SUMO1 haploinsufficiency can cause cleft lip and palate. I. Saadi¹, F.S. Alkuraya¹, J.J. Lund¹, A. Turbe-Doan¹, C.C. Morton², R.L. Maas¹. 1) Division of Genetics, Department of Medicine; 2) Departments of Obstetrics & Gynecology, Reproductive Biology, and Pathology, Brigham & Women's Hospital and Harvard Medical School, Boston, MA.
   Human birth defects represent an example of a complex genetic disease, in which multiple genetic and environmental factors play a role. Cleft lip with or without cleft palate (CL/P) is among the most common human birth defects, with an incidence between 1/500 and 1/2000 births, depending on the population. Monogenic forms and variants in several genes have been identified that contribute to CL/P, but the full spectrum of such genes and whether and how they interact is unknown. As part of the Developmental Genome Anatomy Project (DGAP), we have studied de novo balanced translocation cases with CL/P to discover genes involved in CL/P that may be difficult to determine by other means. We ascertained a patient with isolated cleft lip and palate and a 46,XX,t(2;8)(q33.1;q24.3)dn that disrupts SUMO1 in intron 2. To validate the role of SUMO1 in clefting, we confirmed expression of Sumo1 in the developing mouse lip and palate. We also established a hypomorphic Sumo1 gene-trap allele, Sumo1^Gt , that produces an incompletely penetrant cleft palate phenotype. Furthermore, the Sumo1 hypomorphic allele interacts genetically with a loss-of-function allele for Eya1, a gene involved in palatogenesis in mouse and human, such that Sumo1^Gt/+/Eya1^+/- mice show a significant increase in the incidence of cleft palate (17%) compared to Sumo1^Gt/+ (5%) (p<0.008, Fisher exact test) or Eya1^+/- (0%). We also demonstrate that Eya1 is sumoylated at multiple sites in vivo, and thus joins a growing list of proteins implicated in clefting that are sumoylated, including Msx1, Satb2, and Pax9. Sumoylation is a post-translational modification in which a SUMO peptide is transferred to a target protein, altering its transcriptional activity, nuclear localization or other functional properties. Our data suggest that through its potential impact on a number of proteins involved in palatogenesis, SUMO1 haploinsufficiency may reduce the overall genetic threshold required to cause CL/P.