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PSI_MIC

Abstract:

Cleft lip with/without cleft palate (CL/P) is one of the most common congenital facial anomalies with an incidence of approximately 1 in 700 births. Orofacial clefts can be responsible for a major social and psychological burden in the lives of the patients and their families. These patients require a long and multidisciplinary follow-up until adulthood, including several surgical procedures, speech therapy, orthodontics, genetic counseling and psychological assistance. CL/P can be associated with other birth defects in the context of a syndrome, but in the majority of cases (70%), they are an isolated finding. The heterogeneous orofacial cleft group can be subdivided in two classes: CL/P and isolated cleft palate (CP), based on differences in prevalence and embryological etiology. Non-syndromic cleft lip with/without cleft palate (NSCL/P) is a complex condition, meaning that the phenotype is not caused by one gene. Instead, a combination of multiple genes and environmental factors are responsible for the etiology of NSCL/P. This complex etiology causes the heterogeneous phenotype in complex conditions and contributing etiologic factors are difficult to identify. Although tremendous advancements were obtained in the knowledge about etiology of NSCL/P, it remains important to find new approaches to fully unravel the genetic etiology of NSCL/P. Therefore, more accurate phenotyping is vital before doing linkage or association studies. It is possible that certain subphenotypes or endophenotypes are present in unaffected family members of patients with NSCL/P. When doing a linkage analysis or association study, it is essential to take these endophenotypes into account. Relatives showing subphenotypes or endophenotypes cannot be considered to be fully ‘unaffected’. Further, accurate phenotyping of syndromic and non-syndromic patients with orofacial clefts can be helpful to delineate more homogeneous subgroups in this diverse patient population. Facial phenotyping has evolved from direct anthropometry (measuring the face by physical contact with the subject) to indirect anthropometry on images of the face. Indirect measurements were originally conducted on 2D pictures, but a face is intrinsically a 3D object, so the implementation of 3D facial imaging was a logical evolution. 3D scanning preserves the 3D nature of the face and pose variations can be corrected by rotating and translating the images, so the face can be seen from every possible viewpoint. The main advantage of using images is that these can be collected for repeatable measurements, allowing calculation of inter-rater variability and future analysis. Besides the advancement of acquisition techniques to capture facial morphology, the analytic methods to describe facial shape must also gain in resolution, precision, and power. Facial shape is often still described using only a sparse set of specific biological landmarks, which are indicated manually on each facial shape. Several extensions to spatially-dense landmarks have been proposed, including quasi-landmarks (mesh-based representation across the whole surface) and semi-landmarks (landmarks relative to other landmarks). For instance, the anthropometric mask has been proposed recently to expand the classic or true landmarks to 10000 spatially-dense quasi-landmarks. The main advantage of a spatially-denser set of landmarks is that they provide more coverage and therefore a fuller description of shapes. A statistical shape analysis can always be performed to represent the typical variability between similarly shaped objects. Shape regression for example, is a useful technique to investigate the effect of an independent variable of interest on facial morphology. In this dissertation, the focus lies on the description of facial morphology in families with a history of syndromic and non-syndromic orofacial clefting. Ultimately, the goal of this scientific work is to gain insight in the interaction between phenotype and genotype in one of the most frequent congenital conditions: orofacial clefting. The investigation of endophenotypes (phenotypic features associated with a condition without being a deviaton from the standard, and considered to be an expression of underlying susceptibility genes) is a valuable approach to gain more insight in the etiology of NSCL/P. The concept of endophenotypes is not new in research on the etiology of orofacial clefts. Most studies on endophenotypes in this field have focused on subtle variations in facial morphology of unaffected relatives. Furthermore, differences between relatives of NSCL/P patients and controls have been observed in the anatomy of the orbicularis oris muscle, nasolabial fold continuity, and occurrence of lip whorls. Our research identified a broadening of the upper facial area and midfacial retrusion as facial characteristics in a diverse group of unaffected relatives of patients with NSCL/P. Patients with NSCL/P and their unaffected relatives also showed reduced olfactory function in comparison with a control group, which was associated differences in the central olfactory structures (olfactory bulb and sulcus). Relatives with a diminished smell capacity also showed a smaller upper nasal area, compared to relatives with a normal smell capacity. There is thus a variation in facial morphology in different subgroups of unaffected relatives. This suggests that the biological pathway resulting in a fusion deficit of the facial prominences in their offspring is different. Different susceptibility genes may increase the chance of non-fusion of the maxillary and medial nasal prominences during embryogenesis via different biological pathways. Second, facial morphology is objectively described in patients with an isolated CP or Pierre Robin Sequence (PRS) and compared with a matched unaffected control group, using 3D facial morphometrics. Further, differences in facial morphology between patients with isolated CP and patients diagnosed with PRS are investigated. Our results show that both patient groups show retrusion of the chin, and this effect was more pronounced in the patients with PRS. On top of this effect, patients with PRS show additional facial characteristics and significantly more tooth agenesis. In this study, we showed that 3D facial imaging and morphometrics is an excellent tool to differentiate between two very similar phenotypes, such as CP and PRS. Although classically used for this type of analysis, this differentiation was not possible based on cephalometric analysis. One of the most common microdeletion syndromes, associated with cleft palate, is 22q11.2 deletion syndrome (22q11.2DS). We investigated the 3D facial morphology of patients with an atypical 22q11.2 deletion and linked it to its underlying genetic etiology. Our observations show that haploinsufficiency of TBX1 is not sufficient to cause facial dysmorphology associated with 22q11.2DS, in contradiction to the general belief. CRKL however is a good candidate gene. We conclude that advanced 3D morphometrics, as used in the described studies, is an excellent method to accurately describe and objectively compare facial morphology associated with orofacial clefting. This is a first step in the investigation of the correlation between facial morphology associated with orofacial clefting and underlying susceptibility genes. By setting-up large, international and multidisciplinary collaborations, we are convinced that this approach can lead to a better insight in the phenotype-genotype interaction in orofacial clefting.