REPORT LTBP3 Pathogenic Variants Predispose Individuals to Thoracic Aortic Aneurysms and Dissections Dong-chuan Guo, 1 Ellen S. Regalado, 1 Amelie Pinard, 1 Jiyuan Chen, 1 Kwanghyuk Lee, 2 Christina Rigelsky, 3 Lior Zilberberg, 4 Ellen M. Hostetler, 1 Micheala Aldred, 3 Stephanie E. Wallace, 1 Siddharth K. Prakash, 1 University of Washington Center for Mendelian Genomics, Suzanne M. Leal, 2 Michael J. Bamshad, 5 Deborah A. Nickerson, 5 Marvin Natowicz, 3 Daniel B. Rifkin, 4 and Dianna M. Milewicz 1, * The major diseases affecting the thoracic aorta are aneurysms and acute dissections, and pathogenic variants in 11 genes are confirmed to lead to heritable thoracic aortic disease. However, many families in which multiple members have thoracic aortic disease do not have alterations in the known aortopathy genes. Genes highly expressed in the aorta were assessed for rare variants in exome sequencing data from such families, and compound rare heterozygous variants (p.Pro45Argfs*25 and p.Glu750*) in LTBP3 were identified in affected members of one family. A homozygous variant (p.Asn678_Gly681delinsThrCys) that introduces an additional cysteine into an epidermal growth factor (EGF)-like domain in the corresponding protein, latent TGF-b binding protein (LTBP-3), was identified in a sec- ond family. Individuals with compound heterozygous or homozygous variants in these families have aneurysms and dissections of the thoracic aorta, as well as aneurysms of the abdominal aorta and other arteries, along with dental abnormalities and short stature. Het- erozygous carriers of the p.Asn678_Gly681delinsThrCys variant have later onset of thoracic aortic disease, as well as dental abnormal- ities. In these families, LTBP3 variants segregated with thoracic aortic disease with a combined LOD score of 3.9. Additionally, hetero- zygous rare LTBP3 variants were found in individuals with early onset of acute aortic dissections, and some of these variants disrupted LTBP-3 levels or EGF-like domains. When compared to wild-type mice, Ltbp3 À/À mice have enlarged aortic roots and ascending aortas. In summary, homozygous LTBP3 pathogenic variants predispose individuals to thoracic aortic aneurysms and dissections, along with the previously described skeletal and dental abnormalities. Thoracic aortic aneurysms involving the aortic root and ascending aorta are typically asymptomatic as they progressively enlarge and eventually lead to acute ascending aortic dissections in the absence of surgical repair of the aortic aneurysm. These thoracic aortic dis- eases can be triggered by mutations in single genes, either in association with syndromic features (e.g., in Marfan syn- drome [MFS, MIM: 154700]) or in the absence of syndromic features. Heritable thoracic aortic disease (HTAD) is a genetically heterogeneous condition typically inherited in an autosomal-dominant manner, and there is sufficient evidence to classify mutations in 11 genes as causative. 1,2 These HTAD genes encode proteins involved in aortic smooth muscle cell (SMC) contraction, the extra- cellular matrix (ECM), and transforming growth factor- beta (TGF-b) signaling. 3–12 Despite identification of these HTAD genes, the causative genes in the majority of families with thoracic aortic disease have not been identified to date. TGF-b signaling is initiated when the cytokine binds to the cell-surface TGF-b type II receptor, which then re- cruits and phosphorylates the TGF-b type I receptor. The type I receptor phosphorylates SMAD2 and SMAD3 (mothers against decapentaplegic homolog 2 and 3 (MIM: 601366 and MIM: 603109, respectively), which form a complex with SMAD4 and translocate to the nu- cleus to alter gene transcription. Mutated HTAD genes in this pathway include genes encoding one of the three TGF-b ligand family members, TGFB2 (TGF-b2; MIM: 190220), the TGF-b cellular receptors, TGFBR2 (MIM: 190182), TGFBR1 (MIM: 19018), and SMAD3. Pathogenic variants in these genes have been shown or are predicted to decrease cellular TGF-b signaling as a result of haploin- sufficiency (e.g., nonsense or frameshift mutations) or missense variants that disrupt the protein function (e.g., decrease the intracellular kinase activity of the TGF-b receptors). 13,14 To identify pathogenic variants in novel genes predis- posing to HTAD, we obtained whole-exome sequencing data on affected probands and family members from 271 unrelated HTAD families in which a HTAD gene mutation had not been identified (dbGap: phs000693.v5.p1). We collected blood or saliva samples from both affected and unaffected family members after obtaining approval from the institutional review board at the University of Texas Health Science Center at Houston. All participants provided informed consent. Gene variants identified by whole-exome sequencing were filtered for further analyses on the basis of the following criteria: (1) vari- ants that altered amino acids in proteins, including 1 Department of Internal Medicine, University of Texas Health Science Center at Houston McGovern Medical School, Houston, TX 77030, USA; 2 Department of Molecular and Human Genetics, Center for Statistical Genetics, Baylor College of Medicine, Houston, TX 77030, USA; 3 Genomic Medicine Institute, Cleveland Clinic Foundation, Cleveland, OH 44195, USA; 4 Department of Cell Biology, New York University School of Medicine, New York, NY 10016, USA; 5 Department of Genome Sciences, University of Washington, Seattle, WA 98195, USA *Correspondence: dianna.m.milewicz@uth.tmc.edu https://doi.org/10.1016/j.ajhg.2018.03.002. 706 The American Journal of Human Genetics 102, 706–712, April 5, 2018 Ó 2018 American Society of Human Genetics.