465 REVIEW J Med Genet: first published as 10.1136/jmg.2004.029181 on 1 June 2006. Downloaded from What you can learn from one gene: GLI3 L G Biesecker ............................................................................................................................... J Med Genet 2006;43:465–469. doi: 10.1136/jmg.2004.029181 The study of patients with rare multiple congenital anomaly several genes that cause abnormal phenotypes in the human when mutated (for example, SHH, syndromes can provide illuminating insights into normal PTC1, and CBP).7 development and the pathogenesis of congenital The understanding of the pathogenesis of GLI3 anomalies. The GLI3 gene is a particularly good example was markedly facilitated by analyses of model organisms and comparing the model organism as it illuminates the phenomena of pleiotropy, data with the human data. The bifunctional phenocopies, syndrome families, and evolutionary nature of GLI3 was a hypothesis based on two conservation of pathogenesis, and raises questions about observations: the biological function of cubitus interruptus (ci, the homologue of the GLI gene how diagnoses are conceptualised. These topics are family in Drosophila) and the position of trunca- reviewed in turn, in the context of the clinical and biological tion mutations in GLI3 in humans. The key data derived from patients with mutations in GLI3 and biological insight was made by the Kornberg group, when they showed that ci, which nor- experimental work in model systems. mally localises to the cytoplasm, is under the ........................................................................... regulation of hedgehog (hh, the fly homologue of the vertebrate hedgehog gene family, sonic, nterruption of the GLI3 gene (OMIM 165240) Indian, and desert hedgehog).89In the presence by translocations in 7p13 was originally of hh, the 155 kDa ci protein was translocated to Idescribed in 1991 in a small series of patients the nucleus and activated downstream genes, with the Greig cephalopolysyndactyly syndrome whereas in the absence of hh, ci was proteoly- (GCPS, OMIM 175700),1 which is inherited in an tically processed to an N-terminal 75 kDa form autosomal dominant pattern. This syndrome that repressed downstream genes. The human comprises the dyad of polydactyly and craniofa- genetics insight was provided by the observation cial anomalies2 (table 1). Although the polydac- that mutations in GLI3 that cause Pallister-Hall tyly is classically described as preaxial, the more syndrome are frameshift or nonsense truncating common presentation is preaxial polydactyly of mutations and that these mutations occur in a the feet and postaxial polydactyly of the hands particular region of the protein.510 Intriguingly, (crossed polydactyly, type I, OMIM 174700).3 The when one aligns the human GLI3 and the fly ci craniofacial anomalies of GCPS include macro- proteins, it appears that the human truncation http://jmg.bmj.com/ cephaly and hypertelorism with a broad nasal mutations that cause PHS occur exclusively in bridge. Early reports of the disorder included the domains of the protein that are carboxy- craniosynostosis as a manifestation, but it has terminal of the predicted protein processing subsequently become clear that this is an cleavage point of the ci gene. This suggested uncommon manifestation of the disorder. that PHS truncation mutations perturb the SHH After the molecular delineation of GCPS, related regulation of GLI3 processing and that linkage analysis of two families with autosomal GLI3 is bifunctional, as is ci. Subsequent data on October 3, 2021 by guest. Protected copyright. dominant Pallister-Hall syndrome (PHS, OMIM have shown that this is correct,11 and that PHS 146510) was undertaken.4 The phenotype of PHS truncation mutations generate a constitutive includes central or postaxial polydactyly, gain of function mutation of a GLI3 repressor hypothalamic hamartoma, bifid epiglottis or protein that is likely to be independent of SHH laryngeal cleft, and pulmonary segmentation controlled post-translational regulation. anomalies, and this disorder is also inherited in Thus GLI3 is a component of an evolutionarily an autosomal dominant pattern. This phenotype conserved developmental cassette or module that ....................... thus had very little clinical overlap with GCPS. In is used to accomplish various tasks throughout spite of that lack of phenotypic overlap, PHS the developing organism. For example, the SHH/ Correspondence to: mapped to 7p13 and was eventually shown to be GLI pathway is used to specify the neural tube, Dr Leslie Biesecker, National Human Genome allelic to GCPS, with several GLI3 mutations craniofacial structures, the limb, the lung, and 5 Research Institute, 49 described