Point mutations, or single-base substitutions, are the most common form of mutations

Any nucleotide in the genomic sequence may be substituted with another, either through transition between two purine or pyrimidine bases or transversion between a purine base and a pyrimidine base. Many point mutations are silent functionally. Those that occur within introns and other noncoding sequences may not have any functional effect. Since many amino acids are encoded by more than a single genetic code, many base substitutions within exons have no effect on the translation products. In some instances, mutations may not affect the function of the products even when they result in changes in the primary amino acid sequence. Statistically, however, a majority of point mutations that alter the primary amino acid sequence of the translation products result in impaired gene function at various stages. If a point mutation affects the function of a regulatory sequence, the gene may not be transcribed properly. If a mutation is at or near an intron—exon junction, processing of the primary transcript may be abnormal and an intron may remain or exons may be skipped in the processed mRNA. Sometimes a point mutation creates a consensus sequence for splicing which generates abnormally spliced mRNA. Loss of the polyadenylation signal also will result in abnormally processed mRNA. Within the coding sequence, if a mutation affects the initiation codon, no translation product can be generated. Single-base substitution can change the codon to another amino acid, termed a sense mutation, or generate a stop codon, termed a nonsense mutation. A sense mutation can alter the folding of the protein; affect sites important in post-translational modifications, such as the proteolytic cleavage, phosphorylation and glycosylation sites; or modify the translation products in other ways. Statistically, most of these will result in loss of the function of the translation product.

Many point mutations appear to occur randomly. Fidelity of DNA polymerase and slipped mispairing at the time of DNA duplication are often suggested as the main mechanisms for random point mutations [14]. However, there is one mechanism that appears to occur frequently. The sequence CpG is known as a mutation hot spot because spontaneous chemical methylation and deamination result in C-to-T and G-to-A transitions. Many disease-causing point mutations are known at the “CpG island.”

Deletion, insertion and duplication also are frequent causes of many genetic disorders

Segments of the genomic sequence, ranging in size from a single nucleotide to regions of the chromosome large enough to be visible microscopically, can be either deleted, inserted or duplicated. Deletions or insertions that are not multiples of three and that occur within the protein coding sequence result in a frameshift in translation. The amino acid sequence downstream from the mutation is then abnormal and a stop codon is encountered 21 or 22 codons downstream, on the average. Larger deletions or insertions naturally will affect the structure and, consequently, the function of the gene more drastically. These larger deletions or insertions are often readily detectable by appropriate restriction digestion of genomic DNA and Southern analysis. The mechanisms for these mutations are multiple. Relatively small deletions or insertions can occur during DNA replication as the result of slippage mispairing or homologous and unequal recombination, particularly between repetitive sequences, such as the Alu element. Rare retrotransposition may also be responsible for some of the large insertions [14].

Gene duplication can be considered as a large-scale insertion. This mechanism appears to have played an important role in the genomic evolution of higher organisms. If both the original and the duplicated genes are functional, the organism will acquire a double dose of that gene. However, most of the time, the duplication is not complete and can be the cause of a genetic disorder. For example, the mld mutant mouse has a duplicated and partially inverted myelin basic protein gene in close proximity to the native gene. This results in a decreased level and an abnormal temporal schedule of myelin basic protein synthesis with a clinical phenotype milder than that of the allelic shiverer mutant [17].

Trinucleotide expansion is one of the most important categories of mutation underlying neurodegenerative disorders

The dynamically expanding trinucleotide repeat is an important class of abnormality that causes some genetic disorders [15,16]. It was first reported in fragile X syndrome in 1992. The expanding triplets can be CAG, CGG, GCC, CTG or GAA. As of this writing, there are at least a dozen genetic disorders which have triplet expansions as the underlying gene abnormality (Table 40-1). They are commonly classified into two groups, those with very long expansions that are not translated and those with moderate CAG expansions that result in a polyglutamine stretch in the translation products. The repeating sequence in the first group can be extremely long, from a few hundred to thousands. The repeating sequence in the second group is usually moderate, approximately 40 to 120 triplets. Most of the disorders in the second group are inherited as dominant traits. Thus, acquired toxic function is often postulated as the pathogenetic mechanism. The location may be within the coding sequence, on either side of the coding sequence but within exons, in the promoter region or within an intron. For unknown reasons, all of these disorders are neuromuscular in phenotype. In some disorders, such as myotonic dystrophy and spinocerebellar atrophy 1, a positive correlation is found between the length of the repeat and the clinical severity of the disease. The exact mechanism for the expansion is not known. The rule of the triplet repeat expansion has been extended to include a repeated stretch of 12 nucleotides in the cystatin B gene in progressive myoclonus epilepsy of the Unverricht type [18]. We can expect that more genetic disorders will be found in the near future that are due to dynamic expansions of repeated nucleotide sequences within the genome.

Table 40-1

Genetic Neuromuscular Disorders Caused by Dynamic Expansion of Nucleotide Repeats.