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Clinical Chemistry 54: 1908-1913, 2008. First published September 11, 2008; 10.1373/clinchem.2008.113126
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(Clinical Chemistry. 2008;54:1908-1913.)
© 2008 American Association for Clinical Chemistry, Inc.

Molecular Diagnostics and Genetics

TaqMan-Based Real-Time PCR for Genotyping Common Polymorphisms of Haptoglobin (HP1 and HP2)

Mikiko Soejima1 and Yoshiro Koda1,a

1 Department of Forensic Medicine and Human Genetics, Kurume University School of Medicine, Kurume, Japan

aAddress correspondence to this author at: Department of Forensic Medicine and Human Genetics, Kurume University School of Medicine, Kurume 830-0011, Japan. E-mail ykoda{at}med.kurume-u.ac.jp.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Background: The haptoglobin gene (HP) has 2 common codominant alleles (HP1 and HP2) that account for 3 phenotypes. HP2 is generated by a 1.7-kb intragenic duplication of HP1.

Methods: We used the real-time TaqMan PCR system to develop an effective method for HP genotyping that allows us to evaluate the relative number of copies of the HP2 allele–specific junctional region of the 1.7-kb gene duplication (HP2) by comparing the intensity of the amplification signals to those of the HP promoter region (HP5'), which was used as the internal control. The difference in threshold cycles ({Delta}Ct) between HP2 and HP5' was used to assess HP2 copy number. In addition, the assay detects the HP deletion (HPdel) at the same time.

Results: The mean 2–{Delta}{Delta}Ct values (the HP2/HP5' ratio) obtained from 123 samples of known HP genotypes clearly differentiated 2 nonoverlapping intervals that correspond to the HP genotypes. Ratios for HP2/HP1 samples ranged from 0.34–0.50, HP2/HP2 samples ranged from 0.79–0.98, and the absence of an HP2 allele signal was defined as HP1/HP1. We simultaneously detected HPdel. The assay produces results in <1 h.

Conclusions: The TaqMan-based real-time PCR method was successfully applied to HP genotyping. The method is easy to use in a molecular diagnosis laboratory, and its robustness and rapidity make it suitable for high-throughput analysis of large populations.


   Introduction
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 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Haptoglobin (HP)1 is a glycoprotein found in the plasma of all vertebrates. It binds hemoglobin to prevent both iron loss and kidney damage during hemolysis. Humans have a polymorphism in the gene encoding the protein because of 2 codominant HP (haptoglobin) alleles, HP1 and HP2, that give rise to the 3 common HP phenotypes, HP1, HP2-1, and HP2 (1)(2). The HP2 allele appears to have been generated by a 1.7-kb intragenic duplication of exons 3 and 4 of the HP1 allele. The frequencies of the HP1 and HP2 genotypes vary worldwide depending on the ethnic group, with the HP1 frequency varying from about 0.07 in parts of India to >0.7 in parts of West Africa and South America (3).

Because of its polymorphic nature, HP has been used as a genetic marker for identifying parentage and individuals, and several variant phenotypes have been reported, one of which, HP Johnson, occurs at low frequency in various populations (3). The molecular weight of the {alpha} chain is larger (23 kDa) than that encoded by HP2, and Southern blotting has shown 3 tandem repeats of the same 1.7-kb DNA segment found in the HP2 gene duplication (4). Another variant, HP2-1 modified (HP2-1M), which exhibits greater HP1 protein production than HP2 because of a mutation in one of the interleukin-6 response elements in the promoter of the HP2 allele, occurs at relatively high frequencies in African populations (5)(6)(7). In addition, HPdel, which lacks an approximately 28-kb segment of chromosome 16 that extends from the HP promoter region to exon 5 of the haptoglobin-related gene, has been identified (8)(9). This allele has been found only in East and Southeast Asian populations (9)(10)(11)(12)(13).

Both HP1 and HP2 alleles have been associated with susceptibility to diabetic, cardiovascular, inflammatory, and autoimmune diseases (2). Such associations may be explained by functional differences between the subtypes in the binding of hemoglobin and its rate of clearance from the plasma. Recent studies have identified profound differences between the antioxidant and immunomodulatory properties of the HP1 and HP2 polypeptides (14). There are also corresponding reports of negative associations with disease (2). Thus, development of a simple, high-throughput HP genotyping method is needed to facilitate these association studies.

