Cleft Lip and Palate

M. T. Cobourne

• Publisher: S. Karger
• Year: 2012
• ISBN:
  9783318021080
• Topic: Medicine
• Subtopic: Dentistry

Cleft lip and palate is a complex, multifactorial and relatively common craniofacial disorder, which arises because of disrupted facial development in the embryo. The manifestations of this condition can be life-long and associated with significant morbidity. In the last decade, progress has been made in our understanding of how clefts of the lip and palate arise in human populations, and laboratory studies are beginning to elucidate the molecular mechanisms that control development of the lip and palate. In addition, advances in surgical and medical care, and long-term rehabilitation are improving outcome and quality of life for affected individuals. Written by international experts in their respective fields, this publication covers in detail the epidemiology and genetic basis of cleft lip and palate, the developmental biology of lip and palate formation and provides current concepts in the management of patients affected by this condition. Thus, the book provides a contemporary overview of the epidemiology, aetiology and treatment of cleft lip and palate, and will be of use to a wide range of individuals, including students, biologists and clinicians, who have an interest in this subject.

Cobourne MT (ed): Cleft Lip and Palate. Epidemiology, Aetiology and Treatment.
Front Oral Biol. Basel, Karger, 2012, vol 16, pp 32–51

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The Mouse as a Developmental Model for Cleft Lip and Palate Research

Amel Gritli-Linde

Department of Oral Biochemistry, Sahlgrenska Academy at the University of Gothenburg, Göteborg, Sweden

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Abstract

Vertebrate and invertebrate model organisms are essential for deciphering biological processes. One of these, the mouse, proved to be a valuable model for understanding the etiopathogenesis of a vast array of human diseases, including congenital malformations such as orofacial clefting conditions. This small mammal's usefulness in cleft lip and palate research stems not only from the striking anatomical and molecular similarities of lip and palate development between human and mouse embryos, but also from its amenability to experimental and genetic manipulation. Using some recent studies as illustrative examples, this review describes different ways of generating and exploiting mouse models to study normal and abnormal development of the lip and palate. Despite a few surmountable disadvantages of using the mouse, numerous mutants have revealed a growing number of molecular key players and have pointed at a tight and complex molecular control during each step of lip and palate development.

Copyright © 2012 S. Karger AG, Basel

The mouse has transcended its humble role as a tiny part of the ecosystem to become a prominent player in the war against human ailments such as cancer, congenital malformations as well as neurological, metabolic, immunological, degenerative and age-related diseases. While early embryologists and teratologists used the rat and the rabbit as mammalian models, these were overshadowed by the mouse following the advent of mouse genetics, the generation of inbred strains and the availability of scores of spontaneous, chemically and radiation-induced mouse mutants. Added to these advantages are the facts that mice have relatively short generation times, are prolific, small in size, and can be maintained in a cost-effective way. In the last decade of the 20th century, development of innumerable and increasingly innovative and powerful molecular and genetic tools enabling genetic manipulation of the mouse, combined later on with the completion of the mouse genome sequence, catapulted the use of the mouse in biomedical research to a new era. In parallel, mouse strain repositories and information resources, such as sequence, gene expression and phenotype databases, emerged and continue to facilitate the endeavor of scientific and industrial communities. In addition to mice with spontaneous mutations, thousands of mouse models of human diseases have been generated by different methods, including transgenesis, ethylnitrosourea (ENU)- or transposon-induced mutagenesis, gene-targeting and gene-trapping technologies. Work by international consortia is underway with the challenging goal of disabling the 20,000 protein encoding genes in the mouse genome. Recently, a novel and powerful technology enabled, so far, the generation of more than 9,000 reporter (lacZ)-tagged conditional alleles in mouse embryonic stem cell lines, and is aiming at generating reporter-tagged conditional mutations in all murine protein-coding genes within the next few years [1].

