Agnathia-Otocephaly Complex Due to a De Novo Deletion in the OTX2 Gene

Abstract

Agnathia-otocephaly complex (AOC) is a rare and usually lethal malformation typically characterized by hypoplasia or the absence of the mandible, ventromedial and caudal displacement of the ears with or without the fusion of the ears, a small oral aperture with or without a tongue hypoplasia. Its incidence is reported as 1 in 70,000 births and its etiology has been attributed to both genetic and teratogenic causes. AOC is characterized by a wide severity clinical spectrum even when occurring within the same family, ranging from a mild mandibular defect to an extreme facial aberration incompatible with life. Most AOC cases are due to a de novo sporadic mutation. Given the genetic heterogeneity, many genes have been reported to be implicated in this disease but to date, the link to only two genes has been confirmed in the development of this complex: the orthodenticle homeobox 2 (OTX2) gene and the paired related homeobox 1 (PRRX1) gene. In this article, we report a case of a fetus with severe AOC, diagnosed in routine ultrasound scan in the first trimester of pregnancy. The genetic analysis showed a novel 10 bp deletion mutation c.766_775delTTGGGTTTTA in the OTX2 gene, which has never been reported before, together with a missense variant c.778T>C in cis conformation.

Keywords: AOC; OTX2; agnathia; clinical exome sequencing; de novo variant; otocephaly; synotia and proboscis.

Figure 1

Ultrasound findings of the fetus at 11 weeks gestation. Sagittal sections (top two images) showing the absence of the mandible and the axial sections (lower two images) showing the asymmetry of the maxillary bones and a protruding tubular structure (proboscis) from the lower part of the face.

Figure 2

Front and profile of the fetus at 13 weeks after a medical abortion, showing cyclopia, agnathia, synotia and the presence of a proboscis.

Figure 3

(A) Visualization of the result of the clinical exome sequencing (CES) showing the identified variant in the OTX2 gene by Integrative Genomics Viewer (IGV) software (https://software.broadinstitute.org/software/igv/ accessed on 1 August 2022). c.778T>C and c.766_775delTTGGGTTTTA variants were in a heterozygous status in the proband. (B) Sanger sequencing confirmed the identified OTX2 variants, which were not present in his parents.

Figure 4

Gel image showing the PCR products of exon 5 of the OTX2 gene. The resultant PCR products using DNA from a control (in this case the mother of fetus), showed a single amplified DNA fragment of 290 bp, as expected of a normal allele, whereas the proband showed two bands comprising an aberrant band (280 bp), that was smaller than that of the mother. Sanger sequencing shows the two different alleles in the proband, highlighting the presence in cis of the two identified variants in the OTX2 gene.

Figure 5

(A) Predicted structure and protein features of the wild type OTX2 protein. (B) Predicted structure and protein features of the frameshift variant Leu256ThrfsTer43 on the OTX2 protein. Predicted structure was obtained using the trRosetta tool (https://yanglab.nankai.edu.cn/trRosetta/ access on 15 November 2022), instead the predicted protein features were calculated using the PredictProtein tool (https://predictprotein.org/ access on 15 November 2022).