RNA splicing

Case details

A 45-year- old female of Indian origin reported for a routine checkup. She had no complaints and was in good health; however she reported a family history of anemia. Her complete blood count demonstrated mild anemia and the attending physician suspected thalassemia minor in the patient. Thalassemia is most often due to a defect in RNA splicing, which is best explained by which of the following statements?

A) A high molecular weight precursor is trimmed to a short and mature m RNA.

B) The coding regions found on the introns are rearranged and joined together.

C) The introns are removed and the exons are joined together.

D) The terminal non coding sequences are removed.

E) The mRNA gains stability by 5′ capping and 3’poly A tailing.

The correct answer is –C.

Intons or intervening sequences are the RNA sequences which do not code for the proteins. These introns are removed from the primary transcript in the nucleus, exons (coding sequences) are ligated to form the mRNA molecule, and the mRNA molecule is transported to the cytoplasm.

The steps of splicing are as follows-

i) Role of small nuclear RNA (sn RNA)  and Spliceosome

The molecular machine that accomplishes the task of splicing is known as the spliceosome. Spliceosome consists of the primary transcript, five small nuclear RNAs (U1, U2, U5, U4, and U6) and more than 60 proteins. Collectively, these form a small ribonucleoprotein (snRNP) complex, sometimes called a “snurp” (snRNPs) (Figure-1).Snurps are thought to position the RNA segments for the necessary splicing reactions. These facilitate the splicing of exon segments by forming base pairs with the consensus sequence at each end of the intron. Although the sequences of nucleotides in the introns of the various eukaryotic transcripts—and even those within a single transcript—are quite heterogeneous, there are reasonably conserved sequences at each of the two exon-intron (splice) junctions and at the branch site, which is located 20–40 nucleotides upstream from the 3′ splice site.

Spliceosome assembly

Figure-1- Showing spliceosome assembly at the splice site.  

ii) Mechanism of excision of introns

The binding of snRNPs brings the sequences of the neighboring exons in to the correct alignment for splicing. The 2′-OH group of an adenosine (A) residue (known as the branch site) in the intron attacks and forms a phosphodiester bond with the phosphate at the 5′ end of the intron 1.The newly- feed 3’OH of the upstream exon 1 then forms a phosphodiester bond with the 5’end of the downstream exon 2.The excised intron is released as a “lariat” structure, which is degraded (Figure-2).

Mechanism of splicing

Figure-2- showing the process of splicing.

After removal of all the introns, the mature m RNA molecules leave the nucleus by passing in to the cytosol through pores in to the nuclear membrane.

Clinical significance

1) Antibodies against snRNPs

In systemic Lupus Erythematosus (SLE), an auto immune disease, the antibodies are produced against host proteins, including sn RNPs.

2) Mutations at the splice site

Mutations at the splice site can lead to improper splicing and the production of aberrant proteins.For example some cases of Beta thalassemia are as a result of incorrect splicing of beta globin m RNA due to mutation at the splice site.

Biological significance

Alternative Splicing

Alternative patterns of RNA splicing are adapted for the synthesis of tissue-specific proteins.

The pre-m RNA molecules from some genes can be spliced in two or more alternative ways in different tissues. This produces multiple variations of the m RNA and thus diverse set of proteins can be synthesized from a given set of genes (Figure-3).

For example- Tissue specific tropomyosins are produced from the same primary transcript by alternative splicing. Alternative splicing and processing results in the formation of seven unique α -tropomyosin mRNAs in seven different tissue .

Alternative splicing

Figure-3- Showing the mechanism of alternative splicing.

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