Haplotype-sharing analysis implicates chromosome 7q36 harboring DPP6 in familial idiopathic ventricular fibrillation

Abstract

Idiopathic Ventricular Fibrillation (IVF) is defined as spontaneous VF without any known structural or electrical heart disease. A family history is present in up to 20% of probands with the disorder, suggesting that at least a subset of IVF is hereditary. A genome-wide haplotype-sharing analysis was performed for identification of the responsible gene in three distantly related families in which multiple individuals died suddenly or were successfully resuscitated at young age. We identified a haplotype, on chromosome 7q36, that was conserved in these three families and was also shared by 7 of 42 independent IVF patients. The shared chromosomal segment harbors part of the DPP6 gene, which encodes a putative component of the transient outward current in the heart. We demonstrated a 20-fold increase in DPP6 mRNA levels in the myocardium of carriers as compared to controls. Clinical evaluation of 84 risk-haplotype carriers and 71 noncarriers revealed no ECG or structural parameters indicative of cardiac disease. Penetrance of IVF was high; 50% of risk-haplotype carriers experienced (aborted) sudden cardiac death before the age of 58 years. We propose DPP6 as a gene for IVF and increased DPP6 expression as the likely pathogenetic mechanism.

Figure 1

Pedigrees of the Three IVF Families Used for the Haplotype-Sharing Analysis Affected subjects (SCD or resuscitated after IVF) are shown as filled circles (females) or squares (males). A slash indicates that the subject is deceased, and a dot indicates obligate carrier. The numbers after SCD or IVF indicate the age of the subject at the time of the event. Individuals included in the analysis are indicated with an arrow. Numbers inside symbols indicate the number of individuals present of that sex and status; for example, individual B-1 has four unaffected sisters.

Figure 2

Mapping of the IVF Locus (A) Result of the haplotype-sharing analysis. The length of the shared segment in number of SNPs is plotted, and the largest shared haplotype, of 301 contiguous SNPs on chromosome 7q36, is indicated by an arrow. (B) Shared 7q36 chromosomal region, discovered after initial haplotype-sharing analysis and after extended haplotyping in additional families. The position of the markers and genes (in Mb) is shown, according to the human genome build 36.3. Isoforms 1, 2, and 3 of DPP6 are also known as isoforms L, S, and K, respectively. Only validated genes are indicated.

Figure 3

Haplotypes in the Ten IVF Families Haplotype analysis in the ten families (A to J) with the risk haplotype. Probands are labeled −1. Affected subjects (SCD or resuscitated after IVF) are shown as filled circles (females) or squares (males). A slash indicates that the subject is deceased, and a dot indicates an obligate carrier. Individuals included in the haplotype-sharing analysis are indicated (B-1, A-1, A-2, C-1), as well as individuals from whom a cardiac biopsy was used for expression studies (A-2, B-1, H-2, I-1, and J-1). The risk haplotype is colored black. Families E, G, H, and I carry recombinants, which are indicated with an arrow. The smallest shared segment, based on these recombinations, is indicated with a black bar next to the haplotype. From individuals labeled with an asterisk, DNA was extracted from paraffin-embedded tissues, and the haplotypes of individuals labeled with a “+” were inferred from their children.

Figure 4

Expression Analysis of the DPP6 Gene (A) Reverse Transcriptase PCR on biopsies of three patients heterozygous for SNP rs3807218. The contribution of the A allele, which is on the risk haplotype, is increased relative to that of the G allele. (B) DPP6 mRNA expression in cardiac biopsies from affected individuals, relative to HPRT expression. The quantification of all DPP6 isoforms with primers in exon 6 and 8 was performed on five patient samples, and isoform-specific analysis was performed on two patient samples. Bars represent standard errors.

Figure 5

A 12-Lead ECG Recording of a Resuscitated Male No specific conduction or repolarization abnormalities are observed. Regular sinus rhythm is disturbed by spontaneous monomorphic extrasystoles (indicated with an asterisk) with a short coupling interval, a left bundle branch block morphology, and a superior axis. The fourth extrasystole initiates ventricular fibrillation, for which defibrillation was required (not shown).

Figure 6

Survival The survival curve of risk-haplotype carriers (black) and noncarriers (gray), demonstrating a median survival of 58 years for individuals carrying the high-risk haplotype.