Even Nature Edits: RNA Modifications

Dr. Stephanie Spohn, UVM Alumna and post-doctoral fellow at the University of Michigan, writes:

I think many would agree that editing a piece of writing is far easier (in most cases) than creating a whole new document. While I often think this is true when writing in my own life and research, the idea that editing is easier than constructing something novel likely holds true in the biological world as well. In the keynote address of the 2016 Neuroscience, Behavior and Health (NBH) Forum at the University of Vermont on January 22nd, we were fortunate to hear a talk from Dr. Ronald B. Emeson, Ph.D., from Vanderbilt University School of Medicine on this very phenomenon. At Vanderbilt, Dr. Emerson is a professor in many departments and is Principal Investigator of a lab that focuses on RNA editing in the nervous system in normal health and disease. His keynote address focused, in part, on the role of RNA editing in a model of the rare genetic disorder, Prader-Willi Syndrome.

To better understand this jump from RNA to a clinical syndrome, we should backtrack and get a brief introduction to RNA. RNA is ribonucleic acid and is the intermediate step between DNA and proteins. DNA, the double stranded blueprint for an organism, is transcribed into RNA, the single stranded template. RNA is then translated into proteins; three RNA code for one amino acid, which are then strung together to make a peptide or protein. This general scheme is shown in Figure 1 below, which demonstrates the central dogma of DNA to RNA to protein.

Steph Figure 1

At both the levels of transcription and translation the template can be processed or changed. Common RNA modifications include splicing (removing parts), capping (protects the 5’ end), adding a PolyA tail (protects the 3’ end), and editing. Editing is an alteration in the primary sequence that is different from splicing. The RNA edit of choice for Dr. Emeson’s talk was the adenosine to inosine (A to I) base modification. In DNA and RNA, bases pair with complementary bases, such that C (cytosine) pairs with G (guanosine) and A pairs with T (thymine, or uracil, U, in the case of RNA). In this type of RNA editing, adenosine is deaminated (a reaction that replaces an amine group with a carbonyl group) to inosine, and I is read as G by the translational machinery. This editing at the RNA level may change which amino acid a three base unit encodes, and thus, change the protein and its structure.

One gene that undergoes A to I RNA editing is the 5HT2C gene, which encodes for the serotonin 2C receptor. In the brain, 5HT2C receptors have been implicated in sleep, feeding behaviors, and obesity. This receptor’s DNA has 5 distinct points in which A to I edits are found, resulting in a potential 32 RNA templates and 24 different protein isoforms (variations).

In an initial experiment, Emeson’s group used wild-type animals (those with no A to I edits in the 5HT2C gene) and fully edited animals (all 5 sites edited) and compared their messenger RNA (mRNA) expression. They saw no changes in mRNA expression, but decided to look at protein expression as well and they saw nearly 7000% more protein in the fully edited animals!1 Based on the observed phenotype (outward expression of genes), it turns out that these fully edited 5HT2C receptor animals are a potential model of Prader-Willi Syndrome.

Prader-Willi Syndrome (PWS) is a rare genetic disorder that initially causes a failure to thrive, but after the first year, is accompanied by morbid obesity (often leading to bodyweights over 500 lbs).2 Aside from body composition, those with PWS frequently have low IQs (~70), poor muscle tone, short stature, and incomplete sexual maturation.2 There is no cure for PWS, and treatments tend to be intervention-based. Children with PWS often undergo physical therapy to increase muscle tone, speech and occupational therapy, and highly structured school or educational settings. The greatest barrier is obesity. Therapies to address this include recombinant growth hormone to encourage linear growth and increased muscle mass, as well as potentially lessen a preoccupation with food. Additionally, there are clinical trials underway to find medication to lessen the insatiable hunger that PWS patients experience.

Let’s get back to edited RNA and what an edited 5HT2C gene might be doing in PWS. The 5HT2C receptor is encoded on the X chromosome and is found in the central nervous system. Activity of the 5HT2C receptor causes reduced physical activity, reduced feeding, and is implicated in normal neuroendocrine function.3 The fully edited 5HT2C receptor has significantly reduced kinetic activity.1 3 What could be linking the 5HT2C gene on the X chromosome and the PWS genes found on chromosome 15?

This leads to discussing yet another type of RNA: snoRNA. snoRNA is small nucleolar RNA, a type of small non-coding RNA usually involved in modifications to other types of RNA. It appears that snoRNA 115 (also called SNORD115 or snoRNA HBII-52), previously thought to be an orphaned or unmatched snoRNA because it did not code for any known ribosomal RNA, is complementary to 5HT2C mRNA. In typical cases, snoRNA 115 regulates alternative splicing or editing by binding to a silencing element that encourages full 5HT2C gene transcription without RNA editing. Patients with PWS do not have snoRNA 115 because it is encoded in the same region as the genes silenced or missing in PWS (Figure 2), and therefore can produce a 5HT2C with the option to edit.4 5

Screen Shot 2016-04-21 at 2.33.29 PM

Collectively, this keynote address presented the concept that RNA is not merely a traditional blueprint that proteins are built to, but a dynamic and edit-able document that can rewrite a physiological outcome. The Emeson lab’s findings that the A to I edits in the 5HT2C gene in mice created a model that shared many features of PWS in people, such as early life failure to thrive, reduced muscle tone, and increased adulthood hunger. Furthermore, there’s a connection between the edits of 5HT2C genes and a snoRNA found within the PWS locus that modulates RNA editing at that site, strengthening the argument that RNA editing can drastically change physiology.

  1. Morabito MV, Abbas AI, Hood JL, et al. Mice with altered serotonin 2C receptor RNA editing display characteristics of Prader-Willi syndrome. Neurobiology of disease 2010;39(2):169-80.
  2. Angulo MA, Butler MG, Cataletto ME. Prader-Willi syndrome: a review of clinical, genetic, and endocrine findings. Journal of endocrinological investigation 2015;38(12):1249-63.
  3. Hoyer D, Hannon JP, Martin GR. Molecular, pharmacological and functional diversity of 5-HT receptors. Pharmacol Biochem Behav 2002;71(4):533-54.
  4. Kishore S, Stamm S. The snoRNA HBII-52 Regulates Alternative Splicing of the Serotonin Receptor 2C. Science 2006;311(5758):230-32.
  5. Elena G, Bruna C, Benedetta M, et al. Prader-willi syndrome: clinical aspects. Journal of obesity 2012;2012:473941.

 

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