A novel KCNQ1 nonsense variant in the isoform-specific first exon causes both jervell and Lange-Nielsen syndrome 1 and long QT syndrome 1: a case report
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According to previous KCNQ1 (potassium channel, voltage gated, KQT-like subfamily, member 1) gene screening studies, missense variants, but not nonsense or frame-shift variants, cause the majority of long QT syndrome (LQTS; Romano-Ward syndrome [RWS]) 1 cases. Several missense variants are reported to cause RWS by a dominant-negative mechanism, and some KCNQ1 variants can cause both Jervell and Lange-Nielsen Syndrome (JLNS; in an autosomal recessive manner) and LQTS1 (in an autosomal dominant manner), while other KCNQ1 variants cause only JLNS. The human KCNQ1 gene is known to have two transcript isoforms (kidney isoform and pancreas isoform), and both isoforms can form a functional cardiac potassium channel.
Here, we report a novel nonsense KCNQ1 variant causing not only JLNS, but also significant QTc prolongation identical to RWS in an autosomal dominant manner. Our case study supports that haploinsufficiency in the KCNQ1 gene is causative of significant QTc prolongation identical to RWS. Interestingly, the nonsense variant (NM_000218.2:c.115G > T [p.Glu39X]) locates in exon 1a of KCNQ1, which is a kidney-isoform specific exon. The variant is located closer to the N-terminus than previously identified nonsense or frame-shift variants.
To the best of our knowledge, this is the first report showing that a nonsense variant in exon 1a of KCNQ1, which is the kidney-isoform specific exon, causes JLNS. Our findings may be informative to the genetic pathogenesis of RWS and JLNS caused by KCNQ1 variants.
KeywordsCase report Haploinsufficiency Jervell and Lange-Nielsen Syndrome (JLNS) KCNQ1 (potassium channel Voltage gated, KQT-like subfamily, member 1) Long QT syndrome (LQTS) Romano-Ward syndrome (RWS) Phenotype variety
the Genome Reference Consortium
Implantable cardioverter defibrillator
Induced pluripotent stem cells
Jervell and Lange-Nielsen Syndrome
Potassium voltage-gated channel subfamily E regulatory subunit 1
Potassium voltage-gated channel subfamily E regulatory subunit 2
Potassium voltage-gated channel subfamily H member 2
Potassium voltage-gated channel subfamily J member 2
Potassium voltage-gated channel subfamily J member 5
Potassium voltage-gated channel subfamily Q member 1
Long QT syndrome 1
Long QT syndrome
National Center for Biotechnology Information
Polymerase chain reaction
Sodium voltage-gated channel beta subunit 4
Sodium voltage-gated channel alpha subunit 5
Variants in the KCNQ1 gene can cause two hereditary variants of congenital long-QT syndrome (LQTS). One variant is known as long QT syndrome 1 (LQT1) and the other is severe Jervell and Lange-Nielsen syndrome 1 (JLNS1). The typical mode of LQT1 inheritance is autosomal dominant, whereas JLNS1 shows autosomal recessive inheritance or sporadic cases of compound heterozygosity .
In LQTS1, the variants may produce different effects in the multimeric cardiac potassium channel. Defective and wild-type protein subunits may coassemble and exert a dominant negative effect on the potassium channel function. Alternatively, some mutant subunits may not coassemble with the wild-type proteins, resulting in a loss of function (haploinsufficiency) . LQTS1 can be caused by loss-of-function variants in the KCNQ1-encoded cardiac potassium channel . Homozygous gene variants in KCNQ1, or compound heterozygous gene variants, may cause the recessive JLNS variant, which is characterized by deafness.
KCNQ1 exhibits different functions in different tissues, which accounts for KCNQ1 variants causing the two syndromes. When KCNQ1 is coupled with the beta-subunit KCNE1, it repolarizes cardiac action potentials [7, 8]. Importantly, transepithelial potassium transport in the inner ear is also associated with the KCNQ1 protein , which is why KCNQ1 variants may cause deafness in JLNS1 . Deafness in JLNS is characterized as congenital, bilateral and sensorineural hearing loss .
Both frameshift/nonsense variants and missense/splice site variants may cause LQT1 and JLNS1 [10, 11]. Therefore, a precise genotype-phenotype correlation in LQT1 and JLNS1 is not established, which complicates both genetic counseling and clinical risk evaluation in carriers [9, 12, 13, 14, 15, 16]. A frameshift variant (NM_000218.2[KCNQ1]:c.567dupG ) is reported to cause not only JLNS1 when homozygous, but also causes severe LQT1 when heterozygous. Nevertheless, nonsense and frameshift variants that are generally associated with a non-penetrant phenotype (no symptoms, QTc normal or borderline) have been identified in heterozygous carriers in JLNS1 families . For example, skipping of KCNQ1 exon 1, the first common exon of the kidney and pancreas isoforms (Fig. 1), causes JLNS1 when homozygous, but exhibits an asymptomatic cardiac phenotype with normal QTc interval when heterozygous .