in patients with PHS. many others. This developmental module is Convent Drive Room generally used to specify positional or polarity 4A80, Bethesda, MD EVOLUTIONARY CONSERVATION information within part of the developing 20892-4472; leslieb@ helix.nih.gov ILLUMINATES GLI3 FUNCTION embryo. A well known example of this is in the The GLI3 protein is a zinc finger transcription developing limb bud where SHH functions as a Received factor that is expressed early in development. morphogen in the zone of polarising activity 13 December 2005 This transcription factor regulates downstream to establish anterior-posterior polarity. In this Revised version received genes by direct binding to specific sequences in 13 December 2005 6 Accepted for publication the promoter region of target genes. The GLI3 Abbreviations: ACLS, acrocallosal syndrome; GCPS, 17 December 2005 protein is a downstream mediator of the sonic Greig cephalopolysyndactyly syndrome; PHS, Pallister- ....................... hedgehog pathway, and this pathway includes Hall syndrome www.jmedgenet.com 466 Biesecker Table 1 Features of Greig cephalopolysyndactyly syndrome and overlapping syndromes J Med Genet: first published as 10.1136/jmg.2004.029181 on 1 June 2006. Downloaded from Manifestation GCPS GPCS-CGS PHS ACLS Preaxial polydactyly +++ +++ +++ Cutaneous syndactyly +++ +++ +++ Macrocephaly +++ ++ +++ Hypertelorism ++ ++ +++ Mental retardation + +++ + +++ Seizures ++ + ++ Postaxial polydactyly ++ ++ ++ ++ Central polydactyly +++ Hypothalamic hamartoma +++ Bifid epiglottis ++ ACLS, acrocallosal syndrome GCPS, Greig cephalopolysyndactyly syndrome; CGS, contiguous gene syndrome; PHS, Pallister-Hall syndrome. pathway, SHH functions in a cell autonomous fashion each other. For example, PHS has substantial clinical overlap through a cell surface receptor complex of patched (PTC1) with Bardet-Biedl syndrome, McKusick-Kaufmann syn- and smoothened (SMO) (reviewed by Villavicencio et al7). As drome, and several forms of the oral-facial-digital syndromes. is true for many developmental cassette pathways, SHH In contrast, GCPS can be difficult to distinguish from triggers a derepression of a negative modulation of the frontonasal dysplasia spectrum, Opitz trigonencephaly syn- pathway. That is, binding of SHH to PTC1 releases the drome, and others. It is ironic that PHS is easy to distinguish negative interaction of PTC1 on SMO, which normally from GCPS in the clinic (see table 1). Part of the challenge negatively modulates the downstream pathway members. with the differential of GCPS is that the cardinal features of The net effect of SHH binding to the cell surface complex is that disorder are relatively non-specific. Preaxial polydactyly an activation of the pathway. Downstream from this is recognised in more than 25 syndromes and as a non- complex, the cassette includes a complex of proteins that syndromic entity (see below) and hypertelorism is recognised are tethered to a cytoplasmic complex and this includes GLI3. in more than 100 syndromes.12 13 It must be emphasised that It is at this point that that the post-translational regulation of although both GCPS and PHS are clinically distinct, both GLI3 occurs (as described above). have a wide range of severity and it is incorrect to assert that Thus GLI3 functions as a bifunctional mediator of the SHH one disorder is more severe than the other. This misconcep- pathway, either activating or repressing the transcription of tion was fostered because early reports of PHS were downstream genes. This is in contrast to most transcription unusually severe,14 whereas early reports of GCPS were factors, which function as either activators or repressors (but relatively mild.15 The recognition that the two phenotypes not both). The modulatory function of most other transcrip- each manifested a full range of severity meant that they could tion factors is attributable either to their presence or absence, not be placed on a single continuum of severity and that but they are not bifunctional. This means that GLI3 has other mechanisms must be invoked to explain the pheno- multiple distinct biological functions and those functions are types. This led to the pursuit of the bifunctional model, as related to the distinct phenotypes that are caused by different delineated above. http://jmg.bmj.com/ classes of mutations in the gene. Thus distantly related lines In spite of this ready clinical distinction of the two of research (Drosophila basic biology and clinical molecular disorders, an argument was made that the allelism dictated genetics) have converged to provide a coherent pathogenic that PHS and GCPS should be considered as a phenotypic basis of two distinct but allelic human malformation continuum and should together be redesignated