Identification of E3 Ubiquitin Ligase, UBR5, as a Potential Interactor of TTC7A and PI4KIIIα

Mutations in Tetratricopeptide Repeat Domain 7A (TTC7A) results in severe intestinal and immune disease phenotypes, one of them being Very Early Onset Inflammatory Bowel Disease (VEO-IBD). TTC7A is part of an evolutionarily conserved pathway and acts as a scaffolding protein recruiting Phosphatidylinositol 4-kinase Type III Alpha (PI4KIIIα) to the plasma membrane (PM). This conserved pathway helps PI4KIIIA phosphorylate phosphatidylinositol (PI) to PI 4-phosphate (PI4P) at the PM. The PIPs downstream of PI4P have been implicated in maintaining cell polarity and survival. Previously, mass spectrometry was performed to identify potential TTC7A binding partners. Ubiquitin protein ligase E3 component N-recognin 5 (UBR5) was identified as one of the top TTC7A binding proteins. Results show co-immunoprecipitation of UBR5 with TTC7A along with mutants (E71K, Q526X, A832T) identified at The Hospital for Sick Children. Results also suggest that UBR5 may be part of the PI4KIIIA-FAM126A-TTC7A complex (and therefore implicate UBR5 in PM PI4P synthesis).M.Sc

TTC7A: Steward of Intestinal HealthSummary

The increasing incidence of pediatric inflammatory bowel disease, coupled with the efficiency of whole-exome sequencing, has led to the identification of tetratricopeptide repeat domain 7A (TTC7A) as a steward of intestinal health. TTC7A deficiency is an autosomal-recessively inherited disease. In the 5 years since the original description, more than 50 patients with more than 20 distinct disease-causing TTC7A mutations have been identified. Patients show heterogenous intestinal and immunologic disease manifestations, including but not limited to multiple intestinal atresias, very early onset inflammatory bowel disease, loss of intestinal architecture, apoptotic enterocolitis, combined immunodeficiency, and various extraintestinal features related to the skin and/or hair. The focus of this review is to highlight trends in patient phenotypes and to consolidate functional data related to the role of TTC7A in maintaining intestinal homeostasis. TTC7A deficiency is fatal in approximately two thirds of patients, and, as more patients continue to be discovered, elucidating the comprehensive role of TTC7A could show druggable targets that may benefit the growing cohort of individuals suffering from inflammatory bowel disease. Keywords: Inflammatory Bowel Disease, Monogenic, Very Early Onset IBD, Genetics, Whole-Exome Sequencing, Multiple Intestinal Atresia, PI4K, Primary Immunodeficienc

The E3 ubiquitin ligase UBR5 interacts with TTC7A and may be associated with very early onset inflammatory bowel disease

Very early onset inflammatory bowel disease (VEOIBD) denotes children with onset of IBD before six years of age. A number of monogenic disorders are associated with VEOIBD including tetratricopeptide repeat domain 7A (TTC7A) deficiency. TTC7A-deficiency is characterized by apoptotic colitis in milder cases with severe intestinal atresia and immunodeficiency in cases with complete loss of protein. We used whole exome sequencing in a VEOIBD patient presenting with colitis characterized by colonic apoptosis and no identified known VEOIBD variants, to identify compound heterozygous deleterious variants in the Ubiquitin protein ligase E3 component N-recognin 5 (UBR5) gene. Functional studies demonstrated that UBR5 co-immunoprecipitates with the TTC7A and the UBR5 variants had reduced interaction between UBR5 and TTC7A. Together this implicates UBR5 in regulating TTC7A signaling in VEOIBD patients with apoptotic colitis.AMM is funded by a Canada Research Chair (Tier 1) in Pediatric IBD, CIHR Foundation Grant and NIDDK (RC2DK118640) Grant. AMM, SBS, CK, DK are supported by the Leona M. and Harry B. Helmsley Charitable Trust. CK and DK are supported by the Collaborative Research Consortium SFB1054 project A05.Ye