Here, we report on a JLNS1 patient with a homozygous nonsense variant in exon 1a of KCNQ1 in a family exhibiting LQT1. Thus, this is the first report that a nonsense variant in the kidney isoform-specific exon (exon 1a) can cause JLNS1.
The father (I-1) and mother (I-2) of the proband were first cousins. There is no history of sudden unexplained syncope or death of children or adults in the immediate family members, despite the prolonged QTc of the children.
Clinical evaluation of the proband and her family members, and blood collection
Clinical evaluation and consultation of the proband and her family members were performed at Chiba University Hospital. Clinical phenotypes were deduced from the clinical history, physical examinations, and ECG. Blood samples were collected from the proband and her family members following genetic counseling, and written informed consent was obtained prior to sample collection.
Genomic DNA was isolated from peripheral blood lymphocytes according to established protocols at our laboratory . Entire coding exons, including the intronic boundaries of the genes, of KCNQ1 (NCBI ref: NM_000218) and other LQT causative genes (KCNH2, SCN5A, KCNE1, KCNE2, KCNJ2, SCN4B, KCNJ5) were amplified by polymerase chain reaction (PCR), according to established protocols in our laboratory. Briefly, 30–100 ng of genomic DNA was subjected to PCR amplification with DNA polymerase (PrimeSTAR GXL DNA Polymerase; Takara Bio Inc., Kusatsu, Japan) and primer sets.
The amplicons were subjected to conventional sequencing with Sanger sequencers (Applied Biosystems 3730/3130 DNA analyzers; Thermo Fisher Scientific, Waltham, MA, USA). The sequence data were processed with Gene Codes Sequencher Software (Takara Bio Inc.) and mapped to the human genome sequence (build GRCh37/hg19).
Genetic analysis was performed to screen all coding exons and the exon–intron boundaries of the KCNQ1 gene (NCBI ref: NM_000218.2, NP_000209.2) with concurrent screening of other LQT causative genes (KCNH2, SCN5A, KCNE1, KCNE2, KCNJ2, SCN4B, KCNJ5). We detected a novel homozygous nonsense variant, NM_000218.2:c.115G > T (p.Glu39X, in exon 1a), in the KCNQ1 gene of the proband, as well as a homozygous common variant (NM_000218.2:c.1343C > G, p.Pro448Arg) (Additional file 1: Table S1). Genetic screening of her mother (I-2) and children (III-1 and III-2) revealed that they were heterozygous for the nonsense variant (Fig. 2). Her husband (II-3) was also screened and found to be heterozygous for the common variant (NM_000218.2:c.1343C > G, p.Pro448Arg). The proband is a child from a first-cousin marriage, and we have concluded the homozygous nonsense variant in the proband is the cause of her JLNS1. The proband was negative for pathogenic variants in other LQT causative genes, including the KCNE1 gene (Additional file 1: Table S1).
Discussion and conclusions
To the best of our knowledge, this is the first report showing that a nonsense variant in exon 1a of KCNQ1, which is the kidney-isoform specific exon, causes JLNS. The novel NM_000218.2:c.115G > T (p.Glu39X) variant is located closer to the N-terminus than previously identified pathogenic variants. This nonsense variant can cause not only JLNS, but also significant QTc prolongation that is identical to RWS.
A homozygous common variant (NM_000218.2:c.1343C > G, p.Pro448Arg) was also detected in the KCNQ1 gene of the proband. This common variant is reported to be highly frequent in Asian populations, including the Chinese and Japanese (14 to 28% allele frequency [20, 21, 22]), and may have an effect on the channel current . The p.Pro448Arg common variant is reported to increase the channel current of normal channels, while having lesser effects on the current of mutant channels . Therefore, although its effect is not negligible, the p.Pro448Arg common variant does not strongly influence the JLNS/LQTS syndrome. Indeed, the son (III-1 in Fig. 2) of the proband was heterozygous both for p.E39X and p.P448R, which would have been inherited in cis from the proband. The son (III-1) has an equally convincing LQTS phenotype to his sister (III-2), who has inherited the two variants on one chromosome from her mother as well as the p.P448R common variant from her father.
