Detection of mutation delta F508 in the cystic fibrosis gene using allele-specific PCR primers and time-resolved fluorometry

Research advances in molecular mechanisms underlying the pathogenesis of cystic fibrosis: From technical improvement to clinical applications (Review)

Cystic fibrosis (CF) is a chronic disease causing severe impairment to the respiratory system and digestive tracts. Currently, CF is incurable. As an autosomal recessive disorder, the morbidity of CF is significantly higher among Caucasians of European descent, whereas it is less pervasive among African and Asian populations. The disease is caused by identical mutations (homozygosity) or different mutations (heterozygosity) of an autosomal recessive mutation at position 7q31.2-q31.1 of chromosome 7. Diagnostic criteria and guidelines work concurrently with laboratory detection to facilitate precise CF detection. With technological advances, the understanding of CF pathogenesis has reached an unprecedented level, allowing for increasingly precise carrier screening, more effective early stage CF intervention and improved prognostic outcomes. These advances significantly increase the life quality and expectancy of patients with CF. Given the numerous improvements in the field of CF, the current review summarized the technical advances in the study of the molecular mechanisms underlying CF, as well as how these improvements facilitate the clinical outcomes of CF. Furthermore, challenges and obstacles to overcome are discussed.

Development of quantitative polymerase chain reaction assays for allelic discrimination of gangliosidoses in cats

OBJECTIVE

To develop quantitative PCR (qPCR) assays with allele-specific primers to provide a rapid and accurate diagnostic and screening test for the 3 mutations identified as causes of gangliosidoses in domestic cats.

SAMPLE POPULATION

DNA samples obtained from archived feline blood samples submitted for GM1 and GM2 testing.

PROCEDURES

A qPCR assay was developed for each mutation to monitor the efficiency of PCR amplification. Results were determined on the basis of the fluorescent intensity of DNA staining.

RESULTS

Samples from 60 cats were screened by use of the 3 qPCR assays. Of these, 59 qPCR results agreed with the sequence-derived genotypes. The phenotype (affected) for the other cat agreed with results for the qPCR assay, which indicated that interpretation of the sequence-based result was incorrect.

CONCLUSIONS AND CLINICAL RELEVANCE

The qPCR assays offer a sensitive, rapid, and reproducible technique for allelic discrimination without the need for complicated processing steps, such as hybridization or sequencing, after PCR procedures. These assays may prove beneficial for a rapid diagnosis of gangliosidoses in cats and could also provide a means for reliable large-scale screening for the carrier state, thereby accelerating the eradication of these debilitating diseases from feline populations.

Factor V Leiden mutation screened by PCR and detected with lanthanide-labeled probes

The Factor V Leiden mutation is an important human polymorphism, responsible for increased risk of venous thrombosis in heterozygotes as well as homozygotes. Therefore, screening is a useful possibility, and many detection systems have been described for PCR products. We have developed a simplified and robust assay using oligonucleotide probes for normal and mutant sequences, labeled with europium and samarium, respectively, and measured by time-resolved fluorescence. Populations consisting of 233 Welsh and 148 Irish subjects were examined by both restriction fragment length polymorphism (RFLP) analysis and our assay. The allele frequency was 14/466 in the Welsh and 5/296 in the Irish population, in line with other surveys of European populations. Results were not obtained in 2/381 samples by RFLP, compared with 1/381 with our method. We conclude that our method represents an improved system capable of considerable throughput at reasonable cost.

