Large fragment Bst DNA polymerase for whole genome amplification of DNA from formalin-fixed paraffin-embedded tissues

With growing interest in the genomic characteristics of various human tumors and a steep increase in the availability of genomic tests for both clinical and research purposes, the amount of genomic DNA available from biological samples may limit the practicality of genomic analysis. Having been used for decades, formalin-fixed paraffin-embedded (FFPE) tissues comprise the most common form of human tissue samples archives. Therefore, it is desirable to establish a whole genome amplification (WGA) method specifically for DNA extracted from FFPE tissues. Two main approaches for WGA have been developed: thermocycling protocols and isothermal amplification methods.

Several thermocycling protocols have been used, including the degenerate oligonucleotide primed-polymerase chain reaction (DOP-PCR) [1–4], primer extension preamplification (PEP) [5–7], tagged-PCR (T-PCR) [8], and single cell comparative genomic hybridization (SCOMP), also known as linker adaptor-PCR [9, 10]. What common in all these protocols are the PCR principle of temperature-dependent cyclic amplification, and the use of primers with a random sequence to allow for multiple binding sites. They differ in primer design and the sequence of temperature changes. Their amplification magnitude is a few hundredfold and the size of their DNA product ranges from 200–3000 bases. Each technique has its advantages and limitations, varying from incomplete genomic coverage to preferences for certain DNA length (e.g. shorter alleles in DOP-PCR [4]), and inconsistency in the magnitude of amplification and elaborated protocol (SCOMP).

Isothermal amplification methods refer to Hyperbranched Strand Displacement Amplification (HSDA), which is also known as Multiple-strand Displacement Amplification (MDA) [11–13]. MDA is based on two principles [14–16]: (1) the ability of the polymerase to cause strand-displacement, and (2) random initiation points using random primers. The 5' end of each strand is displaced by another upstream strand that is growing in the same direction. Displaced single strands are targeted by new random priming events. As more DNA is generated by strand displacement, an increasing number of random priming events occur, forming a network of hyperbranched DNA structures of high molecular weight. As the reaction proceeds, thousands or even millions of copies of the original DNA are generated. Two enzymes are capable of catalyzing MDA: Φ29 DNA polymerase and the large fragment of Bacillus stearothermophilus (Bst DNA polymerase, large fragment). Previous work has shown that MDA using Bst DNA polymerase on intact DNA (e.g. DNA isolated from fresh or snap-frozen tissue) gives rise to robust amplification with a representational distortion of less than threefold [14–16]. We have investigated the feasibility of MDA on DNA from FFPE tissue using the Bst DNA polymerase, and evaluated the magnitude of representational distortion using quantitative realtime PCR (QPCR) and whole genome tiling array comparative genomic hybridization (CGH) representing complete coverage of the human genome.

The Bst DNA polymerase yielded a median 990-fold (range 613–1618) of DNA amplification (Figure 1). The reaction efficiency for commercial DNA and DNA extracted from snap-frozen samples was comparable and achieved a median amplification of 803-fold (range 613 – 1043), whereas the FFPE derived DNA was amplified slightly more, with a median amplification of 1035-fold (range 839 – 1618). The amplification of the negative control also generated DNA product, which was consistent in amount to the other samples. Amplification of DNA from the human pancreatic ductal epithelium (HPDE) cell line yielded 1422 ± 310- and 1560 ± 144-fold changes without and with DNA shearing, respectively. DNA replication products were of very high molecular weight as they were larger than 23 Kb (Figure 2).

QPCR analysis revealed similar findings in all samples, whether of normal tissue or tumoral nature, carrying known gene copy number abnormalities. All tested genes were found in all FFPE and Bst amplified samples, and their relative gene copy number was within 3-fold range of non-amplified samples (Figure 3 to 5). In normal lung samples, the expected copy number ratio of any given gene to GAPDH was 1. The values shown in Figure 3 are the average ratios of the five samples tested before and after Bst amplification. The average ratios of the non-amplified samples were close to 1 (range 1.08–1.26), while those following amplification were somewhat higher (range 1.20–2.00) but within 3-fold (Figure 3). For neuroblastoma xenografts where the N-myc gene is highly amplified, the representational distortion introduced by Bst amplification was negligible relative to the magnitude of gene amplification (Figure 4). For genes with low amplification levels such as the Skp2 gene in NSCLC (Skp2/PIK3R1 ≤ 6), an increase in gene copy was detected following Bst amplification with a bias of up to 3-fold (Figure 5). It should be noted that in two of the non-amplified samples (NSCLC no. 6 and 7), the Skp2/PIK3R1 ratio was lower than 3, and therefore within the bias range. Nevertheless, it was detectable after Bst amplification.

In the negative control, the amplification reaction produced substantial amounts of DNA. However, no genes were ever detected by QPCR, indicating that the measured product was the result of a spurious amplification of the primers.

