Metabolic treatment of an electrical disease? Beneficial APD/QT prolongation by L-Carnitine in transgenic SQT1 rabbit models

Background

Short-QT syndrome 1 (SQT1) is a genetic cardiac channelopathy caused by gain-of-function mutations (KCNH2-N588K) in HERG/IKr, that leads to shortened QT-interval, increased risk for arrhythmias and sudden cardiac death (SCD). An acquired form of SQTS has been described in patients with primary (genetic) carnitine-deficiency, indicating that carnitine might affect cardiac repolarization.

Purpose

We aimed to investigate potential beneficial (APD/QT-prolonging) effect of L-Carnitine in (genetic) SQTS using transgenic SQT1 rabbits that mimic the human disease phenotype.

Methods

Effects of L-carnitine on cardiac repolarisation were assessed in adult wildtype (WT) and transgenic SQT1 rabbits (KCNH2-N588K) using in vivo ECG and ex vivo Langendorff-perfused whole-heart or isolated ventricular cardiomyocyte action potential (AP) recordings. Effects on ion currents were assessed by whole-cell patch-clamping.

Results

In vivo, the heart-rate corrected QT index (QTi) was prolonged significantly by L-carnitine both in WT (QTi, baseline 102.7%±4.9 vs. L-carnitine 106.9%±6.2, p<0.05, n=12) and SQT1 (QTi, baseline 94.8%±7.4 vs. L-carnitine 99.5%±8.2, p<0.05, n=13), leading to normalisation of QTi in SQT1. Ex vivo, whole-heart monophasic and cellular APs were also significantly prolonged by L-carnitine in WT and SQT1 (change in monophasic APD75, ms, WT +13.9±4.4, SQT1 +9.9±7.0; change in cellular APD90, %, WT +10.4%, SQT1 +10.4%, all p<0.05). As underlying mechanisms, we identified acute effects on the main repolarizing ion currents IKr and IKs: IKr-steady, which is significantly increased in SQT1 contributing to accelerated repolarization, was reduced by L-carnitine (WT: −23%, SQT1: −16%). Moreover, L-carnitine accelerated the deactivation kinetics of IKr - while no change was observed in IKr-tail. In addition, IKs-steady was decreased by L-carnitine in SQT1 and WT cardiomyocytes.

Conclusion

L-carnitine prolongs/normalizes QT and APD in transgenic SQT1 rabbits by decreasing the pathologically increased IKr-steady and also IKs-steady and may therefore serve as potential future anti-arrhythmic therapy in SQTS.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): German Research Foundation (DFG)

The prothrombin G20210A polymorphism in patients with myocardial infarction

To investigate a possible association between the human prothrombin gene G20210A polymorphism and coronary artery thrombosis, we screened 172 consecutive patients with ischaemic heart disease admitted for coronary arteriography. The patients were divided into two groups on the basis of their clinical history and examination of their hospital records: 66 patients with a definite previous myocardial infarction, and 106 with angina-like chest pain but no evidence of myocardial infarction. The overall frequency of the G20210A polymorphism was 0.011, four out of the 172 patients being heterozygous for the mutation. The allelic frequency was 0.015 in the group with myocardial infarction and 0.009 in the group without myocardial infarction (P = 0.622). The results of this study suggest that the single nucleotide polymorphism at position 20210 of the prothrombin gene is unlikely to be a risk factor for coronary thrombosis.

Platelet membrane glycoprotein Ibalpha gene -5T/C Kozak sequence polymorphism as an independent risk factor for the occurrence of coronary thrombosis

OBJECTIVE

To explore the potential of the GPIbalpha gene variable number tandem repeat (VNTR) and -5T/C Kozak polymorphisms to act as independent risk factors for myocardial infarction.

METHODS

256 patients aged 33-80 years (180 caucasian, 76 Indian Asian) were recruited at cardiac catheterisation for any diagnostic indication, and divided into two groups: group A, with confirmed previous myocardial infarction evident on ECG or ventriculogram (88 patients, 79 men, 9 women) and group B, with no evidence of myocardial infarction (168 patients, 101 men, 67 women).

