PyroMark Q24 Advanced increases both read length and reliability of methylation analysis at positions later in the sequence. This example demonstrates 135 nucleotide dispensations and the accurate analysis of 16 different CpG positions in a single PyroMark Q24 Advanced CpG reaction. To accurately analyze all of these sites with PyroMark Q24, one would need to run 3 separate assays.
A DNA segment is amplified, and the strand to serve as the Pyrosequencing template is biotinylated. After denaturation, the biotinylated single-stranded PCR amplicon is isolated and allowed to hybridize with a sequencing primer. The hybridized primer and single-stranded template are incubated with the enzymes DNA polymerase, ATP sulfurylase, luciferase, and apyrase, as well as the substrates adenosine 5' phosphosulfate (APS) and luciferin.
The first deoxribonucleotide triphosphate (dNTP) is added to the reaction. DNA polymerase catalyzes the addition of the dNTP to the squencing primer, if it is complementary to the base in the template strand. Each incorporation event is accompanied by release of pyrophosphate (PPi), in a quantity equimolar to the amount of incorporated nucleotide.
ATP sulfurylase converts PPi to ATP in the presence of adenosine 5' phosphosulfate (APS). This ATP drives the luciferase-mediated conversion of luciferin to oxyluciferin that generates visible light in amounts that are proportional to the amount of ATP. The light produced in the luciferase-catalyzed reaction is detected by CCD sensors and seen as a peak in the raw data output (Pyrogram). The height of each peak (light signal) is proportional to the number of nucleotides incorporated.
Apyrase, a nucleotide-degrading enzyme, continuously degrades unincorporated nucleotides and ATP. When degradation is complete, another nucleotide is added.
Addition of dNTPs is performed sequentially. It should be noted that deoxyadenosine alpha-thio triphosphate (dATPαS) is used as a substitute for the natural deoxyadenosine triphosphate (dATP), since it is efficiently used by the DNA polymerase, but not recognized by the luciferase. As the process continues, the complementary DNA strand is elongated, and the nucleotide sequence is determined from the signal peaks in the Pyrogram trace.
Quantification of CpG methylation directly following a T homopolymer or between T homopolymers is especially challenging. Methylation levels are determined by the ratio of converted C nucleotides to unconverted C nucleotides following bisulfite treatment, and since the C to T conversion often leads to long T homopolymer stretches in an amplicon, this scenario occurs often. PyroMark Q24 Advanced enables accurate quantification of CpG position after or between T homopolymers.This example shows the analysis of a CpG site within a stretch of 8 T nucleotides.
Read length is a critical factor for analysis of unknown sequences, often limiting the reliable analysis to 40–80 bases. The improved chemistry and algorithm of PyroMark Q24 Advanced significantly increases the possible read length and provides higher accuracy. PyroMark Q24 (upper panel) and PyroMark Q24 Advanced (lower panel) were used for de novo sequencing using the same assay. Blue bars indicate reliably detected bases; yellow indicates bases that might have been determined correctly but should be checked by the user; red indicates unreliable readings.
Since mutations and single nucleotide polymorphisms (SNPs) are rarely in directly neighboring positions, common Pyrosequencing chemistry might require separate assays for analysis of more than one mutation site. The new chemistry of PyroMark Q24 Advanced allows much longer runs, enabling reliable analysis of additional mutations within one run. This example shows the analysis of a 10:90 mixture of wild-type and mutated EGFR. Even after 60 dispensations, the analysis is exact.
The components of the PyroMark Q24 Advanced system are designed to make the Pyrosequencing workflow straightforward and efficient. Each step, from assay design to PCR amplification and preparation of sequencing templates, is supported by software, kits, reagents, and sample prep instrumentation optimized for Pyrosequencing.
Though small in size, PyroMark Q24 Advanced manages all steps necessary to rapidly analyze up to 24 samples. Simply load your samples and reagents, upload your run file, and walk away. The instrument dispenses reagents and nucleotides to each well with precision. Light signals emitted are detected by 24 CCD sensors — one sensor per well — thereby eliminating signal crossover.
PyroMark Q24 Advanced is designed as a stand-alone instrument, which makes it easy to place anywhere in the lab. Data are stored on the instrument hard drive and can be viewed on the instrument screen during a run. Additionally, all files are stored on the supplied USB stick, giving the user the flexibility to analyze data on any computer with PyroMark Q24 Advanced Software installed.
The PyroMark Q24 Vacuum Workstation enables conversion of PCR products into the single-stranded DNA for Pyrosequencing. Exposure of the PCR amplicons to a series of optimized solutions denatures and washes the DNA. This process is carried out for 24 samples in parallel and takes only a few minutes.