Previous studies [23, 24] have reported a nonsense variant (NM_000218.2:c. 153 C > G, p.Tyr51X, in exon 1a) in a RWS patient. This patient appears to be the only other RWS patient harboring a nonsense variant in exon 1a; however, the patient has not been described in detail, besides her/his ethnicity being white [23, 24]. Although exon 1a is the kidney-isoform specific exon, the obvious QTc prolongation shown in our patient (III-1, III-2) leads to the reasonable conclusion that nonsense variants in exon 1a can cause RWS. Previous studies have reported several missense variants located in exon 1a (NM_000218.2:c.217C > A, p.Pro73Thr , c.1A > G, p.Met1Val and c.170G > T, p.Gly57Val  c.19C > T, p.Pro7Ser , c.136G > A, p.Ala46Thr ) that can cause RWS. A frameshift variant in exon 1a (NM_000218.2:c.151dupT, p.Tyr51Leufs*234) has also been reported [26, 27]. Although exon 1a is specific to the kidney isoform, variants in exon 1a can nonetheless cause RWS. According to the recent ACMG guidelines for the interpretation of sequence variants and pathogenicity , both the p.Glu39X and p.Tyr51X variants exhibit “very strong” evidence for pathogenicity.
Several papers have reported that families with JLNS exhibit variants affecting both KCNQ1 alleles, although family members harboring just one variant do not exhibit RWS symptoms or QTc prolongation [4, 12]. Conversely, it has been reported that some JLNS family members with heterozygous variant in KCNQ1, including our cases, show RWS symptoms or QTc prolongation . In summary, some KCNQ1 variants can cause both JLNS (in an autosomal recessive manner) and RWS (in an autosomal dominant manner), while several KCNQ1 variants cause only JLNS in an autosomal recessive manner.
According to a comprehensive LQTs gene screening study [23, 24], the majority of RWS cases are caused by missense variants, rather than nonsense or frame-shift variants. In general, variants can cause disease by either haploinsufficiency or dominant-negative mechanisms. Several missense variants are reported to cause RWS by a dominant-negative mechanism . It is reasonable to conclude that dominant-negative KCNQ1 variants can cause both RWS and JLNS.
An amino acid substitution in or nearby functional domains may cause loss of function or dominant negative effects similar to missense variants. The functional domains in the KCNQ1 gene are encoded in the middle (transmembrane domains) and downstream (subunit assembly domain) exons , which likely accounts for the majority of pathogenic variants being located in the middle and downstream exons.
On the other hand, nonsense variants in the region proximal to the N-terminus of exon 1a of KCNQ1 can cause significant QTc prolongations identical to RWS in our current and previous cases . Nonsense variants in KCNQ1 are understood to be causative of haploinsufficiency for gene function . Our case study supports that haploinsufficiency in the KCNQ1 gene is causative of significant QTc prolongation identical to RWS.
Furthermore, our case is the first demonstration of a variant in the kidney-isoform specific exon being causative of JLNS. Several genetically engineered mouse models of JLNS have been created, and two KCNQ1-knockout models have been described [29, 30]. Exon 1 (Fig. 1), the first common exon of the kidney and pancreas isoforms, is engineered in both knockout models. A JLNS family reported by Zehelein et al.  is similar to these models because the described variant is located in exon 1. Interestingly, significant QT prolongation is not observed in one model strain , while both deafness and QT prolongation are observed in the other KCNQ1-knockout model . Our case is clearly a JLNS instance with both deafness and QT prolongation; however, corresponding animal models based on engineering of the kidney-isoform specific exon have not been developed.
As mentioned above, we have reported a unique variant of KCNQ1 (NM_000218.2:c.115G > T, p.Glu39X) that can cause not only JLNS, but also significant QTc prolongation identical to RWS. This nonsense variant may be informative to the genetic pathogenesis of RWS and JLNS caused by KCNQ1 variants. In the absence of corresponding animal models, iPS cell technology  has enabled the study of cell biology with gene variants. In the very near future, we aim to generate iPS cells with the KCNQ1 variant described herein.
Words of thanks are owed to Noriko Utsumi (Kazusa DNA Research Institute) for technical assistance.
This work was supported by JSPS KAKENHI Grant Number 15K08611.
Availability of data and materials
All data supporting our findings are included in the manuscript.
All authors confirmed that they have contributed to this paper. MN coordinated the study, evaluated the molecular and medical data, and wrote the manuscript. MU, RE, NS and YK performed specialized medical examinations of the family. EU is the referring genetic counselor. TI and KM are the referring physicians. OO performed molecular analyses, and evaluated the data. FN coordinated the study and wrote the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Consent for publication
The participants included in this study signed a written informed consent to publish their data (the proband and her husband signed on the behalf of the children).
Ethics approval and consent to participate
Blood samples were collected from the proband and her family members following genetic counseling. Written informed consent was obtained prior to sample collection for publication of this case report and any accompanying images. A copy of the written consent has been made available for review by the editor of this journal. The genetic testing and the study were approved by the Human Ethics Committee of Chiba University.
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