An improved real time PCR method for simultaneous detection of C282Y and H63D mutations in the HFE gene associated with hereditary hemochromatosis

HFE-linked hereditary hemochromatosis (HH) is one of the most common inherited diseases among individuals of Northern European ancestry. Two sites of point mutations in the HFE gene--C282Y and H63D--are associated with greater than 90% of HH cases. We have developed a sensitive real time PCR (TaqMan) 5'-nuclease assay for single nucleotide polymorphism (SNP) detection using novel DNA chemistry, and successfully applied this method to detect these mutations. Fluorogenic PCR probes, chemically modified with a minor groove binding agent to increase duplex stability, were used in single and multiplex probe closed tube formats. The probes were tested in two commercially available thermocycling fluorimeters (the Light Cycler and the ABI Prism 7700). Comparison of the results obtained from the analysis of 43 samples showed no discrepancies between our 5' nuclease assay and the restriction length polymorphism analysis, which is routinely used in hospitals. The reported real time PCR technology is ideal for the clinical setting as it is sensitive, eliminates the labor and supply costs of post-PCR steps, reduces the risk of crossover contamination, minimizes sources of error, and can be fully automated.

A retrospective anonymous pilot study in screening newborns for HFE mutations in Scandinavian populations

We have retrospectively analyzed 837 random anonymized dried blood spot (DBS) samples from neonatal screening programs in Scandinavia for mutations in HFE, the candidate gene for hemochromatosis. We have found C282Y allele frequencies of 2.3% (+2.0%) (-1.3%) in Greenland, 4.5%+/-1.9% in Iceland, 5.1%+/-2.3% in the Faeroe Islands, and 8.2%+/-2.7% in Denmark. The high prevalence of HFE mutations in Denmark suggests that population screening for the C282Y mutation could be highly advantageous in terms of preventive health care. Long-term follow-up evaluation of C282Y homozygotes and H63D/C282Y compound heterozygotes will give an indication of the penetrance of the mutations.

Mutation detection by TaqMan-allele specific amplification: application to molecular diagnosis of glycogen storage disease type Ia and medium-chain acyl-CoA dehydrogenase deficiency

We have devised an allele-specific amplification method with a TaqMan fluorogenic probe (TaqMan-ASA) for the detection of point mutations. Pairwise PCR amplification using two sets of allele-specific primers in the presence of a TaqMan probe was monitored in real time with a fluorescence detector. Difference in amplification efficiency between the two PCR reactions was determined by "threshold" cycles to differentiate mutant and normal alleles without post-PCR processing. The method measured the efficiency of amplification rather than the presence or absence of end-point PCR products, therefore allowing greater flexibility in designing allele-specific primers and an ample technical margin for allelic discrimination. We applied the TaqMan-ASA method to detect a prevalent 727G>T mutation in Japanese patients with glycogen storage disease type Ia and a common 985A>G mutation in Caucasian patients with medium-chain acyl-CoA dehydrogenase deficiency. The method can be automated and may be applicable to the DNA diagnosis of various genetic diseases.

The frequency of an inactivating point mutation (566C-->T) of the human follicle-stimulating hormone receptor gene in four populations using allele-specific hybridization and time-resolved fluorometry

We have described previously in the Finnish population an inactivating point mutation (566C-->T) in the human FSH receptor (FSHR) gene. In women, this mutation causes hypergonadotropic ovarian failure with arrest of follicular maturation and infertility, whereas in men, there is variable suppression of spermatogenesis, but no absolute infertility. To determine whether the same FSHR mutation occurs in other populations, its frequency was determined in Finland, Switzerland, Denmark, and the Chinese population of Singapore. The mutation was screened for using genomic DNA extracted from whole blood or dried blood spots. Exon 7 of the FSHR gene was first amplified using a pair of biotinylated primers. The PCR products were then immobilized on streptavidin-coated microtitration wells and hybridized using short allele-specific oligonucleotide probes labeled with europium. Time-resolved fluorometry was used for europium signal detection. To test the reliability of this method, 40 isolated DNA samples and 35 dried blood spot samples were blindly tested for the 566C-->T FSHR mutation. The analyses yielded identical results with denaturing gradient gel electrophoresis and allele-specific restriction enzyme digestion of the same samples, thus demonstrating the reliability of the tested method. Automation of this procedure allows the screening of large numbers of samples, which was subsequently carried out to investigate the frequency of the 566C-->T mutation in the study populations. A total of 4981 samples from the above-mentioned 4 countries were analyzed. The frequency of the 566C-->T mutation was 0.96% for all Finnish samples (n=1976), with a strong enrichment of the mutant allele in the northeastern part of the country. Only 1 mutation carrier was identified in the samples from Switzerland (n=1162), whereas none was found in samples from Denmark (n=1094) and the Singapore Chinese (n=540). These results suggest that the 566C-->T mutation of the FSHR gene is enriched in Finland, but is uncommon in other populations.

