REVIEWS Cleft lip and palate: understanding genetic and environmental influences Michael J. Dixon*, Mary L. Marazita‡, Terri H. Beaty§ and Jeffrey C. Murray|| Abstract | Clefts of the lip and/or palate (CLP) are common birth defects of complex aetiology. CLP can occur in isolation or as part of a broad range of chromosomal, Mendelian or teratogenic syndromes. Although there has been marked progress in identifying genetic and environmental triggers for syndromic CLP, the aetiology of the more common non-syndromic (isolated) forms remains poorly characterized. Recently, using a combination of epidemiology, careful phenotyping, genome-wide association studies and analysis of animal models, several distinct genetic and environmental risk factors have been identified and confirmed for non-syndromic CLP. These findings have advanced our understanding of developmental biology and created new opportunities for clinical translational research. Nares Clefts of the lip and/or palate (CLP) are immediately overwhelming majority involve only the upper lip and/ The nostrils or nasal passages recognizable disruptions of normal facial structure. or palate. Further, approximately 70% of cases of CLP Although not a major cause of mortality in developed occur as isolated entities with no other apparent cogni- Primary palate countries, CLP does cause considerable morbidity to tive or craniofacial structural abnormalities; this is com- The anterior portion of the affected children and imposes a substantial financial monly termed ‘isolated, non-syndromic CLP’. Because palate including the bony 1 component in humans. risk for families with a concomitant societal burden . the defects arise early in embryological development, Individuals with CLP may experience problems with have a complex aetiology (with both genetic and envi- feeding, speaking, hearing and social integration that ronmental contributions) and modest recurrence rates, can be corrected to varying degrees by surgery, dental it has proven difficult to identify specific aetiologic fac- *Faculty of Medical and treatment, speech therapy and psychosocial interven- tors. A combination of epidemiologic, candidate gene Human Sciences, Manchester tion. CLP is aetiologically heterogeneous, and this has and genome-wide studies, plus analysis of animal mod- Academic Health Sciences Centre, Michael Smith crucial implications for understanding the biology of els, has recently provided deeper insights into the causes Building, University of facial development, how environmental risks interact of non-syndromic CLP. Manchester, Oxford Road, with genetic factors and how we can incorporate known With the advent of the genomics era, there have been Manchester M13 9PT, UK. aetiologic variables to improve clinical care. Recent suc- major advances in the identification of causative genetic ‡Center for Craniofacial and Dental Genetics, Department cesses in genome-wide linkage and association studies mutations underlying syndromic forms of CLP (see the of Oral Biology, School of have identified novel loci that are significantly associated OMIM website for further information). By contrast, Dental Medicine, Department with CLP2–6. Researchers are currently striving to iden- there has been less progress in advancing our under- of Human Genetics, Graduate tify the aetiologic variants at these novel loci to under- standing of the genetic aetiology of non-syndromic School of Public Health, stand the developmental disturbances leading to CLP. CLP owing to its genetic heterogeneity, departure from University of Pittsburgh, Pittsburgh, Pennsylvania This knowledge should eventually result in improved Mendelian inheritance patterns, the limited availabil- 14219, USA. prevention, treatment and prognosis for individuals with ity and expense of genomic tools and the necessity for §Department of Epidemiology, these conditions. very large data sets. However, the recent development School of Public Health, Development of the lip and palate is outlined in FIG. 1. of innovative approaches to phenotyping and powerful, Johns Hopkins University, Baltimore, Maryland 21205, The common forms of CLP involve disruption of tissue cost-effective genomic tools, together with extrapolation USA. planes above the lip, extending into the nares and/or from studies of syndromic forms of CLP, have increased ||Departments of Pediatrics, the palate (hard and/or soft) (FIG. 2). Fogh-Andersen our understanding of non-syndromic CLP. Because of its Biology and Epidemiology, and Fraser7,8 noted that clefts involving the anterior particular challenges, in this Review we focus on non- University of Iowa, structures (lip and primary palate) could be separated syndromic CLP and we summarize syndromic forms Iowa 52242, USA. Correspondence to