Array CGH genome profiles of the Bst-amplified DNA from two NSCLC tumours were similar to profiles obtained using their respective non-amplified DNA (Figure 6A). Hybridization of Bst-amplified samples against Bst-amplified reference DNA allowed for the identification of genomic changes that were below 3-fold change. For each of the array CGH clones, the ratio of sample to reference signal defines the changes in gene content of a given tumour. This ratio should not change for each clone when an optimal WGA method is used. Figure 6B illustrates that the correlation of such ratios was near ideal (1:1) between the un-amplified and Bst-amplified DNA from NSCLC 8; similar finding was found in NSCLC 9. The four CGH arrays had variable quality; therefore a different number of human bacterial artificial chromosome (hBAC) clones was evaluated for each pair of arrays hybridized to non-amplified and amplified DNA (Table 2). Both NSCLC samples had normal gene content in more than half of the hBAC clones, and detected changes were of low gene-dosage. Analysis using the aCGH-Smooth software showed that 58.3–84.3% of the clones had a matching call (normal gene content/amplification/deletion) following Bst amplification. The percentage of matching clones correlated with the level of genomic changes: the higher the un-amplified sample/reference ratio was (especially > 3), the more likely it was to be detected correctly following Bst amplification. NSCLC 8, which had higher level of genomic gains compared to NSCLC 9 (as reflected by the highest ratio of un-amplified sample/reference of 2.9 vs. 2.1), also exhibited a higher percentage of hBAC clones with matching profiles (84.3% vs. 58.3% respectively). The level of disagreement between the paired arrays was expressed by the number of clones that changed after Bst amplification. Both NSCLC samples had more amplified than deleted clones. Likewise, more amplified than deleted clones were undetected after Bst amplification. However, based on the percentage of originally amplified/deleted clones, gene deletions were more prone to escape detection following Bst MDA (95.83–100%) compared to gene amplifications (67.22–92.31%).

In contrast to the successful Bst MDA of FFPE DNA, repeated attempts to amplify FFPE DNA using Φ29 DNA polymerase failed. Although the latter yielded 568 ± 342-fold DNA amplification and the reaction product was visible on a gel, QPCR of genes successfully validated on Bst MDA products consistently failed on Φ29 DNA polymerase WGA products. To rule out the inadequacy of FFPE DNA or of the Φ29 DNA polymerase reaction, we repeated the QPCR reactions on non-amplified FFPE DNA and DNA from frozen tissue before and after Φ29 DNA polymerase amplification and were able to detect all genes.

Lage and Dean et al [14, 15] reported that MDA demonstrates high-amplification potential and excellent loci representation with less than 3-fold bias. Our study showed for the first time that Bst MDA is feasible and reliable for WGA even on FFPE DNA. We have demonstrated that in three groups of FFPE samples (normal lung tissue, neuroblastoma xenografts and NSCLC), the relative content of different genes was maintained following the amplification. If a bias existed, it was limited to a 3-fold change. We deliberately monitored the content of genes that are located on separate, unrelated regions of the genome to provide a good estimation of the overall amplification process. Array CGH data further supports the adequacy of Bst MDA on FFPE DNA. Hybridization against Bst-amplified reference DNA allows detection of genomic changes that are even below 3-fold change.

In our study, median amplification ranged from 803- to 1035-fold, and was higher for FFPE samples than for intact DNA isolated from snap-frozen tissue or commercial DNA. This is more than the 250-fold reported previously [14]. The discrepancy might be attributed to the method used for quantitation of the template and products (PicoGreen DNA quantitation vs. NanoDrop spectrophotometry). The wide range in amplification yield may be due to variability in DNA quality and tissue fixation. A possible explanation for the higher yield in FFPE compared to intact DNA could be the preferential amplification of shorter DNA fragments by the Bst polymerase [17, 18]. However, the partial shearing of intact genomic DNA did not result in a significant change of the amplification yield. Yet, our results contradict previous mathematically-based predictions of a lower yield with sheared DNA [14].

Our QPCR analysis of gene copy content of FFPE samples following Bst amplification demonstrated up to a 3-fold change with respect to the non-amplified samples, fitting earlier reports of up to 3-fold representational bias [14, 15]. To compare the bias resulting from various WGA methods is challenging, as the reported characteristics of each method depend on the initial amount of DNA template used, as well as on the application under investigation. The representational bias can be implied from the reported range of efficiency rates for amplification of DNA sequences of several microsatellite loci. When performed in single cells, DOP-PCR efficiency rate ranged from 0–10% and was inferior to PEP-PCR and improved PEP-PCR (I-PEP-PCR) that ranged from 0–20% and 20–50%, respectively [6]. Dean et al [15] specifically compared the representational bias of three WGA methods and reported a 10³–10⁶ representational bias with DOP-PCR, 10²–10⁴ bias with PEP-PCR, and less than a 3-fold bias with MDA, which remained almost constant between 100- to 100,000-fold amplification.

It is known that MDA by either Φ29 or Bst DNA polymerases gives products even in the absence of DNA template. This is thought to result from spurious amplification of the primers. Lage et al [14] reported that background DNA synthesis was completely suppressed when modified primers with two 5'-nitroindole (universal base) residues were used. We were unable to eliminate primer amplification reactions despite the use of modified primers. However, this spurious primer amplification appears to be tolerable, since tested genes were consistently detected by QPCR in all the Bst-amplified samples but not in the amplified negative control.

The two enzymes used for MDA, Φ29 DNA polymerase and the large fragment of Bacillus stearothermophilus, have distinct qualities. Where as Bst DNA polymerase is devoid of the 3'→5' exonuclease, Φ29 DNA polymerase holds this proofreading activity. Therefore, Φ29 DNA polymerase has a lower error rate, more efficient amplification reactions and appears to be more suitable for additional sequencing studies. However, Bst DNA polymerase seemed to demonstrate greater fidelity for copy number than Φ29 DNA polymerase. One reason is the significantly reduced activity of Φ29 DNA polymerase at the telomeres. In contrast, Bst DNA polymerase can switch templates and is therefore less affected by the proximity of genes to the telomere [14]. Thus, Bst MDA may be a better choice for WGA in array-CGH studies. It was suggested that MDA (by either Φ29 or by large fragment Bst DNA polymerase) is appropriate for array-CGH from the point of view of both sequence fidelity and representation distortion with the following guidelines. First, amplification should be < 1000-fold to keep the representation distortion less than threefold [14–16]; second, only gene copy number changes that are minimally threefold can be reliably detected [14], and finally, array-CGH study design should include amplification of the sample of interest and reference genomic DNA under identical conditions to minimize biases [14]. Based on our results, we also recommend that these guidelines should be adopted whenever Bst MDA is followed by QPCR for gene copy number evaluation. Copy number changes detected following Bst amplification are reliable only if they are higher than the 3-fold representational distortion range. Thus, it is expected that high copy number changes (e.g. N-myc in neuroblastoma xenografts) will be easily detected, since even with 3-fold amplification bias, the change in copy number is conspicuous compared to normal gene content. Although low copy number changes (e.g. Skp2 in NSCLC) are also detectable, the difference in copy number relative to normal gene content may be attenuated with 3-fold representation distortion.

Our array CGH results using FFPE DNA are remarkably similar to results previously obtained on intact DNA. With 1000-cell experiments, Lage et al reported a concordance between the amplified and non-amplified DNA array CGH of 53.6–83.3% [14]. We found concordance of 58.3–84.3% when Bst amplification was applied to 10 ng of DNA starting material. Lage stated that altered loci with relatively high gene-dosage alterations were detected with high reproducibility. Likewise, we observed that gene amplifications were consistently detected, even when smaller than 3-fold, yet the detection sensitivity correlated with the level of genomic changes. We noted that gene deletions were prone to be missed following Bst amplification and similar findings can be seen in the data presented by Lage et al. Array properties, the quality of the amplified DNA, the length of deleted areas and the low level of gene content change in deletions, which is strongly affected by the amplification representation bias introduced by MDA, all contribute to the reduced detection ability of deletions following Bst amplification. Altogether, we found that Bst MDA on FFPE DNA is reliable for following genome profiling by array CGH, in particular for the detection of gene amplification.

Prior to 2003, there were very few studies on the use of MDA compared to the widely reported PCR-based WGA [19]. However, there has recently been a growing interest in the method, as reflected by the increasing number of papers published during the past two years. MDA was reported to succeed in a variety of applications, including sequencing [20, 21], microsatellite marker analysis [22, 23], SNP analysis [24, 25], genotyping [26] and array-CGH [14, 27]. All these studies used the Φ 29 DNA polymerase, and all but one [28] used DNA isolated from fresh or snap-frozen tissue samples. Like others [26], we have failed in our attempts to amplify FFPE samples with the Φ 29 polymerase. Although the reaction yielded DNA that was 500-fold greater than the initial amount, no genes could be detected by QPCR and results were consistent with a spurious amplification of the primers. On the other hand, Wang et al [28] reported a successful Φ 29 amplification of FFPE-derived DNA after the addition of a preliminary restriction enzyme fragmentation step. The modified protocol was named Restriction and Circularization Aided Rolling Circle Amplification (RCA-RCA). Our work is the first to describe the use of the large fragment of Bst DNA polymerase for WGA of FFPE DNA.

Aside from PCR-based techniques and MDA, a novel approach to WGA is the T7-based linear amplification of DNA (TLAD). TLAD appears free of sequence and length-dependent biases, and is thus applicable to FFPE DNA. However, this technique requires purification following each step and is therefore laborious and vulnerable to sample loss [19, 29].

We have shown that the large fragment of Bst DNA polymerase is suitable for WGA of DNA extracted from FFPE tissues, with an expected representational distortion of up to threefold. Amplified DNA may be used for the detection of gene copy number changes by QPCR and genome profiling by array CGH.

The expected application, magnitude of findings and the limits of the method are factors that need to be considered when WGA is chosen. The virtues of Bst DNA polymerase use for WGA should be emphasized: the method is efficient, technically easy, gives high yield and is suitable for FFPE DNA. We believe that in time, Bst DNA amplification will become part of routine molecular laboratory work for research and clinical purposes.