Testing the face shape hypothesis in twins discordant for nonsyndromic orofacial clefting

What’s Shape Got to Do With It? Examining the Relationship Between Facial Shape and Orofacial Clefting

Nonsyndromic orofacial clefts belong to a class of congenital malformations characterized by a complex and multifactorial etiology. During early facial development, multiple factors can disrupt fusion leading to a cleft; this includes the shape of the embryonic face. The face shape hypothesis (FSH) of orofacial clefting emerged in the 1960s, influenced by morphological differences observed within affected families, comparative studies of mouse models, and advances in modeling genetic liability for complex traits in populations. For the past five decades, studies have documented changes in the shape or spatial arrangement of facial prominences in embryonic mice and altered post-natal facial shape in individuals at elevated risk for orofacial clefting due to their family history. Moreover, recent studies showing how genes that impact facial shape in humans and mice are providing clues about the genetic basis of orofacial clefting. In this review, I discuss the origins of the FSH, provide an overview of the supporting evidence, and discuss ways in which the FSH can inform our understanding of orofacial clefting.

The Influence of Sex and Ancestry on Three-Dimensional Palate Shape

ABSTRACT

Modern human palate shape has been reported to vary by sex and ancestry, but limitations in the methods used to quantify shape and in population coverage have led to inconsistent findings. In the present study, the authors aim to characterize the effects of sex and ancestry on normal-range three-dimensional palate shape through landmark-based morphometrics.Three-dimensional digital dental casts were obtained and landmarked from 794 adults of European (n = 429), African (n = 295), and East Asian (n = 70) ancestry. Principal component analysis was conducted to identify patterns of shape variation present in our cohort, and canonical variates analysis was performed to test for shape differences between sexes and ancestries.Principal component analysis showed that 3 principal components, explaining 76.52% of variance, linked higher palatal vault with either a relative reduction in anteroposterior or mediolateral dimensions. Canonical variates analysis showed that males had wider and shorter palates with more posteriorly located maximum vault depth than females. Individuals of African ancestry, having higher vaults with more posteriorly located maximal depths, also had wider and shorter palates, whereas individuals of European ancestry had narrower and longer palates with more anteriorly located maximum vault depths. Individuals of East Asian ancestry showed the shallowest vaults.It was found that both sex and ancestry influence palate shape, suggesting a possible genetic component underlying this variation. Additionally, our findings indicate that vault height tends to co-vary with anteroposterior or mediolateral dimensions. Further investigation of these morphological patterns may shed light on possible links to common congenital anomalies such as orofacial clefting.

PRICKLE1 × FOCAD Interaction Revealed by Genome-Wide vQTL Analysis of Human Facial Traits

The human face is a highly complex and variable structure resulting from the intricate coordination of numerous genetic and non-genetic factors. Hundreds of genomic loci impacting quantitative facial features have been identified. While these associations have been shown to influence morphology by altering the mean size and shape of facial measures, their effect on trait variance remains unclear. We conducted a genome-wide association analysis for the variance of 20 quantitative facial measurements in 2,447 European individuals and identified several suggestive variance quantitative trait loci (vQTLs). These vQTLs guided us to conduct an efficient search for gene-by-gene (G × G) interactions, which uncovered an interaction between PRICKLE1 and FOCAD affecting cranial base width. We replicated this G × G interaction signal at the locus level in an additional 5,128 Korean individuals. We used the hypomorphic Prickle1 Beetlejuice (Prickle1 Bj ) mouse line to directly test the function of Prickle1 on the cranial base and observed wider cranial bases in Prickle1 Bj/Bj . Importantly, we observed that the Prickle1 and Focadhesin proteins co-localize in murine cranial base chondrocytes, and this co-localization is abnormal in the Prickle1 Bj/Bj mutants. Taken together, our findings uncovered a novel G × G interaction effect in humans with strong support from both epidemiological and molecular studies. These results highlight the potential of studying measures of phenotypic variability in gene mapping studies of facial morphology.

Parents of Children With Nonsyndromic Orofacial Clefting Show Altered Palate Shape

OBJECTIVE

The unaffected relatives of individuals with nonsyndromic orofacial clefts have been shown to exhibit subtle craniofacial differences compared with the general population. Here, we investigate whether these morphological differences extend to the shape of the palate.

DESIGN

We conducted a geometric morphometric analysis to compare palate shape in the clinically unaffected parents of children with nonsyndromic cleft lip with or without cleft palate and adult controls of European, Asian, and African ancestry. We conducted pairwise group comparisons using canonical variates analysis, and then confirmed and characterized findings of shape differences using Euclidean distance matrix analysis.

