Digeorge Syndrome: an Enigma in Mice and Men
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MOLECULAR MEDICINE TODAY, JANUARY 2000 (VOL. 6) Letters DiGeorge syndrome: an enigma in mice and men One out of 4000 children are born with cardiac and syndrome, including the pharyngeal arches, car- to the phenotype. The precise role of Ufd1l and facial abnormalities and are missing part of chro- diac outflow tract and aortic arch arteries (specif- Cdc45l in vertebrates remains unknown as well; mosome 22 (22q11.2). This condition, often called ically, the fourth aortic arch artery). A small 20 kb however, in yeast they are involved in a ubiquitin- DiGeorge syndrome (DGS), is the most common deletion in a ‘non-deleted’ DiGeorge patient (JF) dependent degradation pathway and cell cycle human deletion syndrome and the second most encompassing portions of UFD1L and a neigh- regulation, respectively. Numerous studies sug- common genetic cause of congenital heart de- boring gene, CDC45L, provided further evidence gest convergence of degradation and cell cycle fects1. Thymic and parathyroid anomalies are that UFD1L and/or CDC45L might be involved in pathways, raising the possibility that Ufd1l, Cdc45l often present, together with characteristic heart the etiology of 22q11 deletion syndrome6. As and perhaps HIRA function in a common pathway and facial defects. Because of the diverse clinical Novelli points out, this is not evidence that loss of during development. Whether heterozygosity of features, it is possible that more than one gene on one copy of UFD1L alone is sufficient to cause the the three genes in combination disrupts neural 22q11.2 contributes to the observed phenotype. phenotype; however, the results would suggest crest development will soon be testable in gen- However, many of the embryonic regions affected that a molecular pathway involving UFD1L con- etically engineered mice. in 22q11 deletion are derived from a common mi- tributes to the 22q11 phenotype. As the molecular basis for the 22q11 deletion gratory population of cells that arise from the The recently published contribution by Baldini’s syndrome becomes clearer, how will our findings neural folds (neural crest cells), suggesting that group7 has further clarified the potential etiologies impact individuals and families who have endured disruption of a molecular pathway regulating of 22q11 deletion syndrome. By generating a the hardships associated with chronic and often neural crest development might account for many mouse mutant lacking 14 of the genes in the pre- life-threatening disease? Unfortunately, fetal aspects of 22q11 deletion syndrome. sumed ‘DiGeorge critical region’ (DGCR), they gene therapy approaches remain in the distant fu- Substantial resources have been devoted to were able to recapitulate defects of the cardiac out- ture. However, the wide variance of clinical fea- identifying the genes involved in this syndrome. flow tract and fourth aortic arch artery observed in tures in individuals with 22q11 deletion might in- Genetic analyses of hundreds of patients suggest the syndrome. Unfortunately, no other phenotypic dicate that secondary genetic or environmental that the size of 22q11 deletion, typically 3 million features were found, and even the arch defects factors influence the final phenotype. Thus, regu- base pairs, does not correlate with the type of de- were incompletely penetrant (25%). It remains to lation of secondary factors during fetal develop- fects observed. As a result, the human genetic be determined whether genes outside this region ment in individuals genetically predisposed to ab- focus has shifted to studying patients with charac- are important for other features of the syndrome, normal neural crest development might allow teristic phenotypic features but without a de- or if species-specific factors influence the discor- attenuation of the disease. It is our hope that con- tectable 22q11 deletion. Surprisingly, extensive dant phenotypes observed in mice and men. Their tinued efforts to understand molecular pathways mutation analysis of such ‘non-deleted’ patients deletion (Df1) encompassed Ufd1l and Cdc45l but in mouse and chick neural crest development will has failed to reveal even a single individual, world- also removed 12 other genes. Interestingly, het- ultimately lead to novel approaches for preven- wide, who harbors a point mutation in any of the erozygosity of Ufd1l alone was not sufficient to tion of cardiac and craniofacial birth defects. Until genes (which number close to 30) in the commonly cause aortic arch defects in mice, although loss of then, scientists, clinicians and patients will con- deleted region. This finding might suggest that the both alleles of Ufd1l resulted in embryonic lethal- tinue to experience the roller coaster of hope and vast majority of non-deleted patients have genetic ity. As Baldini suggests, further studies will be re- despair that have become a hallmark of the 22q11 or epigenetic causes of neural crest disruption that