Base substitution mutations

Lesch-Nyhan syndrome, an overproduction hyperuricemia characterized by frequent episodes of uric acid hthiasis and a bizarre syndrome of self-mutilation, reflects a defect in hypoxanthme-guanine phosphoribo-syl transferase, an enzyme of purine salvage (Figure 34—4). The accompanying rise in intracellular PRPP results in purine overproduction. Mutations that decrease or abohsh hypoxanthine-guanine phosphoribosyltrans-ferase activity include deletions, frameshift mutations, base substitutions, and aberrant mRNA splicing. 

Studies with mutated (base substitution) 30-mer synthetic oligonucleotides revealed differences for 3, 2 and 1 mismatched strands. The type of interaction that disappears and the possible new interactions that are generated when a base substitution occurs are of importance. One mismatch is detected even if it is located extreme far from the electrode, being the attenuation of the signal lower than when the substitution occurs near the electrode surface (Fig. 26.5). Moreover,... 

Mutations in protein-coding sequences Point mutations (base substitutions)... 

Given a large population of individuals, a considerable number of sequence variants can be found for a protein. These variants are a consequence of mutations in a gene (base substitutions in DNA) that have arisen naturally within the population. Gene mutations lead to mutant forms of the protein in which the amino acid sequence is altered at one or more positions. Many of these mutant forms are neutral in that the functional properties of the protein are unaffected by the amino acid substitution. Others may be nonfunctional (if loss of function is not lethal to the individual), and still others may display a range of aberrations between these two extremes. The severity of the effects on function depends on the nature of the amino acid substitution and its role in the protein. These conclusions are exemplified by the more than 300 human... 

Tvrdik T et al Molecular characterization of two deletion events involving Alu-sequences, one novel base substitution and two tentative hotspot mutations in the hypoxanthine phosphori-bosyltransferase gene in five patients with Lesch-Nyhan-syndrome. Hum Genet 1998 103 3H. 

The base substitutions are primarily transversions at G C base pairs and the available evidence suggests that these mutations are induced by apurinic sites which are generated as secondary consequences of the initial alkylation event. The significance of these results in the context of carcinogenesis is briefly considered. 

On the other hand, the observation that 95% of the UV induced base substitution mutations arose at the very sites (pyrimidine-pyrimidine sequences) where the major fraction of UV damage is deposited suggested that at least the UV induced mutations were targeted (24). 

We have not directly determined the relative frequencies of frameshift mutations and other mutational events in comparison to the base substitution mutations. However, based on the high frequency of nonsense mutations (11%) among all lacl mutants induced by BPDE and because nonsense mutations are monitorable at less than one-fifth of the lacl codons and, even then, only via certain base pair substitutions, we believe that base substitutions account for a major fraction of mutations induced by BPDE. 

Point mutations can occur when one base is substituted for another (base substitution). Substitution of another purine for a purine base or of another pyrimidine for pyrimidine is called a transition, while substitutions of purine for pyrimidine or pyrimidine for purine are called transversions. Both types of base substitution have been identified within mutated genes. These changes lead to a codon change which can cause the wrong amino acid to be inserted into the relevant polypeptide and are known as mis-sense mutations. Such polypeptides may have dramatically altered properties if the new amino acid is close to the active center of an enzyme or affects the three-dimensional makeup of an enzyme or a structural protein. These changes, in turn, can lead to change or reduction in function, which can be detected as a change in phenotype of the affected cells. 

A base substitution can also result in the formation of a new inappropriate terminator (or non-sense) codon, and are thus known as non-sense mutations. The polypeptide formed from such mutated genes will be shorter than normal and is most likely to be inactive. Owing to the redundancy of the genetic code, about a quarter of all possible base substitutions will not result in an amino acid replacement and will be silent mutations. 

Base substitutions and frameshift changes occur spontaneously and can be induced by radiation and chemical mutagens. It is apparent that the molecular mechanisms resulting in these changes are different in each case, but the potential hazards associated with mutagens capable of inducing the different types of mutation are equivalent. 

The presence of mutator plasmids. Excision-deficient strains containing pKMIOl have a higher spontaneous mutation rate at both base substitution and frameshift loci than excision-proficient strains. 

Forward Mutation Tests. Forward mutation is an endpoint that may arise from various events, including base substitutions, frameshifts, DNA deletions, and so on, as mentioned earlier. 

It is now believed that a substantial proportion of the single nucleotide substitutions causing human genetic disease are due to misincorporation of bases during DNA replication. Which proofreading activity is critical in determining the accuracy of nuclear DNA replication and thus the base substitution mutation rate in human chromosomes ... 

Molecular analysis of the fibrillin gene in these family members identified the mutation associated with Marfan syndrome as a single base substitution at the 3 end of exon 2. The result of this substitution was a deletion of the entire exon 2 firom the mRNA, and consequently 41 amino acids from the fibrillin protein. The mutation was most likely a... 

Point mutations or single base substitutions are classified as transitions or transversions. 

The base substitutions can be either a change from one purine or pyrimidine to another, which is a transition or a change of a purine for a pyrimidine and vice versa, which is a transversion. A chemical change in the base or formation of an adduct, which changes the nature of the base(s), could cause a substitution at replication. The position of the adduct on the particular DNA base would determine the type of mutation. 

Mutations in the coding sequences, which will include genes for specific proteins, will potentially cause adverse changes in the gene and therefore the phenotype. Chemicals can interact with DNA in a variety of ways including adduct formation, base substitutions (transitions or transversions), and base additions or deletions. 

In a study by Sisk et al. (1994), male B6C3Fj lad transgenic mice were exposed by inhalation to 0, 62.5, 625 or 1250 ppm [0, 138, 1380 or 2760 mg/m ] butadiene for four weeks (6 h per day, five days per week). Animals were killed 14 days after the last exposure and lad mutants were recovered from the DNA according to established protocols. A 2.5- and 3-fold increase in the lad mutant frequency was observed in the bone marrow of mice exposed to 625 or 1250 ppm butadiene, respectively, compared with air-exposed control mice. DNA sequence analysis of lad mutants recovered from the bone marrow of mice exposed to 625 ppm butadiene demonstrated that there was a shift in the spectrum of base substitution mutations at A T base pairs in butadiene-exposed mice (6/26, 23%), compared to air control mice (2/45, 4%). Recio and Meyer (1995) examined the lad mutational spectrum in the bone marrow of mice exposed to 1250 ppm butadiene in the above study. DNA sequence analysis of lad mutants revealed an increase in mutations at A T base pairs (9/49, 20%) similar to that observed by Sisk et al. (1994). 

When a mutation introduces a termination codon in the interior of a gene, translation is prematurely halted and the incomplete polypeptide is usually inactive. These are called nonsense mutations. The gene can be restored to normal function if a second mutation either (1) converts the misplaced termination codon to a codon specifying an amino acid or (2) suppresses the effects of the termination codon. Such restorative mutations are called nonsense suppressors they generally involve mutations in tRNA genes to produce altered (suppressor) tRNAs that can recognize the termination codon and insert an amino acid at that position. Most known suppressor tRNAs have single base substitutions in their anticodons. 

---