Several methods for phenotyping HP1 and HP2 have been described. High-pressure gel-permeation chromatography and gel electrophoresis methods rely on differences in the molecular sizes of HP1, HP2-1, and HP2 for typing (14)(15). More recently, a single-chain antibody-based ELISA test was also established (16). In addition, several HP-genotyping methods based on conventional PCR have been developed; however, these PCR-based methods need to amplify a relatively large DNA fragment to detect the 1.7-kb duplicated regions, are time-consuming, and require laborious post-PCR processing steps (8)(17)(18). On the other hand, real-time PCR is a high-throughput, rapid, and sensitive method. It also eliminates post-PCR processing of PCR products, reducing the chances of carryover contamination. TaqMan assays (Applied Biosystems) use both amplification with gene-specific primers and fluorescence detection of target-specific probes. This method allows evaluation of RNA production or DNA genotyping of not only single-nucleotide polymorphisms but also gene dosage (19)(20)(21).

We recently developed a TaqMan-based real-time PCR method that allows differentiation of HP and HPdel alleles by detecting the junctional region of the gene deletion and the promoter region of a gene that is deleted in HPdel (22). To detect the junctional region of the 1.7-kb intragenic duplication of HP2, we added the HP2 probe and primers in the TaqMan-based real-time PCR method mentioned above. This new method allows differentiation of HP1 and HP2 alleles by evaluation of the relative numbers of copies at the junctional region of the 1.7-kb intragenic duplication in HP2 via comparison with the intensity of the amplification signals from an internal control (the promoter region of this gene).


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
The ethics committee of Kurume University School of Medicine approved this study. We used genomic DNA from 123 randomly selected Ghanaians from Accra and 7 Japanese individuals whose HP phenotypes and genotypes had previously been examined (10).

We performed real-time PCR to detect 3 regions: (a) the region encompassing the breakpoint of the partial gene duplication in intron 4 of the HP2 allele (HP2), (b) the 5' region of HP exon 1 as an internal control to compare the intensity of the amplification signals (HP5'), and (c) the region encompassing the HPdel breakpoint (HPdel) to detect the HPdel allele at the same time. The 20-µL PCR reaction contained 6.1 pg to 100 ng genomic DNA, 10 µL of Premix Ex TaqTM (Perfect Real Time) (Takara Bio), and the following primers and TaqMan probes at the indicated concentrations: HP2-F and -R primers (300 nmol/L), HP2 probe (83 nmol/L) for the detection of HP2, HP5'-F and -R primers (150 nmol/L), HP5' probe (42 nmol/L) for the detection of the HP 5' region, HPdel-F and -R primers (300 nmol/L), and HPdel probe (83 nmol/L) for the detection of HPdel. The PCR temperature profile was 95 °C for 30 s followed by 40 cycles of denaturation at 95 °C for 5 s and annealing and extension at 60 °C for 30 s. The positions and sequences of the HP2-F and -R primers and the HP2 probe are as follows (GenBank no. M10935): HP2-F (5'-GGAGCTGCTCTGCACATCAA-3', the reverse of the sequence of base pairs 4547–4566); HP2-R (5'-CCCTTTCAATGAATTTCAGGGA-3', sequence from base pairs 4448–4469); and HP2 probe (5'-CAL Fluor Orange 560-ACCCCGAATAGAAGCTCGCGAACTGTA-BHQ1-3', the reverse of the sequence of base pairs 4511–4537). The positions and sequences of the HP5'-F and -R primers, the HP5' probe, the HPdel-F and -R primers, and the HPdel probe have previously been described (22). All oligonucleotides were synthesized by Biosearch Technologies. The relative positions of the HP2 and HP5' probes in the HP1 and HP2 alleles are shown in Fig. 1 . To increase the fluorescence signals, we carried out the PCR and signal detection in Sorenson ultraAmp PCR semiskirted 96-well white plates (Nippon Genetics). To monitor the progress of amplification, we measured the fluorescence at the end of each cycle with an Mx3000P system (Stratagene) equipped with excitation/emission filters of 492/516 nm (FAM), 535/555 nm (CAL Fluor Orange 560), and 585/610 nm (CAL Fluor Red 610). Data were collected and analyzed with MxProTM software (version 4.00; Stratagene).


Figure 1
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  		Figure 1. Structures of the HP1 and HP2 alleles and relative positions of the HP2 and HP5' probes.

The change in threshold cycle ({Delta}Ct) of each sample was calculated as the Ct value of HP5' (control) minus the Ct value of HP2 (target). The {Delta}{Delta}Ct of each sample was obtained by subtracting the {Delta}Ct value of the sample from the {Delta}Ct value of the reference. The {Delta}Ct reference value was obtained with 5 ng genomic DNA from an HP2/HP2 individual. The HP2/HP5' ratio of each sample was calculated as 2–{Delta}{Delta}Ct sample.


   Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
triplex pcr for hp2, hp5', and hpDEL
The region chosen as the target sequence is located in the intronic sequence 5' of the junction region of the gene duplication of the HP2 allele (intron 4; Fig. 1Up ). The HapMap and NCBI databases have not reported any single-nucleotide polymorphisms within this region. Although we could have used any of several genomic regions as an internal control for relative quantification of the HP2-specific copy number, we selected the HP 5' region as an internal control for 2 reasons: (a) We previously used the HP 5' region to distinguish the HPdel allele (22), and (b) we might miscategorize individuals with HPdel, particularly HP2/HPdel individuals as HP2/HP1, if we selected another region as the internal control. Thus, we used triplex PCR to detect the relative numbers of copies of the HP2 allele and the HPdel allele by adding the HP2-F primer, HP2-R primer, and HP2 probe in the previously described TaqMan-based method, although the duplex PCR (using the HP5' and HP2 probes) seems to be adequate for HP genotyping in many populations other than those of East and Southeast Asia. In this assay system, we observed amplification of the HP 5' region (CAL Fluor Red 610 signal) without amplification of the HP2 allele–specific region (CAL Fluor Orange 560 signal) or the HPdel allele–specific region (FAM signal) in the HP1/HP1 DNA sample (Fig. 2A ), whereas we observed amplification of the HP2 allele–specific region and the HP 5' region without amplification of the HPdel allele–specific region in the HP2/HP1 and HP2/HP2 DNA samples (Fig. 2 , B and C). We observed amplification of the HP 5' region and the HPdel allele–specific region without amplification of the HP2 allele–specific region in the HP1/HPdel DNA sample (Fig. 2D ) and obtained all 3 signals in the HP2/HPdel DNA sample (Fig. 2E ). Only the FAM signal was detected in the HPdel/HPdel DNA samples (Fig. 2F ).


Figure 2
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  		Figure 2. Real-time amplification plots for HP2, HP5', and HPdel with genomic DNA targets of various genotypes: HP1/HP1 (A), HP2/HP1 (B), HP2/HP2 (C), HP1/HPdel (D), HP2/HPdel (E), and HPdel/HPdel (F).

Red, blue, and green curves represent HP2, HP5', and HPdel, respectively. dR, baseline-corrected raw fluorescence.

pcr efficiencies for hp2 and hp5'
To determine the common HP genotypes, we then performed quantitative real-time PCR assays with TaqMan probes to evaluate the relative numbers of copies of the HP2-specific sequence. We evaluated the effect of DNA quantity on PCR efficiency in a dilution series of genomic DNA (100, 25, 6.25, 1.56, 0.39, 0.098, 0.024, and 0.0061 ng/reaction) from HP2/HP1 or HP2/HP2 individuals. The calculated efficiency rates of the PCR for the HP2-specific sequence were 108.9% and 102.3% for templates from HP2/HP1 and HP2/HP2 individuals, respectively, and those for HP5' were 107.0% and 102.2% for templates from HP2/HP1 and HP2/HP2 individuals, respectively (Fig. 3 ). Because the efficiencies for both regions were very similar, the estimates of relative copy number obtained with these 2 Ct values were considered valid. In addition, the PCR reaction required <1 h. Thus, this method permits evaluation for both the common HP genotypes and the HPdel allele. This triplex PCR system is particularly suitable for East and South Asian populations, where the HPdel is observed.


Figure 3
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  		Figure 3. HP2 and HP5' calibration curves calculated from Ct data for 6.1 pg to 100 ng DNA template from HP2/HP1 and HP2/HP2 individuals.

Red and blue lines represent calculations for means of 6 data points for HP5' (CAL Fluor Red 610) and HP2 (CAL Fluor Orange 560), respectively. Eff, efficiency; dR, baseline-corrected raw fluorescence.

validation of the assay
To determine the quantity of genomic DNA necessary to obtain correct results, we calculated the results obtained with serially diluted samples of genomic DNA from HP2/HP2 or HP2/HP1 individuals as PCR templates. Table 1 in the Data Supplement that accompanies the online version of this article at http://www.clinchem.org/content/vol54/issue11 presents mean (SD) Ct values for 0.0061–100 ng (2–33 000 copies of the control region) of genomic DNA from HP2/HP2 and HP2/HP1 samples (n = 6). The ranges of {Delta}Ct values (the HP5' Ct value minus the HP2 Ct value) were –0.79 to –0.91 for HP2/HP2 and –1.85 to –2.07 for HP2/HP1. As expected, the variances of Ct values are higher with lower amounts of genomic DNA (0.0061 and 0.024 ng, i.e., 2–8 copies); however, mean {Delta}Ct values did not differ appreciably at genomic DNA amounts >0.0061 ng. We observed no significant differences in Ct values between samples of the same genotype at genomic DNA amounts between 0.1–100 ng. Thus, the quantity of template DNA was not critical in this method. To obtain reference {Delta}Ct values, we performed real-time PCR with 5 ng genomic DNA from an HP2/HP2 individual and obtained reference {Delta}Ct values of –0.73 to –0.86 (data not shown).