Detection of the hereditary hemochromatosis gene mutation by real-time fluorescence polymerase chain reaction and peptide nucleic acid clamping

Hereditary hemochromatosis (HH), an iron overload disease, is the most common known inheritable disease. The most prevalent form of HH is believed to be the result of a single base-pair mutation. We describe a rapid homogeneous mutation analysis method that does not require post-polymerase chain reaction (PCR) manipulations. This method is a marriage of three emerging technologies: rapid cycling PCR thermal cyclers, peptide nucleic acid (PNA) probes, and a new double-stranded DNA-selective fluorescent dye, Sybr Green I. The LightCycler is a rapid thermal cycler that fluorometrically monitors real-time formation of amplicon with Sybr Green I. PNAs are DNA mimics that are more sensitive to mismatches than DNA probes, and will not serve as primers for DNA polymerases. PNA probes were designed to compete with PCR primers hybridizing to the HH mutation site. Fully complemented PNA probes at an 18:1 ratio over DNA primers with a mismatch result in suppression of amplicon formation. Conversely, PNA probes with a mismatch will not impair the binding of a complementary primer, culminating in amplicon formation. A LightCycler-based rapid genetic assay has been developed to distinguish HH patients from HH carriers and normal individuals using PNA clamping technology.

Simple triple-label detection of seven cystic fibrosis mutations by time-resolved fluorometry

We describe a simple hybridization assay performed in microtitration wells with use of DNA probes labeled with three different lanthanide chelates for detection of seven mutations that cause cystic fibrosis. The assay is based on DNA amplification of four fragments containing the mutations (ΔF508, G1717→A, G542X, R553X, 3905 insertion T, W1282X, and N1303K) by PCR, followed by hybridization with short, allele-specific oligonucleotide probes labeled with europium, terbium, or samarium chelates. Because the technology makes it possible to hybridize three DNA probes simultaneously in one reaction, all 14 mutation-related alleles were detected in a total of five reaction wells. Blood spot specimens, obtained from children with cystic fibrosis, their parents, and their siblings, have been assayed, and for all the probes the positive signal-to-noise ratios are >10. Solution hybridization utilizing triple-label time-resolved fluorometry combined with PCR is a suitable procedure for large-scale screening and automation.

Solid-phase synthesis of chelate-labelled oligonucleotides: application in triple-color ligase-mediated gene analysis

Oligonucleotides labelled with detectable groups are essential tools in gene detection. We describe here the synthesis of pyrimidine deoxynucleotide-building blocks, modified at their C-5 position with a protected form of a strongly chelating agent. These reagents can be used to introduce multiple metal ions into oligodeoxynucleotides during standard oligonucleotide synthesis. The chelating functions form strongly fluorescent complexes with europium ions, characterized by a wide separation between the excitation and emission spectra. Moreover, the long decay time of the fluorescence permits sensitive time-resolved fluorescence measurements. The chelates also have the stability required to function in triple-color assays involving europium, samarium, and terbium ions. We demonstrate the application of these reagents for ligase-based gene analysis reactions.

Detection of point mutations by solid-phase methods

Several techniques exist that permit the efficient distinction among characterized DNA sequence variants. In this review we discuss a number of such analytic procedures. These techniques all take advantage of a variety solid supports to prepare and analyze reaction products. The described diagnostic principles are now being applied for the development of miniaturized assay formats, suitable for automated detection of large sets of sequences in clinical samples.