Received: 26 April 2017 | Revised: 17 December 2017 | Accepted: 23 March 2018 DOI: 10.1002/ajmg.a.38718 ORIGINAL ARTICLE The ontogeny of Robin sequence Robrecht J. H. Logjes1 | Corstiaan C. Breugem1 | Gijs Van Haaften2 | Emma C. Paes1 | Geoffrey H. Sperber3 | Marie-Jose H. van den Boogaard2 | Peter G. Farlie4 1 Department of Plastic and Reconstructive The triad of micrognathia, glossoptosis, and concomitant airway obstruction defined as “Robin Surgery, University Medical Center Utrecht, sequence” (RS) is caused by oropharyngeal developmental events constrained by a reduced stoma- Wilhelmina Children’s Hospital Utrecht, Utrecht, The Netherlands deal space. This sequence of abnormal embryonic development also results in an anatomical 2Department of Genetics, Center for configuration that might predispose the fetus to a cleft palate. RS is heterogeneous and many Molecular Medicine, University Medical different etiologies have been described including syndromic, RS-plus, and isolated forms. For an Center Utrecht, Utrecht, The Netherlands optimal diagnosis, subsequent treatment and prognosis, a thorough understanding of the embryol- 3 Faculty of Medicine and Dentistry, ogy and pathogenesis is necessary. This manuscript provides an update about our current University of Alberta, Alberta, Canada understanding of the development of the mandible, tongue, and palate and possible mechanisms 4Royal Children’s Hospital, Murdoch involved in the development of RS. Additionally, we provide the reader with an up-to-date sum- Children’s Research Institute, Parkville, Australia mary of the different etiologies of this phenotype and link this to the embryologic, developmental, and genetic mechanisms. Correspondence Robrecht J. H. Logjes, Department of KEYWORDS Plastic and Reconstructive Surgery, University Medical Centre Utrecht, and embryology, genetics, glossoptosis, micrognathia, cleft palate, Pierre Robin sequence, Robin Wilhelmina Children’s Hospital, PO Box sequence 85090, 3508 AB Utrecht, The Netherlands. E-mail: [email protected] 1 | INTRODUCTION airway obstruction) that should be included in the diagnosis of RS (Breugem et al., 2016). The triad of micrognathia, glossoptosis, and concomitant neonatal RS has an incidence of 1 in 8,000–14,000 newborns (Bush & airway obstruction, currently known as Robin sequence (RS), was first Williams, 1983; Printzlau & Andersen, 2004; Vatlach, Maas, & Poets, described by St. Hilaire in 1822, by Fairbain in 1846, and by Shukowsky 2014) and the majority of cases may be associated with a syndrome, a in 1911(Fairbairn, 1846; Randall, 1977; St-Hilaire & Buchbinder, 2000). chromosomal abnormality, or other additional anomalies, but may also The subsequent description in 1923 by the French Stomatologist Pierre occur as an isolated entity (Breugem & Mink van der Molen, 2009; Robin led to the eponymous definition of the condition (Robin, 1923). Holder-Espinasse et al., 2001; Izumi, Konczal, Mitchell, & Jones, 2012; The condition was named Pierre Robin syndrome for nearly half a cen- Xu et al., 2016). tury, before better understanding lead to the identification of multiple Symptoms of RS include varying degrees of upper airway obstruc- etiologies that could result in the same clinical findings, which does not tion and feeding problems, possibly leading to subsequent life- occur in a syndrome (St-Hilaire & Buchbinder, 2000). Instead of syn- threatening respiratory and cardiac sequelae and failure to thrive when drome, the term “sequence” was introduced, since the micrognathia not adequately treated (Costa et al., 2014; Van den Elzen, Semmekrot, subsequently resulted in glossoptosis and upper airway obstruction. It Bongers, Huygen, & Marres, 2001). Mortality rates of 1–26% have became widely accepted that the condition should be called “Pierre been described (Costa et al., 2014; Kaufman et al., 2016). Robin sequence” and that the prior anomaly, the mandibular growth Numerous treatment options have been developed and vary restriction, is pathogenetically heterogeneous (Sadewitz, 1992). In cur- according to severity. Conservative interventions such as prone- or rent literature, the disorder is most commonly described as “Robin side-positioning techniques, placement of a nasopharyngeal airway or sequence” (RS) (Breugem & Courtemanche, 2010; Breugem & Mink van pre-epiglottic baton, and continuous positive airway pressure (CPAP), is der Molen, 2009). Recently, an international consensus was achieved primarily applied (Abel, Bajaj, Wyatt, & Wallis, 2012; Amaddeo et al., regarding the three features (micrognathia, glossoptosis, and upper 2016; Buchenau et al., 2007; Evans et al., 2011; Poets & Bacher, Am J Med Genet. 