Navigation Links
Researchers discover critical detail of cellular defense against genetic mistakes
Date:4/17/2008

Researchers are closing in on a completed diagram of how human cells protect themselves against constant genetic mistakes that contribute to most diseases, according to a study to be published in the April 18 edition of the journal Cell.

The blueprint for the human body is encoded in genes. Gene expression is the process by which those blueprints are converted into proteins that make up the bodys structures and send its signals. When molecular biologists began analyzing the complete set of human genes (the human genome) in 2001, one surprise was that humans have as few as 30,000 genes when, given their complexity, they should have more than 100,000. How can humans have one-fifth as much genetic material as wheat, for instance, or share one quarter of their genes with fish?

One answer is that humans do more with fewer genes. While genes consist of chains of deoxyribonucleic acids (DNA), they are put into practice by chains of ribonucleic acid chains (RNA), which are modified copies of DNA. Messenger RNA (mRNA) is transported to cellular factories called ribosomes that receive instructions for building proteins by reading mRNA templates, a process called translation. Remarkably, about 75 percent of human genes code for more than one protein through a process called alternate RNA splicing. Unfortunately, the more intricate the splicing process, the greater the opportunity for error. More than one-third of alternatively spliced mRNAs are flawed, and must be destroyed before they can cause harm. Thus, cellular processes that detect and eliminate processing errors are vitally important to effective gene expression.

In recent years, researchers at the University of Rochester Medical Center have revealed the existence of a natural surveillance system called nonsense-mediated mRNA decay (NMD) that determines which mRNAs are fit to serve as protein templates and sees to the destruction of those with flaws. Researchers hope to tweak the process such that it catches more genetic errors in some cases, or leaves more templates for helpful proteins in place in others, based on the disease at hand. To do so will require a highly detailed knowledge of the NMD pathway.

The current results uncover a critical and previously unappreciated step during the natural process that finds flaws in mRNAs, said Lynne E. Maquat, Ph.D., J. Lowell Orbison Endowed Chair and professor of Biochemistry & Biophysics at University of Rochester Medical Center, director of the University of Rochester Center for RNA Biology and lead author of the Cell piece. This work has important implications for our understanding of how one of the human cells most important activities, protein synthesis, undergoes quality control.

An Elegant Process Emerges

Over time, genes evolve to show changes in their makeup. Some changes, or mutations, have no impact, some provide advantages making organisms more likely to survive, and others cause disease. One frequently occurring, damaging class of mutation is the inclusion of premature stop reading signals (stop codons) within mRNAs. Called nonsense mutations, they order the process to stop reading part way through the genetic instructions. Such mutations result in the building of incomplete, disabled proteins that sabotage natural processes by competing for spots usually held by their full-length counterparts, or by simply not working. Mutations of this type cause genetic syndromes and contribute to many diseases, including cancer. Since truncated proteins are potentially hazardous, the NMD pathway has evolved to eliminate the mRNAs that encode them.

From studying genetic diseases, Maquat theorized seven years ago that there must be two types of translation, the process by which instructions encoded in mRNAs are read during protein building. An early pioneer round checks all newly built mRNAs for errors, and initiates NMD when errors are detected. Subsequent steady-state rounds then direct the mass production of normal proteins based on NMD-approved mRNAs. Over time, the Maquat lab, along with other labs, has identified a number of protein complexes that form during the intricate process by which cells analyze each mRNA for flaws.

In the past, her team showed, for instance, which proteins bind to each end of mRNA during the pioneer and subsequent steady-state rounds of translation, and how the pioneer round, cap-binding protein promotes the recognition and decay of flawed mRNAs. The team also demonstrated how other complexes that identify flawed mRNAs form near exon-exon junctions, the places where each must read section of the mature mRNA template is joined to the next by RNA splicing.

Past work by Maquats team further revealed that much of the NMD quality review depends on the physical spacing of proteins bound to the mRNA chain. If a stop reading signal occurs too far ahead of the final exon in the chain, as marked by an exon-exon junction complex (EJC), the cell concludes that the stop codon has mistakenly fallen in the middle of a set of instructions. These mRNAs are degraded. They also found that the EJC contains human up-frameshift (UPF) proteins that play a role in NMD.

In their latest search for detail, Maquat and colleagues determined that the delivery of a given faulty mRNA to the degradation machinery requires first the active shutdown (translational repression) of protein building based on that mRNA. In the studys key finding, experiments revealed that repression of protein synthesis during NMD is controlled the attachment of phosphate groups to human UPF1, researchers said. Human cells have evolved such that phosphorylation, the attachment of phosphate groups to proteins, is used in many scenarios like a switch to turn processes on or off.

Based on their findings, Maquat and colleagues propose the following new model for NMD: When a nonsense stop codon is detected, UPF1 together with the enzyme that directs its phosphorylation interacts with the EJC. The same step makes possible the attachment of phosphate groups to UPF1. Once phosphorylated, UPF1 interacts directly with and inhibits the function of eukaryotic initiation factor 3 (eIF3), which would otherwise direct protein building based on that mRNA sequence.

