No abstract

Background Brugada syndrome (BrS) is an arrhythmogenic disorder that has been linked to mutations in SCN5A, the gene encoding for the pore-forming α-subunit of the cardiac sodium channel. Typically, BrS mutations in SCN5A result in a reduction of sodium current with some mutations even exhibiting a dominant-negative effect on wild-type (WT) channels thus leading to an even more prominent decrease in current amplitudes. However, there is also a category of apparently benign (“atypical”) BrS SCN5A mutations that in vitro demonstrates only minor biophysical defects. It is therefore not clear how these mutations produce a BrS phenotype. We hypothesized that similar to dominant-negative mutations atypical mutations could lead to a reduction in sodium currents when co-expressed with WT to mimic the heterozygous patient genotype. Methods and Results WT and “atypical” BrS mutations were co-expressed in HEK293 cells, showing a reduction in sodium current densities similar to typical BrS mutations. Importantly, this reduction in sodium current was also seen when the atypical mutations were expressed in rat or human cardiomyocytes. This decrease in current density was the result of reduced surface expression of both mutant and WT channels. Conclusions Taken together, we have shown how apparently benign SCN5A BrS mutations can lead to the ECG abnormalities seen in BrS patients through an induced defect that is only present when the mutations are co-expressed with WT channels. Our work has implications for risk management and stratification for some SCN5A-implicated BrS patients.

Glucocorticoids increase beta 2-adrenergic responsiveness and receptor density in the lung, but the underlying mechanisms have not been clearly elucidated. To determine whether changes in beta 2-adrenergic receptor gene expression are involved in vivo, we measured beta 2-adrenergic receptor mRNA levels and beta 2-adrenergic receptor density in lungs from Sprague-Dawley rats treated with a daily injection of dexamethasone (1 mg/kg subcutaneously) for 1, 3, or 7 days. Animals were sacrificed either 2 or 24 h after receiving the last injection. beta 2-Adrenergic receptor mRNA levels were significantly (p < .05) elevated compared to saline-treated controls in the lungs of animals sacrificed 2 h after dexamethasone injection for 1 day (174 +/- 12%), 3 days (236 +/- 18%), and 7 days (220 +/- 11%). Receptor mRNA levels measured 24 h after dexamethasone injection did not differ significantly from the control group. Induction of beta 2-adrenergic receptor mRNA by dexamethasone was transient, since no significant cumulative or sustained increase in receptor mRNA levels was observed during the study period. Treatment with dexamethasone increased beta 2-adrenergic receptor density as expected, but no significant increase in receptor density was detected until 24 h after the third daily injection of dexamethasone, when levels reached 2045 +/- 150 fmol/mg protein compared to 1292 +/- 34 fmol/mg protein in the control group. Receptor density then remained at this elevated level through 7 days of treatment. These results show that dexamethasone up-regulates both the beta 2-adrenergic receptor and its mRNA in vivo in the lung. The induction of beta 2-adrenergic receptor mRNA levels indicates that glucocorticoids may regulate receptor density in the lung through modulation of gene expression. However, the difference between the time course of induction for the beta 2-adrenergic receptor and its mRNA suggests that additional translational or post-translational mechanisms may also be involved.

Although central to the susceptibility of adult diseases characterized by abnormal rhythmogenesis, characterizing the genes involved is a challenge. We took advantage of the C57BL/6J (B6) trait of hypoxia-induced periodic breathing and its absence in the C57BL/6J-Chr 1(A/J)/NaJ chromosome substitution strain to test the feasibility of gene discovery for this abnormality. Beginning with a genetic and phenotypic analysis of an intercross study between these strains, we discovered three quantitative trait loci (QTLs) on mouse chromosome 1, with phenotypic effects. Fine-mapping reduced the genomic intervals and gene content, and the introgression of one QTL region back onto the C57BL/6J-Chr 1(A/J)/NaJ restored the trait. mRNA expression of non-synonymous genes in the introgressed region in the medulla and pons found evidence for differential expression of three genes, the highest of which was apolipoprotein A2, a lipase regulator; the apo a2 peptide fragment (THEQLTPLVR), highly expressed in the liver, was expressed in low amounts in the medulla but did not correlate with trait expression. This work directly demonstrates the impact of elements on mouse chromosome 1 in respiratory rhythmogenesis.