Compound heterozygous GLI3 variants in siblings with thyroid hemiagenesis

The GLI3 is a gene located on 7p14.1 chromosome, encoding a protein of cytoplasmatic localization, known to play a key role in development which activates patched Drosophila homolog gene expression. This evolutionary conservative gene is expressed in multiple organs, mainly ovaries, placenta, endometrium, and presents moderate expression in the thyroid gland. The Gli3 protein is a transcription factor and negative regulator of Sonic hedgehog (Shh) signaling [8, 9], governing the symmetric lobulation of the thyroid. Shh gene knock-out mice present thyroid developmental failure resembling THA in humans [10], usually accompanied by other severe developmental anomalies, i.e., holoprosencephaly, trachea-esophageal fistula, and lung hypoplasia, while in humans such genetic variant is lethal [11]. Patients with Williams syndrome were also reported to have THA [1]. The microdeletion of 7q11.23 region in these patients harbors the FZD9 gene that interacts with Shh and suggests contribution of this pathway. GLI3 is essential for human development. To date the role of GLI3 in embryogenesis was demonstrated for brain, lungs and limbs [9]. The GLI3-associated syndromes encompass polydactyly, central nervous system, and lungs pathologies, associating in Greig cephalopolysyndactyly and Pallister-Hall syndromes, presenting only autosomal dominant mode of inheritance [12]. In this report, none of the family members has clinical features of the mentioned GLI3-associated syndromes. Also, none of the probands’ offspring (obligatory heterozygotes) presented isolated THA.

This study represents the first report indicating a potential involvement of GLI3 in the thyroid development. The pathogenic effect on phenotype was documented for variant p.Gly727Arg (HGMD: CM990705) affecting an evolutionary conserved locus in the gene and segregating in a familial case of postaxial polydactyly being present in all affected family members [12]. Even though seven relatives were manifesting polydactyly and condition showed autosomal dominant inheritance, the variant was reported in population databases as very rare and therefore was classified as variant of unknown significance (VUS).

In our study, we deny its major causative impact also in regard of THA as a single separate factor (heterozygous GLI3 allele defect in healthy offspring of probands). The second alteration p.Gln1347Lys is located 233 amino acid residues apart from the end of GLI3 canonical transcript, before two independent transactivation domains TA1 (aa 1376–1580) and TA2 (aa 1044–1322) [13]. The change is predicted to be pathogenic in all used algorithms, and VUS according to ACMG (one control individual was reported for this position, but presenting different nucleotide substitution). More distally located mutations (AA1359–1486) were reported to be causative for at least seven cases of severe autosomal dominant Greig cephalopolysyndactyly syndrome [14]. This change was detected also in probands’ daughter who had normal thyroid on US. The case of compound heterozygosity has never been evidenced for GLI3 and apparently milder THA phenotype detected in our study is surprising. However, we could not perform thyroid US in two out of five probands’ offspring. This constitutes a limitation of the study, same as that unaffected siblings of the probands refused to undergo genetic testing.

Mutations in exon 7 of GLI3 have been demonstrated also to cause other congenital malformation characterized by hypoplastic or absent tibia–hemimelia. Multiple similarities between THA and hemimelia should be mentioned here: occurs unilaterally, often as an isolated condition (rarely in syndromic cases), while family history is usually negative. Deimling et al. reported an exon 7 deletion of GLI3 in two patients with bilateral hemimelia, which causes truncated GLI3 protein that is deprived of DNA-binding domain and unable to repress SHH activity to the limb bud [15]. Interestingly, an asymptomatic mother of the proband was a carrier of the deletion, thus, authors came to a conclusion that tibial hemimelia may be inherited in an autosomal dominant mode with incomplete penetrance.

As potential mechanism how mutation of GLI3 affects the SHH signaling we postulate a disequilibrium between the repressor and activator forms of the gene, what leads to lack of negative regulation of Shh. Shh is mostly expressed in epithelia therefore regulatory disturbances would have an effect particularly in organs derived from that kind of tissue. As evidenced for pituitary development [16, 17], the Shh pathway significantly contributes to the processes of morphogenesis and cell specification but both processes are not coupled. The consequences of emanation of the signals throughout developing tissue include not only a presence of a certain molecules but also an amount of signals that is locally expressed. Although in case of the reduction (or even absence) of Gli3 regulatory function, several alternative signaling cascades can be triggered (other Gli family molecules, i.e., Gli2 as well as recently evidenced Glis3) in order to compensate lack of Gli3. That would explain disturbances in thyroid shaping while maintaining the proper function of the tissue.

Interestingly, another associated transcription factor contributing to the Shh pathway—GLIS3 (GLI-Similar protein 3) has recently been demonstrated to influence thyroid development. GLIS3 zebrafish knock-out shows a reduced number of thyroid follicles and an increase of the TSHβ subunit, resembling THA [18]. In humans, homozygous mutations in GLIS3 lead to a multiple malformation syndrome encompassing CH. Rare heterozygous GLIS3 missense variants were identified in 18 out of 177 patients with CH; half of them presented TDA (agenesis, hypoplasia, or ectopy) [19]. It was demonstrated that GLIS3 may bind to the GLI consensus sequence GACCACCCAC, what suggests a possible cross-talk between the GLI and GLIS3 signaling pathways [20].

Comparison of mice Gli3 and human GLI3 reveals that all domains demonstrate high similarity except the transactivation domain. Our mutations were identified in a proteasomal cleavage site of the protein (p.Gly727Arg) that showed 95% evolutionary conservation and the second one p.Gln1347Lys in a region bearing activatory function, presenting 76% similarity.

GLI3 haploinsufficiency has been noted in a wide range of human syndromes with a diverse severity, while phenotypic variability and incomplete penetration may be explained by coexistent mutations of SHH regulators. This may be one of the explanations for lack of extremities malformations, but the presence of an inherited THA in our family. Several mouse phenotypes associated with mutations of Gli3 affected the skeleton, eye, tail, extra toes, brachyphalangy, anterior digit deformity, and polydactyly. According to Radhakrishna et al. these mutants display phenotypic variability of expression, and there is no clear effect of the site of mutation on the phenotype. The authors proposed that all phenotypes associated with GLI3 mutations may be called “GLI3 morphopathies”. Hence there is no evident phenotype-genotype correlation of GLI3 mutations [12]. According to the results of our study, GLI3 morphopathies may include THA.