Designing PCR Primers for the Amplification-Refractory Mutation System

Analysis of single nucleotide polymorphisms (SNPs) has been and will be increasingly utilized in various genetic disciplines, particularly in studying genetic determinants of complex diseases. Such studies will be facilitated by rapid, simple, low cost and high throughput methodologies for SNP genotyping. One such method is reported here, named tetra-primer ARMS-PCR, which employs two primer pairs to amplify, respectively, the two different alleles of a SNP in a single PCR reaction. A computer program for designing primers was developed. Tetra-primer ARMS-PCR was combined with microplate array diagonal gel electrophoresis, gaining the advantage of high throughput for gel-based resolution of tetra-primer ARMS-PCR products. The technique was applied to analyse a number of SNPs and the results were completely consistent with those from an independent method, restriction fragment length polymorphism analysis.

In two randomly selected human genomes, 99.9% of the DNA sequence is identical. The remaining 0.1% of DNA contains sequence variations. The most common type of such variation is called a single-nucleotide polymorphism, or SNP. SNPs are highly abundant, stable, and distributed throughout the genome. These variations are associated with diversity in the population, individuality, susceptibility to diseases, and individual response to medicine. Recently, it has been suggested that SNPs can be used for homogeneity testing and pharmacogenetic studies and to identify and map complex, common diseases such as high blood pressure, diabetes, and heart disease. Consistent with this proposal is the identification of the patterns of SNPs in conditions such as diabetes, schizophrenia, and blood-pressure homeostasis. Although these studies have provided insight into the nature of human sequence variation, it is not known at present whether these variations are truly significant toxicologically and pharmacologically. Moreover, it is possible that most complex, common disorders are caused by the combined effects of multigenes and nongenetic environmental factors (multifactorial). Therefore, it is likely that sequence variation alone is not sufficient to predict the risk of disease susceptibility, particularly in homeostatic organisms like humans. Nevertheless, these variants may provide a starting point for further inquiry.

The A3243G mitochondrial tRNA leu(UUR) point mutation causes mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome, the most common mitochondrial DNA (mtDNA) disorder, and is also found in patients with maternally inherited diabetes and deafness syndrome (MIDD). To correlate disease manifestation with mutation loads, it is necessary to measure the percentage of the A3243G mtDNA mutation. To reliably quantify low proportions of the mutant mtDNA, we developed a real-time amplification refractory mutation system quantitative PCR (ARMS-qPCR) assay. We validated the method with experimental samples containing known proportions of mutant A3243G mtDNA generated by mixing known amounts of cloned plasmid DNA containing either the wild-type or the mutant sequences. A correlation coefficient of 0.9995 between the expected and observed values for the proportions of mutant A3243G in the experimental samples was found. Evaluation of a total of 36 patient DNA samples demonstrated consistent results between PCR-restriction fragment length polymorphism (RFLP) analysis and real-time ARMS-qPCR. However, the latter method was much more sensitive for detecting low percentages of mutant heteroplasmy. Three samples contained allele-specific oligonucleotide-detectable but RFLP-undetectable mutations. The real-time ARMS-qPCR method provides rapid, reliable, one-step quantitative detection of heteroplasmic mutant mtDNA.