RESULTS

Significant differences in palate shape were detected in unaffected mothers (but not fathers) compared to demographically matched controls. The differences in shape were ancestry-specific; mothers of Asian-derived and African-derived ancestry showed wider and shorter palates with higher posterior palatal vaults, while mothers of European-derived ancestry showed narrower palates with higher anterior palatal vaults.

CONCLUSIONS

Our findings suggest that altered palate shape is a subclinical phenotypic feature, which may be indicative of elevated orofacial cleft risk. The risk phenotype varied by sex and ancestry, suggesting possible etiologic heterogeneity among demographic groups. Understanding the genetic basis of these informative palate shape traits may reveal new genes and pathways relevant to nonsyndromic orofacial clefting.

Symmetry and fluctuation of cell movements in neural crest-derived facial mesenchyme

In the face, symmetry is established when bilateral streams of neural crest cells leave the neural tube at the same time, follow identical migration routes and then give rise to the facial prominences. However, developmental instability exists, particularly surrounding the steps of lip fusion. The causes of instability are unknown but inability to cope with developmental fluctuations are a likely cause of congenital malformations, such as non-syndromic orofacial clefts. Here, we tracked cell movements over time in the frontonasal mass, which forms the facial midline and participates in lip fusion, using live-cell imaging of chick embryos. Our mathematical examination of cell velocity vectors uncovered temporal fluctuations in several parameters, including order/disorder, symmetry/asymmetry and divergence/convergence. We found that treatment with a Rho GTPase inhibitor completely disrupted the temporal fluctuations in all measures and blocked morphogenesis. Thus, we discovered that genetic control of symmetry extends to mesenchymal cell movements and that these movements are of the type that could be perturbed in asymmetrical malformations, such as non-syndromic cleft lip.

This article has an associated ‘The people behind the papers’ interview.

Highlighted Article: Live imaging of the chick embryo face followed by mathematical analysis of mesenchymal cell tracks captures novel fluctuations between states of order/disorder as well as symmetry/asymmetry, revealing developmental instabilities that are part of normal morphogenesis.

The Intersection of the Genetic Architectures of Orofacial Clefts and Normal Facial Variation

Unaffected relatives of individuals with non-syndromic cleft lip with or without cleft palate (NSCL/P) show distinctive facial features. The presence of this facial endophenotype is potentially an expression of underlying genetic susceptibility to NSCL/P in the larger unselected population. To explore this hypothesis, we first partitioned the face into 63 partially overlapping regions representing global-to-local facial morphology and then defined endophenotypic traits by contrasting the 3D facial images from 264 unaffected parents of individuals with NSCL/P versus 3,171 controls. We observed distinct facial features between parents and controls across 59 global-to-local facial segments at nominal significance (p ≤ 0.05) and 52 segments at Bonferroni corrected significance (p < 1.2 × 10-3), respectively. Next, we quantified these distinct facial features as univariate traits in another dataset of 8,246 unaffected European individuals and performed a genome-wide association study. We identified 29 independent genetic loci that were associated (p < 5 × 10-8) with at least one of the tested endophenotypic traits, and nine genetic loci also passed the study-wide threshold (p < 8.47 × 10-10). Of the 29 loci, 22 were in proximity of loci previously associated with normal facial variation, 18 were near genes that show strong evidence in orofacial clefting (OFC), and another 10 showed some evidence in OFC. Additionally, polygenic risk scores for NSCL/P showed associations with the endophenotypic traits. This study thus supports the hypothesis of a shared genetic architecture of normal facial development and OFC.

Whole-genome sequencing in a pair of monozygotic twins with discordant cleft lip and palate subtypes

OBJECTIVE

Orofacial clefts (OFCs) are common and etiologically complex birth defects. This study explored potential genetic differences in a pair of Japanese monozygotic (MZ) twins with different forms of OFC using whole-genome sequencing.

SUBJECTS AND METHODS

One co-twin (MZ-1) presented with nonsyndromic bilateral cleft lip and palate; the other co-twin (MZ-2) had nonsyndromic bilateral cleft lip and unilateral left-sided cleft alveolus. Neither parent had an OFC. Craniofacial morphologic features and potential genetic differences were compared using standard cephalometry and whole-genome sequencing, respectively.

RESULTS

Morphologically, MZ-1 had a smaller vertical mandibular height, compared to MZ-2. However, no discordant genetic differences were detected. Moreover, both twins and their parents harbored rare candidate gene variants (GRHL3; TPM1) considered to be associated with OFCs.

CONCLUSION

The observed differences between MZ-1 and MZ-2 in craniofacial morphology assessed by cephalograms might be directly attributable to the effects of the OFC on growth and/or differences in surgical history, given the lack of any differences in genetic background. However, comparisons of discordant MZ twins should continue to identify novel candidates that might control OFC or that might partly explain the missing heritability for this common birth defect, in addition to understanding craniofacial growth and development.