DiGeorge syndrome, Tbx1, and retinoic acid signaling come full circle.

Among the most well-characterized cardiac congenital malformations are those associated with DiGeorge syndrome (DGS)/velocardiofacial syndrome. DGS is mainly caused by heterozygous deletion of a region of chromosome 22q11.2 and is characterized by cardiac conotruncal malformations, aortic arch anomalies, dysmorphic face and hypoplasia of the thymus and parathyroid.1 This spectrum of anomalies is attributed to defects in neural crest–derived structures and associated tissues. The T-box–containing transcription factor TBX1 has been identified as responsible for cardiovascular, thymic, and parathyroid phenotypes of DGS.2 Alterations in retinoic acid (RA) metabolism are similarly associated with cardiac conotruncal, aortic arch, and pharyngeal structural abnormalities similar to DGS.3 In this issue of Circulation Research, Ryckebusch et al4 establish a new functional link between Tbx1 and RA signaling in the regulation of aortic arch anomalies in a mouse model of DGS. A characteristic feature of DGS is malformation of the pharyngeal arch arteries (PAAs), particularly of the fourth PAA, which leads to interrupted aortic arch type B (IAA-B). Mice heterozygous for Tbx1 exhibit defects in the development in the fourth PAA related to aortic arch anomalies including IAA.5,6 Ryckebusch et al define molecular and cellular mechanisms underlying aortic arch anomalies in Tbx1 / mice.4 The fourth PAA in embryonic day 10.5 embryos is derived from neural crest cells and is absent or hypomorphic in Tbx1 / embryos. This PAA anomaly is accompanied by neural crest migration defects as well as inhibition of vascular smooth muscle (VSM) differentiation. Both neural crest migration and VSM differentiation related to PAA defects are controlled by a precise balance of RA signaling and Tbx1 expression. Studies reported here and elsewhere support a mutual inhibition of RA signaling and Tbx1 expression (Figure). Reduced RA signaling with heterozygosity of retinaldehyde dehydrogenase 2 (raldh2) improves fourth PAA development in Tbx1 / embryos through restoration of normal neural crest migration trajectories and increased VSM differentiation. The Tbx1-RA interaction does not appear to affect second heart field derivatives but specifically contributes to neural crest-related anomalies. A significant conclusion from this work is that decreased RA signaling in a mouse model of DGS ameliorates aortic arch anomalies. Together, these studies provide important mechanistic insights into the molecular regulatory interactions that lead to common cardiac malformations in the human population. Extensive efforts by multiple research groups led to the identification of TBX1 as a disease causing gene in DGS.2 Mice lacking Tbx1 have severe PAA anomalies, as well as cardiac conotruncal, pharyngeal, and facial defects related to DGS.5,7,8 However, Tbx1 / heterozygous mice exhibit less severe PAA defects than those observed with the 22q11.2 deletion.6 Therefore, it is hypothesized that modifier genes contribute to DGS phenotypes associated with Tbx1 heterozygosity. This idea is supported by the variable spectrum of malformations and penetrance of DGS, even in the same family. There is increasing evidence that RA signaling is a critical modifying factor in DGS. Vitamin A (retinoic acid) deficiency has long been associated with cardiovascular malformations in humans and rodents.9,10 Increased or decreased RA signaling in mice leads to DGS phenotypes evident in pharyngeal arch anomalies, including IAA, and cardiac conotruncal defects.3 Mice with decreased expression of raldh2, a rate-limiting enzyme in RA synthesis, exhibit DGS-related PAA defects.3 Likewise, disruption of either RA signaling or loss of Tbx1 expression leads to similar pharyngeal, craniofacial, and cardiovascular anomalies related to abnormal development of neural crest or second heart field derivatives. Evidence from multiple systems supports a mutually repressive regulatory interaction between RA signaling and Tbx1 in the control of neural crest migration and differentiation in the pharyngeal arches and cardiac outflow tract (OFT) (Figure). The study by Ryckebusch et al4 shows that Tbx1 expression is increased in raldh2 / embryos or in pharyngeal arch tissue treated with the RA inhibitor disulfiram. Similarly, Tbx1 expression is repressed by RA treatments. Previous studies in chicken embryos also showed that RA treatment represses Tbx1 expression in the developing pharyngeal arches.11 Mice lacking Tbx1 have increased expression of raldh2, as well as reduced expression of RA inactivation enzymes cyp26a1, -b1, and -c1, with an overall increase in RA signaling evident in ectopic activation of RARE reporter gene.12,13 Overall, these data demonstrate that Tbx1 represses RA signaling, whereas increased RA signaling leads to decreased Tbx1 function. Therefore, a precise balance of Tbx1 function and RA signaling is necessary for normal pharyngeal and cardiac conotruncal development. The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Division of Molecular Cardiovascular Biology, Cincinnati Children’s Hospital Medical Center, Ohio. Correspondence to Katherine E. Yutzey, Division of Molecular Cardiovascular Biology, Cincinnati Children’s Hospital Medical Center ML7020, 240 Albert Sabin Way, Cincinnati, OH 45229. E-mail [email protected] (Circ Res. 2010;106:630-632.) © 2010 American Heart Association, Inc.