Normally, eIF3 drives a key change in a complex (40S/Met-tRNAiMet/mRNA) that consists of mRNA and part of a functional ribosome. The binding of phosphorylated UPF1 to eIF3 prevents this complex from going on to form a complex (80S/Met-tRNAiMet/mRNA) that is capable of driving translation and consists of mRNA and the completed functional ribosome.

The team corroborated the importance of eIF3 as a target for translational repression during NMD using an experiment with an mRNA sequence from cricket paralysis virus. Where human cells use eIF3 to initiate translation, the cricket virus mRNA sequence does not. Researchers found that the non-eIF3 translation initiation directed by the cricket virus sequence in mammalian cells was resistant to NMD, and thus that eIF3 is a must for the translational repression that makes NMD possible.

In Maquats model of NMD, phospho-UPF1 not only inhibits the pioneer round of translation so that the translational machinery "falls away" from the flawed mRNA at hand, but also recruits degradative enzymes to that mRNA.

Along with Maquat, the study was authored by post-doctoral associates Olaf Isken, Yoon Ki Kim and Nao Hosoda under the auspices of the Medical Center. Greg L. Mayeur and John W.B. Hershey from the Department of Biological Chemistry at the University of California at Davis provided important reagents and advice. This work was supported by the National Institutes of Health.

Our study provides the first evidence that translational repression does indeed occur during NMD in mammalian cells, Maquat said. One implication of these results is that we have a new target by which the decay of faulty mRNA can be prevented. In cases where a nonsense codon occurs in a gene supplying an essential protein, and thus causes disease via protein shortage, we may be able to design drugs that suppress related decay. That could restore the supply of an mRNA that can direct the cell to synthesize full-length, functional protein.


'/>"/>

Contact: Greg Williams
Greg_Williams@urmc.rochester.edu
University of Rochester Medical Center
Source:Eurekalert

Related biology news :

1. Researchers identify proteins involved in new neurodegenerative syndrome
2. Texas researchers and educators head for Antarctica
3. MGH researchers describe new way to identify, evolve novel enzymes
4. University of Pennsylvania researchers develop formula to gauge risk of disease clusters
5. U of MN researchers discover noninvasive diagnostic tool for brain diseases
6. U of Minnesota researchers discover noninvasive diagnostic tool for brain diseases
7. Researchers discover new strategies for antibiotic resistance
8. Researchers find new taste in fruit flies: carbonated water
9. Binghamton University researchers investigate evolving malaria resistance
10. UIC researchers find promising new targets for antibiotics
11. Researchers develop simple method to create natural drug products
Post Your Comments:
*Name:
*Comment:
*Email:
(Date:2/2/2016)... Va. , Feb. 2, 2016   ... award from the U.S. Army Research Office and ... the range and sensitivity of the company,s ... Past Accounting Mission and, more generally, defense-related DNA ... DNA phenotyping capabilities (predicting appearance and ancestry from ...
(Date:2/1/2016)... wallet ( www.wocketwallet.com ) announces the launch of a new video featuring ... Las Vegas , where Joey appeared at the Wocket booth ... , where Joey appeared at the Wocket booth to meet and ... at the Consumer Electronics Show (CES2016) in Las Vegas ... fans. --> --> The video ...
(Date:1/25/2016)... , Jan. 25, 2016  Glencoe Software, the ... pharma and publication industries, will provide the data management ... Centre (NPSC). ... Phenotypic analysis measures ... whole organisms, allowing comparisons between states such as health ...
Breaking Biology News(10 mins):
(Date:2/5/2016)... , February 5, 2016 Amarantus ... biotechnology company focused on developing products for Regenerative Medicine, ... Rare Pediatric Disease Designation (RPDD) from the US Food ... with MANF. MANF was previously granted orphan drug designation ... --> Amarantus BioScience Holdings, Inc. (OTCQB: ...
(Date:2/4/2016)... CA (PRWEB) , ... February 04, 2016 , ... ... enterprise talent development and compliance training, today announced an interactive FDA compliance ... Playbook™. The RAPS (Regulatory Affairs Professional Society) accredited interactive course on Morf ...
(Date:2/4/2016)... , Feb. 4, 2016  Sangamo BioSciences, Inc. ... editing, announced today that Edward Lanphier , Sangamo,s ... on the progress of Sangamo,s ZFP Therapeutic ® ... strategy at 2:40 pm ET on Thursday, February 11, ... Global Healthcare Conference. The conference is being held in ...
(Date:2/4/2016)... (PRWEB) , ... February 04, ... ... conference presented by Bloomsburg University’s Digital Forensics Club, takes place February 5-6 ... two-day event features 20+ speakers and activities such as workshops and competitions ...
Breaking Biology Technology: