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After being produced, the stability and distribution of the different transcripts is regulated (post-transcriptional regulation) by means of RNA binding protein (RBP) that control the various steps and rates of the transcripts: events such as alternative splicing, nuclear degradation (exosome), processing, nuclear export (three alternative pathways), sequestration in P-bodies for storage or degradation and ultimately translation. These proteins achieve these events thanks to a RNA recognition motif (RRM) that binds a specific sequence or secondary structure of the transcripts, typically at the 5’ and 3’ UTR of the transcript.
Modulating the capping, splicing, addition of a Poly(A) tail, the sequence-specific nuclear export rates and in several contexts sequestration of the RNA transcript occurs in eukaryotes but not in prokaryotes. This modulation is a result of a protein or transcript which in turn is regulated and may have an affinity for certain sequences.
- Capping changes the five prime end of the mRNA to a three prime end by 5'-5' linkage, which protects the mRNA from 5' exonuclease, which degrades foreign RNA. The cap also helps in ribosomal binding.
- Splicing removes the introns, noncoding regions that are transcribed into RNA, in order to make the mRNA able to create proteins. Cells do this by spliceosomes binding on either side of an intron, looping the intron into a circle and then cleaving it off. The two ends of the exons are then joined together.
- Addition of poly(A) tail otherwise known as polyadenylation. That is, a stretch of RNA that is made solely of adenine bases is added to the 3' end, and acts as a buffer to the 3' exonuclease in order to increase the half life of mRNA. In addition, a long poly(A) tail can increase translation. Poly(A)-binding protein (PABP) binds to a long poly(A) tail and mediates the interaction between EIF4E and EIF4G which encourages the initiation of translation.
- RNA editing is a process which results in sequence variation in the RNA molecule, and is catalyzed by enzymes. These enzymes include the Adenosine Deaminase Acting on RNA (ADAR) enzymes, which convert specific adenosine residues to inosine in an mRNA molecule by hydrolytic deamination. Three ADAR enzymes have been cloned, ADAR1, ADAR2 and ADAR3, although only the first two subtypes have been shown to have RNA editing activity. Many mRNAs are vulnerable to the effects of RNA editing, including the glutamate receptor subunits GluR2, GluR3, GluR4, GluR5 and GluR6 (which are components of the AMPA and kainate receptors), the serotonin2C receptor, the GABA-alpha3 receptor subunit, the tryptophan hydroxlase enzyme TPH2, the hepatitis delta virus and more than 16% of microRNAs. In addition to ADAR enzymes, CDAR enzymes exist and these convert cytosines in specific RNA molecules, to uracil. These enzymes are termed 'APOBEC' and have genetic loci at 22q13, a region close to the chromosomal deletion which occurs in velocardiofacial syndrome (22q11) and which is linked to psychosis. RNA editing is extensively studied in relation to infectious diseases, because the editing process alters viral function.
- mRNA Stability can be manipulated in order to control its half-life, and the poly(A) tail has some effect on this stability, as previously stated. Stable mRNA can have a half life of up to a day or more which allows for the production of more protein product; unstable mRNA is used in regulation that must occur quickly.
microRNA mediated regulation
MicroRNAs (miRNAs) appear to regulate the expression of more than 60% of protein coding genes of the human genome. As reviewed by Farazi et al., if an miRNA is abundant it can behave as a “switch,” turning some genes on or off. However, altered expression of many miRNAs only leads to a modest 1.5- to 4-fold change in protein expression of their target genes. Individual miRNAs often repress several hundred target genes. Repression usually occurs either through translational silencing of the mRNA or through degradation of the mRNA, via complementary binding, mostly to specific sequences in the 3' untranslated region of the target gene's mRNA. The mechanism of translational silencing or degradation of mRNA is implemented through the RNA-induced silencing complex (RISC).
This area of study has recently gained more importance due to the increasing evidence that post-transcriptional regulation plays a larger role than previously expected. Even though protein with DNA binding domains are more abundant than protein with RNA binding domains, a recent study by Cheadle et al. (2005) showed that during T-cell activation 55% of significant changes at the steady-state level had no corresponding changes at the transcriptional level, meaning they were a result of stability regulation alone.
Furthermore RNA found in the nucleus is more complex than that found in the cytoplasm: more than 95% (bases) of the RNA synthesized by RNA polymerase II never reaches the cytoplasm. The main reason for this is due to the removal of introns which account for 80% of the total bases. Some studies have shown that even after processing the levels of mRNA between the cytoplasm and the nucleus differ greatly.
Developmental biology is a good source of models of regulation, but due to the technical difficulties it was easier to determine the transcription factor cascades than regulation at the RNA level. In fact several key genes such as nanos are known to bind RNA but often their targets are unknown. Although RNA binding proteins may regulate post transcriptionally large amount of the transcriptome, the targeting of a single gene is of interest to the scientific community for medical reasons, this is RNA interference and microRNAs which are both examples of posttranscriptional regulation, which regulate the destruction of RNA and change the chromatin structure. To study post-transcriptional regulation several techniques are used, such as RIP-Chip (RNA immunoprecipitation on chip).
microRNA role in cancer
Deficiency of expression of a DNA repair gene occurs in many cancers (see DNA repair defect and cancer risk and DNA repair epigenetics in cancer). Altered microRNA (miRNA) expression that either decreases accurate DNA repair or increases inaccurate microhomology-mediated end joining (MMEJ) DNA repair is often observed in cancers. Deficiency of accurate DNA repair may be a major source of the high frequency of mutations in cancer (see mutation frequencies in cancers). Repression of DNA repair genes in cancers by changes in the levels of microRNAs may be a more frequent cause of repression than mutation or epigenetic methylation of DNA repair genes.
For instance, BRCA1 is employed in the accurate homologous recombinational repair (HR) pathway. Deficiency of BRCA1 can cause breast cancer. Down-regulation of BRCA1 due to mutation occurs in about 3% of breast cancers. Down-regulation of BRCA1 due to methylation of its promoter occurs in about 14% of breast cancers. However, increased expression of miR-182 down-regulates BRCA1 mRNA and protein expression, and increased miR-182 is found in 80% of breast cancers.
In another example, a mutated constitutively (persistently) expressed version of the oncogene c-Myc is found in many cancers. Among many functions, c-Myc negatively regulates microRNAs miR-150 and miR-22. These microRNAs normally repress expression of two genes essential for MMEJ, Lig3 and Parp1, thereby inhibiting this inaccurate, mutagenic DNA repair pathway. Muvarak et al. showed, in leukemias, that constitutive expression of c-Myc, leading to down-regulation of miR-150 and miR-22, allowed increased expression of Lig3 and Parp1. This generates genomic instability through increased inaccurate MMEJ DNA repair, and likely contributes to progression to leukemia.
To show the frequent ability of microRNAs to alter DNA repair expression, Hatano et al. performed a large screening study, in which 810 microRNAs were transfected into cells that were then subjected to ionizing radiation (IR). For 324 of these microRNAs, DNA repair was reduced (cells were killed more efficiently by IR) after transfection. For a further 75 microRNAs, DNA repair was increased, with less cell death after IR. This indicates that alterations in microRNAs may often down-regulate DNA repair, a likely important early step in progression to cancer.
- RNA interference
- Cis-regulatory element
- Glossary of gene expression terms
- Wormbook.org on RNA-binding protein
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