A phenotype-based forward genetic screen identifies Dnajb6 as a sick sinus syndrome gene

Abstract

Previously we showed the generation of a protein trap library made with the gene-break transposon (GBT) in zebrafish (Danio rerio) that could be used to facilitate novel functional genome annotation towards understanding molecular underpinnings of human diseases (Ichino et al, 2020). Here, we report a significant application of this library for discovering essential genes for heart rhythm disorders such as sick sinus syndrome (SSS). SSS is a group of heart rhythm disorders caused by malfunction of the sinus node, the heart's primary pacemaker. Partially owing to its aging-associated phenotypic manifestation and low expressivity, molecular mechanisms of SSS remain difficult to decipher. From 609 GBT lines screened, we generated a collection of 35 zebrafish insertional cardiac (ZIC) mutants in which each mutant traps a gene with cardiac expression. We further employed electrocardiographic measurements to screen these 35 ZIC lines and identified three GBT mutants with SSS-like phenotypes. More detailed functional studies on one of the arrhythmogenic mutants, GBT411, in both zebrafish and mouse models unveiled Dnajb6 as a novel SSS causative gene with a unique expression pattern within the subpopulation of sinus node pacemaker cardiomyocytes that partially overlaps with the expression of hyperpolarization activated cyclic nucleotide gated channel 4 (Hcn4), supporting heterogeneity of the cardiac pacemaker cells.

Circadian regulation of sinoatrial nodal cell pacemaking function: Dissecting the roles of autonomic control, body temperature, and local circadian rhythmicity

“…We modeled CRBT as a simplified sinus function that peaks at ZT18, while actual experimental recordings of CRBT could feature additional complexities with surges in BT immediately after waking up and before sleep, and potential phase advances in aged mice [ 17 ]. Inherited from the baseline model [ 48 , 51 , 75 ], specific molecular identities of several ion channels (e.g., I st , I Nab , I Cab ) remain unknown, and a biochemical description of beta-adrenergic and cholinergic signaling pathways and their interactions is absent. In addition, our model is limited by the lack of a detailed transcription-translation feedback loop model for LCR in SANC, owing to the scope of this study and the limited availability of experimental data.…”

DNAJB6 isoform specific knockdown: Therapeutic potential for limb girdle muscular dystrophy D1

“… 30 Additionally, even DNAJB6 haploinsufficiency might have deleterious effects. 13 Although human mutation databases contain frameshift and nonsense mutations scattered throughout DNAJB6 , there seems to be a strong selection against loss-of-function variants based on gnomAD constraint metrics such as the probability of loss of function intolerance and the observed/expected ratio. It remains to be determined how much loss of expression can be safely tolerated while at the same time, how much reduction of the mutant allele is minimally required for benefit.…”

“… 4 Recently, DNAJB6 haploinsufficiency was associated with a sick sinus syndrome phenotype in mice with no structural cardiac changes. 13 Therapeutic approaches will, therefore, need to address the dominant mechanism of DNAJB6 mutations, but avoid the potential deleterious effects of complete knockdown.…”

DNAJB6 Isoform Specific Knockdown: Therapeutic Potential for LGMDD1

“…3 Recently, DNAJB6 haploinsufficiency was associated with a sick sinus syndrome phenotype in mice with no structural cardiac changes. 12 Therapeutic approaches will therefore need to address the dominant mechanism of DNAJB6 mutations, but avoid potential deleterious effects of complete knockdown.…”

“…3 Recently, DNAJB6 haploinsufficiency was associated with a sick sinus syndrome phenotype in mice with no structural cardiac changes. 12 Therapeutic approaches will therefore need to address the dominant mechanism of DNAJB6 mutations, but avoid potential deleterious effects of complete knockdown. Given the B isoform appears to preferentially contribute to disease pathogenesis, and the possibility of a dominant negative or toxic gain of function mechanism in LGMDD1, selective reduction of DNAJB6b levels could be a potential therapeutic target.…”