Truncated SALL1 Impedes Primary Cilia Function in Townes-Brocks Syndrome

Abstract

Townes-Brocks syndrome (TBS) is characterized by a spectrum of malformations in the digits, ears, and kidneys. These anomalies overlap those seen in a growing number of ciliopathies, which are genetic syndromes linked to defects in the formation or function of the primary cilia. TBS is caused by mutations in the gene encoding the transcriptional repressor SALL1 and is associated with the presence of a truncated protein that localizes to the cytoplasm. Here, we provide evidence that SALL1 mutations might cause TBS by means beyond its transcriptional capacity. By using proximity proteomics, we show that truncated SALL1 interacts with factors related to cilia function, including the negative regulators of ciliogenesis CCP110 and CEP97. This most likely contributes to more frequent cilia formation in TBS-derived fibroblasts, as well as in a CRISPR/Cas9-generated model cell line and in TBS-modeled mouse embryonic fibroblasts, than in wild-type controls. Furthermore, TBS-like cells show changes in cilia length and disassembly rates in combination with aberrant SHH signaling transduction. These findings support the hypothesis that aberrations in primary cilia and SHH signaling are contributing factors in TBS phenotypes, representing a paradigm shift in understanding TBS etiology. These results open possibilities for the treatment of TBS.

LUZP1, a novel regulator of primary cilia and the actin cytoskeleton, is altered in Townes-Brocks Syndrome

“…We have previously shown that a truncated and mislocalized form of SALL1 present in TBS individuals (SALL1 275 ) can interact aberrantly with cytoplasmic proteins. (18). LUZP1 was found among the most enriched proteins in the SALL1 275 interactome.…”

“…LUZP1 localization in human cells and proximity labelling experiments suggested that this protein might associate with centrosome-related proteins. We previously found that SALL1 275 -YFP interacted with the centrosome-associated ciliogenesis suppressors, CCP110 and CEP97 (18), so we checked whether LUZP1 may also interact with these factors. Indeed, LUZP1-YFP interacts with CCP110 and CEP97 in both WT HEK 293FT (293 WT ) and in a TBS model cell line, 293 335 (Figure 3C, lanes 5 and 7, respectively, and Figure 3-figure supplement 1) (18).…”

“…Townes-Brocks Syndrome (TBS1 [MIM: 107480]) is an autosomal dominant genetic disease caused by mutations in SALL1 , characterized by the presence of imperforate anus, dysplastic ears, thumb malformations, and often with renal and heart impairment, among other symptoms (16, 17), features seen in the ciliopathic spectrum. It has been recently demonstrated that primary cilia defects are contributing factors to TBS aetiology (18). Truncated SALL1, either by itself or in complex with the SALL1 full length form (SALL1 FL ), can interact with CCP110 and CEP97.…”

“…Truncated SALL1, either by itself or in complex with the SALL1 full length form (SALL1 FL ), can interact with CCP110 and CEP97. As a consequence, those negative regulators disappear from the MC and ciliogenesis is promoted (18). Truncated SALL1 likely interferes with multiple factors to give rise to TBS phenotypes.…”

“…Truncated SALL1 likely interferes with multiple factors to give rise to TBS phenotypes. Here we focus on LUZP1, a leucine-zipper motif containing protein that was identified by proximity proteomics as an interactor of truncated SALL1 (18).…”