Myelomeningocele exomes with WNT trafficking genes disrupted
Disrupting the process of WNT synthesis or secretion has the potential to directly and indirectly inhibit one or more of the three downstream WNT signaling pathways. WNT ligands are expressed, processed, and secreted into extracellular space before finding their target cell membrane, where they can bind several cell surface receptors (Angers & Moon, 2009) (Fig 3).
In the Hispanic mutational burden comparison, the X chromosome genePORCN associated with risk for myelomeningocele. PORCN is necessary for the post-translational modification of the WNT3A ligand.PORCN belongs to an evolutionarily conserved gene family termed “Porcupine,” whose members code for Wnt processing proteins across species (Caricasole, Ferraro, Rimland, & Terstappen, 2002; Tanaka, Okabayashi, Asashima, Perrimon, & Kadowaki, 2000). In humans, the PORCN protein catalyzes O-palmitoylation of WNT3A’s Ser209 residue, allowing WNT3A to leave the endoplasmic reticulum (Gao & Hannoush, 2014; Takada et al., 2006) with the help of Wntless/WLS (Coombs et al., 2010). Therefore, the likely deleterious variants found in PORCN in our MA myelomeningocele population may prevent WNT3A from leaving the endoplasmic reticulum, ultimately downregulating all WNT signaling pathways in the target cell because less WNT3A would be secreted. Previously, PORCN ’s potential role in the development of NTDs has been suggested in a mouse study where heterozygous constitutive inactivation of its homolog Porcn caused open neural tubesin utero (W. Liu et al., 2012). Myelomeningocele has been documented in a person with focal dermal hypoplasia (FDH), a rare congenital disorder associated with mutations in PORCN (Peters, Perrier, & Haber, 2014). Furthermore, PORCN is differentially expressed in the caudal human neural tube during closure (Krupp et al., 2012). Three myelomeningocele subjects (two females and one male) carried the three RDVs NC_000023.10:g.48371088G>A p.(Gly223Ser), NC_000023.10:g.48371104G>A p.(Arg228His), and NC_000023.10:g.48374165C>G p.(Ala336Gly), all in highly conserved functional regions of PORCN (Rios-Esteves, Haugen, & Resh, 2014). The male who carries NC_000023.10:g.48371088G>A may constitute a homozygote loss of PORCN function. Although a p.(Arg228Cys) has been described as benign in FDH subjects, the NC_000023.10:g.48371104G>A p.(Arg228His) variant we report is different because it is predicted to be damaging by multiple functional analysis algorithms (Leoyklang, Suphapeetiporn, Wananukul, & Shotelersuk, 2008). The NC_000023.10:g.48374165C>G p.(Ala336Gly) variant is located within one of the transmembrane domains of PORCN that forms the substrate transportation pore to provide substrate for acylation of WNT in the ER lumen. The NC_000023.10:g.48374165C>G p.(Ala336Gly) variant is located in between several FDH variants that cause amino acid substitutions such as p.(Leu331Arg), p.(Ser337Arg) and p.(Ala338Pro) (Fokkema et al., 2011).
The carboxypeptidase E (CPE) gene on chromosome 4, which may affect binding of WNT3A to the target cell’s receptor, associated with myelomeningocele in the MA population. The peptide encoded by full-length CPE interacts with WNT3A and its receptor Frizzled (FZD) to decrease WNT/β-catenin signaling in a proteasome-dependent manner in human cells (Skalka, Caspi, Caspi, Loh, & Rosin-Arbesfeld, 2013). Disruption of this protein’s function, therefore, may upregulate the WNT/β-catenin pathway.