(Circulation. 1997;95:565-567.)
© 1997 American Heart Association, Inc.
Articles |
the Laboratory of Molecular Medicine, Institute of Medical Science, University of Tokyo (T.T., Y.N.); Second Department of Internal Medicine, Gunma University School of Medicine (R.N.); First Department of Internal Medicine, Yamagata University School of Medicine (H.T.); First Department of Internal Medicine, School of Medicine, Kanazawa University (S.T.); Third Department of Internal Medicine, School of Medicine, Nagasaki University (K. Yano); Department of Pediatrics, School of Medicine, Juntendo University, Tokyo (K. Yabuta); Department of Pediatrics, Shimane Medical University (N.H.); Department of Pediatrics, Takamatsu Municipal Hospital, Kagawa (O.N.); First Department of Internal Medicine, School of Medicine, Niigata University (A.S.); Division of Cardiology, Department of Medicine, Kawasaki Medical School, Okayama (T.S.); Department of Cardiology, Metropolitan Kiyose Children's Hospital, Tokyo (H.K.); and Department of Internal Medicine III, University of Tokyo School of Medicine (Y.Y.), Japan.
Correspondence to Toshihiro Tanaka, MD, Laboratory of Molecular Medicine, Institute of Medical Science, University of Tokyo, 4-6-1, Shirokanedai, Minato-ku, Tokyo 108, Japan. E-mail toshitan@ims.u-tokyo.ac.jp.
| Abstract |
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Methods and Results Thirty-two Japanese families with LQTS were brought together for screening for mutations. Genomic DNA from each proband was examined by the polymerase chain reactionsingle-strand conformation polymorphism technique followed by direct DNA sequencing. In four of the families, comprising 16 patients, mutations were identified in KVLQT1; five other families (9 patients) segregated mutant alleles of HERG. All 25 of these patients carried the specific mutations present in their respective families, and none of 80 normal individuals carried these alleles. Mutations were confirmed by endonuclease digestion or hybridization of mutant allelespecific oligonucleotides. No mutation in SCN5A was found in any family.
Conclusions We identified nine different mutations among 32 families with LQTS. Eight of these were novel and account for 25% of all types of mutations reported to date. Such a variety of mutations makes it difficult to screen high-risk groups using simple methods such as endonuclease digestion or mutant allelespecific amplification.
Key Words: genes intervals genetics death, sudden torsade de pointes tachyarrhythmia
| Introduction |
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For patients with clinical symptoms, ß-adrenergic blocking agents are the first choice for treatment. Approximately 80% of LQTS patients can be managed effectively with these drugs, but the remaining 20% do not respond.5 Hence, if the different responses reflect mutations at different sites within a given gene or mutations in different genes, genetic diagnosis might provide information to govern the choice of an appropriate therapeutic approach.
Recently, three genes responsible for LQTS were identified. Two encode potassium-channel subunits (KVLQT16 and HERG7 ) and the other encodes a sodium-channel subunit (SCN5A8 ). So far, 12 mutations in KVLQT1,6 9 9 in HERG,7 10 11 and 3 in SCN5A8 12 have been identified. Although there seems to be one mutational hot spot (nucleotide 212 in KVLQT1; 8 of 34 LQTS families), it would not enable us to screen many people by means of endonuclease digestion or mutant allelespecific amplification methods.13
In the present study, we report mutations in KVLQT1 and HERG among 9 of 32 Japanese families examined.
| Methods |
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Mutational Analysis
DNA from the proband of each family was analyzed by polymerase chain reactionsingle-strand conformation polymorphism (PCR-SSCP). Primers for genomic amplification and sequencing were described previously.6 7 8 Cycling conditions were 35 cycles of 94°C for 30 seconds, 58°C for 30 seconds, and 72°C for 60 seconds. Each reaction mixture was diluted with 95% formamide dye, incubated at 80°C for 5 minutes, and applied to a 6% polyacrylamide gel containing 0.5x Tris-borate-EDTA buffer and 5% glycerol. Electrophoresis was performed at 4°C. Gels were dried and autoradiographed with intensifying screens.
For DNA sequencing, genomic DNA (20 ng) from each patient showing an aberrant conformer in PCR-SSCP analysis was amplified in a 25-mL PCR reaction as above. After removal of dNTPs and primers by columns, each sample was subjected to cycle sequencing using a dye terminator cycle-sequencing kit (Perkin-Elmer) according to the supplier's instructions. Electrophoresis was performed with the use of a 377 DNA sequencer (Perkin-Elmer). When a new recognition site of an endonuclease was made by a mutation, PCR products of DNAs from all available family members and from 80 normal individuals were digested by that enzyme and applied to a nondenaturing polyacrylamide gel for confirmation of the mutation. For additional confirmation, we also performed hybridization of mutant allele-specific oligonucleotides.17
| Results |
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| Discussion |
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Only 12.5% of our panel of 32 Japanese families with LQTS were shown to have mutations in KVLQT1, an unexpected finding in view of the linkage studies indicating that more than half of all LQTS patients may have mutations in KVLQT1.6 Possible reasons for the discrepancy include the following: (1) some families are too small to define the responsible genetic loci by linkage analysis; (2) because the entire coding region of KVLQT1 has not yet been cloned, mutational analysis cannot investigate the entire gene; and (3) another gene(s) responsible for LQTS may be located close to KVLQT1.
The relationship between KVLQT1 mutations and clinical symptoms is not yet clear. Patients with mutated SCN5A may be more likely to benefit from Na+ channel blockers,18 and an increase in serum potassium is shown to correct abnormalities of repolarization duration, T-wave morphology, QT/RR slope, and QT dispersion.19 Our preliminary, unpublished data may indicate that among symptomatic patients, those who have a mutated KVLQT1 allele show a better response to ß-blocking agents; this requires additional validation.
It also remains a mystery why a single family may contain clinically symptomatic and asymptomatic carriers of a mutant LQTS allele, all of them exhibiting prolongation of the QTc interval. It is likely that factors other than genetic ones, such as a hormonal environment that may cause an imbalance between activities of the sympathetic and parasympathetic nerves, may generate variations in the clinical phenotype. This issue is highly important for understanding the mechanism of ventricular arrhythmias.
| Acknowledgments |
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Received October 31, 1996; revision received December 6, 1996; accepted December 9, 1996.
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