In this study we investigated whether in plants, like in mammals, components of the nuclear cap-binding protein complex (CBC) are involved in nonsense-mediated mRNA decay (NMD). We selected several genes producing at least two alternatively spliced mRNA variants: one with a premature termination codon (PTC+) and another without it (PTC-). For each gene the PTC+/PTC- ratio was calculated using RT-PCR and direct sequencing in four Arabidopsis thaliana lines: wild type, the NMD mutant atupf3-1 and two CBC mutants: cbp20 and abh1. Whereas in the NMD mutant the ratios of PTC+/PTC- splice variants were higher than in wild-type plants, the two CBC mutants investigated showed no change in the PTC+/PTC- ratios. Our results suggest that neither CBP20 nor CBP80 is involved in NMD in A. thaliana.
Linear models based on proportionality between variables have been commonly applied in biology and medicine but in many cases they do not describe correctly the complex relationships of living organisms and now are being replaced by nonlinear theories of deterministic chaos. Recent advances in molecular biology and genome sequencing may lead to a simplistic view that all life processes in a cell, or in the whole organism, are strictly and in a linear fashion controlled by genes. In reality, the existing phenotype arises from a complex interaction of the genome and various environmental factors. Regulation of gene expression in the animal organism occurs at the level of epigenetic DNA modification, RNA transcription, mRNA translation, and many additional alterations of nascent proteins. The process of transcription is highly complicated and includes hundreds of transcription factors, enhancers and silencers, as well as various species of low molecular mass RNAs. In addition, alternative splicing or mRNA editing can generate a family of polypeptides from a single gene. Rearrangement of coding DNA sequences during somatic recombination is the source of great variability in the structure of immunoglobulins and some other proteins. The process of rearrangement of immunoglobulin genes, or such phenomena as parental imprinting of some genes, appear to occur in a random fashion. Therefore, it seems that the mechanism of genetic information flow from DNA to mature proteins does not fit the category of linear relationship based on simple reductionism or hard determinism but would be probably better described by nonlinear models, such as deterministic chaos.
Gene transcription leads to the generation of pre-mRNA molecules which contain both coding sequences (exons) and intervening non-coding sequences (introns). The primary transcript needs further processing which involves the excision of introns and ligation of exons. This process is called RNA splicing. Nearly all primary transcripts undergo alternative forms of splicing, which may lead to exon skipping or intron inclusion in the final mRNA. Thus, the translation of alternatively spliced RNA molecules results in the formation of slightly different proteins, which may, in some cases, exert antagonistic activity. Splicing is a multistage process which is conducted by a complex machinery comprising small nuclear RNA molecules and many proteins called splicing factors. The process undergoes precise regulation by means of cis acting internal RNA sequences and trans acting protein factors which may either enhance or silence the splicing of an exon. Many diseases are associated with aberrations of alternative splicing and its modulation may be used therapeutically, e.g. for the treatment of spinal muscular atrophy (SMA) or Duchenne muscular dystrophy (DMD). This article presents current knowledge of the ways of pharmacological modulation of alternative splicing. The focus is on the use of those therapeutics which have been already approved for clinical application or have entered clinical trials. The chemistry and mechanism of action of specific splice switching oligonucleotides is presented. Nusinersen promotes exon inclusion during splicing of SMN2 and is used for SMA treatment. On the other hand, eteplirsen is an oligonucleotide promoting exon skipping during splicing of mutated DMD and has been conditionally approved for DMD treatment. Moreover, small molecule modulators of alternative splicing (e.g. branaplam) are also described. The dynamic developments in this field should result in the approval of new drugs acting by the modulation of alternative splicing in the nearest future.
PL
Powstająca w wyniku transkrypcji genu cząsteczka pre-mRNA zawiera odcinki kodujące (eksony) poprzedzielane odcinkami niekodującymi (intronami). Pierwotny transkrypt wymaga obróbki, która polega między innymi na wycinaniu intronów i łączeniu eksonów. Proces ten nazywany jest składaniem RNA. W przypadku niemal wszystkich transkryptów, składanie pre-mRNA przebiega w komórkach alternatywnymi drogami, na przykład prowadząc do wykluczenia jednego z eksonów z ostatecznego transkryptu, lub włączenia jednego z intronów. Proces składania RNA przeprowadzany jest przez skomplikowaną maszynerię złożoną z małych cząsteczek RNA i białek, i podlega precyzyjnej regulacji. Wiele chorób związanych jest z nieprawidłowościami alternatywnego składania RNA, a modulacja tego procesu może być wykorzystana terapeutycznie, między innymi w leczeniu rdzeniowego zaniku mięśni (SMA, ang. spinal muscular atrophy) czy dystrofii mięśniowej Duchenne’a (DMD). Celem artykułu jest przedstawienie wiedzy na temat farmakologicznych możliwości wpływania na proces składania RNA. Nacisk położono na scharakteryzowanie tych terapeutyków, które zostały już zarejestrowane do użytku klinicznego, lub które są w trakcie zaawansowanych badań klinicznych. Zaprezentowano budowę chemiczną i mechanizm działania oligonukleotydowych przełączników składania RNA. Nusinersen stymuluje włączanie eksonu 7 w trakcie składania SMN2 i jest wykorzystywany w leczeniu SMA. Eteplirsen, który stymuluje pomijanie eksonu 51 w trakcie składania zmutowanych wariantów DMD, został warunkowo dopuszczony do leczenia DMD w USA. Opisano także drobnocząsteczkowe modulatory alternatywnego składnia RNA, np. branaplam. Dynamiczny rozwój tego obszaru badań stwarza szansę na wprowadzone w najbliższej przyszłości do lecznictwa kolejnych leków, których mechanizm działania oparty będzie na modulacji alternatywnego składania RNA.
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