Introduction
Myelomeningocele is the most common neural tube defect (NTD), with a prevalence of 3.2 per 10,000 births (Zaganjor et al., 2016). Affected people are born with both meninges and spinal cord exposed through a cleft in their vertebral column. People with myelomeningocele ordinarily survive with the appropriate medical care but frequently live with comorbidities such as Chiari malformation type II, sensory and motor issues below the opening, and more (Copp et al., 2015).
Myelomeningocele is a multifactorial disease, with evidence suggesting genetic susceptibilities play an important contributing role. Although maternal folate deficiency and gestational diabetes are both risk factors for NTDs, not all cases are explained by the environment of the fetus. One study indicates that only 27.6% of myelomeningocele cases can be attributed to known risk factors (Agopian, Tinker, Lupo, Canfield, & Mitchel, 2013). In fact, the heritability estimate of myelomeningocele in humans is 0.6 (Woolf, 1975). There is an increasing number of naturally occurring and lab-generated knockout mice with disruption of at least 372 genes exhibiting NTD phenotypes in mouse models (Salbaum & Kappen, 2010), illustrating the role that genetic mutation can have on this family of disorders in vertebrates.
One possible pathway affected in people with myelomeningocele is planar cell polarity (PCP). Aside from controlling other morphological events across many species (Henderson, Long, & Dean, 2018), PCP regulates convergent extension during embryogenesis which is necessary for correct neural tube closure in vertebrates (Nikolopoulou, Galea, Rolo, Greene, & Copp, 2017). In humans with NTDs, variants predicted to impair protein function have been found in PCP pathway genes (Juriloff & Harris, 2012). PCP is one branch of the larger group of WNT signaling pathways (Chu & Sokol, 2016; Wu & Mlodzik, 2017; W. Yang et al., 2017). Some genes from the WNT signaling pathways outside PCP are also implicated in the development of NTDs in humans (Allache et al., 2015; Lei et al., 2015).
Given previous evidence that genes involved in WNT signaling contribute to NTDs in humans and model organisms, we aimed to comprehensively evaluate rare, likely deleterious, coding variants within all WNT signaling pathway genes. To do so, we leveraged a gene-based mutational burden analysis, which provides the following advantages: it does not require multiplex family data, it lends potentially more power than single-variant approaches, and it has been successfully applied to the publicly-available datasets we chose as controls in another study (Guo, Plummer, Chan, Hirschhorn, & Lippincott, 2018). We hypothesize that genes within the WNT signaling pathways harbor rare deleterious variants (RDVs) that are overrepresented in myelomeningocele subjects.