A human homeotic transformation resulting from mutations in PLCB4 and GNAI3 causes auriculocondylar syndrome

Abstract

Auriculocondylar syndrome (ACS) is a rare, autosomal-dominant craniofacial malformation syndrome characterized by variable micrognathia, temporomandibular joint ankylosis, cleft palate, and a characteristic "question-mark" ear malformation. Careful phenotypic characterization of severely affected probands in our cohort suggested the presence of a mandibular patterning defect resulting in a maxillary phenotype (i.e., homeotic transformation). We used exome sequencing of five probands and identified two novel (exclusive to the patient and/or family studied) missense mutations in PLCB4 and a shared mutation in GNAI3 in two unrelated probands. In confirmatory studies, three additional novel PLCB4 mutations were found in multigenerational ACS pedigrees. All mutations were confirmed by Sanger sequencing, were not present in more than 10,000 control chromosomes, and resulted in amino-acid substitutions located in highly conserved protein domains. Additionally, protein-structure modeling demonstrated that all ACS substitutions disrupt the catalytic sites of PLCB4 and GNAI3. We suggest that PLCB4 and GNAI3 are core signaling molecules of the endothelin-1-distal-less homeobox 5 and 6 (EDN1-DLX5/DLX6) pathway. Functional studies demonstrated a significant reduction in downstream DLX5 and DLX6 expression in ACS cases in assays using cultured osteoblasts from probands and controls. These results support the role of the previously implicated EDN1-DLX5/6 pathway in regulating mandibular specification in other species, which, when disrupted, results in a maxillary phenotype. This work defines the molecular basis of ACS as a homeotic transformation (mandible to maxilla) in humans.

Figure 1

PLCB4 and GNAI3 Mutations Lead to Severe Mandibular Hypoplasia and Ear Dysplasia (A) Facial photos of proband S001 (PLCB4 p.Tyr623Cys) (B) S008 (GNAI3 p.Gly40Arg) with severe micrognathia and the classic “question mark” ear (inset). Below each photo is the respective three-dimensional computed tomography (3D CT) scan, which shows the severe mandibular hypoplasia associated with mandibular body and ramus dysplasia.

Figure 2

Homeotic Transformation of the Mandible to a Maxillary Phenotype (A) Microglossia and lateral soft-tissue projections at the tongue base as seen intraoperatively in our index case S011 (GNAI3 p.Gly40Arg). (B) Excessive soft-tissue projections emanating from the medial surface of the lower jaw. (C) 3D CT demonstrates severe mandibular hypoplasia, with fusion of the lateral mandibular process to the temporozygomatic suture (red arrow). (D) Medial view of a parasagittal section of the skull reveals complete fusion of the left mandibular ramus with the medial and lateral pterygoid plates (within red dashed circle); inset depicts the plane of the section. (E) Control CT image of an age- and sex-matched individual showing normal mandible anatomy. (F) CT image of individual S001 with a PLCB4 p.Tyr623Cys substitution. (G) A composite CT image generated with photo-editing software in which the mandible from panel F was copied, inverted, and placed over the control maxilla in panel G. The resultant image shown here depicts the remarkable alignment of the ACS mandible with the control maxilla, zygomatic arch, and midfacial structures, demonstrating transformation to a maxillary identity (a homeotic transformation).

Figure 3

Conservation and Substitution Modeling of PLCB4 and GNAI3 (A) PLCB4 sequence alignment at the highly conserved catalytic domains. Alignments were performed between human, Drosophila, and fungus protein sequences with the use of ClustalW2. Conservation among vertebrates, invertebrates, and fungi suggests evolutionary importance for these residues. Identified substitutions are depicted in green boxes; residues conserved between humans, Drosophila, and fungi are depicted in yellow; conservative substitutions are in orange; and divergent residues are in white. Protein sequences used for alignment of PLCB4 include Homo sapiens (NP_877949.2), Drosophila (no receptor potential A, isoform D, NP_001014720.1), and Aspergillus clavatus NPRL 1 (1-phosphatidylinositol-4,5-bisphosphate phosphodiesterase 1 [XP_001268930.1]). (B) Sequence alignment (similar to panel A) of the GNAI3 catalytic region with the p.Gly40Arg ACS subsitution. Protein sequences used for alignment of GNAI3 include Human (NP_006487.1), Drosophila (G protein α_i subunit 65A, NP_477502.1), and Saccharomyces cerevisiae (Gpa2p, NP_010937.1). (C) PLCB4 structure depicting the four missense substitutions (red residues). In this panel and in panel E, other missense variants observed in controls are shown as light gray spheres. (D) Magnification of the active site of PLCB4 demonstrates variant residues that contribute directly to catabolic release of inositol triphosphate (IP3, gray molecule) and disrupt catalytic function without destabilizing protein folding. These residues are presumed to inhibit MAPK signaling without affecting other protein interactions. Calcium ion is depicted in purple. Hydrogen bonds between amino-acid side chains, calcium ion, and IP3 are shown as blue lines, and ionic metal bonds are shown as green dashed lines. (E) GNAI3 structure showing the p.Gly40Arg mutant (red) residue. (F) Magnification of the GNAI3 active site shows the Gly40Arg mutant residue (pink lines) reaching across a previously undescribed cavity (transparent green), consistent in all three GNAI3 crystal structures, to stabilize the RapGapII-recognized active conformation of guanosine triphosphate-activated switch III (black ribbon; switch I in gray ribbon, switch II in white ribbon) by forming a hydrogen bond network (pink lines) between the Gly40Arg residue, a nearby main chain carbonyl (blue ribbon), and stabilizing the active confirmation of switch III through interaction with the key switch residues (red and white sticks). Switch III is recognized by RapGapII and would lead to inhibition of the MAPK pathway activation through Rap (Figure 4).

Figure 4

Hypothesized Pathway for Mandibular Patterning Associated with Mutations in the Endothelin-DLX5/6 Pathway Several reports support the premise that mutations in the endothelin pathway cause homeotic transformation of the lower (ventral) jaw to a maxillary (dorsal) identity. Mutations in mice of Edn1, Ednra, the G protein α_q subunit, Dlx5/Dlx6, and most recently Hand2, each result in a small ventral jaw with dorsal jaw features. Mutations in zebrafish plcb3 lead to the same homeotic transformation. In this figure, cartoons of fish, mice, and humans indicate gene family members in this pathway that, when mutated, result in a homeotic transformation of the dorsal and ventral jaws (left: PLCB4 loss-of-function, right: GNAI3 gain-of-function). The arrow with a dashed line indicates hypothetical signaling between α_q and PLCB4. Figure adapted from Marinissen and Gutkind.