Interactions between planar cell polarity genes cause diverse neural tube defects

Abstract

Neural tube defects (NTDs) are among the commonest and most severe forms of developmental defect, characterized by disruption of the early embryonic events of central nervous system formation. NTDs have long been known to exhibit a strong genetic dependence, yet the identity of the genetic determinants remains largely undiscovered. Initiation of neural tube closure is disrupted in mice homozygous for mutations in planar cell polarity (PCP) pathway genes, providing a strong link between NTDs and PCP signaling. Recently, missense gene variants have been identified in PCP genes in humans with NTDs, although the range of phenotypes is greater than in the mouse mutants. In addition, the sequence variants detected in affected humans are heterozygous, and can often be detected in unaffected individuals. It has been suggested that interactions between multiple heterozygous gene mutations cause the NTDs in humans. To determine the phenotypes produced in double heterozygotes, we bred mice with all three pairwise combinations of Vangl2Lp, ScribCrc and Celsr1Crsh mutations, the most intensively studied PCP mutants. The majority of double-mutant embryos had open NTDs, with the range of phenotypes including anencephaly and spina bifida, therefore reflecting the defects observed in humans. Strikingly, even on a uniform genetic background, variability in the penetrance and severity of the mutant phenotypes was observed between the different double-heterozygote combinations. Phenotypically, Celsr1Crsh;Vangl2Lp;ScribCrc triply heterozygous mutants were no more severe than doubly heterozygous or singly homozygous mutants. We propose that some of the variation between double-mutant phenotypes could be attributed to the nature of the protein disruption in each allele: whereas ScribCrc is a null mutant and produces no Scrib protein, Celsr1Crsh and Vangl2Lp homozygotes both express mutant proteins, consistent with dominant effects. The variable outcomes of these genetic interactions are of direct relevance to human patients and emphasize the importance of performing comprehensive genetic screens in humans.

Digenic variants of planar cell polarity genes in human neural tube defect patients

“…It is estimated that up to 70% of the risk of NTDs is attributed to genetic factors [6]; yet the main predisposing genetic factors for human NTDs remain unknown. Given that NTDs present a mainly sporadic pattern and exhibit a relatively high prevalence across the world, it is thought that the etiology of these disorders represents a multifactorial oligogenic or polygenic pattern of inheritance, together with an important role for non-genetic factors such as the environment [2]. …”

“…Single-nucleotide variants (SNVs) in the core PCP genes CELSR1, FZD6, PRICKLE1, DVL2, VANGL1 and VANGL2 , and the PCP associated genes SEC24B, DACT1, FUZ and SCRIB [7,17–23] have been proposed as human NTD risk factors. In contrast to the association of mouse PCP gene mutations with craniorachischisis in homozygotes, most of the variants identified in human NTDs are heterozygous variants, and the PCP-mutation-associated NTD phenotypes vary widely ranging from the “open NTD” of craniorachischisis, myelomeningocele, and anencephaly, to the “closed NTD” of lipomyelomeningocele, lipomyelocele, and lipoma [2,7]. Unlike mouse models, variants in single genes may not explain the genetic mechanism underlying the development of human NTDs.…”

“…Actually, mouse studies showed that several digenic combinations involving the core PCP gene Vangl2 and other genes ( Sec24b, Sfrp1/Sfrp2/Sfrp5, Dvl3, Scrib, Celsr1, Ptk7, Vangl1 ), most of which were double heterozygotes, could cause either open spina bifida or exencephaly or craniorachischisis, in contrast to only the craniorachischisis phenotype when homozygous mutants were detected in some of these PCP genes [2,24–28]. Additionally, double mutants of Scrib Crc/+ and Celsr1 Crsh/+ , Dvl2 − / − and Dvl3 +/ − , Dvl2 +/ − and Dvl3 − / − induced a phenotype of craniorachischisis in mice [2,26]. Although these findings raise the possibility of similar genetic combinatorial mechanisms in human NTDs, little evidence was available in support of this hypothesis in human studies except a small chohort study including 90 patients with cranial NTDs in England [29].…”

Development and dynamics of cell polarity at a glance

“…In mouse, null mutations of Scrib -resulting in the circletail mouse mutant -cause craniorachischisis (Murdoch et al, 2003), as do mutations of the core PCP genes Celsr1 and Vangl2. Strikingly, mice heterozygous for these mutations -alone or in combination -also exhibit craniorachischisis (Murdoch et al, 2014). The human genetics of neural tube defects is complex and poorly understood, and the mouse studies reveal that both genetic background and environmental effects lead to complexities, such as phenotypes that are incompletely penetrant or variably expressive.…”

Genetic and functional analysis of SHROOM1-4 in a Chinese neural tube defect cohort

“…PCP molecules have been recognized as linking these two morphogenetic processes in the neural tube (Copp and Greene 2010; Nishimura et al 2012). Genetic modification of PCP core genes in mouse models, such as Vangl2 and Celsr1 , lead to severe NTDs, indicating that there is a strong connection between PCP signaling and risk for NTDs (Murdoch et al 2014). PCP-dependent convergent extension is crucial for NTC, and loss of function (LoF) alleles of Vangl2 (Greene et al 1998), Celsr1 (Curtin et al 2003), Frizzled3 ( Fz3 ) (Wang et al 2006), Frizzled6 ( Fz6 ) (Wang et al 2006), Dvl1 and Dvl2 (Etheridge et al 2008) cause craniorachischisis.…”