1. IntroductionNonsyndromic cleft lip only (NSCLO) and nonsyndromic cleft lip and palate (NSCLP), that combined are denominated nonsyndromic cleft lip with or without cleft palate (NSCL±P), are the most common craniofacial defect in the world [1]. Around 1 out of 700 live births worldwide are affected, but large variations, depending on population origin, are observed [1]. Asians and Native Americans have the highest occurrence (1:500), followed by Europeans (1:1000), while Africans have the lowest incidence (1:2500) [2,3]. The incidence fluctuates between 1:650 and 1:2700 live births in Brazil due to the population’s ethnic diversity [4,5]. Despite the fact that NSCL±P has a multifactorial and complex genesis, our knowledge about the genetic and environmental players is still incomplete [6]. As a complex polygenic disorder with clear influence of the ancestral origin, it is essential to characterize the critical genetic variants in development pathways that interfere with normal lip and palate embryogenesis, defining common and population-specific risk variants and understanding the contribution for the etiology of malformation.In an original genome-wide scan, Chiquet et al. revealed a strong linkage between CRISPLD2 and multiplex families with NSCL±P [7]. Follow up studies confirmed that Crispld2 is expressed in the lateral palatine processes during palatogenesis, and single-nucleotide polymorphisms (SNPs) within the CRISPLD2 gene are associated with NSCL±P risk [8,9]. In zebrafish, Crispld2 silencing impaired the neural crest cell migration, resulting in both jaw and palatal abnormalities in a dose-dependent manner [10], which was associated, among other putative candidates, with dysregulated expression of FOS (Fos proto-oncogene, which encodes a leucine zipper protein that can dimerize with proteins of the JUN family, thereby forming the transcription factor complex AP-1), CASP8 (caspase 8) and MMP2 (matrix metalloproteinase 2) [11]. Although putative roles for those genes are expected during orofacial development, only MMP2 has been described as essential for normal palatogenesis [12]. In our previous study, associations of CRISPLD2 rs4783099 and rs8061351 variants with NSCL±P were detected, with rs8061351 association being driven by participants with high African genomic ancestry [13]. Indeed, the influence of the intense admixed ancestry of the Brazilian population, mainly from three different roots (Amerindians, Europeans and Africans), in NSCL±P susceptibility has been previously reported [14,15,16].In this study, we investigated the association of SNPs in FOS (rs1046117), CASP8 (rs3769825) and MMP2 (rs243836) with NSCL±P risk in a Brazilian population using an ancestry-structured case-control approach. The SNP-SNP epistatic interactions of CRISPLD2 variants (rs1546124, rs8061351, rs2326398 and rs4783099), which were previously studied by us [13], with those SNPs on NSCL±P susceptibility were also assessed. 3. ResultsIndividual variations in the genetic ancestry proportions were detected, but the groups were not statistically different (Figure 1). All groups showed a higher prevalence of European ancestry compared to African and Amerindian. Regarding sex, the frequency of males was significantly higher in NSCL±P (n = 451, 56.3%, ppTable 1).The FOS rs1046117 polymorphism revealed significant associations (Table 2). The frequency of the C allele was significantly higher in patients with NSCLO than in individuals of the control group (23.5% vs. 19.6%, p = 0.004), with the OR in heterozygotes of 1.59 (95% CI: 1.16–2.18, p = 0.003) and dominant model of 1.50 (95% CI: 1.10–2.02, p = 0.007). The variant C allele was also more frequent in NSCL±P than in controls (22.5% vs. 19.6%), yielding an OR of 1.19 (95% CI: 1.00–1.41) and a nominal p value of 0.04, which did not resist to correction for multiple comparison tests. This same trend was observed for the TC heterozygotes, with an OR of 1.25 (95% CI: 1.01–1.55, p = 0.04), and the dominant genetic model, with an OR of 1.25 (95% CI: 1.02–1.53, p = 0.03), that were more frequent in NSCL±P compared to controls, but the significance did not remain after application of Bonferroni correction for multiple tests. There was no evidence of allelic or genotypic associations of CASP8 rs3769825 (Table 3) and MMP2 rs243836 (Table 4) with the susceptibility to NSCL±P, NSCLO and NSCLP. The stratified analysis of the samples by genomic ancestry (patients with high European ancestry and patients with high African ancestry) showed no significant results at a Bonferroni threshold (Supplementary Tables S1–S3).As the SNPs rs1546124, rs8061351, rs2326398 and rs4783099 in CRISPLD2 were previously analyzed in this same case-control sample [13], we sought to verify whether SNP-SNP interactions among variants in CRISPLD2 and genes of its pathway could increase the prediction risk for nonsyndromic orofacial clefts. All possible combinations of pairs were analyzed, but only those with nominal p values are depicted in Table 5 (pCASP8 rs3769825 and FOS rs104617 (pperm = 0.01) and CASP8 rs3769825 and CRISPLD2 rs8061351 pperm = 0.02) were found to be significant after permutation tests. The FOS rs1046117 interactions with MMP2 rs243836 (pperm = 0.03) and with CASP8 rs3769825 (pperm = 0.05) potentially increased the risk of NSCLO, whereas the interactions between rs8061351 in CRISPLD2 with CASP8 rs3769825 (pperm = 0.02) or with MMP2 rs243836 (pperm = 0.04) increased the risk for NSCLP. 