quired to clarify the role of UFD1L in the 22q11.2 deletion syndrome. are unrelated to 22q11. Adding further complexity deletion phenotype. Whether heterozygosity of to the human genetic approach are the ten re- Ufd1l and Cdc45, as suggested by patient JF, is References ported patients that harbor large, atypical deletions sufficient to cause aortic arch defects in mice is 1 Emanuel, B.S. et al. (1998) The genetic basis of of 22q11 that do not overlap with one another2. currently being tested in our laboratory. conotruncal cardiac defects: The chromosome Two recent reports have begun to take alterna- Ultimately, the most important information will 22q11.2 deletion, in Heart Development (R.P. tive approaches to identifying the genes involved come from understanding the function of the vari- Harvey and N. Rosenthal, eds) pp. 463–478, in this enigmatic syndrome. Our group had been ous genes in the commonly deleted region. HIRA, Academic Press studying the function of a transcription factor, a gene located 50 kb away from UFD1L, encodes 2 Novelli, G. et al. (2000) Individual haploinsufficient dHAND, required for survival of the types of neural a protein that interacts with the homeodomain pro- loci and the complex phenotype of the DiGeorge crest-derived cells affected in 22q11 deletion syn- tein Pax3, and might play a functional role in syndrome. Mol. Med. Today 6, 10–11 drome3–5. In a screen for genes downregulated in neural crest development in chick embryos8,9. The 3 Srivastava, D. et al. (1995) A new subclass of bHLH dHAND mutant mice, we identified UFD1L, which yeast homolog of HIRA (Tup1) plays a role in cell proteins required for cardiac morphogenesis. had previously been mapped to 22q11 by Novelli’s cycle-dependent histone regulation. Although nei- Science 270, 1995–1999 group. We subsequently demonstrated specific ther Baldini’s nor our studies have addressed 4 Srivastava, D. et al. (1997) Regulation of cardiac UFD1L mRNA expression in the neural-crest-de- HIRA’s role in 22q11 deletion, it remains conceiv- mesoderm and neural crest by the bHLH protein, rived structures most affected in 22q11 deletion able that functional disruption of HIRA contributes dHAND. Nat. Genet. 16, 154–160 1357-4310/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S1357-4310(99)01641 13 Letters MOLECULAR MEDICINE TODAY, JANUARY 2000 (VOL. 6) 5 Thomas, T. et al. (1998) A signaling cascade in- 8 Magnaghi P. et al. (1998) HIRA, a mammalian Depts of Pediatric Cardiology and Molecular volving endothelin-1, dHAND and Msx1 regulates homologue of Saccharomyces cerevisiae tran- Biology and Oncology, University of Texas development of neural crest-derived branchial arch scriptional co-repressors, interacts with Pax3. Nat. Southwestern Medical Center at Dallas, 6000 mesenchyme. Development 125, 3005–3014 Genet. 20, 74–77 Harry Hines Boulevard, Dallas, 6 Yamagishi, H. et al. (1999) A molecular pathway re- 9 Farrell M.J. et al. (1999) HIRA, a DiGeorge syn- TX 75235-9148, USA. vealing a genetic basis for human cardiac and cran- drome candidate gene, is required for cardiac out- iofacial defects. Science 283, 1158–1161 flow tract septation. Circ. Res. 84, 127–135 7 Lindsay, E.A. et al. (1999) Congenital heart disease Tel: +1 214 648 1246 in mice deficient for the DiGeorge syndrome region. Deepak Srivastava MD Fax: +1 214 648 1196 Nature 401, 379–383 Assistant Professor e-mail: [email protected] UFD1L is not the monogenic basis for heart defects associated with the CATCH phenotype* It is clearly possible that some aspects of the im- further reinforced the rejection of a defect in forward to generate a fragment spanning the portant and highly variable CATCH phenotype* UFD1L as ‘the cause’; using the Cre–Lox system, deleted region in order to clarify the breakpoints. are attributable to loss of a single allele. However, they have generated mice that are deleted for a We are not aware of these results being pub- we and others have devoted much effort to 1.2 Mb section of the murine region homologous lished. The one certainty is that this story is not searching for mutations in candidate genes in to the human deleted region, from Es2 to Ufd1l. yet complete. individuals with features suggestive of the Mice heterozygous for the Lox site inserts, in CATCH phenotype, without success until the ex- which the only genetic defect is disruption of Es2 References citing report by Yamagishi et al.1 of a 20 kb de- and Ufd1l, do not develop heart defects, but hemi- 01 Yamagishi, H. et al. (1999) A molecular pathway re- letion involving the first three exons of UFD1L. zygous cases manifest the classic heart malfor- vealing a genetic basis for human cardiac and cran- The fact that this gene shows appropriate ex- mations associated with DiGeorge syndrome, in iofacial defects. Science 283, 1158–1161 pression and was absent in a large series of particular, hypoplasia or apparent absence of the 02 Novelli, G. et al. (2000) Individual haploinsufficient cases was important supportive evidence but not fourth arch artery.