Nonsense mutation

A nonsense mutation is a genetic mutation in a DNA sequence that results in a shorter, unfinished protein product. DNA is a chain of many smaller molecules called nucleotides. During protein formation, DNA (or RNA) nucleotide sequences are read three nucleotides at a time in units called codons, and each codon corresponds to a specific amino acid or stop signal. Stop codons are also called nonsense codons because they do not code for an amino acid and instead signal the end of protein synthesis. Thus, nonsense mutations occur when a premature nonsense or stop codon is introduced in the DNA sequence. Nonsense mutation: A mutation in a base in the DNA that prematurely stops the translation. of messenger RNA (mRNA) resulting in a polypeptide chain that ends prematurely and a protein product that is truncated and incomplete and usually non-functional.

The nonsense mutation converts a codon (a triplet of bases) that encodes an amino acid into a stop codon, one that specifies the termination of translation. There are three nonsense codons (UAG, UAA, and UGA) in mRNA. Catching such a defective messenger RNA (mRNA) before a truncated and potentially harmful protein is synthesized is the job of what is called the nonsense-mediated mRNA decay (NMD) pathway. A nonsense mutation occurs when the sequence of nucleotides in DNA is changed in a way that stops the normal sequence of amino acids in the final protein. In central dogma of biology, DNA is transposed into RNA, which is then translated into a protein. In the DNA, each amino acid is designated by a series of three nucleotides, called a codon.

There are around 21 amino acids which can be designated by this system. There are also two other important signals, “START” and “STOP”. These signals allow the ribosome assembling the protein to know where to begin, and where to end. This completely changes the structure of the protein, because anything after the “STOP” signal is ignored. The ribosome snips off the incomplete protein, and goes on its way. Without the remainder of the amino acid chain, the protein may function and form completely differently than before.

The vast majority of mutations are deleterious, meaning they cause a decrease in the overall fitness and reproductive success of the organism. A nonsense mutation would fall into this category if the mutation affected an important functional protein. Imagine if the nonsense mutation was found in the DNA which coded for an ion channel protein.

Cystic fibrosis is a genetic disorder caused by a nonsense mutation which does exactly that. The protein affected by the nonsense mutation in cystic fibrosis is a regulator protein for ion channels. Without the ability to properly move ions, people with cystic fibrosis often have respiratory problems caused by a mucous build up due to the unregulated ions in their system. Duchene muscular dystrophy is another disease cause by a nonsense mutation, and there are many more examples. Below is a chart of several point mutations, or mutations of a single nucleotide.

A nonsense mutation can be seen in the middle. In this case, the original codon read “TTC”. This called for an mRNA with the codon “AAG”, which then produced a lysine in the amino acid chain. A nonsense mutation would change the first “T” to an “A”. This makes the first codon “ATC”. The corresponding mRNA segment, “UAG”, is a signal to the ribosome to stop the chain. Unlike any of the other mutations, this ends the chain entirely.

Nonsense mutation represents about 10% of the mutations responsible for genetic related pathologies attracted the attention of researchers and industries to specifically correct them. Various strategies have been developed in order to correct nonsense mutations. Some of these strategies are nonsense mutation-specific, and some can apply to different categories of mutations. Finally, some will affect the genomic DNA and some will target the RNA only, excluding the ethical question of heredity.

Nonsense mutations are involved in about 10% of patients with genetic diseases. These genetic diseases can enter in the rare pathology category, or in the frequent disease class, making nonsense mutation therapies of interest for a significant number of patients. Among the rare pathology category, Duchenne muscular dystrophy, cystic fibrosis, and spinal muscular atrophy (SMA) will be described. The rate of nonsense mutations among all these pathologies is variable, from one gene to another, or from one tissue to another. However, the mechanism of gene silencing occurring as a consequence of a nonsense mutation is shared by all patients harbouring a nonsense mutation in the gene responsible for their pathology.

Describe in detail with suitable examples.

    DNA: 5' - ATG ACT CAC CGA GCG CGA AGC TGA - 3'

         3' - TAC TGA GTG GCT CGC GCT TCG ACT - 5'

   mRNA: 5' - AUG ACU CAC CGA GCG CGA AGC UGA - 3'

Protein:      Met Thr His Arg Ala Arg Ser Stop

Suppose that a nonsense mutation was introduced at the fourth triplet in the DNA sequence (CGA) causing the cytosine to be replaced with thymine, yielding TGA in the DNA sequence. Since TGA is transcribed-then-translated as UGA, the resulting transcript and protein product would be:

  DNA: 5' - ATG ACT CAC TGA GCG CGA AGC TGA - 3'

         3' - TAC TGA GTG ACT CGC GCT TCG ACT - 5'

   mRNA: 5' - AUG ACU CAC UGA GCG CGU AGC UGA - 3'

Protein:      Met Thr His Stop

The remaining codons of the mRNA are not translated into amino proteins because the stop codon is prematurely reached during translation. This can yield a truncated abbreviated protein product, which quite often lacks the functionality of the normal, non-mutant protein. Many organisms—including humans and lower species, such as yeast—employ a nonsense-mediated mRNA decay pathway, which degrades mRNAs containing nonsense mutations before they are translated into non-functional polypeptides. Examples of diseases in which nonsense mutations are known to be among the causes include Cystic fibrosis (caused by the G542X mutation in the cystic fibrosis transmembrane conductance regulatorgene), Duchenne muscular dystrophy (dystrophin), Beta thalassaemia (β-globin), Hurler syndrome