Opitz GBBB syndrome with total anomalous pulmonary venous connection: A new MID1 gene variant

Abstract

Background: Opitz GBBB syndrome (GBBB) is an X-linked disease characterized by midline defects, including congenital heart defects. We present our diagnostic approach to the identification of GBBB in a consanguineous family in which two males siblings were concordant for a total anomalous connection of pulmonary veins and minor facial dysmorphias.

Methods: Targeted exome sequencing analysis of a 380-gene panel associated with cardiovascular disease was performed on the propositus. Interpretative analysis of the exome results was conducted, and 3D models of the protein changes were generated.

Results: We identified a NM_000381.4:c.608G>A;p.(Arg203Gln) change in MID1, affecting the conformation of the B-box 2 domain of the protein, with a zinc finger structure and associated protein interactions. This clinical phenotype is consistent with GBBB; however, the type of congenital heart disease observed in this case has not been previously reported.

Conclusion: A new likely pathogenic variant on MID1 c.608G>A was found to be associated with Opitz GBBB syndrome.

Keywords: Opitz GBBB syndrome; congenital heart defects; total anomalous pulmonary venous connection.

FIGURE 1

(a) Pedigree showing the consanguineous union between the parents of the proband. (b) Alignment of total exome reads spanning exon 2 of MID1 gene, showing the C>T transition causing the Arg203Gln mutation in the patient. (c) Sanger sequencing trace showing the C>T hemizygous transition in the proband and the same heterozygous variant in IV1 and V3 hemizygous for C allele. (d) Global alignment of the MID1 orthologs from several species showing high conservation of the arginine (R) residue. The zebrafish shows a histidine [H] residue and a charged polar amino acid. (e) Echocardiogram subject V4, prior to corrective surgery. Subcostal view visualizing the pulmonary veins collector (PV) arriving at the dilated coronary sinus (CS). (f): Subcostal view visualizing both atria (RA, LA) and atrial septal defect (arrow) with right to left shunt. (g) Echocardiogram of the propositus (V5) prior to corrective surgery, suprasternal view showing the pulmonary veins (PV) reaching the vertical vein (VV) and pulmonary veins collector (PVC) is observed (dotted line).

FIGURE 2

(a) The three‐dimensional structure of E3 ubiquitin‐protein ligase Midline‐1, as determined by molecular modeling and the spatial locations of surface mutations. Eight natural variants of the protein have been reported in the literature, with mutations at positions 266 (C→R, in warm pink), 295 (L→P, in olive), 391, and 392 (LC→R, in deep blue), 438 (missing, in yellow), 534 (V→VFIDSGRHL, in violet purple), 536 (I→T, in deep teal), and 626 (L→P, in orange). The position corresponding to Arg 203 is shown in red. (b) Alignment of the B‐box 2 domain of the wild‐type E3 ubiquitin‐protein ligase Midline‐1 (cyan) and the mutant protein (red). Located in this domain is a zinc finger (at positions 172–212, which numbers correspond to those in the PDB file 2JUN), consisting of an alpha‐helix and an antiparallel beta‐sheet. (c) The zinc ion (deep blue) is coordinated by histidine 178 (H178) and three cysteines (C175, C195, and C198). The mutation of a charged amino acid (Arg, R) by an amidic amino acid (Gln, Q) implies both the shortening of the length of the side‐chain and changes in the physicochemical and acid–base properties of the residue. The mutation by Q necessarily implies the destabilization of the zinc finger region, mainly in its interaction with the imidazole ring of H178. The modification of the chemical environment generates structural destabilization. The main detrimental effect of this is a decrease in the specificity of the zinc finger. A more long‐distance effect that may result is the abolition of molecular recognition by other biomolecules.