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The Role of MAGEL2 in Ubiquitination Pathways and its Contribution to Prader-Willi and Schaaf-Yang Syndromes

  • Author / Creator
    Carias, Karin Vanessa
  • The MAGEL2 gene is implicated in two neurodevelopmental disorders: Prader-Willi syndrome (PWS) and Schaaf-Yang syndrome (SYS). PWS is characterized by intellectual disability, obesity, poor muscle tone, distinct facial features, excessive daytime sleepiness and nighttime wakening. PWS is a sporadic multigene chromosomal deletion disorder involving the 15q11-q13 region, which encompasses MAGEL2. SYS is caused by mutations in MAGEL2 and has characteristics that overlap with PWS. MAGEL2 belongs to the MAGE (Melanoma Antigen Gene) family of proteins, which are encoded by over 60 genes in the human genome. MAGE proteins are divided into type I that are predominantly expressed in reproductive tissues and tumors, and type II that are expressed in various tissues throughout the human body and have emerging roles in development and disease. Various MAGE proteins have been identified as interacting partners of the ubiquitination system. MAGEL2 acts as a modulator of ubiquitination, a reversible post-translational modification. Ubiquitination is a multistep process that tags proteins with a 76 amino acid protein called ubiquitin. This tag targets proteins for downstream events or for proteasomal degradation. MAGEL2 modifies the ubiquitination levels of components of the ubiquitination system or substrate proteins through interactions with E3 ubiquitin ligases responsible for substrate recognition or with deubiquitinating enzymes. For my thesis, I explored the role of MAGEL2 in ubiquitination and investigated how the loss of MAGEL2 could contribute to symptoms seen in PWS and SYS through its role in the regulation of ubiquitination.Firstly, I investigated the cellular role of MAGEL2 in ubiquitination of proteins involved in circadian rhythm. Circadian rhythm is the endogenous oscillation of physiological and cellular functions over a roughly 24-hour period. Children with PWS have excessive daytime sleepiness and experience sleep disturbances. Magel2 knockout mice have abnormal circadian rhythm. This leads us to our hypothesis that MAGEL2 aids in the regulation of circadian rhythm output. I found that MAGEL2 interacts with the E3 ubiquitin ligase RBX1, the deubiquitinase USP7, and two proteins that are critical for circadian rhythm, Cryptochrome 1 and 2 (CRY1 and CRY2). I determined that MAGEL2 modulates the ubiquitination of CRY1 protein levels and that co-expression of CRY1 and MAGEL2 results in less CRY1 protein in the cytoplasm. The regulation of circadian rhythm relies on the tightly controlled levels of core circadian proteins, including CRY1. The abnormal regulation of CRY proteins through perturbed ubiquitination, secondary to the loss of MAGEL2 could cause circadian rhythm output irregularities in mice and humans with MAGEL2 deficiency.Secondly, I investigated the role of MAGEL2 in the ubiquitination of proteins in the BBSome. The BBSome is a multi-protein complex involved in the function of cilia, which are organelles that are present on almost all mammalian cells. The BBSome functions in ciliary membrane biogenesis and mediates protein/receptor trafficking to the ciliary and plasma membranes. The genes encoding BBSome proteins are mutated in people with Bardet-Biedl syndrome, a ciliopathy that has many overlapping phenotypes with PWS and SYS, such as obesity, intellectual disability, delayed development, and congenital muscle weakness. MAGEL2 regulates endosomal protein recycling through its interactions with E3 ubiquitin ligases TRIM27 and RNF41 and deubiquitinase USP7. MAGEL2 modified ubiquitination and trafficking of the leptin receptor. The BBSome is also important for recycling of the leptin receptor. I investigated the overlap between the function of MAGEL2 and the BBSome in trafficking of the leptin receptor given the similarities in phenotypes observed when these genes are mutated. Ubiquitination of BBS2, a BBSome component is required for BBSome degradation. I identified TRIM32 and TRIM27 as E3 ubiquitin ligases responsible for the degradation of BBS2, a vital protein in the BBSome complex. I identified MAGEL2 as a modifier of the ubiquitination of BBS2 by TRIM32 and TRIM27. MAGEL2 may reduce ubiquitination of the BBSome and stabilize this protein complex, important for leptin receptor recycling. Overall, I show MAGEL2 modulates the ubiquitination of substrate proteins, CRY1 and BBS2. Dysregulation of ubiquitinated proteins due to loss of MAGEL2 function could contribute to phenotypes seen in people with PWS, SYS, and related neurodevelopmental disorders such as BBS. My thesis work aids in the understanding of how MAGEL2 (and MAGE proteins in general) can modulate ubiquitination processes to regulate protein stability and fine tune intracellular trafficking.

  • Subjects / Keywords
  • Graduation date
    Fall 2019
  • Type of Item
    Thesis
  • Degree
    Doctor of Philosophy
  • DOI
    https://doi.org/10.7939/r3-fe86-e477
  • License
    Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.