In the present study, we demonstrate an association between a common TGFBR2 polymorphism (rs9838682) and risk of SCA due to malignant ventricular arrhythmias (VA) in the setting of CAD. Rigorously phenotyped cases were collected, with documented presence or absence of CAD and VA. Compared to healthy controls, adjusted for gender and age differences, the rs9838682 minor allele was associated with increased odds of SCA.
The TGFBR2 gene lies on chromosome 3p22 with 7 exons and encodes for the type II TGFß receptor, a 565-amino acid transmembrane tyrosine kinase with a calculated molecular mass of approximately 60 kD.25
In gastrointestinal tissues, TGFBR2 acts as a tumor suppressor gene and approximately 30% of colorectal cancers carry TGFBR2 mutations.26
In the cardiovascular system, mutations resulting in abnormal splicing in TGFBR2 lead to loss of function of TGFß signaling activity on extracellular matrix formation and cause Marfan syndrome.27
Interestingly, tissues derived from affected individuals show increased expression of collagen.
An important substrate for ventricular arrhythmias that leads to SCD post-MI is altered conduction, particularly in the infarct border zone, due to structural remodeling. TGFß plays a key role in the development of cardiac fibrosis.28
In experimental models, ventricular fibrosis increases the incidence of VF initiated by triggered activity.29
After MI, TGFß signaling is altered and plays a central role in infarct healing and cardiac remodeling.13
TGFß1 and TGFß2 are induced early after MI, while TGFß3 shows delayed and prolonged upregulation. 30, 31
SMAD2, 3, and 4 protein expression is upregulated in the scar and border zone area.32
Recent work from our laboratory demonstrates mitigation of cardiac fibrosis and reduction in vulnerability to atrial and ventricular arrhythmias with an antifibrotic agent (pirfenidone) which is known to inhibit TGFß1 signaling.33
TGFBR2 binds all three TGFß ligands, and represents the first critical intracellular step in this complex signaling cascade.17
Genetic variation in TGFBR2 that may affect function may be expected to play a central role in altered downstream effects, such as enhancing collagen and extracellular matrix synthesis,34
and inducing conversion of fibroblasts to myofibroblasts,35
and thus risk for VT/VF, particularly in the setting of CAD.
The TGFBR2 rs9838682 SNP found to be associated with SCA in the setting of CAD is a common polymorphism. This SNP is intronic, lying between exons 3 and 4, thus it may be in LD with the actual functional variant, or it may impact gene splicing, transcription, or regulation. However, limited data are available on its specific functional effects, therefore further study is necessary to fine-map the association signal, identify causative functional variants, and elucidate their impact on TGFBR2 function or levels, downstream signaling, and whether they ultimately affect development of ventricular arrhythmias.
There are notable limitations to our study. We examined 12 genes in the TGFß signaling pathway but a number of other genes, in particular TGFB1, in the complex cascade were not explored. In addition, we selected only common SNPs captured on the Affymetrix 6.0 DNA SNP Array, thus rarer polymorphisms and insertion/deletion variants were not assessed by our study. Stringent control for the number of statistical tests substantially reduces power, in particular given the number of SCA cases, such that some associations in the examined SNPs may have been missed. Furthermore, because only Caucasian cases and controls were examined, further studies are needed to determine whether the association with TGFBR2 SNP can be generalized to SCAs in other ethnicities. Healthy controls were used, thus a confounding association with CAD or other intermediate phenotypes such as CHF or hypertension cannot be ruled out. These results may also represent an association between SCA and other variants (in TGFBR2 or other genes) in LD with this SNP. Finally and most importantly, the possibility that this association is due to chance should be considered, thus validation of this finding in independent populations is critical.
The strengths of our study include the rigorously phenotyped and adjudicated SCA cases, compared to healthy controls without SCD. Because of our requirement for documented VA in all SCA cases, the possibility of association with other causes of sudden death is avoided. Because we used shared controls, principal components analysis was employed to exclude the possibility of population stratification as a cause of spurious association.