In genetics and cell biology, repression is a mechanism often used to decrease or inhibit the expression of a gene. Removal of repression is calledderepression. This mechanism may occur at different stages in the expression of a gene, all resulting with increasing the overall RNA or protein products. Dysregulation of derepression mechanisms might result in altered gene expression patterns, which may lead to negative phenotypic consequences, such as disease.
Transcription can be repressed in a variety of ways, and also therefore can be derepressed in different ways. A common mechanism isallosteric regulation, when asubstrate binds arepressor protein and causes it to undergo aconformational change. If the repressor is bound upstream of a gene, for example in an operator sequence, then it would be repressing the gene's expression. This conformational change would take away the repressor’s ability to bind DNA, thus removing its repressive effect on transcription.[1]
Another form of transcriptional derepression useschromatin remodeling complexes. For transcription to occur,RNA polymerase needs to have access to thepromoter sequence of the gene or it cannot bind the DNA. Sometimes these sequences are wrapped aroundnucleosomes or are in condensedheterochromatin regions, and are therefore inaccessible. Through different chromatin remodeling mechanisms, these promoter sequences can become accessible to the RNA polymerase, and transcription becomes derepressed.[2]
Transcriptional derepression may also occur at the level oftranscription factor activation. Certain families of transcription factors are non-functional on their own because theiractive domains are blocked by another part of the protein.[3] The substrate binding to this second, regulatory domain causes a conformational change in the protein to allows access to the active domain.[3] This lets the transcription factor bind to DNA and serve its function, thus derepressing the transcription factor.
Derepression oftranslation increases protein production without altering the levels ofmRNA in the cell.miRNAs are a common mechanism of translation repression, binding to the mRNA through complementary base pairing to silence them.[4] CertainRNA binding proteins have been shown to targetuntranslated regions of the mRNAs and upregulate the translation initiation rates by alleviating the repressive miRNA effects.[5]
An example is theauxin mediated derepression of the auxin response factor family of transcription factors in plants. These auxin response factors are repressed by Aux/IAA repressors. In the presence of auxin, these Aux/AII proteins undergoubiquitination and are then degraded.[6][7] This derepresses the auxin response factors so they may carry out their functions in the cell.
Alzheimer’s is aneurodegenerative disease involving progressive memory loss and other declines in brain function. One common cause of familial Alzheimer’s is mutation in thePSEN1 gene.[8] This gene encodes a protein that cleaves certain intracellular peptides which, once free in thecytoplasm, promoteCBPdegradation. Mutations inPSEN1 decrease its production or ability to cleave proteins. This derepresses the CBP proteins, and allows them to perform their function of upregulating transcription of their target genes.[8]
Rett syndrome is aneurodevelopmental disorder involving deterioration of learned language and motor skills,autism, and seizures starting in infancy. Many cases of Rett syndrome are associated with mutations inMECP2, a gene encoding a transcriptional repressor.[8] Mutations in this gene decrease the levels of MeCP2 binding to different promoter sequences, resulting in their overall derepression. The increased expression of these MeCP2 regulated genes inneurons contribute to the Rett syndrome phenotype.[8][9]
This syndrome is associated with increased susceptibility to tumors and growth abnormalities in children. A common cause of this syndrome is a mutation in animprint control region near theIgf2 gene.[9] This imprint control region is normally bound by aninsulator on the maternalallele, which represses anenhancer from acting on theIgf2 gene. This insulator is absent on the paternal allele and allows it access to the gene. Mutations in this imprint control region inhibit the insulator from binding, which derepresses enhancer activity on the maternal Igf2 gene. This abnormal derepression and increase in gene expression can result in Beckwith-Wiedemann syndrome.[9]