MAT2A mutations predispose individuals to thoracic aortic aneurysms

Abstract

Up to 20% of individuals who have thoracic aortic aneurysms or acute aortic dissections but who do not have syndromic features have a family history of thoracic aortic disease. Significant genetic heterogeneity is established for this familial condition. Whole-genome linkage analysis and exome sequencing of distant relatives from a large family with autosomal-dominant inheritance of thoracic aortic aneurysms variably associated with the bicuspid aortic valve was used for identification of additional genes predisposing individuals to this condition. A rare variant, c.1031A>C (p.Glu344Ala), was identified in MAT2A, which encodes methionine adenosyltransferase II alpha (MAT IIα). This variant segregated with disease in the family, and Sanger sequencing of DNA from affected probands from unrelated families with thoracic aortic disease identified another MAT2A rare variant, c.1067G>A (p.Arg356His). Evidence that these variants predispose individuals to thoracic aortic aneurysms and dissections includes the following: there is a paucity of rare variants in MAT2A in the population; amino acids Glu344 and Arg356 are conserved from humans to zebrafish; and substitutions of these amino acids in MAT Iα are found in individuals with hypermethioninemia. Structural analysis suggested that p.Glu344Ala and p.Arg356His disrupt MAT IIα enzyme function. Knockdown of mat2aa in zebrafish via morpholino oligomers disrupted cardiovascular development. Co-transfected wild-type human MAT2A mRNA rescued defects of zebrafish cardiovascular development at significantly higher levels than mRNA edited to express either the Glu344 or Arg356 mutants, providing further evidence that the p.Glu344Ala and p.Arg356His substitutions impair MAT IIα function. The data presented here support the conclusion that rare genetic variants in MAT2A predispose individuals to thoracic aortic disease.

Copyright © 2015 The American Society of Human Genetics. Published by Elsevier Inc. All rights reserved.

Figures

Figure 1

Identification and Characterization of MAT2A Rare Variants in Families Affected by Thoracic Aortic Disease (A) TAA059 family pedigree. The legend indicates the designations for disease and mutation status of family members. The age at diagnosis of aortic aneurysm (dx), age at death (d), or age at last aortic imaging are shown in years. Individuals with aortic dilatation measuring ≥4.2 cm or Z scores of ≥2 were marked as affected. A diagonal line across a symbol indicates that the individual is deceased, an arrow indicates the proband, a single asterisk indicates an individual whose DNA was used for whole-genome linkage analysis, and a red circle indicates the individuals whose DNA was used for exome sequencing. Individual IV:5, marked by symbol †, has had stable aortic-root measurements around the upper limit of normal for 6 years; the ascending aorta is normal. (B) Profile of the parametric multipoint LOD score for segregation of TAAD with SNPs across the human genome in family TAA059. (C) Amino acid alignment of MAT2A sequences containing the rare variants identified in this study. (D) Schematic representation of MAT2A. The boxes represent exons 1–9, and the UTRs and the open reading frame are designated. The MAT2A rare variants identified in this study are above the gene diagram, and the rare variants identified in the ESP database are below. Blue letters designate variants predicted to be possibly or probably damaging by PolyPhen-2 analysis, and black letters designate variants predicted to be benign. (E) X-ray crystallographic structure of MAT IIα (PDB identifier 2P02) shows positions of Glu344 (E344) and Arg356 (R356, designated in pink) relative to the SAM binding site. Analysis of hydrogen bonds and non-bonded contacts of the respective residues and others in the vicinity was performed with PYMOL. Arg356 is located near the SAM pocket and is part of a hydrogen-bonding network involving residues Glu128 (E128), Asp129 (D129), Ser325 (S325), and Asp354 (D354, all designated in blue) and a water molecule (Wat). Structural elements that we propose as part of a “cantilever system” are shown in green and blue and include Glu344 and Arg356; the rest of the monomer is in cyan. Part of a second monomer is also seen (pale gray).

Figure 2

Aortic Pathology Associated with Aneurysms in Individuals with MAT2A Variants Compared with the control aorta, aortas from affected individuals showed medial degeneration upon Movat staining, which showed increased proteoglycan deposition (blue), focal mild fragmentation of elastic fibers (black), and a decreased number of cells (red). Immunostaining for α-actin confirmed the mild focal loss of SMCs.

Figure 3

Phenotypic Spectrum after mat2aa MO Injection and mRNA Rescue Zebrafish were phenotyped with light microscopy 72 dpf after mat2aa MO injection in the zebrafish Tg(flk1:EGFP.) Representative images are shown. (A) Normal phenotype of control-MO-injected zebrafish showing minimal pericardial effusion and no tail defects; a moderately affected morphant with a large pericardial effusion and small eyes; and a severely affected morphant with a large pericardial effusion, small eyes, and a curly tail or severe tail curvature. (B) At 3 dpf after mat2aa MO injection, significant defects in the development of the aortic arches were observed. (C) Zebrafish mat2aa MO injection resulted in significant defects on embryonic development. Co-injection of wild-type (WT) MAT2A mRNA, in comparison with the substitution mRNA, significantly reduced defects of cardiovascular development.