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  		Table 1. HP genotyping of 123 Ghanaian samples with a TaqMan assay.

We then examined 123 DNA samples from Ghanaians whose HP phenotypes and genotypes had previously been determined (10). HP5' Ct values ranged from 26.6–29.3, and we obtained no HP2 Ct values for the 35 HP1/HP1 individuals (n = 105). This absence of an HP2 signal was defined as indicating an HP1/HP1 genotype. Mean (SD) {Delta}Ct values for 57 HP2/HP1 individuals were –1.99 (0.11) (range, –1.64 to –2.52; n = 171), and those for 31 HP2/HP2 individuals were –1.00 (0.11) (range, –0.62 to –1.29; n = 93) (data not shown). We then calculated the HP2/HP5' ratio with the {Delta}{Delta}Ct value of each sample. Table 1Up shows the average of 6 independent assays (3 independent assays of each sample). In each assay, the quantitative results obtained with the present real-time PCR assay were concordant with those obtained with previous phenotype and genotype assays. We observed no overlap between the ranges of values that corresponded to the HP2/HP2 and HP2/HP1 genotypes. The mean (SD) HP2/HP5' ratios were 0.44 (0.036) for 57 HP2/HP1 individuals and 0.88 (0.071) for 31 HP2/HP2 individuals.

Although we have examined only a single individual for each of the HPdel genotypes (i.e., HP1/HPdel, HP2/HPdel, and HPdel/HPdel), the results of 12 replicate assays were identical, suggesting that our application of the present method to genotyping of the common HP alleles and to detecting HPdel was successful.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
We have developed an effective method for genotyping common HP polymorphisms with the real-time TaqMan PCR system; however, the main limitation of this method, compared with Southern blotting, phenotyping, and conventional PCR methods, is misidentification of rare variants such as HP Johnson and HP2-1M (4). The frequency of the HP Johnson allele is very low in many populations (3), whereas the incidence of HP2-1M can be as high as 10% (in Africans), which is not negligible (6)(7). In fact, we have 15 samples from individuals with the HP2-1M phenotype but without HP Johnson in the sample of 123 Ghanaians. Because the present method calculated only the copy number of the HP2-specific sequence relative to that of the internal control sequence, we identified HP2-1M as HP2/HP1 in the present real time PCR method. In addition, we may score an HP Johnson allele as HP2/HP2. Overcoming this miscategorization seems to be difficult with the present assay system. On the other hand, if instead of the HPdel probe we were to use a probe in the assay system that includes the substitution of the causal base at the promoter that is responsible for the HP2-1M phenotype (A-61C), we would be able to identify HP2-1M in African samples. Thus, the assay design can be customized in accordance with the minor alleles present in a particular population. Our method may be useful for rapid and high-throughput genotyping of common HP alleles for association studies.


   Acknowledgments
 
Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.

Authors’ Disclosures of Potential Conflicts of Interest: Upon manuscript submission, all authors completed the Disclosures of Potential Conflict of Interest form. Potential conflicts of interest:

Employment or Leadership: None declared.

Consultant or Advisory Role: None declared.

Stock Ownership: None declared.

Honoraria: None declared.

Research Funding: This work was supported by grants-in-aid for Scientific Research from the Ministry of Health, Labour and Welfare of Japan.

Expert Testimony: None declared.

Role of Sponsor: The funding organizations played no role in the design of study, choice of enrolled patients, review and interpretation of data, or preparation or approval of manuscript.

Acknowledgments: We thank Katherine Ono for the English editing of this manuscript.


   Footnotes
 
1 Nonstandard abbreviations: HP, haptoglobin; HP2–1M, HP2–1 modified; Ct, threshold cycle; HP2, region encompassing the breakpoint of the partial gene duplication in intron 4 of the HP2 allele; HP5', 5' region of HP exon 1; HPdel, region encompassing the HPdel breakpoint.


   References
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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