What is the impact of a novel MED12 variant on syndromic conotruncal heart defects? Analysis of case report on two male sibs

Conotruncal heart defects (CTDs), account for approximately 30% of congenital heart disease (CHDs). They are characterized by anomalies of the cardiac outflow tract or of the great arteries [1]. These include common arterial trunk (CAT) and aorto-pulmonary window (APW), as well as double outlet right ventricle (DORV), tetralogy of Fallot (TOF) with or without pulmonary atresia (PA), transposition of the great arteries (TGA), interrupted aortic arch (IAA).

CAT is characterized by a single great artery arising from the ventricles, giving origin to aorta, coronary arteries and pulmonary arteries, and by a ventricular septal defect (VSD) [2]. According to the Collett and Edwards’ Classification [3], CAT is distinguished in: type I when a single pulmonary trunk arises from the truncus arteriosus, type II when two pulmonary branches arise separated but close from the posterior wall of the truncus arteriosus, type III when two pulmonary branches arise separated from lateral walls of the truncus arteriosus. Type IV now is defined pulmonary atresia with ventricular septal defect (PAVSD).

Aorto-pulmonary window (APW) is a communication between the ascending aorta and the pulmonary artery in the presence of separate semilunar valves [4]. It was classified by Richardson into three types [5]: in type I the defect between the aorta and the main pulmonary artery is immediately above the sinuses of Valsalva, in type II the defect is located more distally between the ascending aorta and the origin of the right pulmonary artery and in type III there is anomalous origin of the right pulmonary artery from the ascending aorta.

Both CAT and APW are determined by the failure of septation of the physiologic arterial common trunk into aorta and pulmonary artery trunk during development of the ventricular outlets and proximal arterial segment of the heart tube.

We report on a novel variant in the MED12 gene, detected by Next Generation Sequencing (NGS), associated with recurrent conotruncal heart defects, CAT and APW respectively, and facial dysmorphisms in an Italian family.

The prenatal finding of recurrent CTDs, truncus arteriosus and aorto-pulmonary window respectively, associated with facial dysmorphisms in an Italian family gave us the suspicious of a syndromic CHDs¸ inducing the search for a possible genetic cause.

Congenital heart defects (CHDs) are structural defects due to abnormal cardiac development in embryos. Approximately 70% of CHDs present as an isolated malformation (non-syndromic CHD), while 30% are syndromic because in association with other extra-cardiac anomalies [6]. Causes of CHDs are genetic in 8%, environmental in 2% and multifactorial or not determined in about 90% of cases [7]. The genetic mechanisms responsible for CHD are complex, heterogeneous and incompletely understood: approximately 8–12% is due to chromosomal anomalies, 3–25% to copy number variation and 3–5% to single gene defect [8]. These genetic variants, de novo or inherited, concern genes involved in normal cardiac development, altering signal transduction, transcriptional regulation and encoding proteins. In some cases the same genes have pleiotropic effects on other organs [8] and this explains syndromic CHDs.

Clark EB [9] classified the CHDs according to the pathogenesis: conotruncal defects are classified among the anomalies of the migration of the ectomesenchyme, in which there is an anomalous migration of component of the secondary/anterior heart field. During heart development the outflow tract develops by adding cells from the pharyngeal mesoderm called the secondary/anterior cardiac field [10], whereas the aorto-pulmonary septum develops by condensation of cardiac neural crest (CNC) cell – derived mesenchyme [11]. Alteration or loss of these developmental processes leads to conotruncal anomalies. The increasing studies of the genetic etiology of CHDs allowed the identification of many genes involved in specific morphogenetic mechanisms and responsible of specific cardiac phenotype, creating a link between cause, pathogenetic mechanism and type of malformation [12]. Conotruncal heart diseases are associated with various chromosomal pathologies and monogenic syndromes: the most frequent associations of the CAT are with DiGeorge syndrome, interstitial duplication of 8q and CHARGE syndrome, whereas the APW can be associated to VACTERL association and CHARGE syndrome [13]. Among the genetic variants associated with conotruncal defects, the MED12 gene does not currently appear. MED12 gene is located on chromosome X and encodes a subunit of the Mediator Complex, called mediator complex subunit 12, which is crucial in RNA polymerase II transcription [14]. Zebrafish models have shown that Med12, being co-regulator of specific transcription factors, is necessary during development: med12 zebrafish mutants showed defects in neural crest formation, chondrogenesis and organogenesis of the brain, liver, pancreas and kidney [15, 16]. Zebrafish and mouse MED12-mutant demonstrated the involvement of the MED12 gene in heart development [17, 18].

Our study reports for the first time the finding of variant in the MED12 gene associated with syndromic CHD, specifically associated with facial dysmorphisms. A probable explanation of the role of the MED12 gene in determining conotruncal defect would be found in the molecular mechanisms involved in embryonic development of the heart: Med12 regulates gene-specific functions during development and it is necessary for correct Wnt/β-catenin signaling and Wnt/planar cell polarity (PCP) pathway, as demonstrated on mouse models by Rocha et al. [18] The consequent alteration of the Wnt pathway and of the PCP signaling would therefore be responsible for the lack of migration of the CNC cells during the formation of the aorto-pulmonary septum. Schleiffarth et al [19] have indeed shown that the Wnt signal, in particular Wnt5a, is necessary for the correct development of the aorto-pulmonary septum and that its loss is responsible for the development of conotruncal defects.

The variant reported in our study is not described in the international literature (PubMed/Medline) and in the Catalog of Human Genes and Genetic Disorders (OMIM) and it is currently to be considered VOUS (Variants Of Uncertain Significance). The normal phenotype of the mother carrying the variant on the X chromosome in heterozygosity and the recurrence of the pathological phenotype in fetuses both male with hemizygous state increase the suspicion that this variant is the cause of the pathology in these family patients and that it may explain an X-linked transmission of syndromic conotruncal defects, contributing to increase the phenotypic range of MED12-related disorders. Therefore it will have important implications for genetic and preconception counseling, family planning and clinical management.

In conclusion, despite we know many genetic variations involved in the development of CHDs and specifically of CTDs, in most patients the genetic defects remain unknown [1]. The latest investigation techniques such as next generation sequencing (NGS) could increase our knowledge about CHD genetic etiology. The variant c.887G > A (p.Arg296Gln) in MED12 gene is a likely pathogenic VOUS and it could shed new light on the possible pathogenesis of conotruncal heart defects.