J.C.M. on both genetic and embryological grounds from those (which are genetically tractable) only briefly. We discuss e-mail: [email protected] involving only the secondary palate. Although there are important epidemiologic clues, environmental contri- doi:10.1038/nrg2933 many disruptions affecting the craniofacial complex, the butions, genetic architecture and issues of phenotyping, NATURE REVIEWS | GENETICS VOLUME 12 | MARCH 2011 | 167 © 2011 Macmillan Publishers Limited. All rights reserved REVIEWS CD E /GFKCNPCUCN RTQEGUUGU (TQPVQPCUCN RTQOKPGPEG .CVGTCNPCUCN RTQEGUUGU /CZKNNCT[ RTQEGUUGU 0CUCNRKVU 1TCNECXKV[ #NCG /CPFKDWNCT RTQEGUUGU FG H 2TKOCT[ RCNCVG +PEKUKXG HQTCOGP 0CUCN (WUGF UGRVWO RCNCVG 5GEQPFCT[ RCNCVCN UJGNXGU Figure 1 | Development of the lip and palate. Schematic diagrams of the development of the lip and palate in humans. a | The developing frontonasal prominence, paired maxillary processes and paired mandibular0CVWTG4G XKGYUprocesses^)GPGVKEU surround the primitive oral cavity by the fourth week of embryonic development. b | By the fifth week, the nasal pits have formed, which leads to the formation of the paired medial and lateral nasal processes. c | The medial nasal processes have merged with the maxillary processes to form the upper lip and primary palate by the end of the sixth week. The lateral nasal processes form the nasal alae. Similarly, the mandibular processes fuse to form the lower jaw. d | During the sixth week of embryogenesis, the secondary palate develops as bilateral outgrowths from the maxillary processes, which grow vertically down the side of the tongue. e | Subsequently, the palatal shelves elevate to a horizontal position above the tongue, contact one another and commence fusion. f | Fusion of the palatal shelves ultimately divides the oronasal space into separate oral and nasal cavities. Figure is modified, with permission, from REF. 137 © (2009) John Wiley and Sons Ltd. gene discovery and insights into molecular pathogen- Historically, CLP has been divided into cleft palate esis. We also speculate about the implications of these only and cleft lip with or without cleft palate (CL/P)7,8. findings for estimating recurrence, finding new clinical However, recent epidemiologic data suggest that cleft associations building on advances in imaging and using lip only may have unique aetiologic features, including large databases to examine long-term outcomes. strong genetic associations, whereas some individuals with cleft palate only show evidence of subclinical cleft Challenges in studying CLP lip11–15. Nevertheless, this broad sub-division of anatom- Epidemiology. CLP affects approximately 1 in 700 live ical defects is consistent with the distinct developmental births, with wide variability across geographic origin, origins of the lip/primary palate versus the secondary racial and ethnic groups, as well as environmental expo- palate. Furthermore, separate cellular and genetic aeti- sures and socioeconomic status. In general, Asian and ologies for CL/P and cleft palate only are consistent Native American populations have the highest reported with the general observation that these two conditions birth prevalence rates, which are often as high as 1 in 500. do not segregate in the same family, although excep- European-derived populations have intermediate prev- tions have been reported for families with aetiologic alence rates at approximately 1 in 1,000, and African- mutations in specific genes (for example, tumour pro- derived populations have the lowest prevalence rates at tein p63 (TP63), msh homeobox 1 (MSX1), interferon approximately 1 in 2,500. These observations suggest regulatory factor 6 (IRF6) and fibroblast growth factor that the relative contribution of individual susceptibil- receptor 1 (FGFR1))16–20. Approximately 70% of all cases ity genes may vary across different populations6,9,10. The of CL/P and 50% of cases of cleft palate only are con- frequency of CLP also differs by gender and laterality: sidered to be non-syndromic21–23. The remaining cases there is a 2:1 male to female ratio for clefts involving the are composed of a wide range of malformation syn- Secondary palate lip, approximately a 1:2 male to female ratio for clefts of dromes, including over 500 Mendelian syndromes (see Posterior or soft palate the palate only and a 2:1 ratio of left to right sided clefts the OMIM website for further information) as well as in humans. among unilateral cleft lip cases. those arising secondary to chromosomal or teratogenic 168 | MARCH 2011 | VOLUME 12 www.nature.com/reviews/genetics © 2011 Macmillan Publishers Limited. All rights reserved REVIEWS # 7RRGTNKR *CTFRCNCVG 5QȎRCNCVG 7XWNC 7PKNCVGTCN C DEF $KNCVGTCN G HIJ $ C/KETQHQTO%. D.GȎWPKNCVGTCN%. E.GȎWPKNCVGTCN%.RNWU%2