720_731delinsTTTCAGATGTTCTCCCCAG). Evaluation of the IDS activity of two hemizygous variants identified in the same patient, p.R101C and p.R468Q, by expression of IDS with the individual mutations in COS 7 cells indicated that only the p.R468Q mutation affected IDS protein activity. Two exonic mutations, c.257C>T (p.P86L) and c.418G>A, were found to activate multiple cryptic splice sites, resulting in aberrantly spliced transcripts. Thus, MPS II in Thailand is caused by a diverse set of defects affecting both IDS protein production and activity.

Molecular genetics of long-QT syndrome

Recent discoveries of genes involved in long-QT syndrome (LQTS) have led to extensive progress in understanding the molecular basis for this disorder of syncope and sudden cardiac death secondary to ventricular arrhythmias. The emerging unifying theme is that all genes identified to date encode either structural or regulatory subunits for ion channels involved in cardiac repolarization. Defects have been identified in the KCNQ1, HERG, and KCNE1 genes, whose proteins form the K+ channels for the slowly and rapidly inwardly rectifying K+ currents IKs and IKr. Depending on their location and copy number, mutations of KCNQ1 and KCNE1 can cause either autosomal dominant Romano-Ward syndrome or autosomal recessive Jervell and Lange-Nielsen syndrome. The cardiac sodium channel gene, SCN5A, is also mutated in some Romano-Ward cases to produce defects in INa, the cardiac inward Na+ current. The fact that multiple genes are involved and that most LQTS mutations are "private" or "family-specific" complicates molecular diagnosis of LQTS which, currently, is limited to a small number of research laboratories. In future, genotypic determination of LQTS patients and their family members will hopefully lead to improved gene-specific prognostic determinations and therapeutic interventions.

Globalization of DNA-based prenatal diagnosis for recessive dystrophic epidermolysis bullosa

Globalization of economies and improvements in international telecommunications has led to increased demand for better access to the latest developments in healthcare, wherever they may be available. In this report, we describe the first case from Thailand of DNA-based prenatal testing of a mother at risk for recurrence of severe recessive dystrophic epidermolysis bullosa (RDEB), whose affected child had died in early childhood. In the absence of previous access to prenatal diagnostic tests, the mother had undergone several terminations for fear of having another affected child. To prevent this happening again, DNA from the mother and her consanguineous partner was sent from Bangkok to a specialist laboratory at St John's Institute of Dermatology in London and screened for pathogenic mutations in the COL7A1 gene: both individuals were shown to be heterozygous carriers of a splice-site mutation, c.2440G --> C. In a subsequent pregnancy, amniocentesis was performed at 18 weeks' gestation in Bangkok, and fetal DNA was extracted and sent to London for analysis. Restriction endonuclease digestion of the amplified fetal DNA revealed the wild-type COL7A1 sequence only, and 5 months later, a clinically unaffected boy was born. This case represents the first example of DNA-based prenatal diagnosis for RDEB in Thailand and illustrates the benefits for patients in establishing international links with diagnostic centres with technological expertise that is not widely available in certain countries.

Molecular characterization of type 3 (neuronopathic) Gaucher disease in Thai patients

Gaucher disease is an autosomal recessive lysosomal storage disorder due to deficiency of the lysosomal enzyme glucocerebrosidase. Three clinical phenotypes, type 1, nonneuronopathic; and types 2 and 3, acute and subacute neuronopathic are recognized. The incidence of Gaucher disease in the Thai population is unknown, but likely under-diagnosed. We performed molecular analysis in four patients, from three sibships, with type 3 Gaucher disease. Four mutant glucocerebrosidase (GBA) alleles were identified including two novel splice site mutations, IVS6-1G>C and IVS9-3C>G; both are predicted to result in truncated protein products, p.F255fsX256, and p.K464fsX487 and p.S463fsX480, respectively. One patient, homozygous for the L444P point mutation, had a "Norbottnian-like" phenotype, with more severe visceral involvement, kyphosis, barreled chest, and no neurological involvement other than supranuclear gaze palsy. These molecular studies of neuronopathic Gaucher disease will provide additional genotype-phenotype correlation particularly in non-Caucasian population.

Strokes in Thai children : etiology and outcome

In Asian countries, specific etiology and outcome of stroke in children are rarely reported. During January 1979 to December 1997, 68 children with stroke, admitted to the Department of Pediatrics, Ramathibodi Hospital, Bangkok were reviewed for etiology and outcome; 38 patients (22 males) had ischemic stroke and 30 (20 males) had hemorrhagic stroke. Severe headache, vomiting, disturbance of consciousness and papilledema were prominent presentations of hemorrhagic stroke. Bleeding from vascular anomalies of the brain (AVM) was the most common etiology of hemorrhagic stroke while septic and non-septic emboli from congenital and acquired heart diseases were the most common cause of ischemic stroke. The mortality rate was 7% and 9% in hemorrhagic and ischemic strokes, respectively. Complete recovery was observed in 52% of cases of hemorrhagic stroke but in only 26% of ischemic stroke. Hemiparesis was the most common residual neurological deficit with higher occurrence in ischemic stroke.

Case report of a Thai male cystic fibrosis patinet with the 1898+ 1G-->T splicing mutation in the CFTR gene: a review of East Asian cases. Mutations in brief no. 196. Online

Cystic fibrosis (CF) is the most common fatal autosomal recessive multisystem disorder, which occurs mainly in European-derived populations. The incidence of CF varies between 1 in 2000 to 3000 live-births in various ethnic groups. The disease is rare in East Asians. Here we report a 9 year old Thai male patient, who was diagnosed to have CF based on recurrent pneumonia, a slow weight gain, pancreatic insufficiency and repeatedly elevated sweat chloride levels by two different methods. A comprehensive genetic analysis showed the splicing mutation, 1898+ 1G-->T, which was apparently of maternal origin. Literature search found 39 documented cases of CF patients in East Asians. CFTR (MIM# 602421) genotyping was performed in 14 patients including our patient and in 9 of them a CF allele was identified. The findings seem to indicate that the splicing mutations, 1898+ 1G-->T and 1898+ 5G-->T are more common in East Asian CF patients.