Loss of CHSY1, a Secreted FRINGE Enzyme, Causes Syndromic Brachydactyly in Humans via Increased NOTCH Signaling

Abstract

We delineated a syndromic recessive preaxial brachydactyly with partial duplication of proximal phalanges to 16.8 Mb over 4 chromosomes. High-throughput sequencing of all 177 candidate genes detected a truncating frameshift mutation in the gene CHSY1 encoding a chondroitin synthase with a Fringe domain. CHSY1 was secreted from patients' fibroblasts and was required for synthesis of chondroitin sulfate moieties. Noticeably, its absence triggered massive production of JAG1 and subsequent NOTCH activation, which could only be reversed with a wild-type but not a Fringe catalytically dead CHSY1 construct. In vitro, depletion of CHSY1 by RNAi knockdown resulted in enhanced osteogenesis in fetal osteoblasts and remarkable upregulation of JAG2 in glioblastoma cells. In vivo, chsy1 knockdown in zebrafish embryos partially phenocopied the human disorder; it increased NOTCH output and impaired skeletal, pectoral-fin, and retinal development. We conclude that CHSY1 is a secreted FRINGE enzyme required for adjustment of NOTCH signaling throughout human and fish embryogenesis and particularly during limb patterning.

A Novel Ribosomopathy Caused by Dysfunction of RPL10 Disrupts Neurodevelopment and Causes X-Linked Microcephaly in Humans

“…Previous studies have shown D. rerio to represent a useful surrogate to study neurodevelopmental defects in humans (Komoike et al 2010;Tian et al 2010;Bicknell et al 2011;Golzio et al 2012;Wan et al 2012;Beunders et al 2013; Dauber et al 2013;Schaffer et al 2014). Although reported previously to be expressed ubiquitously (Thisse and Thisse 2004), in situ hybridization of rpl10 riboprobes in 2 dpf embryos demonstrated enriched expression in the anterior vs. posterior structures, particularly at the midbrain-hindbrain boundary ( Figure S1).…”

Mechanisms in Endocrinology: Notch signaling in skeletal health and disease

“…CHSY1 encodes a transmembrane protein containing a fringe domain, and cultured fibroblasts from affected individuals exhibited Notch activation secondary to upregulation of the ligand encoded by JAG1 . Recombinant CHSY1 inhibited Notch signaling in vitro , indicating that excessive activation of Notch signaling is responsible for the disease (53) . Chsy1 null mice display abnormalities in digit patterning, although the phenotype appears to be secondary to dysregulation of Indian hedgehog and transforming growth factor β, and not of Notch signaling (54) .…”

Notch signaling: simplicity in design, versatility in function

“…How the Notch receptor is modified by glycans is the subject of much research (Stanley and Okajima, 2010), and it will be interesting to see which modifications are required for basic Notch function and which confer ligand-specific effects. For example, a secreted Fringe protein, chondroitin sulfate synthase 1 (CHSY1), has recently been identified that appears to suppress Notch signaling; loss of function of CHSY1 leads to hyperactivation of Notch signaling and Notch gain-of-function phenotypes (Tian et al, 2010 .…”

“…How the Notch receptor is modified by glycans is the subject of much research (Stanley and Okajima, 2010), and it will be interesting to see which modifications are required for basic Notch function and which confer ligand-specific effects. For example, a secreted Fringe protein, chondroitin sulfate synthase 1 (CHSY1), has recently been identified that appears to suppress Notch signaling; loss of function of CHSY1 leads to hyperactivation of Notch signaling and Notch gain-of-function phenotypes (Tian et al, 2010 .Restricting the distribution of Notch ligands and receptors to specific areas within cells can also contribute to signaling specificity, as it may allow only certain combinations of cells in a larger cellular cluster to engage in Notch signaling. This is observed in Drosophila sensory organ development, a model system that relies on Notch signaling to generate lateral inhibition…”