RESULTS

There was no significant difference in race, age, hypertension, smoking status, or family history between the infarct and non-infarct groups, though there was a significant difference in sex (89.8% male in group A, 60.1% male in group B, p < 0.001). Genotype analysis showed a strong association between the GPIbalpha Kozak homozygous TT genotype and the occurrence of myocardial infarction (group A: TT 85.2%, TC 12.5%, CC 2.3%; group B: TT 67.3%, TC 32.7%, p = 0.001). No significant association was found between myocardial infarction and the GPIbalpha VNTR, although analysis of the CC VNTR genotype against all other GPIbalpha VNTR genotypes showed a marginal association with myocardial infarction (p = 0.059). There was no association between the Kozak sequence polymorphism (p = 0.797) or GPIbalpha VNTR (p = 0.714) and the degree of vessel disease.

CONCLUSIONS

The homozygous TT Kozak genotype may be a significant factor in the outcome of coronary artery disease completed by myocardial infarction. Conversely, the Kozak C allele in the heterozygous state TC may confer some protection against myocardial infarction.

Factor VIII gene mutations found by a comparative study of SSCP, DGGE and CMC and their analysis on a molecular model of factor VIII protein

Screening of the factor VIII (FVIII) gene which spans 186 kb and codes for 26 exons, was originally hampered by its size but is now feasible because rapid DNA scanning methodologies have been developed. The present study for the first time directly compares the three most widely applied screening methods, denaturing gradient gel electrophoresis (DGGE), single-stranded conformational polymorphism (SSCP) and chemical mismatch cleavage (CMC) for their sensitivity of mutation detection in a selected group of ten haemophilia A patients. Nine of these patients are known to be cross-reacting material positive and eight exhibited a mild to moderate phenotype. Of the ten patients screened, we identified mutations in nine by all three screening methods. Of the mutations characterised, two are previously unpublished. T to C (S373P) and G to A (D525N). In one mildly affected haemophiliac, we identified a second T to C sequence change in the 5' untranslated region at -601 bp, probably having no effect on FVIII gene expression. Modelling studies were performed on those mutations lying within the A domains of FVIII (D525N, R527W, I566T) to study the possible effect of these mutations on structure and/or function. When the three methods are performing optimally and have been standardised, our experience is that CMC and DGGE are equally efficient at sequence variation detection while SSCP is slightly less sensitive.

Moderation of hemophilia A phenotype by the factor V R506Q mutation

Although many examples of unrelated hemophilia A patients carrying identical point mutations in the factor VIII (FVIII) gene have been reported, the clinical phenotype is not always the same among patients sharing the same molecular defect. Possible explanations for this discrepancy include undetected additional mutations in the FVIII gene or coinheritance of mutations at other genetic loci that modulate FVIII function. We report molecular genetic analysis of potential modifying genes in two sets of unrelated patients carrying common FVIII missense mutations but exhibiting different levels of clinical severity. Both mutations (FVIII R1689C and R2209Q) are associated with severe hemophilia A in some patients and mild/moderate disease in others. The common von Willebrand disease type 2N mutation (R91Q) was excluded as a modifying factor in these groups of patients. However, analysis of the recently described factor V (FV) R506Q mutation (leading to activated protein C resistance) identified a correlation of inheritance of this defect with reduced hemophilia A severity. Two moderately affected hemophilia A patients, each with either of two FVIII gene mutations, were heterozygous for FV R506Q, whereas two severely affected patients and two moderately affected patients were homozygous normal at the FV locus. Our results suggest that coinheritance of the FV R506Q mutation may be an important determinant of clinical phenotype in hemophilia A and that modification of the protein C pathway may offer a new strategy for the treatment of FVIII deficiency.

