Mutations in SIX1 Associated with Branchio-oto-Renal Syndrome (BOR) Differentially Affect Otic Expression of Putative Target Genes

Abstract

Several single-nucleotide mutations in SIX1 underlie branchio-otic/branchio-oto-renal (BOR) syndrome, but the clinical literature has not been able to correlate different variants with specific phenotypes. We previously assessed whether variants in either the cofactor binding domain (V17E, R110W) or the DNA binding domain (W122R, Y129C) might differentially affect early embryonic gene expression, and found that each variant had a different combination of effects on neural crest and placode gene expression. Since the otic vesicle gives rise to the inner ear, which is consistently affected in BOR, herein we focused on whether the variants differentially affected the otic expression of genes previously found to be likely Six1 targets. We found that V17E, which does not bind Eya cofactors, was as effective as wild-type Six1 in reducing most otic target genes, whereas R110W, W122R and Y129C, which bind Eya, were significantly less effective. Notably, V17E reduced the otic expression of prdm1, whereas R110W, W122R and Y129C expanded it. Since each mutant has defective transcriptional activity but differs in their ability to interact with Eya cofactors, we propose that altered cofactor interactions at the mutated sites differentially interfere with their ability to drive otic gene expression, and these differences may contribute to patient phenotype variability.

A gradient border model for cell fate decisions at the neural plate border

“…Within these epiblast modules, unsurprisingly we note the expression of important placodal and non-neural genes Tfap2c , Homer2 and the BMP inhibitor Bambi . Alongside these markers, we also find the pluripotency marker Sall4 and Tspan13 , with the latter shown to be positively regulated by Six1 (Mehdizadeh et al, 2021). A recent study in chick suggests that based on transcriptional profiling a ‘pre-border’ state can already be defined prior to gastrulation.…”

“…GM21 displays similar dynamics but is restricted to non-neural clusters somewhat earlier (HH5; Figure 3A). Although some genes within these epiblast modules have been described as non-neural/PPR specific based on in situ hybridisation ( Tfap2c , Bambi , Homer2 , Tspan13 ) (Bell et al, 2004; Hintze et al, 2017; Mehdizadeh et al, 2021; Reichert et al, 2013; Rothstein & Simoes-Costa, 2020), others are found to be expressed within the early neural territory ( Epcam , Sall4 , Cited4 ) (Barembaum & Bronner-Fraser, 2007; Bell et al, 2004; Trevers et al, 2021) (Figure 3–Source Data 1).…”

“…GM21 displays similar dynamics but is restricted to non-neural clusters earlier at HH5. Although some genes within these epiblast modules have been described as non-neural/PPR specific based on in situ hybridisation (Tfap2c, Bambi, Homer2, Tspan13) (Bell et al, 2004;Hintze et al, 2017;Mehdizadeh et al, 2021;Reichert et al, 2013;Rothstein & Simoes-Costa, 2020), others are found to be expressed within the early neural territory (Epcam, Sall4, Cited4) (Barembaum & Bronner-Fraser, 2007;Bell et al, 2004;Trevers et al, 2021) In summary, modelling the expression dynamics of selected gene modules provides new insight into the sequential activation of fate-specific developmental programmes, as well as proposing key differences in the mechanism by which neural, neural crest and placodal cells are specified. We show, using an unbiased gene module approach, that a placodal gene module is activated first, followed by neural and neural crest modules.…”

Advances in Understanding the Pathogenesis of Craniofacial Birth Defects

“…The authors posit that these cell lines will serve as a reliable, patient-specific platform with which to understand the cellular processes that are affected in MS. The variants associated with another craniofacial syndrome, Branchio-Oto-Renal (BOR) syndrome, were investigated by Mehdizadeh et al [ 12 ]. They expressed the homologues of four different human variants of SIX1 (BOR) in wild-type Xenopus embryos to model this autosomal dominant disease in which patients express one wild-type allele and one mutant allele.…”

“…Tavares and Moody are coauthors of one of the publications included in the special issue [ 12 ]. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.…”