A trans-Regulatory Code for the Forebrain Expression of Six3.2 in the Medaka Fish

Ubiquitylation of BBSome is required for ciliary assembly and signaling

Bardet-Biedl syndrome (BBS) is a ciliopathy characterized by retinal degeneration, obesity, renal abnormalities, postaxial polydactyly, and developmental defects. Genes mutated in BBS encode for components and regulators of the BBSome, an octameric complex that controls the trafficking of cargos and receptors within the primary cilium. Although both structure and function of the BBSome have been extensively studied, the impact of ubiquitin signaling on BBSome is largely unknown. We identify the E3 ubiquitin ligase PJA2 as a novel resident of the ciliary compartment and regulator of the BBSome. Upon GPCR-cAMP stimulation, PJA2 ubiquitylates BBSome subunits. We demonstrate that ubiquitylation of BBS1 at lysine 143 increases the stability of the BBSome and promotes its binding to BBS3, an Arf-like GTPase protein controlling the targeting of the BBSome to the ciliary membrane. Downregulation of PJA2 or expression of a ubiquitylation-defective BBS1 mutant (BBS1^K143R ) affects the trafficking of G-protein-coupled receptors (GPCRs) and Shh-dependent gene transcription. Expression of BBS1^K143R in vivo impairs cilium formation, embryonic development, and photoreceptors' morphogenesis, thus recapitulating the BBS phenotype in the medaka fish model.

Common genetic causes of holoprosencephaly are limited to a small set of evolutionarily conserved driver genes of midline development coordinated by TGF-β, hedgehog, and FGF signaling

Here, we applied targeted capture to examine 153 genes representative of all the major vertebrate developmental pathways among 333 probands to rank their relative significance as causes for holoprosencephaly (HPE). We now show that comparisons of variant transmission versus nontransmission among 136 HPE Trios indicates some reported genes now lack confirmation, while novel genes are implicated. Furthermore, we demonstrate that variation of modest intrinsic effect can synergize with these driver mutations as gene modifiers.

Identification and characterization of variants and a novel 4 bp deletion in the regulatory region of SIX6, a risk factor for primary open-angle glaucoma

Background

Primary open‐angle glaucoma (POAG) is a complex disease of multigenic inheritance and the most common subtype of glaucoma. SIX6 encodes a transcription factor involved in retina, optic nerve, and pituitary development. Previous studies showed a genetic association between the SIX6 locus and POAG, identifying risk alleles. Whether these alleles are present also in the south Indian population is unclear.

Methods

To address this question, the SIX6 gene and an already characterized and highly conserved SIX6 enhancer (Ch14:60974427‐60974430) were sequenced in two south Indian cohorts, respectively, composed of 65/65 and 200/200 POAG cases/age‐matched controls. We next used Taqman‐based allelic discrimination assay to genotype a common variant (rs33912345: c.421A>C) and the rs1048372 SNP in two cohorts, respectively, composed of 557/387 and 590/448 POAG cases/age‐matched controls. An additional cohort of 153 POAG cases was subsequently recruited to assess the association of the rs33912345:c.421A>C and rs10483727 variants with more prominent changes in two POAG diagnostic parameters: retinal nerve fiber layer thickness and vertical cup/disc ratio, using spectral domain optical coherence tomography. The activity of the newly identified enhancer variants was assessed by transgenesis in zebrafish and luciferase assays.

Results

We identified two known rare and two common variants in the SIX6 locus and a novel 4 bp deletion in the analyzed enhancer. Contrary to previous studies, we could not establish a significant association between the rs10483727 and rs33912345:c.421A>C variants and PAOG in the south Indian ethnicity but patients carrying the corresponding C or T risk alleles exhibited a dose‐dependent reduction of the thickness of the retinal nerve fiber layer and a significant increase in the vertical cup/disc ratio. Transgenesis in zebrafish and luciferase assays demonstrated that the newly identified 4 bp deletion significantly reduced reporter expression in cells of the retinal ganglion and amacrine layers, where human SIX6 is expressed.

Conclusion

Altogether, our data further support the implication of SIX6 variants as POAG risk factors and implicates SIX6 haploinsufficiency in POAG pathogenesis.