Digital PCR (dPCR) has gained popularity in recent years for cancer research such as rare mutation detection, minimal residual disease, treatment selection, and recurrence monitoring. However, currently available technologies suffer from several limitations such as tedious workflow, long time-to-result, poor multiplexity, and inconsistent reagent digitization, which has severely hindered its broad adoption. We have designed and manufactured a dPCR platform that is capable of consistent sample digitization, thermal cycling and simultaneous interrogation of 20,000 partitions with walk-away workflow.
Application Notes: Absolute Quantification
The Absolute Q dPCR platform and its 80-minute 1-step RT-dPCR technology have broad implications for characterizing infectious diseases beyond COVID-19. The versatile platform can be adapted to a wide range of nucleic acid detection applications requiring absolute quantification. The Absolute Q simplifies dPCR with bestin-class data consistency, a short sample-to-answer time, and flexible multi-color multiplexing capabilities. Combinati aims to lower the barrier to bring dPCR into the lab to accelerate the response to global public health emergencies such as the COVID-19 pandemic.
- Experiment flexibility to use a single consumable up to four times
- Optimization of time allowed for annealing/extension step for a duplex BCR-ABL1 Assay
- Maintained partition consistencies for iterative use
Figure 1. The MAP16 consumable was used four times, using one column per experiment to optimize the annealing/extension time for a FAM/HEX multiplexed assay.
Combinati’s patented Microfluidic Array Partitioning (MAP) technology utilizes fixed microchamber arrays and positive pneumatic pressure to partition reagents and perform digital PCR, instead of using fluid-shearing to generate droplets. Each MAP16 plate consists of a 4-unit by 4-unit grid of dPCR reaction units – each unit containing 20,480 fixed partitions. The plate was designed to enable flexibility – meaning up to 16 samples may be used simultaneously or as few as four units, i.e. one column, can be loaded and run at a time without sacrificing data quality. This flexibility can be useful for applications in which lower throughput for dPCR is desired or iterative assay optimization, demonstrated here, is required.
In this tech note, an iterative test was performed on a single MAP16 plate to optimize the time allowed for the extension step of PCR for a BCR-ABL1 assay, which detects a gene fusion present in 95% of chronic myeloid leukemia patients. To showcase experiment flexibility of the consumable for repeat uses, we ran four sequential dPCR runs, modifying the extension step of PCR to be 0, 15, 30 and 45 seconds. We compared the final calculated concentration of target as well as compared the fluorescent intensity across each condition. To evaluate the integrity of the MAP plate across successive runs, we calculated the total number of partitions analyzed per condition.
We selected the BCR-ABL pDNA calibrant (Sigma, Cat:ERMAD623), a plasmid containing target sequences for both BCR-ABL1 and ABL1. ERM(R) certification of this well-characterized reference material ensures reliability and comparability of the results. We used a published duplex assay targeting the BCR-ABL1 (FAM) and ABL-1 (HEX) sequences respectively1, and prepared the assay using the Combinati 2X MasterMix. Each reaction contained a final target of 500 copies/µL. Using one column at a time, four replicates were run, and the concentration of each target was quantified in copies/µL. The reagent mix recipe and the dPCR protocol are described in Table 1.
Table 1. dPCR reagent preparation
For each partition, both low reagent volume and close proximity to the heated surface contribute to PCR robustness at a variety of extension times. Typically, the suggestion for extension time is approximately one minute per 1000 bases. In this study, we test the performance of the duplex assay at increasing extension time intervals (Table 2). In each of the four successive runs, a different column was utilized to evaluate the effects of changing the time allowed for annealing/extension step, starting at 0 seconds and increasing by 15 seconds with each run (Figure 1).
Table 2. Absolute Q dPCR thermal protocol
The quantification results for both the BCR-ABL1 (FAM) and ABL1 (HEX) targets across the four different extension times (0 seconds, 15 seconds, 30 seconds, and 45 seconds) are shown in Figure 2, together with the representative 2D scatter plots. Extension times at 15 seconds or longer produced accurate quantification, while extension times of 30 seconds or greater provided the best separation between positive and negative partition clusters.
Figure 2A. Concentration of multiplex assay targets in the FAM and HEX channels. Colored bars indicate the various extension time used for each condition. Error bars represent the standard deviation, and mean values are noted at the top of each bar.
Figure 2B. Two-dimensional dPCR scatter plot data from a single representative reaction per condition. Extension time used denoted at the top.
The industry standard “targeted minimum” number of analyzed dPCR partitions is typically 20,000. In addition to consistent quantification across repeated use of the same MAP16 plate, the average total number of partitions analyzed per unit remains well above the targeted minimum at 20,252 (±165) partitions per reaction. Figure 3 denotes the average number of accepted partitions and associated standard deviation for the entire plate used, as well as the average per run. Since each dPCR run for this assay requires 40 cycles of PCR, after the fourth run, the partitions in the last column have been exposed to thermal changes for an aggregate of 160 cycles. Even so, the MAP plate yields consistent numbers of acceptable partitions well above 20,000 per unit even in later runs (Figure 3).
Figure 3. Figure 3. Total partitions accepted for analysis by Combinati |Q| Analysis software for one MAP16 plate run 4 separate times to test the effect of extension time on dPCR assay performance. Results from all 4 runs are shown in the first column, and the results of individual runs are shown in subsequent columns. Each point represents the total partition yield from one dPCR unit.
Microfluidic Array Partitioning (MAP) technology enhances the dPCR. With a simple workflow and highly consistent performance, the MAP plate enables flexibility in experimental design and optimization of dPCR assay conditions without sacrificing robustness.