4. DiscussionEvidence has suggested that CRISPLD2, and genes in its pathway, may play important roles during craniofacial development, and the polymorphic variants in these genes may influence their function, contributing to nonsyndromic orofacial cleft susceptibility [7,8,9,10,11,12,13]. We have demonstrated, in a previous study, the association of CRISPLD2 variants and NSCL±P risk, with a clear influence on the individual genomic ancestry [13]. This influence was observed in other studies, with CRISPLD2 representing a candidate gene for Caucasian, Hispanic, African and Chinese populations [7,8,9,20,21], but not for individuals of Italian or Indian ancestry [22,23]. The current study explored whether polymorphic variants in CRISPLD2-pathway genes such as FOS, CASP8 and MMP2 individually or interacting with CRISPLD2 contribute to NSCL±P susceptibility in the Brazilian population. Although in CASP8 and MMP2, SNPs were not associated with the risk of NSCL±P, a clear tendency between the risk allele of FOS rs1046117 and NSCL±P was observed. Further stratified analysis revealed that the FOS rs1046117 C allele, TC heterozygous genotype and TC/CC genotype, representing the dominant model, significantly increased the risk of NSCLO, but not of NSCLP.Chiquet et al. demonstrated that Fos is abundantly expressed in the orofacial region during zebrafish development, and the FOS rs1046117 C allele is significantly associated with an increased risk of NSCL±P in non-hispanic white families [11]. In a dimeric form with JUN, ATF or MAF, FOS forms the AP-1 transcription complex, which is implicated in the control of proliferation, differentiation, apoptotic cell death and many other important events associated with both normal development and tumorigenesis [24,25]. The rs1046117, characterized by a T to C transition at nucleotide position 252, represents a synonymous genetic variant with no alteration on the amino-acid sequence of the protein (Reference SNP Report, https://www.ncbi.nlm.nih.gov/snp/?term=rs1046117, accessed on 13 December 2022). However, the predicting effect of rs104117 on protein function was verified in the sorting intolerant from tolerant (SIFT) [26] and combined annotation-dependent depletion (CADD) [27], and both software revealed damaging scores (0.122 for SIFT and 0.867 for CADD) for the protein function in the presence of the variant allele. Consequently, a covered impact of polymorphism on the gene function, including translation efficiency and RNA stability, or affecting the AP-1 complex structure and subsequently its activity, is possible. It still is possible that this SNP belongs to a region within the gene that acts as a cis-regulatory element regulating the transcription of neighboring genes. On the other hand, as rs1046117 belongs to a large linkage disequilibrium block, its association detected in this study may potentially rely on a causative variant in this block.In the past, genetic studies in nonsyndromic orofacial clefts have mostly focused on the analysis of individual SNPs. Although this approach has allowed the discovery of important candidates for nonsyndromic orofacial cleft risk, it clearly does not uncover potential interactions among them. SNP-SNP interaction analysis applied in this study, representing epistasis, revealed important interactions that predicted the risk of nonsyndromic orofacial clefts. The significant interactions containing FOS rs1046117 with CASP8 rs3769825 in both NSCL±P and NSCLO or with MMP2 rs243836 in NSCLO classified correctly high-risk and low-risk genotypes. Although MMP2 is expressed during craniofacial development, including normal palate fusion process, and MMP2 knockout mice display many craniofacial defects due to dysregulated osteoblast and osteoclast differentiation [28], the specific expression of MMP2 during lip development has never been described. However, due to its importance to extracellular matrix remodeling, facilitating angiogenesis and cellular migration [12,29,30], the expression of MMP2 during lip development is expected. MMP2 expression levels are known to be regulated by the AP-1 transcription factor, which is composed by association of Fos and Jun family members, and a large variety of cytokines and growth factors can trigger cell signaling culminating in MMP2 promoter activation by the convergence at the AP-1 [31]. Although evidence from both animal and human studies support a role for MMP2 as a candidate gene in the occurrence of nonsyndromic orofacial clefts, only one study has explored its genetic variants in NSCL±P risk [32]. During lip and palate morphogenesis, processes such as apoptosis play important roles in different periods, and some craniofacial abnormalities have been attributed to irregular activation of the apoptotic cascade [33,34]. After activation, caspase 8 activates effector caspases, inducing the apoptotic caspase cascade which is essential to medial edge epithelium degradation and subsequent palatal fusion [35]. Collectively, the results suggested that FOS rs1046117 may affect NSCL±P and NSCLO susceptibility by interacting with MMP2 and CASP8.For NSCLP, interactions with significant p values after correction with 1000 permutations involved CRISPLD2 rs8061351 with CASP8 rs3769825 and with MMP2 rs243836, but not with FOS. CRISPLD2 overexpression promoted apoptosis of lung fibroblasts after activation of multiple proapoptotic genes and caspase activities, and also regulated migration and extracellular matrix genes that modulate lung development and repair, including MMP [36]. The loss-of-function strategy using morpholino targeting Crispld2 revealed a direct control of both Casp8 and Mmp2, supporting a regulatory effect of CRISPLD2 in events dependent on interactions between CRISPLD2-CASP8 and CRISPLD2-MMP2 [11]. Together, our findings show that variants in the CRISPLD2 pathway may influence the risk of NSCLP through potential epistatic interaction.

The study has strengths and limitations. Among the strengths we can highlight its multicenter design, enrolling samples from distinct regions of Brazil, which brings a better representation of the Brazilian population, and the use of robust statistical approaches with control for confounding effects including sex and ancestry proportions and application of correction for multiple comparison tests such as Bonferroni threshold and 1000 permutation, which reduce spurious results. The limitations include the test of only one SNP in each of the candidate genes, the lack of characterization of the impact of SNPs on function of the encoded proteins, the limited power in the stratification analyses due to modest sample size, though the effect of FOS SNP was highlighted when NSCLO was separated from NSCLP, and the absence of environmental factors, which could exert important roles under gene-environment interactions.