2018;176A:1349–1368. wileyonlinelibrary.com/journal/ajmga VC 2018 Wiley Periodicals, Inc. | 1349 1350 | LOGJES ET AL. 2011). However, if these are unsuccessful, surgical management such as tongue-lip-adhesion (TLA), mandibular distraction osteogenesis (MDO), subperiosteal release of the floor of the mouth, or tracheos- tomy may be considered (Evans et al., 2011). Since it is well known that the RS is not only pathogenetically heterogeneous, but also phenotypically heterogeneous, it is possible that defining the specific cause could influence the treatment approach or may at least influence the prognosis (Cohen, 1999). The complex consequences of the RS glossopalatognathic malformations require a team of specialists to remedy the impaired airway and orognathic mal- function. This team of specialists might include molecular biologists, FIGURE 1 The mandible divided in skeletal units. Reprinted from geneticists, embryologists, plastic surgeons, pediatricians, otolaryngolo- the textbook Craniofacial Embryogenetics and Development,2nd edition by G.H. Sperber, S.M. Sperber, and G.D. Guttmann with gists, maxillofacial surgeons, dentists, orthodontists, and speech pathol- permission of the publisher, People’s Medical Publishing House— ogists. A better understanding of the etiopathogenesis and subsequent Raleigh, North Carolina [Color figure can be viewed at expectation of the potential mandibular development could result in wileyonlinelibrary.com] better treatment for individual RS-patients. It is important for all physicians involved in the care of children template of the mandible (Amano et al., 2010; Lee et al., 2001; with RS to have a functional understanding of the embryology of RS. Lorentowicz-Zagalak, Przystanska, & Wozniak, 2005; Radlanski et al., The aim of the current review is to focus on the embryology of the pal- 2016). Subsequent interaction of the ectomesenchyme with the ate, mandible and the tongue. In addition, information from molecular mandibular arch epithelium results in an osteogenic membrane pathways and possible underlying syndrome diagnosis could improve between days 36 and 38 of development (Figure 2). This osteogenic our understanding of this phenomenon and will be discussed. membrane lies lateral to the Meckel’s cartilage, which forms between 41 and 45 days of development (Orliaguet, Dechelotte, Scheye, & 2 | EMBRYOLOGY AND PRENATAL Vanneuville, 1993). In the region of the bifurcation of the inferior alve- DEVELOPMENT olar nerve and artery into its mental and incisive branches, a single ossi- fication center for each half of the mandible will develop in the sixth Much of what we currently know about the origins of RS is based week postconception (Sperber et al., 2010b). From here the process of upon work done in zebra fish and murine models (Bhatia et al., 2015; intramembranous ossification, where the ectomesenchymal neural Ghassibe-Sabbagh et al., 2011; Gordon et al., 2014; Swindell et al., crest-derived osteoprogenitor cells differentiate directly into bone, 2015; Tan, Kilpatrick, & Farlie, 2013; Yuan et al., 2012). While this results in formation of the mandibular ramus dorsally and the mandibu- information is important to identify the developmental mechanisms lar body ventrally. Eventually the bony tissue surrounds and invades involved in RS, understanding normal human oral development is the Meckel’s cartilage in a proximal to distal direction and results in necessary to place these mechanistic insights into a clinically relevant perspective (Marques, Barbieri, & Bettiol, 1998). 2.1 | Mandibular development During neural plate folding, cranial neural crest cells, which have the potential to differentiate into bones and connective tissue, will arise in the mid- and hind-brain regions and migrate ventrally to initiate the development of the first pharyngeal arch (which provides the embry- onic maxillary and mandibular prominences) (Parada & Chai, 2015; Sperber, Sperber, & Guttmann, 2010d). Within the mandibular promi- nence, formation of the mandibular division of the trigeminal nerve is followed by the condensation of the ectomesenchyme, the multipotent cells derived from the cranial neural crest (Sperber, Sperber, & Guttmann, 2010b). The process of condensation brings skeletal precursors into close association, thereby increasing cell–cell signaling required to initiate chondrogenesis and forms a primordial anlage for FIGURE 2 Intramembranous mandibular bone forming adjacent to Meckel’s cartilage. Reprinted from the textbook Craniofacial the ensuing skeletal element of the mandible (Hall & Miyake, 2000) Embryogenetics and Development, 2nd edition by G.H. Sperber, S.M. (Figure 1). Sperber, and G.D. Guttmann with permission of the publisher, The first skeletal element formed within the mandibular process is People’s Medical Publishing House—Raleigh, North Carolina Meckel’s