In the field of cleft lip (CL) and cleft palate (CP) research, the mouse, in conjunction with human genetics studies, has been instrumental in providing insights into normal development of the lip and palate and unveiling clues about the genetic, molecular and cellular events that engender these frequent and devastating orofacial defects. Studies of transgenic and mutant mice have pointed at a bewildering number of molecular players during lip and palate development, including receptors, ion channels, signaling molecules, extracellular components, junctional complexes as well as cytoplasmic and nuclear effectors. Some elegant studies have demonstrated genetic interactions, as well as vertical and horizontal connections between signaling pathways. However, much work is still needed, and the challenge ahead is to understand how cells process the dazzling amount of signals they receive from within and from the neighborhood to reach the decision to remain unchanged, change their identity, proliferate, migrate, differentiate or die. Previous comprehensive reviews [2–7] have listed and described in detail the scores of mouse mutants and what they have taught us so far about normal development of the lip and palate as well as the etiopathogenesis of CL with and without CP (CL/CP) or clefting affecting only the secondary palate (CPO). This review will highlight the importance of the mouse as a model organism for CL and CP research, some of the clever genetic tricks used by researchers to bypass hurdles, and discuss experimental needs and the growing number of biological questions by describing a number of previously and newly generated mutants with CL, CL/CP or CPO.

The Mouse as a Model for Cleft Lip and Cleft Palate Research

Lip and palate clefting results from disruptions impacting on cellular migration, proliferation, programmed cell death (apoptosis), extracellular matrix deposition and/or morphogenetic movements. All of these developmental events are crucial during the formation of lip and palate primordia. In contrast to development of the upper lip and primary palate, development of the secondary palate takes place in conjunction with growth and differentiation of other structures in the head and oral cavity, including the craniofacial skeleton and tongue. Therefore, clefting of the secondary palate may be secondary to defects in these structures, especially if the gene is not expressed in the primordia of the palate proper [4, 5, 8]. However, CP can also be associated either with intrinsic disruptions within the palate proper or by a combination of both primary biological alterations within the palatal shelves and defects in other cranial structures [5, 9]. In view of the striking external differences between humans and mice, a non-specialist would question the usefulness of the mouse in CL and CP research. However, during early craniofacial development, mouse and the human embryos bear a striking resemblance and are essentially of similar sizes. In addition, development of the lip and palate are basically similar in the two species. More importantly and notwithstanding the fact that mice and humans share around 99% of their genes [5], numerous genes that have been incriminated as causal factors in human orofacial clefting also engender clefting in mice and vice versa [5].

CL and CP in humans is an end point and knowing what went wrong in the womb, even if the etiological factor has been identified, is impossible. However, mouse models allow us to track down the cellular and molecular events during the different steps of lip and palate development and flesh out the pathogenesis of a given clefting condition. In other words, using the mouse embryo allows us to determine whether clefting is secondary to altered growth of lip or palatal primordia (due to altered cell proliferation, increased apoptosis, or abnormal migration of cranial neural crest cells which make up the bulk mesenchyme of these structures), lack of morphogenetic movements (failure of elevation of the palatal shelves or abnormal invagination of lip primordia), or abnormal fusion (due to perdurance of the transient epithelial seams as a result of abnormally sustained cell proliferation and/or lack of apoptosis, abnormal early adhesion between the epithelia of the abutting primordia that are fated to merge) [4, 5]. Whilst human genetics studies have been, to some extent, successful at identifying culprit genes in several syndromic forms of orofacial clefting, deciphering the etiology of non-syndromic forms of CL and/or CP is a more challenging task, in view of their complex and multifactorial etiology. A gene could be considered as a good candidate for non-syndromic clefting based on its chromosomal location, if it is expressed during the critical steps of lip and palate development, and/or when its dysfunction generates CL and/or CP in mice. Molecular analyses of mutant mice and gene expression profiles in developing murine lip and palate primordia also enable scientists to unveil regulatory networks and to identify new candidate genes that may be involved in human clefting [4, 5].

Biochemical and Genetic Tools

In addition to the availability of different methods for the generation of mice, which model an array of human diseases, including congenital malformations, complementary techniques and tools for mouse studies are abundant and are becoming more and more sophisticated. In developmental biology, methods such as whole-mount staining of embryos or organ primordia for the visualization of transcripts, small molecules, and endogenous or reporter proteins are particularly informative. Similarly, skeletal preparations with alcian blue and alizarin red staining provide clues about the extent of defects in craniofacial skeletal elements. These can be combined with 3...