Myelomeningocele exomes with PCP genes disrupted
Closure of the developing neural tube occurs via a process of convergent extension that is orchestrated by the PCP pathway (Nikolopoulou et al., 2017). Any disruption of PCP has the potential to prevent proper neural tube closure. When WNT ligand binds only the cell surface receptor Frizzled (FZD), then DVL is recruited and the PCP branch of WNT signaling is initiated (Fig 4). Our results reproduce and expand the findings of several current human NTD studies that show damaging variants in PCP genes play a role in the development of NTDs (Juriloff & Harris, 2012).
DVL2 , a human homolog of the Dishevelled gene family, associated with myelomeningocele in the EA population. Dvl2 is a Dvl family protein essential in the PCP pathway (Henderson et al., 2018). Specifically, Dvl2 is required for endocytosis of the activated Wnt receptor, Frizzled (Yu et al., 2007). Frog knockouts for the DVL2 homologXdsh lack convergent extension and consequently display open neural tubes (Wallingford & Harland, 2002). Similarly, mouse knockouts for Dvl2 display a spina bifida phenotype (Hamblet et al., 2002). Also, DVL2 is differentially expressed in the caudal neural tube during neural tube closure (Krupp et al., 2012). Therefore, loss of correct DVL2 function in some EA population subjects may cause myelomeningocele by preventing convergent extension during neural tube closure through disruption of PCP signaling.
PRICKLE2 on chromosome 3 associated with myelomeningocele in the MA population. PRICKLE2 is one of the two vertebrate homologs of the fruit fly’s Prickle (Katoh & Katoh, 2003). As summarized in Y. Yang & Mlodzik, 2015, Prickle and Vangl help establish PCP by directly antagonizing the formation of the Frizzled/FZD-Disheveled/DVL complex (Y. Yang & Mlodzik, 2015). Because this polarity is required for convergent extension, loss ofPRICKLE2 may prevent proper convergent extension and thus failure of the neural tube to close. Another human myelomeningocele association study revealed more potential single-nucleotide polymorphism associations in PRICKLE2 than any other gene among three of the four ethnicities evaluated; however, similar to our own findings, the study failed to establish any strong associations to myelomeningocele in PCP genes (Wen et al., 2010). Similarly, a targeted sequencing study of ninety human cranial NTD cases reported a discovery of likely damaging rare variants in PRICKLE2 as well (Ishida et al., 2018).
CDH2 , which codes for a cadherin cell adhesion protein, associated with myelomeningocele in the MA population. Other cadherin genes such as CELSR1 are expressed diversely in the developing neural tube (Paulson, Prasad, Thuringer, & Manzerra, 2014). Also, mutations in cell adhesion protein genes such as Celsr1 ,EphrinA5 , and EphA7 cause NTD phenotypes in mice (Curtin et al., 2003; Holmberg, Clarke, & Frisen, 2000). Moreover, CDH2itself is already implicated in the cause of NTDs because homozygous mouse knockouts for the homolog Cdh2 displayed a wavy neural tube phenotype (Radice et al., 1997).
The gene FUZ on chromosome 19, whose mouse homolog Fuz is a PCP effector protein required for ciliogenesis by transporting DVL to the cilium (Dai et al., 2011; Gray et al., 2009; Heydeck et al., 2009), is also associated with myelomeningocele in the MA population. Five of the eight MA subjects with qualifying variants in FUZ possessed the variant NC_000019.9:g.50315872C>T p.(Ser78Asn), a variant which was 21 times more frequent in the MA cases than the matched gnomAD exome controls. Murdoch and Copp reviewed the complex relationship between cilia and NTDs (Murdoch & Copp, 2010). Similar to the cilia proteins associated with exencephaly, perhaps FUZ’s transport of DVL to the cilium can also influence neural tube closure in myelomeningocele subjects. Indeed, mice with homozygous loss ofFuz expression display NTD phenotypes such as exencephaly before dying in utero (Gray et al., 2009; Heydeck et al., 2009). Also, multiple human myelomeningocele subjects possessed nonsynonymous mutations in FUZ that were not found in control subjects, and these human variants revealed impaired cilia formation when tested in mouse cell lines (Seo et al., 2011).