Haemophilia A diagnosis by simultaneous analysis of two variable dinucleotide tandem repeats within the factor VIII gene

Haemophilia A is a bleeding disorder caused by defects in the gene coding for the co-factor, factor VIII (FVIII). The few available intragenic restriction fragment length polymorphisms (RFLPs) currently used in carrier detection and prenatal diagnosis of haemophilia A are informative in only about 65% of cases. We earlier reported a multi-allelic dinucleotide tandem repeat, (CA)n, specific to intron 13, which remains the single most informative marker within the FVIII gene. We here report a second informative dinucleotide repeat of the form (GT)n (AG)n, located to intron 22 of the FVIII gene. The polymerase chain reaction (PCR) method was used to examine the variability of the repeat in 60 individuals (75 X-chromosomes) and revealed four alleles. The calculated heterozygosity rate is 45%, and family studies showed X-linked mendelian inheritance. The intron 22 dinucleotide repeat is tightly linked with established RFLPs and tracks with haemophilia A in family studies. We now show that by simultaneous amplification of the intron 13 and 22 repeats using PCR all alleles for both markers are detectable on a single polyacrylamide gel. The information thus obtained from a single multiplexed analysis is greater than from multiple RFLP analyses. Hence, rapid haplotype determination by simultaneous amplification and detection of two intragenic dinucleotide repeats should supersede less informative RFLP analysis.

Double heterozygosity for mutations in the platelet glycoprotein IX gene in three siblings with Bernard-Soulier syndrome

Bernard-Soulier syndrome (BSS) giant platelets have defective and/or deficient glycoprotein (GP) Ib/IX complexes, causing absent ristocetin-induced aggregation, defective interaction with von Willebrand factor, morphologic abnormality, and a clinical bleeding tendency. Recently several mutations have been described in the platelet GPIb alpha gene in individuals exhibiting the BSS phenotype. We have studied a family with classical BSS, and have excluded lesions at the GPIb alpha locus by restriction fragment length polymorphism linkage analysis. Analysis of the genes for two other components of the platelet GPIb:IX complex, namely GPIb beta and GPIX, showed two different missense mutations in the coding region of the GPIX gene: an A-->G transition in codon 21 results in conversion of an aspartic acid to glycine and an A-->G change in codon 45 converts an asparagine residue to serine. Three affected individuals are doubly heterozygous for these mutations, which alter conserved residues in or flanking the GPIX leucine-rich glycoprotein motif. Both mutations create new recognition sites for the enzyme Fnu 4H1; therefore, this enzyme was used to screen 60 normal subjects (120 alleles). Neither mutation was detected in any subject other than direct relatives of the affected individuals. Although low levels of GPIb were demonstrable by both flow cytometry and immunoblot analysis in an affected individual's platelets, there was no evidence of GPIX immunoreactivity. We propose that expression of abnormal GPIX prevents stable assembly of the GPIb/IX complex, causing BSS in the doubly heterozygous individuals in this family.

A G-->A substitution in an HNF I binding site in the human alpha-fetoprotein gene is associated with hereditary persistence of alpha-fetoprotein (HPAFP)

A family displaying hereditary persistence of alpha-fetoprotein (HPAFP) in adult life was detected in an antenatal screening programme for spina bifida. RFLP linkage analysis shows that the trait is linked with the albumin-AFP locus. The molecular mechanism responsible for the post-natal repression of the AFP gene is unknown. We wished to determine the molecular mechanism underlying HPAFP in this family. Sequence analysis of the 5'-flanking sequences of their gene revealed a GA substitution at position -119 associated with the trait. This substitution occurs in a potential HNF I binding site, and increases the similarity of the sequence to a consensus HNF I recognition site. In a competitive gel retardation assay the mutant sequence binds HNF I alpha more tightly than the wild type sequence. Furthermore, 5'-flanking sequences of the human AFP gene containing the G-->A substitution direct a higher level of CAT expression in transfected human hepatoma cells than the wild type sequences. We conclude that the G-->A substitution at position -119 of the AFP gene is the mutation causing HPAFP in this family. These results highlight the importance of this HNF I binding site in the developmental regulation of the AFP gene.

Precipitous rise and fall of SGOT

Three patients are reported whose studies showed a transient but marked elevation of serum transaminase levels with a rapid return toward normal. In two patients this phenomenon was associated with transient episode of hypotension; in the third case this was an incidental finding discovered fortuitously on a routine test. Hepatic ischemia and parenchymal injury may cause transient elevations of serum transaminase levels.