Typical Greig cephalopolysyndactyly syndrome (GCPS) is a rare, congenital limb malformations that is characterized by craniofacial abnormalities; macrocephaly, widely spaced eyes associated with increased interpupillary distance (ocular hypertelorism), and preaxial polydactyly with or without postaxial polydactyly, and cutaneous syndactyly. GCPS is a clinical condition of ‘superfluous fingers’ or ‘extra fingers’ that inherited in an autosomal dominant pattern in the family (Bell, 1953). In general, preaxial polydactyly is found with at least one character among polysyndactyly, macrocephaly or hypertelorism in GCPS patient (Johnston et al., 2005; Balk and Biesecker, 2008; Jamsheer et al., 2012; Rashmi et al., 2014; Rashmi et al., 2016). A high degree of phenotypic and clinical variations is observed in GCPS patients with highly variable penetrance from mild to moderate and severe form, even in affected members (or GCPS patients) within the same family (Debeer et al., 2003; Rashmi et al., 2014; Patel et al., 2017). Developmental delay, intellectual disability, or seizures are uncommon manifestations in GCPS patients and reported in ~<10% cases which is associated with large (>300-kb) deletions containing GLI3 (Johnston et al., 2007). Moreover, hypoplasia or agenesis of the corpus callosum has also been observed in approximately 20% cases of GCPS (Biesecker et al., 2001-2024).
GCPS is caused by GLI3 mutation that results in loss of function of the protein (haploinsufficiency) (Jamsheer et al., 2012; Johnston et al., 2010; Balk and Biesecker, 2008; Biesecker, 2008; Debeer et al., 2003). To date, several pathogenic genetic variants (or mutations) have been detected in the GLI3 gene in more than 200 GCPS patients (Williams et al 1997, Kalff-Suske et al 1999, Debeer et al 2003, Johnston et al 2005, Debeer et al 2007, Schulz et al 2008, Johnston et al 2010, Hurst et al 2011, Jamsheer et al 2012, Patel et al 2014, Démurger et al 2015, Abdullah et al 2019, Siavrienė et al 2019, Khan et al 2021, Polivka et al 2021, Sczakiel et al 2021, Garcia-Rodriguez et al 2022). Allelic heterogeneity is exhibited by the GLI3 gene because GLI3 mutations cause several developmental syndromic and non-syndromic polydactyly abnormalities. Pathogenic variants or mutations in the GLI3 gene cause several congenital malformations including GCPS (OMIM # 175700, Vortkamp et al., 1991) and Pallister-Hall syndrome (PHS; OMIM # 146510, Biesecker et al.,1993), which occur at a relatively high frequency, whereas less commonly acrocallosal syndrome (OMIM #200990), non-syndromic polydactyly type IV (OMIM # 174700), and postaxial polydactyly type A1 (PAPA1) or type B (PAPB) (OMIM # 174200) (Johnston et al., 2010). Moreover, McDonald-McGinn et al. (2010) observed 2 different heterozygous mutations in the GLI3 gene in two unrelated patients with craniosynostosis of the metopic suture resulting in trigonocephaly as well as multiple digital anomalies (165240.0020 and 165240.0021, respectively). Both GCPS and PHS have shown distinct clinical presentations, but are inherited in an autosomal dominant pattern in the family. GCPS is characterized by preaxial polydactyly with polysyndactyly and/or macrocephaly and/or hypertelorism, whereas PHS is characterized by hypothalamic hamartoma, bifid epiglottis, and insertional polydactyly (McDonald et al., 2010). The nontypical clinical features of GCPSs include finger like thumb, mild ichthyosis, sparse hair, and low set ears (Rashmi et al., 2014).
The GLI3 protein is a zinc finger transcription factor that is strongly expressed in limb buds as well as other tissues in the embryo during the early stages of embryonic development. GLI3 is a downstream mediator of the sonic hedgehog (SHH) signaling pathway, a key signaling pathway during embryonic limb development (Cohen, 2010). The SHH/GLI3 gene regulatory network is essential for the expansion of digit progenitor cells in tetrapods (Letelier et al., 2021). In the mouse models, the anterior-posterior polarity of limb buds and dorsal-ventral polarity are regulated by the SHH/GLI3 pathway during the development of neural tubes, craniofacial structures, lungs and several other organs (Cohen, 2010). GLI3 is a bifunctional and SHH mediator protein that functions either as a repressor or activator of the transcription of downstream target genes. SHH signaling also controls and inhibits the processing of the full-length GLI3 protein to its repressor form (GLI3R). This leads to graded GLI3 activator (GLI3A) function in the posterior region and repressor (GLI3R) function in the anterior region of the limb buds (Cohen, 2010; Biesecker, 2006; Ruppert et al., 1988). In the presence of SHH signals, GLI3 protein (full-length) upregulates its target genes, required for essential function during limb development. Conversely, the absence of SHH results in the cleavage of the full-length GLI3 protein, which results in the formation of a repressor. The repressor form of the GLI3 protein downregulates target genes during limb growth and development (Jamsheer et al., 2012; Johnston et al., 2005; Ruppert et al., 1988; Krauß et al., 2009; Shin et al., 1999).
The GLI3 gene shows allelic heterogeneity in patients depending on the position of the mutation in the GLI3 gene. The full-length GLI3 protein contains five highly conserved zinc finger domains. The GLI3 protein contains a trans-activation domain and repressor domain and binds in a sequence-specific manner (Cohen, 2010; Biesecker, 2006; Ruppert et al., 1988). A strong correlation has been found between the position of the mutation and clinical manifestations in patients. Pathogenic, likely pathogenic, frameshift-truncation or deletion mutations in one third of the GLI3 protein cause GCPS syndrome. However, truncating mutations in the middle third of the gene are commonly associated with PHS (Jamsheer et al., 2012; Johnston et al., 2005; Ruppert et al., 1988). GCPS is caused by pathogenic variants of all types, whereas PHS is caused by only truncating variants and one splice variant that generates a frameshift and a truncation mutation. Within the frameshift mutation category, there is a correlation between the position of the truncation and clinical features and/or disorder. In particular, GCPS is caused primarily by pathogenic variants at the 5' position (nt) 1998 and 3' position (nt 3481), whereas PHS is caused exclusively by truncations between nt 1998 and 3481. Interestingly, a single truncating variant in the PHS region can also cause GCPS, which has been observed in six apparently unrelated families (Biesecker et al., 2001-2024).